METHOD AND SYSTEM FOR TRACKING NON-COOPERATIVE OBJECTS USING SECONDARY SURVEILLANCE RADAR

PROCEDE ET SYSTEME POUR SURVEILLER LES OBJETS NON COOPERATIFS A L`AIDE D`UN RADAR SECONDAIRE DE SURVEILLANCE

English Abstract

A method and system for determining a position of a non-cooperative object
using a
reflected signal from Secondary Surveillance Radar (SSR) system is disclosed.
The
method enables the ownship to detect a non-cooperative intruder, an aircraft
which does
not have a functioning transponder. The position and altitude of the non-
cooperative
intruder are determined from reflected signals by a phased array antenna or
mechanically
scanned directional antenna (MSDA) to perform tracking and avoidance of the
non-cooperative object. In the case that a phased array antenna or MSDA is not
available, a
co-altitude assumption is applied to conservatively determine an avoidance
area around
the non-cooperative object, which defines coordinates to be tracked and
avoided by the
ownship.

Claims

CLAIMS:
1. A method for tracking and avoiding a non-cooperative object by an ownship,
comprising:
employing at least one hardware processor for:
detecting a reflected interrogation signal from the non-cooperative object,
the
reflected interrogation signal being an interrogation signal sent from a
secondary
surveillance radar and reflected off the non-cooperative object;
processing the reflected interrogation signal, yielding a processed reflected
interrogation signal; and
determining a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing to track and avoid the non-cooperative
object.
2. The method of claim 1, wherein the processing comprises:
(i) determining a range of durations for time windows, during which the
reflected
interrogation signal may arrive at the ownship;
(ii) integrating the reflected interrogation signal across the time windows
determined in the step (i); and
(iii) identifying and classifying peaks in the integrated reflected
interrogation signal
integrated in the step (ii).
3. The method of claim 2, wherein the integrating the reflected interrogation
signal across
the time windows further comprises:
determining a plurality of sequences of time windows, within which respective
reflected interrogation signals arrive at the ownship, each time window Wi in
a sequence
Wi' having a same duration and a same time delay from a respective start point
for said
each time window; and

for each sequence Wi', processing corresponding samples of the reflected
interrogation signal.
4. The method of claim 2, wherein the determining a range of durations for
time windows
comprises choosing durations to cover a predetermined monitoring distance.
5. The method of claim 2, wherein the integrating the reflected interrogation
signal across
the time windows further comprises determining a number of time windows to be
integrated, based on at least one of the following:
the non-cooperative object being considered stationary for said number of time
windows to be integrated;
an analog-to-digital (ADC) sampling rate;
an expected speed of the non-cooperative object.
6. The method of claim 2, wherein the identifying and classifying peaks
comprises
comparing the reflected interrogation signal and the integrated reflected
interrogation
signal with an interrogation pattern of P1, P2 and P3 pulses generated by the
secondary
surveillance radar.
7. The method of claim 1, wherein the determining comprises:
calculating a range of possible positions of the non-cooperative object from
the
processed reflected interrogation signal;
scanning the range of possible positions of the non-cooperative object; and
detecting the position of the non-cooperative object, based on results of the
scanning.
66

8. The method of claim 7, wherein the calculating the range of possible
positions of the
non-cooperative object comprises calculating a spheroid, wherein the secondary
surveillance radar is at a first focal point of the spheroid, and the ownship
is at a second
focal point of the spheroid, and the non-cooperative object is on the
spheroid.
9. The method of claim 7, wherein the scanning the range of possible positions
comprises:
changing a scan angle along the range of possible positions of the non-
cooperative object;
detecting a strongest signal strength along the range of possible positions of
the
non-cooperative object;
determining a strongest scan angle, corresponding to the strongest signal
strength; and
calculating an altitude of the non-cooperative object from the strongest scan
angle.
10. The method of claim 1, wherein the determining comprises:
applying a co-altitude assumption between the non-cooperative object and the
ownship;
determining an avoidance area around the non-cooperative object, by using the
processed reflected interrogation signal and the co-altitude assumption; and
assuming the position of the non-cooperative object is within the avoidance
area.
11. A system for tracking and avoiding a non-cooperative object by an ownship,
comprising:
67

a memory device for storing computer readable instructions thereon for
execution
by at least one processor, causing the at least one processor to:
detect a reflected interrogation signal from the non-cooperative object, the
reflected interrogation signal being an interrogation signal sent from a
secondary
surveillance radar and reflected off the non-cooperative object;
process the reflected interrogation signal, yielding a processed reflected
interrogation signal; and
determine a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing to track and avoid the non-cooperative
object.
12. The system of claim 11, wherein the computer readable instructions,
causing to
process, further cause the at least one processor to:
(i) determine a range of durations for time windows, during which the
reflected
interrogation signal may arrive at the ownship;
(ii) integrate the reflected interrogation signal across the determined time
windows;
and
(iii) identify and classify peaks in the integrated reflected interrogation
signal.
13. The system of claim 12, wherein the computer readable instructions,
causing to
integrate the reflected interrogation signal, further cause the at least one
processor to:
determine a plurality of sequences of time windows, within which respective
reflected interrogation signals arrive at the ownship, each time window Wi in
a sequence
having a same duration and a same time delay from a respective start point for
said
each time window; and
for each sequence Wi', process corresponding samples of the reflected
interrogation signal.
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14. The system of claim 13, wherein the computer readable instructions,
causing to
process corresponding samples, further cause the at least one processor to
perform one
of the following:
process the corresponding samples coherently;
process the corresponding samples non-coherently.
15. The system of claim 12, wherein the computer readable instructions,
causing to
integrate the reflected interrogation signal across the time windows, further
cause the at
least one processor to determine a number of time windows to be integrated,
based on at
least one of the following:
an analog-to-digital (ADC) sampling rate;
an expected speed of the non-cooperative object.
16. The system of claim 12, wherein the computer readable instructions,
causing to
identify and classify peaks, further cause the at least one processor to
compare the
integrated reflected interrogation signal with an interrogation pattern of P1,
P2 and P3
pulses generated by the secondary surveillance radar.
17. The system of claim 11, wherein the computer readable instructions,
causing to
determine, further cause the at least one processor to:
calculate a range of possible positions of the non-cooperative object from the
processed reflected interrogation signal;
scan the range of possible positions of the non-cooperative object; and
detect the position of the non-cooperative object, based on results of the
scanning.
69

18. The system of claim 17, wherein the computer readable instructions,
causing to scan
the range of possible positions, further cause the at least one processor to:
change a scan angle along the range of possible positions of the non-
cooperative
object;
detect a strongest signal strength along the range of possible positions of
the non-
cooperative object;
determine a strongest scan angle, corresponding to the strongest signal
strength;
and
calculate an altitude of the non-cooperative object from the strongest scan
angle.
19. The system of claim 11, wherein the computer readable instructions,
causing to
determine, further cause the at least one processor to:
apply a co-altitude assumption between the non-cooperative object and the
ownship;
determine an avoidance area around the non-cooperative object, by using the
processed reflected interrogation signal and the co-altitude assumption; and
assume the position of the non-cooperative object is within the avoidance
area.
20. An apparatus for tracking and avoiding a non-cooperative object,
comprising:
a memory device for storing computer readable instructions thereon for
execution
by at least one processor, causing the at least one processor to:
process a reflected interrogation signal, the reflected interrogation signal
being an
interrogation signal sent from a secondary surveillance radar and reflected
off the non-
cooperative object, yielding a processed reflected interrogation signal; and
determine a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing to track and avoid the non-cooperative
object.

CLAIMS:
1. A method for tracking and avoiding a non-cooperative object by an ownship,
comprising:
employing at least one hardware processor for:
detecting a reflected interrogation signal from the non-cooperative object,
the
reflected interrogation signal being an interrogation signal sent from a
secondary
surveillance radar and reflected off the non-cooperative object;
processing the reflected interrogation signal, yielding a processed reflected
interrogation signal; and
determining a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing to track and avoid the non-cooperative
object.
2. The method of claim 1, wherein the processing comprises:
(i) determining a range of durations for time windows, during which the
reflected
interrogation signal may arrive at the ownship;
(ii) integrating the reflected interrogation signal across the time windows
determined in the step (i); and
(iii) identifying and classifying peaks in the integrated reflected
interrogation signal
integrated in the step (ii).
3. The method of claim 2, wherein the integrating the reflected interrogation
signal across
the time windows further comprises:
determining a plurality of sequences of time windows, within which respective
reflected interrogation signals arrive at the ownship, each time window Wi in
a sequence
Wi' having a same duration and a same time delay from a respective start point
for said
each time window; and

for each sequence Wi', processing corresponding samples of the reflected
interrogation signal.
4. The method of claim 2, wherein the determining a range of durations for
time windows
comprises choosing durations to cover a predetermined monitoring distance.
5. The method of claim 2, wherein the integrating the reflected interrogation
signal across
the time windows further comprises determining a number of time windows to be
integrated, based on at least one of the following:
the non-cooperative object being considered stationary for said number of time
windows to be integrated;
an analog-to-digital (ADC) sampling rate;
an expected speed of the non-cooperative object.
6. The method of claim 2, wherein the identifying and classifying peaks
comprises
comparing the reflected interrogation signal and the integrated reflected
interrogation
signal with an interrogation pattern of P1, P2 and P3 pulses generated by the
secondary
surveillance radar.
7. The method of claim 1, wherein the determining comprises:
calculating a range of possible positions of the non-cooperative object from
the
processed reflected interrogation signal;
scanning the range of possible positions of the non-cooperative object; and
detecting the position of the non-cooperative object, based on results of the
scanning.
66

8. The method of claim 7, wherein the calculating the range of possible
positions of the
non-cooperative object comprises calculating a spheroid, wherein the secondary
surveillance radar is at a first focal point of the spheroid, and the ownship
is at a second
focal point of the spheroid, and the non-cooperative object is on the
spheroid.
9. The method of claim 7, wherein the scanning the range of possible positions
comprises:
changing a scan angle along the range of possible positions of the non-
cooperative object;
detecting a strongest signal strength along the range of possible positions of
the
non-cooperative object;
determining a strongest scan angle, corresponding to the strongest signal
strength; and
calculating an altitude of the non-cooperative object from the strongest scan
angle.
10. The method of claim 1, wherein the determining comprises:
applying a co-altitude assumption between the non-cooperative object and the
ownship;
determining an avoidance area around the non-cooperative object, by using the
processed reflected interrogation signal and the co-altitude assumption; and
assuming the position of the non-cooperative object is within the avoidance
area.
11. A system for tracking and avoiding a non-cooperative object by an ownship,
comprising:
67

a memory device for storing computer readable instructions thereon for
execution
by at least one processor, causing the at least one processor to:
detect a reflected interrogation signal from the non-cooperative object, the
reflected interrogation signal being an interrogation signal sent from a
secondary
surveillance radar and reflected off the non-cooperative object;
process the reflected interrogation signal, yielding a processed reflected
interrogation signal; and
determine a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing to track and avoid the non-cooperative
object.
12. The system of claim 11, wherein the computer readable instructions,
causing to
process, further cause the at least one processor to:
(i) determine a range of durations for time windows, during which the
reflected
interrogation signal may arrive at the ownship;
(ii) integrate the reflected interrogation signal across the determined time
windows;
and
(iii) identify and classify peaks in the integrated reflected interrogation
signal.
13. The system of claim 12, wherein the computer readable instructions,
causing to
integrate the reflected interrogation signal, further cause the at least one
processor to:
determine a plurality of sequences of time windows, within which respective
reflected interrogation signals arrive at the ownship, each time window Wi in
a sequence
Wi' having a same duration and a same time delay from a respective start point
for said
each time window; and
for each sequence Wi', process corresponding samples of the reflected
interrogation signal.
68

14. The system of claim 13, wherein the computer readable instructions,
causing to
process corresponding samples, further cause the at least one processor to
perform one
of the following:
process the corresponding samples coherently;
process the corresponding samples non-coherently.
15. The system of claim 12, wherein the computer readable instructions,
causing to
integrate the reflected interrogation signal across the time windows, further
cause the at
least one processor to determine a number of time windows to be integrated,
based on at
least one of the following:
an analog-to-digital (ADC) sampling rate;
an expected speed of the non-cooperative object.
16. The system of claim 12, wherein the computer readable instructions,
causing to
identify and classify peaks, further cause the at least one processor to
compare the
integrated reflected interrogation signal with an interrogation pattern of P1,
P2 and P3
pulses generated by the secondary surveillance radar.
17. The system of claim 11, wherein the computer readable instructions,
causing to
determine, further cause the at least one processor to:
calculate a range of possible positions of the non-cooperative object from the
processed reflected interrogation signal;
scan the range of possible positions of the non-cooperative object; and
detect the position of the non-cooperative object, based on results of the
scanning.
69

18. The system of claim 17, wherein the computer readable instructions,
causing to scan
the range of possible positions, further cause the at least one processor to:
change a scan angle along the range of possible positions of the non-
cooperative
object;
detect a strongest signal strength along the range of possible positions of
the non-
cooperative object;
determine a strongest scan angle, corresponding to the strongest signal
strength;
and
calculate an altitude of the non-cooperative object from the strongest scan
angle.
19. The system of claim 11, wherein the computer readable instructions,
causing to
determine, further cause the at least one processor to:
apply a co-altitude assumption between the non-cooperative object and the
ownship;
determine an avoidance area around the non-cooperative object, by using the
processed reflected interrogation signal and the co-altitude assumption; and
assume the position of the non-cooperative object is within the avoidance
area.
20. An apparatus for tracking and avoiding a non-cooperative object,
comprising:
a memory device for storing computer readable instructions thereon for
execution
by at least one processor, causing the at least one processor to:
process a reflected interrogation signal, the reflected interrogation signal
being an
interrogation signal sent from a secondary surveillance radar and reflected
off the non-
cooperative object, yielding a processed reflected interrogation signal; and
determine a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing to track and avoid the non-cooperative
object.

Description

METHOD AND SYSTEM FOR TRACKING NON-COOPERATIVE OBJECTS USING
SECONDARY SURVEILLANCE RADAR
RELATED APPLICATIONS
The present application is a continuation-in-part of the US patent
application, serial
number 16/276,053 filed on February 14, 2019, which claims benefit from the US
provisional application 62/630,362 filed on February 14, 2018;
the present application also claims benefit from the US provisional
application serial
number 62/885,923 filed on August 13, 2019;
the entire contents of the above noted applications have been incorporated
herein by
reference.
FIELD OF THE INVENTION
The present invention relates to tracking aerial, nautical or ground objects,
and in
particular to tracking and avoiding non-cooperative objects in aviation
systems, which do
not have a transponder, by using a secondary surveillance radar (SSR).
BACKGROUND OF THE INVENTION
Secondary Surveillance Radar (SSR) systems have been used around the world in
air traffic control applications to track positions of an aircraft in the sky
and inform pilots in
other aircraft accordingly. Precision and efficiency of such tracking systems
is particularly
crucial at and near the airports where a higher density of flying objects
(small or large
planes, helicopters, etc) are present. As such, the SSR systems can be
supplemented with
other auxiliary systems. Such an auxiliary system is a Passive Secondary
Surveillance
Radar (PSSR) system that operates as a slave system to the conventional master
SSR
system.
According to the aviation standards, such as the "Minimum Operational
Performance Standards (MOPS) for Air Traffic Control Radar Beacon System
(ATCRBS)
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Airborne Equipment" from the Radio Technical Commission for Aeronautics (RTCA,
Inc.),
an air traffic control system comprises the SSR having a main rotating antenna
transmitting narrow interrogation which are assisted with an omni-directional
antenna
transmitting a related signal. The air traffic control relies on transponders
located in an
aircraft to reply to the interrogation beams to signal their identity as well
as their altitude.
The transponder reply signal is broadcast at another standard frequency (1090
MHz).
Every interrogation message is composed by three pulses, P1, P2 and P3 at a
given
standard frequency (1030 MHz). P1 and P3 pulses are strong when the aircraft
is in the
main antenna beam (main lobe width of 2-3 degrees). Outside of the main lobe,
P1 and
P3 are weaker, and may even be lower than the P2 pulse. This means that a
target object,
for example a target object aircraft, can only receive valid interrogation,
and then responds
when it is in the main lobe of the main antenna beam. P2 pulse also referred
to as Side
Lobe Suppression (SLS) signal is always synchronized with P1 pulse and
transmitted by
the omni-directional antenna (hence referred to as omni signal) exactly 2 ps
after P1
pulse. P3 pulse is used to determine if the current message is a mode A or
mode C
interrogation by delaying with different time intervals (8 us or 21 us) from
P1 pulse. In the
transponder, that aircraft are obliged to have, if the received P2 is weaker
than P1 by 9
dB, the interrogation is responded; or else the interrogation is ignored. The
delay between
a reception of the interrogation pulse and the transmission of the reply or
response is
exactly 3 ps for any transponder. Also, the interrogation time interval is
large enough that a
response to an interrogation will surely be received before the next
interrogation is sent.
The prior art discloses a Passive Secondary Surveillance Radar (PSSR) system
that operates as a slave system to a conventional master SSR system. The PSSR
system,
which comprises an omni-directional antenna, is placed on the ground or on an
aircraft
with known locations relative to the SSR. The SSR interrogation signals are
received at
the PSSR station as well as at a target aircraft. The transponder reply signal
is also
received by the PSSR station. The PSSR uses the received P1-P3 pulses or P2
pulses to
derive the interrogation time of the SSR, and to further calculate the sum of
distances from
the aircraft to the SSR and from the aircraft to the PSSR by measuring the
time it takes to
receive a signal send to the aircraft plus the reply signal.
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The SSR antenna and system have evolved for decades including hardware
modifications to omni-directional antennas and various interrogation patterns,
including
staggered interrogation pattern.
To avoid ambiguity or interference in crowded air space, the SSR normally
staggers the time intervals between successive interrogations in a fixed
pattern. We call
this staggered pattern or pulse repetition frequency (PRF) pattern in this
invention. For
different SSR manufactures and configurations, the staggered pattern may be
different.
Therefore it is important to profile the staggered PRF by using omni-
directional
signal or main beam signal from SSR.
Also an accuracy of the time measurement is important for PSSR applications.
Because a signal travels with a speed of light, so a small amount of error in
time could
result in a large distance error. This is extremely dangerous in a crowed air
space. In this
case, even a GPS based time measurement is not precise or reliable enough for
collision
avoidance if not been properly implemented.
When the target object is not equipped with a transponder which replies to an
SSR
interrogation (non-cooperative target), there should be a method for detecting
its existence
and giving an estimate of its position.
For detecting a non-cooperative target, a primary surveillance radar (PSR) is
normally used. However, it is now fading out of the air traffic control (ATC)
applications
because it provides less information and is less reliable than SSR. The SSR
system also
has longer detection range with less transmitted power because only one-way
propagation
of the microwave signal is needed.
The problem for SSR is that it does not detect non-cooperative targets. It is
a
device meant to transmit 1030 MHz interrogation signal and receive 1090 MHz
reply
signal from the transponder.
Therefore, there is a need in the industry for the development of an improved
method of reusing the function of the SSR to detect a reflected interrogation
signal to
locate an intruder aircraft when it does not have a transponder.
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SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method to detect a
transponder-
equipped aircraft or an aircraft without a transponder accurately and
constantly. In
particular, there is an object of the present invention to provide a method
and system for
detecting target objects for omni-directional antennas that transmit P2 SLS
pulses not
within 360 degrees in azimuth, but cover only limited angle coverage, for
example about
80 degree (or any other limited angle) at the front side and the back side of
the main SSR
antenna, and therefore when P2 pulses are not always available for an observer
during
the rotation of the SSR antenna.
Thus, it is another object of the present invention to provide a method and
system
for detecting target objects when the prior art does not work, for example
when the
ownship is out of the coverage of the SSR main beam and the SLS beam at the
time it
receives a transponder reply from the target object.
According to one aspect of the invention, there is provided a method for
determining a pulse repetition frequency (PRF) pattern for a staggered
interrogation signal
of a Secondary Surveillance Radar (SSR), the method comprising: at a Passive
Secondary Surveillance Radar (PSSR) spaced apart from the SSR: (a) receiving
side lobe
suppression pulses P2 of the staggered interrogation signal comprising pulses
(P1, P2,
P3), the pulses P1, P3 generated by a main narrow-beam antenna of the SSR, and
the
pulse P2 generated by a wide-beam antenna of the SSR, the wide-beam antenna
having
an angular aperture, the pulse P2 synchronized with the pulse P1 and P3 with a
predefined time delay; and provided the PSSR is within the angular aperture of
the wide-
beam antenna: i) receiving a first and second successive P2 pulses, each
having a
respective pulse reception time, determining a first time interval between the
first and
second successive P2 pulses, and storing the first time interval as a time-
ordered
sequence of time intervals; ii) receiving a new P2 pulse and determining a new
time
interval between said new P2 pulse and a last received P2 pulse; iii) provided
said new
time interval does not match the first time interval, adding said new time
interval to the
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time-ordered sequence and repeating the steps (ii) to (iii); and iv) provided
said new time
interval matches the first time interval, and the time ordered sequence starts
repeating
itself from the first time interval and the new time interval, determining the
PRF pattern for
the staggered interrogation sequence of pulses based on those time intervals
that are
between the first time interval and the new time interval.
According to another aspect of the invention there is provided an apparatus
for
determining a pulse repetition frequency (PRF) pattern for a staggered
interrogation signal
of a Secondary Surveillance Radar (SSR)comprising: a memory device in a
Passive
Secondary Surveillance Radar (PSSR) spaced apart from the SSR having a
computer
executable instructions stored thereon, causing a processor to: (a) receive
side lobe
suppression pulses P2 of the staggered interrogation signal comprising pulses
(P1, P2,
P3), the pulses P1, P3 generated by a main narrow-beam antenna of the SSR, and
the
pulse P2 generated by a wide-beam antenna of the SSR, the wide-beam antenna
having
an angular coverage, the pulse P2 synchronized with the pulse P3 with a
predefined time
delay; i) provided the PSSR is within the angular coverage of the wide-beam
antenna,
receive a first and second successive P2 pulses, each having a respective
pulse reception
time, determining a first time interval between the first and second
successive P2 pulses,
and storing the first time interval as a time-ordered sequence of time
intervals; ii) receive a
new P2 pulse and determining a new time interval between said new P2 pulse and
a last
received P2 pulse; iii) provided said new time interval does not match the
first time
interval, add said new time interval to the time-ordered sequence and
repeating the steps
(ii) to (iii); and iv) provided said new time interval matches the first time
interval, and the
time ordered sequence starts repeating itself from the first time interval and
the new time
interval, determine the PRF pattern for the staggered interrogation sequence
of pulses
based on those time intervals that are between the first time interval and the
new time
interval.
According to yet another aspect of the invention, there is provided a method
for
determining the interrogation mode of each interrogation inside the stagger
interrogation
pattern for a SSR, the method comprising: i) receiving successive P1 and P3
pulse pairs,
CA 3072396 2020-02-13

either from the main lobe or the side lobe of the SSR antenna, and determine
the
interrogation mode of each P1-P3 pair; ii) finding the P1-P3 pulse pair
sequence inside the
stored interrogation staggered pattern and marking the matched section of the
stagger
pattern with the corresponding interrogation mode; iii) expand the
interrogation mode of
that section to the whole stagger pattern so that the interrogation mode of
each of the
interrogations inside the staggered pattern can be determined; iv) storing the
staggered
pattern and its corresponding interrogation mode pattern in storage device for
future
calibration; v) expand the staggered pattern and its corresponding
interrogation mode
pattern to the time periods when neither P2 or P1-P3 pair can be received.
According to one more aspect of the invention there is provided an apparatus
for
determining the interrogation mode of each interrogation inside the stagger
interrogation
pattern for a SSR, the apparatus comprising a memory device storing computer
readable
instructions causing a processor to: i) receive successive P1 and P3 pulse
pairs, from the
main lobe or the side lobe of the SSR antenna, and determine the interrogation
mode of
each P1-P3 pair; ii) find the P1-P3 pulse pair sequence inside the stored
interrogation
staggered pattern and mark the matched section of the interrogation pattern
with the
corresponding interrogation mode; iii) expand the interrogation mode of that
section to the
whole staggered pattern so that the interrogation mode of each of the
interrogations inside
the staggered pattern can be determined; iv) store the staggered pattern and
its
corresponding interrogation mode pattern in a storage device for future
calibration; v)
expand the staggered pattern and its corresponding interrogation mode pattern
to the time
periods when neither P2 or P1-P3 pair can be received.
According to yet one more aspect of the invention there is provided a method
for
determining a position of a target object without a transponder, regardless of
the target
object being within the SSR main beam or SLS beam, based on the staggered
interrogation pattern, the method comprising i) receiving the interrogation
signal reflected
from the target object close to the ownship; ii) searching the staggered
pattern and
determining the transmission time of the reflected interrogation; iii)
determining an angle of
arrival of the reflected interrogation using the dual receiving channel; iv)
calculating an
estimated position of the target object using the method used in the PSSR
system.
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According to yet one more aspect of the invention, there is provided a method
for
determining a pulse repetition frequency (PRF) pattern for a staggered
interrogation signal
of a Secondary Surveillance Radar (SSR), the method comprising: at a Passive
Secondary Surveillance Radar (PSSR) spaced apart from the SSR: detecting side
lobe
suppression pulses P2 of the staggered interrogation signal comprising pulses
(P1, P2,
P3), the pulses P1-P3 generated by a main antenna of the SSR, and the pulse P2
generated by a wide-beam antenna of the SSR, the wide-beam antenna having a
beam-
width, the pulses P2 synchronized with the pulses P1-P3 with a predefined time
delay,
comprising provided the PSSR is within the beam-width of the wide-beam
antenna: i)
detecting multiple P2 pulses; ii) forming a time-ordered sequence of P2 pulse
intervals, iii)
determining a repeating sequence of intervals in the time-ordered sequence of
P2 pulse
intervals; and iv) deriving the PRF pattern for the staggered interrogation
signal of the
SSR based on the repeating sequence of intervals.
The method further comprises predicting a transmit time for P1 pulse based on
said PRF pattern provided the PSSR is outside the beam-width of the wide-beam
antenna,
thereby determining the transmit time for the P1 pulse when P2 pulses or P1-P3
pulses
from the wide-beam antenna are not detectable.
The step of detecting of multiples P2 pulses comprises detecting successive P2
pulses. The step of forming a time-ordered sequence of P2 pulse intervals
further
comprises detecting a first and second successive P2 pulses, each having a
respective
pulse detection time, determining a first time interval between the first and
second
successive P2 pulses, and storing the first time interval as the time-ordered
sequence of
P2 pulses.
The step of determining a repeating sequence of intervals in the time-ordered
sequence of P2 pulse intervals further comprises: iii-1) receiving a new P2
pulse and
determining a new time interval between said new P2 pulse and a last received
P2 pulse;
and iii-2) provided said new time interval does not match the first time
interval, adding said
new time interval to the time-ordered sequence of P2 pulse intervals and
repeating the
steps (iii-1) to (iii-2).
7
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Additionally, the step of deriving the PRF pattern for the staggered
interrogation
signal of the SSR based on the repeating sequence of intervals further
comprises:
provided said new time interval matches the first time interval, and the
sequence of
intervals starts repeating itself, determining the PRF pattern based on the
repeating
sequence of intervals.
Furthermore the PRF pattern can be updated by applying statistical processing
or
averaging of the determined PRF pattern.
The present invention allows determining a position of a target object using
the
transmit time of the P1 pulse and/or P3 pulse and a reply message from said
target object
received at said PSSR, wherein said reply message is in response to receiving
the P1
pulse and/or P3 pulse at said target object. Additionally, determining the
position
comprises determining a position of an aerial, nautical or ground object.
The method further comprises determining an interrogation pattern of the PRF
pattern wherein said determining comprises (i) determining an interrogation
sequence of
said main antenna based on P1-P3 pulse combinations; (ii) matching said
interrogation
sequence in said PRF pattern; and (iii)determining the interrogation pattern
of said PRF
pattern.
The method of the present invention further comprises a calibration operation
to
compensate for time drift due to electronics within said PSSR to improve a
time accuracy
of said transmit time of P1 pulse.
According to yet another aspect of the invention, there is provided a method
for
determining the interrogation pattern for a PRF pattern comprising, at a PSSR
spaced
apart from the SSR, the steps of (i) detecting the P1-P3 pulses combination
with or without
P2 pulses (ii) determining the interrogation mode of each pulse combination
(iii) determine
the interrogation mode sequence using the P1-P3 combinations (iv) matching the
combinations in the stagger pattern and (v) determining the interrogation mode
for all
interrogations in the stagger pattern.
8
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The method further comprises determining the PRF pattern, using only the main
antenna signal, when the ownship is too far from the SSR that a SLS signal
cannot be
received.
The present invention also provides a method for a three dimensional (3D)
positioning a target object without a transponder using a reflection of the
interrogation
signal and a phased array receiver based on positioning principles of the
PSSR. A
coherent processing can also be performed on multiple received reflections to
enhance
the signal strength.
According to yet one more aspect of the invention, there is provided an
apparatus
for determining a pulse repetition frequency (PRF) pattern for a staggered
interrogation
signal of a Secondary Surveillance Radar (SSR) comprising:
a memory device having computer executable instructions stored thereon,
causing
a processor to: detect side lobe suppression pulses P2 of the staggered
interrogation
signal comprising pulses P1, P2, P3, the pulses P1 and P3 generated by a main
antenna
of the SSR, and the pulse P2 generated by a wide-beam antenna of the SSR, the
wide-
beam antenna having a beam-width, the pulse P2 synchronized with the pulses P1
and P3
with a predefined time delay, comprising: provided the PSSR is within the beam-
width of
the wide-beam antenna: i) detecting multiple P2 pulses and forming a time-
ordered
sequence of P2 pulse intervals; (ii) determining a repeating sequence of
intervals in said
time-ordered sequence of P2 pulse intervals; and (iii) deriving the PRF
pattern for the
staggered interrogation signal of the SSR based on the repeating sequence of
intervals.
The computer executable instructions further cause the processor to determine
an
interrogation pattern of said PRF pattern based on P1-P3 pulses combinations.
The computer executable instructions also cause the processor to predict a
transmit time for P1 and/or P3 pulse based on said PRF pattern provided the
PSSR is
outside the beam-width of the wide-beam antenna.
The computer executable instructions further cause the processor to determine
a
position of a target object using the transmit time of the P1 and /or P3 pulse
and the
9
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reflection of the interrogation signal from the target, the target object
being one of an
aerial, nautical or ground object.
According to yet one more aspect of the invention, there is provided a method
for
determining a position of a target object, comprising: processing at an
onboard Passive
Secondary Surveillance Radar (PSSR) system, signals received from a Secondary
Surveillance Radar (SSR) to identify a plurality of P2 Pulses, wherein said P2
pulses are
transmitted in a staggered pattern through a wide-beam antenna having a beam-
width and
wherein said PSSR can detect the P2 pulses when it is within said beam-width
of said
wide-beam antenna; forming a time-ordered sequence of P2 pulse intervals from
said P2
pulses; determining a pulse repetition frequency (PRF) pattern of said P2
pulses, based
on an identification of a repeating sequence of intervals in said time-ordered
sequence of
P2 pulse intervals; receiving a reply from the target object wherein said
reply is responsive
to an interrogation signal comprising a P1 pulse sent by the SSR to said
target object;
estimating a transmit time of said P1 pulse interrogation signal based on a
reception time
of said reply and the PRF pattern of the P2 pulses; and determining the target
object
position based on the target object altitude information h contained on said
reply and on a
localization operation using PSSR system location, SSR location, said transmit
time of
said P1 pulse interrogation signal and said reception time of said reply.
An interrogation pattern of said PRF pattern is further determined based on P1-
P3
pulse combinations.
Because the P2 pulse is synchronized to said P1 pulse interrogation signal
with a
predefined time delay, the PRF pattern of the P1 pulses can be determined by
applying a
time shift equal to said predefined time delay to the PRF pattern of the P2
pulses.
According to yet one more aspect of the invention, there is provided a Passive
Secondary Surveillance Radar (PSSR) system for determining a position of a
target
object, comprising: a first receiver for receiving a reply from the target
object wherein said
reply is responsive to an interrogation signal comprising a P1 and a P3 pulse
sent by a
Secondary Surveillance Radar (SSR) to said target object; a second receiver
for receiving
from said SSR a plurality of P2 Pulses, wherein said P2 pulses are transmitted
in a
CA 3072396 2020-02-13

staggered pattern through a wide-beam antenna having a beam-width and wherein
said
second receiver can detect the P2 pulses when it is within the beam-width of
said wide-
beam antenna; and a memory device having computer executable instructions
stored
thereon, causing a processor to: process said plurality of P2 Pulses to form a
time-ordered
sequence of P2 pulse intervals; determine a pulse repetition frequency (PRF)
pattern of
said P2 pulses, based on an identification of a repeating sequence of
intervals in said
time-ordered sequence of P2 pulse intervals; estimate a transmit time of said
P1 pulse
interrogation signal based on a reception time of said reply and the PRF
pattern of the P2
pulses; and determine the target object position based on an altitude
information of the
target object present on said reply and on a localization operation using a
location of the
PSSR system, a location of the SSR, said transmit time of said P1 pulse
interrogation
signal and said reception time of said reply.
The PSSR system comprises a mixer and a local oscillator for translating the
reply
into an intermediate frequency band reply signal and for translating the P2
pulses into an
intermediate frequency band P2 pulses; and a single channel high-speed Analog-
to-Digital
Converter (ADC) for digitizing said intermediate frequency band P2 pulses and
transmitting digitized intermediate frequency band reply signal and digitized
intermediate
frequency band P2 pulses to said processor.
Alternatively the PSSR system may comprise a first mixer and a first local
oscillator for translating the reply into a base band reply signal; a second
mixer and a
second local oscillator for translating the P2 pulse into a base band P2
pulses; and a dual
channel high-speed Analog-to-Digital Converter (ADC) for sampling said base
band reply
signal and said base band P2 pulse and transmitting sampled base band reply
signal and
sampled base band P2 pulses to said processor.
In addition, the location of the PSSR is determined using a GPS unit, the
location
of the SSR being a fixed location known to the PSSR system.
According to yet another aspect of the invention, there is provided a Passive
Secondary Surveillance Radar (PSSR) system in which the second receiver
further
receives a plurality of interrogation signals from said SSR, wherein said
interrogation
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signals are transmitted in a staggered pattern through the main antenna having
a beam-
width and wherein said second receiver can detect the interrogation signals
when it is
within the beam-width of said main antenna (MA); and the computer executable
instructions further cause the processor to process said plurality of
interrogations to form a
time-ordered sequence of interrogation mode; determine an interrogation
pattern of said
PRF pattern, based on matching of the MA interrogation sequence in said time-
ordered
stagger pattern sequence; process said plurality of interrogation signals to
form a rotation
profile of the main antenna of the SSR; wherein said rotation profile and said
interrogation
pattern are used in estimating said transmit time of said P1 pulse.
A Passive Secondary Surveillance Radar (PSSR) can determine the position of a
target object when the target object is in the main (P1, P3-pulse) beam of a
Secondary
Surveillance Radar (SSR) but requires the PSSR to be simultaneously within the
main
(P1, P3-pulse) beam or the wider (p2-pulse) beam of the said SSR. A method for
determining a staggered pattern and interrogation mode pattern from a
staggered
interrogation signal of a SSR is disclosed. This method enables a PSSR to work
not only
inside but also outside the wider P2 pulse beam. At a PSSR spaced apart from
the SSR,
P2 pulses of the staggered interrogation signal (P1, P2, P3) are detected,
where P1 and
P3 are generated by a main narrow-beam antenna of the SSR, and P2 is generated
by a
wide-beam antenna of the SSR baying a beam-width. P2 pulses are synchronized
in time
with P3 pulses. Provided the PSSR is within the beam-width of the wide-beam
antenna,
multiple P2 pulses are detected as time-ordered sequence of P2 pulse
intervals. A
repeating sequence of time intervals in the time-ordered sequence can be
determined,
and the stagger pattern is determined based on the determined repeating
sequence. In
another case, provided the PSSR is too far from the SSR, and P2 pulses are too
weak to
be detected, the staggered pattern can be determined using only the stronger
P1 and P3
pulses from the narrow-beam signal of the main antenna (MA main lobe) using
longer
observation time. The interrogation mode pattern can be determined by
comparing the
said staggered pattern with the narrow-beam P1 and P3 signals. A transmit time
of the P1
and/or P3 pulse is predicted based on said staggered pattern and said
interrogation mode
pattern. When the target object does not have a transponder, the positioning
principle of
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the PSSR can also be used to determine a 3D position of the target object
using the
reflected interrogation signal from the target object, an angle of arrival
measured from a
phase array receiver, and a precise time of the interrogation predicted from
the PRF or
staggered pattern. Corresponding system is also provided.
It is yet another object of the invention to provide a method and system for
tracking
and avoiding a non-cooperative object, for example the non-cooperative object
not having
a transponder, by using reflected interrogation signals, having been sent from
a secondary
surveillance radar, reflected from the non-cooperative object, and detected by
the
ownship.
According to yet another aspect of the invention, there is provided a method
for
tracking and avoiding a non-cooperative object by an ownship, comprising
employing at
least one hardware processor for: detecting a reflected interrogation signal
from the non-
cooperative object, the reflected interrogation signal being an interrogation
signal sent
from a secondary surveillance radar and reflected off the non-cooperative
object,
processing the reflected interrogation signal, yielding a processed reflected
interrogation
signal, and determining a position of the non-cooperative object from the
processed
reflected interrogation signal, thereby allowing the ownship to track and
avoid the non-
cooperative object. The method further comprises tracking and avoiding the non-
cooperative object.
The detecting step of the method comprises capturing the reflected
interrogation
signal by an antenna to generate a captured reflected interrogation signal,
and forwarding
the captured reflected interrogation signal to a 1030 MHz receiver. The
capturing the
reflected interrogation signal comprises one of the following capturing the
reflected
interrogation signal by a directional antenna, capturing the reflected
interrogation signal by
an omni-directional antenna, capturing the reflected interrogation signal by a
directional
antenna and an omni-directional antenna, which are connected by a splitter.
The processing step of the method comprises (i) determining a range of
durations
for time windows, during which the reflected interrogation signal arrives at
the ownship, for
13
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example the durations being comparable to an interrogation time of travel from
a
secondary surveillance radar, SSR, to the ownship, (ii) integrating the
reflected
interrogation signal across the time windows determined in the step (i), and
(iii) identifying
and classifying peaks in the integrated reflected interrogation signal
integrated in the step
(ii).
The integrating the reflected interrogation signal across the time windows
step
further comprises determining a plurality of sequences of time windows, within
which
respective reflected interrogation signals arrive at the ownship, each time
window WI in a
sequence Wi' having a same duration and a same time delay from a respective
start point
for said each time window, and for each sequence WI', processing corresponding
samples
of the reflected interrogation signal. The processing corresponding samples
further
comprises one of the following processing the corresponding samples
coherently,
processing the corresponding samples non-coherently. Additionally, the
determining a
range of durations for time windows comprises choosing durations to cover a
predetermined monitoring distance, for example from about 2km to about 20km
from the
ownship.
The integrating the reflected interrogation signal across the time windows
step
further comprises determining a number of time windows to be integrated, based
on at
least one of the following: the non-cooperative object being considered
stationary for said
number of time windows to be integrated, an analog-to-digital (ADC) sampling
rate, an
expected speed of the non-cooperative object. The identifying and classifying
peaks
comprises comparing the reflected interrogation signal and/or the integrated
reflected
interrogation signal with an interrogation pattern of P1, P2 and P3 pulses
generated by the
SS R.
The determining step of the method comprises calculating a range of possible
positions of the non-cooperative object from the processed reflected
interrogation signal,
scanning the range of possible positions of the non-cooperative object, and
detecting the
position of the non-cooperative object, based on results of the scanning.
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The calculating the range of possible positions of the non-cooperative object
step
comprises calculating a spheroid, wherein the secondary surveillance system is
at a first
focal point of the spheroid, and the ownship is at a second focal point of the
spheroid, and
the non-cooperative object is on the spheroid. The scanning the range of
possible
positions comprises one of the following scanning with a phased array antenna,
scanning
with a mechanically scanned directional antenna (MSDA). Furthermore, the
scanning the
range of possible positions comprises changing a scan angle along the range of
possible
positions of the non-cooperative object, detecting a strongest signal strength
along the
range of possible positions of the non-cooperative object, determining a
strongest scan
angle, corresponding to the strongest signal strength, and calculating an
altitude of the
non-cooperative object from the strongest scan angle.
The determining step of the method comprises applying a co-altitude assumption
between the non-cooperative object and the ownship, determining an avoidance
area
around the non-cooperative object, by using the processed reflected
interrogation signal
and the co-altitude assumption, and assuming the position of the non-
cooperative object is
within the avoidance area. The determining the avoidance area further
comprises one of
the following choosing a size of the avoidance area so that an avoidance time
for avoiding
the non-cooperative object by the ownship is in a range from about 1 second to
about 10
seconds, choosing a size of the avoidance area in accordance with aviation
standards.
The avoidance area may be a cylinder.
It is yet another aspect of the present invention to provide a system for
tracking
and avoiding an non-cooperative object by an ownship, comprising a memory
device for
storing computer readable instructions thereon for execution by at least one
processor,
causing the at least one processor to detect a reflected interrogation signal
from the non-
cooperative object, the reflected interrogation signal being an interrogation
signal sent
from a secondary surveillance radar and reflected off the non-cooperative
object, process
the reflected interrogation signal', yielding a processed reflected
interrogation signal, and
determine a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing the ownship to track and avoid the non-
cooperative
object.
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The computer readable instructions further cause the at least one processor to
track and avoid the non-cooperative object. The computer readable
instructions, causing
to detect, further cause the at least one processor to capture the reflected
interrogation
signal by an antenna to generate a captured reflected interrogation signal,
and forward the
captured reflected interrogation signal to a 1030 MHz receiver. The computer
readable
instructions, causing to capture the reflected interrogation signal, further
cause the at least
one processor to perform one of the following capture the reflected
interrogation signal by
a directional antenna, capture the reflected interrogation signal by an omni-
directional
antenna, capture the reflected interrogation signal by a directional antenna
and an omni-
directional antenna, which are connected by a splitter.
The computer readable instructions, causing to process, further cause the at
least
one processor to a memory device for storing computer readable instructions
thereon for
execution by at least one processor, causing the at least one processor to:
detect a
reflected interrogation signal from the non-cooperative object, the reflected
interrogation
signal being an interrogation signal sent from a secondary surveillance radar
and reflected
off the non-cooperative object, process the reflected interrogation signal,
yielding a
processed reflected interrogation signal, and determine a position of the non-
cooperative
object from the processed reflected interrogation signal, thereby allowing to
track and
avoid the non-cooperative object. The computer readable instructions, causing
to integrate
the reflected interrogation signal, further cause the at least one processor
to determine a
plurality of sequences of time windows, each time window WI in a sequence VVi'
having a
same size and a same time delay within which respective reflected
interrogation signals
arrive at the ownship, and for each sequence VVi', process corresponding
samples of the
reflected interrogation signal. The computer readable instructions, causing to
process
corresponding samples, further cause the at least one processor to perform one
of the
following process the corresponding samples coherently, process the
corresponding
samples non-coherently. The computer readable instructions, causing to
determine a
range of durations for time windows, further cause the at least one processor
to choose
durations to cover a predetermined monitoring distance. The computer readable
instructions, causing to integrate the reflected interrogation signal across
the time
16
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windows, further cause the at least one processor to determine a number of
time windows
to be integrated, based on at least one of the following: the non-cooperative
object being
considered stationary for the number of time windows to be integrated, an
analog-to-digital
(ADC) sampling rate, an expected speed of the non-cooperative object.
The computer readable instructions, causing to identify and classify peaks,
further
cause the at least one processor to compare the the reflected interrogation
signal and/or
the integrated reflected interrogation signal with an interrogation pattern of
P1, P2 and P3
pulses generated by the SSR. The computer readable instructions, causing to
determine,
further cause the at least one processor to calculate a range of possible
positions of the
non-cooperative object from the processed reflected interrogation signal, scan
the range
of possible positions of the non-cooperative object, and detect the position
of the non-
cooperative object, based on results of the scanning.
The computer readable instructions, causing to calculate a range of possible
positions, further cause the at least one processor to calculate a spheroid,
wherein the
secondary surveillance system is at a first focal point of the spheroid, and
the ownship is
at a second focal point of the spheroid, and the non-cooperative object is on
the spheroid.
The computer readable instructions, causing to scan the range of possible
positions,
further cause the at least one processor to perform one of the following scan
with a
phased array antenna, scan with a mechanically scanned directional antenna
(MSDA).
The computer readable instructions, causing to scan the range of possible
positions, further cause the at least one processor to change a scan angle
along the range
of possible positions of the non-cooperative object, detect a strongest signal
strength
along the range of possible positions of the non-cooperative object, determine
a strongest
scan angle, corresponding to the strongest signal strength, and calculate an
altitude of the
non-cooperative object from the strongest scan angle. The computer readable
instructions, causing to determine, further cause the at least one processor
to apply a co-
altitude assumption between the non-cooperative object and the ownship,
determine an
avoidance area around the non-cooperative object, by using the processed
reflected
interrogation signal and the co-altitude assumption, and assume the position
of the non-
17
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cooperative object is within the avoidance area. The computer readable
instructions,
causing to determine the avoidance area, further cause the at least one
processor to
perform one of the following choose a size of the avoidance area so that an
avoidance
time for avoiding the non-cooperative object by the ownship is in a range from
about 1
second to about 10 seconds, choose a size of the avoidance area in accordance
with
aviation standards.
In yet another aspect of the invention, in a system for tracking and avoiding
a non-
cooperative object, having a means for detecting a reflected interrogation
signal from the
non-cooperative object, the reflected interrogation signal being an
interrogation signal sent
from a secondary surveillance radar and reflected off the non-cooperative
object, to
provide an apparatus, comprising a memory device for storing computer readable
instructions thereon for execution by at least one processor, causing the at
least one
processor to process the reflected interrogation signal, yielding a processed
reflected
interrogation signal, and determine a position of the non-cooperative object
from the
processed reflected interrogation signal, thereby allowing the ownship to
track and avoid
the non-cooperative object. The computer readable instructions further cause
the at least
one processor to track and avoid the non-cooperative object.
It yet another aspect of the invention, there is provided an apparatus for
tracking
and avoiding a non-cooperative object, comprising a memory device for storing
computer
readable instructions thereon for execution by at least one processor, causing
the at least
one processor to process a reflected interrogation signal, yielding a
processed reflected
interrogation signal, and determine a position of the non-cooperative object
from the
processed reflected interrogation signal, thereby allowing the ownship to
track and avoid
the non-cooperative object.
The computer readable instructions, causing to process, further cause the at
least
one processor to (i) determine a range of durations for time windows, during
which the
reflected interrogation signal arrives at the ownship, the durations being
comparable to an
interrogation time of travel from a secondary surveillance radar to the
ownship, (ii)
integrate the reflected interrogation signal across the time windows
determined in the step
18
CA 3072396 2020-02-13

(i), and (iii) identify and classifying peaks in the integrated reflected
interrogation signal
integrated in the step (ii). The computer readable instructions, causing to
determine,
further cause the at least one processor to calculate a range of possible
positions of the
non-cooperative object from the processed reflected interrogation signal, and
scan the
range of possible positions of the non-cooperative object, and detect the
position of the
non-cooperative object, based on results of the scanning.
In yet another aspect of the invention, there is provided a method for
tracking and
avoiding a non-cooperative object, comprising employing at least one hardware
processor
for processing a reflected interrogation signal, the reflected interrogation
signal being an
interrogation signal sent from a secondary surveillance radar and reflected
off the non-
cooperative object, yielding a processed reflected interrogation signal, and
determining a
position of the non-cooperative object from the processed reflected
interrogation signal,
thereby allowing the ownship to track and avoid the non-cooperative object.
Thus, an improved method and system for secondary surveillance radar (SSR) for
tracking non-cooperative objects without a transponder have been provided.
DETAILED DESCRIPTION OF THE DRAWINGS
The application contains at least one drawing executed in color. Copies of
this or a
better understanding of the embodiments and/or related implementations
described herein
and to show more clearly how they may be carried into effect, reference will
now be made,
by way of example only, to the accompanying drawings which show at least one
exemplary embodiment and/or related implementation in which:
Figure 1 is a schematic diagram of an SSR 110 system in relation to an ownship
140 and an intruder 160;
Figure 2A illustrates geometry of the configuration of Figure 1 for
calculation of the
position of the intruder 160;
Figure 2B illustrates the signal received by ownship in the configuration
shown by
Figure 2A;
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Figure 2C illustrates a geometry of the configuration of Figure 1 when ownship
is
out of the wide-beam antenna coverage;
Figure 2D illustrates the signal received by ownship in the configuration
shown by
Figure 2C;
Figure 3A is a schematic diagram showing the mobile PSSR system on board the
ownship 140, in relation to the SSR 110;
Figure 3B illustrates various component of a mobile PSSR system;
Figure 4 illustrates a flowchart for determining a position of the target
object;
Figure 5A illustrates a flowchart for obtaining a PRF pattern;
Figure 5B illustrates a flowchart for obtaining an interrogation pattern;
Figure 6A and 6B illustrate diagrams for detecting the P2 pulses;
Figure 7 illustrates an alternative method for determining a position of the
target
object;
Figure 8 illustrates a flowchart for determining the PRF pattern using main
antenna
signals;
Figure 9 is a schematic diagram 900 of the calculation of the intruder's
position
after the interrogation time of the SSR 110 is profiled;
Figure 10 is a schematic diagram 1010 of an avoidance cylinder 401 surrounding
the intruder 160, in relation to the ownship 140;
Figure 11A illustrates a system architecture 1100 of the SSR 110 system;
Figure 11B illustrates an alternative system architecture 1150 of the SSR 110
system;
Figure 11C illustrates a schematic diagram of a receiver unit 320;
Figure 11D illustrates a schematic block diagram 1170 for tracking and
avoiding
non-cooperative objects, or target 160, by an ownship 140;
CA 3072396 2020-02-13

Figure 11E illustrates an expanded schematic block diagram 1170b for
processing
the reflected interrogation signal;
Figure 11F illustrates an expanded schematic block diagram 1170c for
determining
the position of the target 160 from the processed reflected interrogation
signal;
Figure 12 illustrates a schematic diagram 1200 displaying an example of how a
single reflected interrogation signal is used to detect an intruder 160;
Figure 13A is a schematic diagram 1300 of a standard Mode A/C interrogation
message transmitted by the SSR 110;
Figure 13B is a schematic diagram 1350 of how the SSR interrogations and their
corresponding reflections are placed;
Figure 14A shows a signal collection diagram 1400 where the signal is
collected
during 1 s interval;
Figure 14B shows an expanded view of the signal collection diagram 1400,
showing a zoomed view of one of the P2 pulses in Figure 14A;
Figure 14C shows the results of non-coherent integration after being applied
to the
signal collection diagram 1400, taken from Figure 14A;
Figure 15A is a schematic block diagram for processing the reflected
interrogation
signal to determine the 3D position of the intruder 160 for tracking and
avoidance of the
intruder 160;
Figure 15B is a schematic block diagram for processing the reflected
interrogation
signal to determine the 3D position of the intruder 160 for tracking and
avoidance of the
intruder 160, showing the more general method steps from Figure 11D above;
Figure 15C is a schematic block diagram for processing the reflected
interrogation
signal to determine the 3D position of the intruder 160 for tracking and
avoidance of the
intruder 160, showing the more general method steps from Figures 11D and 11E
above;
Figure 15D is a schematic system diagram of the reflection process unit 391;
and
21
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Figure 16 is a schematic diagram 1600 of the masking problem experienced by
the reflected SSR signal, when it is masked by the direct SSR signal.
DETAILED DESCRIPTION OF THE EMBODIMENTS
It will be appreciated that for simplicity and clarity of illustration, where
considered
appropriate, reference numerals may be repeated among the figures to indicate
corresponding or analogous elements. In addition, numerous specific details
are set forth
in order to provide a thorough understanding of the embodiments and/or
implementations
described herein. However, it will be understood by those of ordinary skill in
the art that the
embodiments and/or implementations described herein may be practised without
these
specific details. In other instances, well-known methods, procedures and
components
have not been described in detail so as not to obscure the embodiments and/or
implementations described herein. Furthermore, this description is not to be
considered as
limiting the scope of the embodiments described herein, but rather to describe
the
structure and operation of the various embodiments and/or implementations
described
herein.
It would be beneficial for an aircraft to have a PSSR system onboard to be
able to
detect positions of other aircraft in its vicinity. Preferably, it would be
highly beneficial to
take advantage of the existing systems and infrastructure to do so and in
compliance with
the aviation standards. In this description, the aircraft that carries the on
board PSSR is
referred to as the ownship ("our" aircraft) to distinguish it with the "other"
aircraft (also
referred to as a target object) whose location needs to be determined. The
teachings of
this invention are not limited to detecting aircraft. Any flying object, for
example a drone,
may be detected as long as it is equipped with functioning transponders.
Moreover, in
some embodiments the ownship may be a vehicle on the ground or water which is
a
special case of the most general 3-dimensional (3D) teachings.
Figure 1A illustrates a generic configuration 100 in which the present
invention can
be deployed showing the ownship 140, having a Passive Secondary Surveillance
Radar
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(PSSR) system on board (shown in Figures 3A and 3B), in relation to the SSR
110 and a
target object represented as target object 160.
The PSSR of the ownship 140 is airborne and it works even when the ownship
cannot receive any signals from the SSR. In the generic configuration 100, the
SSR 110
transmits interrogation signals P1, P2 and P3 that can be received at the
target object
160, this transmission path is represented as path 120. P1 and P3 pulses are
transmitted
through a narrow beam antenna of the SSR 110. The interrogation signals
include the side
lobes suppression pulses P2 that the SSR 110 transmits through a wide-beam
antenna
that can be received at the ownship 140. This is represented as path 130. The
target
object 160 broadcast reply is received at the ownship 140 through transmission
path 150
for further processing to derive information necessary to locate and identify
the target
object 160 as will be described hereinafter.
The successive interrogations transmitted by the SSR 110 are not equally
spaced
for modern SSR system. They follow a fixed pulse repetition frequency (PRF)
pattern,
which is called 'staggered PRF'. This PRF pattern needs to be determined
before a
correct interrogation time can be predicted when the ownship is not covered by
the Main
Antenna (MA) and SLS beam. The determination of the PRF or stagger pattern
based on
the main-lobe observation can be slow and unreliable solely because only 7 to
10
interrogations can be observed at the ownship 140 within every rotation of the
SSR 110
antenna. If the PRF pattern is long, it will take a longer time to determine
the PRF pattern,
which slows down the positioning of the target object long enough to cause
midair collision
hazards. A faster way of determining the PRF pattern is to use P2 pulses.
Hundreds of P2
pulses can be observed in each rotation of the SSR 110 antenna, and therefore
the PRF
pattern is very likely to be determined within a small section of each
rotation of the SSR
110 antenna. This greatly increase the speed of the algorithm and hence
improve the
safety of the ownship 140.
Current implementation of the antenna for P2, although referred to in some
literature as omni-directional antenna, is actually a wide-beam antenna
covering about 80
degrees of the front and the back of the MA for a total of about 160 degrees.
It is
understood that teachings of the present invention also apply for any other
limited angle
23
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apart from about 80 degrees, covering less than 180 degrees of the front and
less than
180 degrees of the back of the MA. In such situation, the ownship 140 can only
receive
the P2 when it is in its coverage area or beam-width and hence the ownship 140
in
operation will not receive any reference signal from the SSR for about 200
degrees within
a complete rotation of the SSR 110 antenna. The present application provides a
method to
estimate the P2 pulses transmit times with an incomplete observation of the P2
pulses as
will be described below.
Figure 2A illustrates a geometry of the above configuration in Figure 1 where
the
SSR 110 and ownship 140 are shown as the two focal points. The SSR 110 is
shown to be
on the origin of the XYZ Cartesian coordinate system. Generally in a 3-
dimensional (3D)
space, the surface composed by the points from which the sum of the distances
to the two
focal points is a constant is known to be a spheroid. For the purpose of the
discussion, an
elliptical cross-section of the spheroid on a 2-dimensional (2D) plane shown
is sufficient
because the altitude of the target object can be determined by its Mode C
reply message.
The 2D plane contains the major axis of the 3D spheroid. Mathematically, the
coordinates
of the target object 160 can be obtained from the following equations:
-2b2c + 1j4b4c2 -4(b2 -i-a2 tan' (2n - f3 )(172c + a 217 - a2b2)
x = ___________________________________________________________
2(b2 + tan2 (27r -3))
y = x tan(2rt -Ii)
= -h
where a and b are defined in Figure 2A; c = L/2, h is the altitude of the
target
object 160, and p is the angle from X-axis clockwise to the center of the Main
Antenna
(MA), ranging from 0 to 360 degrees. The above equations are obtained from the
real
spheroid geometry in 3D For cooperative target which has a transponder, the h
in the
formula can be determined by reading the Mode C reply message. For non-
cooperative
target positioning which depends on the reflection of the SSR interrogation
signal, the
determination of altitude h is given later by using a electronically or
mechanically scanned
antenna. Other techniques that can be used to localize the target object 160
include
multinational and triangulation techniques and are well known to those skilled
in the art.
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The Geometry depicted in Figure 2A, illustrates the case where the target
object
160 is within the beam-width or coverage area of the SSR 110 main-lobe while
the
ownship 140 is outside of that radiation field. Additionally because the wide-
beam antenna
coverage is 40 degrees wide around the MA, the ownship 140 is within its
coverage area
and therefore the ownship 140 can see the P2 pulses transmitted by the wide-
beam
antenna of the SSR 110 however ownship cannot see neither P1 or P3 pulses. In
this
geometry the ownship 140 can detect both the P2 pulses and the reply signals
from the
target object 160.
Figure 2B shows the signals received by the ownship 140 with the reply from
the
target object 160 in solid line and the P2 pulse in dashed line corresponding
to an
interrogation signal that trigger the reply. The group of the solid line
pulses is one
complete reply message triggered by the interrogation corresponding to the P2
pulse.
In this geometry the ownship 140 can readily detects the P2 pulses. The method
of
the invention reads the time instances of this P2 pulses and applies the
algorithms
described below to determine the stagger or PRF pattern of the P2 pulses and
therefore
predict the occurrences of the P2 pulses even when it cannot be observed at
the ownship
140. The transmit time of the P1 pulse can then be derived from the
occurrences of the P2
pulses, and transmit time of P3 pulse can also be derived once the
interrogation pattern is
determined.
Figure 2C shows another geometry corresponding to the case where the angle
between the main-lobe of the SSR 110 Main Antenna (MA) and the X-axis is
almost 90
degrees. The wide-beam transmission does not cover the ownship 140 area. In
this
geometry only the 1090 MHz reply from the target object 160 is observed, while
none of
the pulses comprising an interrogation signal is observed. The signals
observed by the
ownship are shown in Figure 2D. The dashed plot is a signal around the 1030
MHz
received by the ownship 140, and a solid plot is the 1090 MHz reply signal. As
can be
seen, the 1030 MHz receiver channel only shows noise, while none of the P1, P2
or P3
pulses are received. In this case, the device has to predict the interrogation
that triggers
the reply received by the ownship 140 using the PRF pattern of the P2 pulses
to be able to
position the target object 160.
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For calculating the sum of the distance dl from the SSR to target object 160
and
the distance d2 from target object to ownship in this case, the time interval
between the
leading edge of the predicted P2 (the transmit time of the P2 pulse can be
predicted for
the case shown in Figure 2D using the algorithm described below) and the reply
message
as shown in Figure 2D should be calculated. Assume the stagger pattern and its
interrogation mode has been determined using the algorithms described below,
and the
time between the assumed P2 pulse to the reply is al s, then the sum distance
can be
dt=d1+d2 calculated as:
dt=c(a1-6e-6-3e-6)+L for Mode A interrogation; and
dt=c(a1-19e-6-3e-6)+L for Mode C interrogation;
where c is the speed of light, L is the distance between the SSR and the
ownship
as shown in Figure 2C. dt is actually the parameter 2a in Figure 2A. The reply
message is
transmitted after the transponder receives the P3 pulse. Therefore, for
different modes,
the reply time that is lagging the P2 pulse time is different. In Mode A
interrogation, the P3
pulse is sent 6 us after the P2 is transmitted, while in Mode C interrogation,
the P3 pulse is
sent 19 us after P2. This is why for different interrogation mode, the formula
above to
calculate the sum distance is different. And for this reason, to profile and
predict the
interrogation mode of each interrogation in the stagger pattern is very
important. The 3 us
in both equations is the fixed transponder delay.
In the reflection case which will be introduced later, there is no transponder
delay
and the interrogation is directly reflected from the target, so the sum
distance is different,
and can be expressed as
dt=c*al+L
Figure 3A illustrates a Passive Secondary Surveillance Radar system (PSSR) 300
embedded in the ownship 140 for detecting transponder equipped aircraft, and
receiving a
signals along the path 130 from the SSR 110, which is situated on the ground
99.
Figure 3B illustrates the PSSR 300 for detecting a target object such as
target
object 160 and determining its positional information.
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The PSSR system 300 comprises a receiver unit 320 for receiving, through an
antenna system (322, 324) SSR mode C, all-call, and roll-call interrogations
signals along
the path 130 comprising P1, P2 and P3 pulses and Mode A/C replies 150 from the
target
object 160. In a preferred embodiment, the receiver unit 320 comprises an omni-
directional antenna 324 (such as a dipole). Since transponders generally use
an omni-
directional antenna, the ownship 140 can always receive reply messages from
the target
object 160. The receiver unit 320 may further comprise a directional antenna
322, for
example for detection of the signals transmitted by the SSR 110 to enhance the
SSR
range when needed. Optionally, a multiple antenna array may be added to the
receiver
unit 320 to estimate the angle of arrival (AOA) of the target object 160 reply
signal, which
is useful for the case when the target object does not have a transponder.
This is shown in
Figures 11A and B.
In Figure 11A, the receiver unit 320 comprises a receiver 325 (1030 MHz)
connected to the omni-directional antenna 324. The directional antenna 322 and
the omni-
directional antenna 324 can also be connected separately to the receiver 325A
(1030
MHz) and a receiver 325B (1030 MHz) controlled by a beam steering unit 326,
respectively, shown in Figure 11B. The directional antenna 322 or to the omni-
directional
antenna 324 may also be connected through a splitter 700, shown in Figure 11C.
The
purpose of the omni-directional antenna is to detect the interrogation signals
transmitted
by the SSR through the narrow-beam antenna (P1, P3) or the SLS signal (P2
pulse)
through the wide-beam antenna of the SSR 110. The 1030 MHz receiver 325 is
tuned to
the 1030 MHz frequency band for receiving and filtering P2 as well as P1 and
P3 signals
in that frequency band.
Back to Figure 3B, the receiver unit 320 comprises also a 1090 MHz receiver
323 tuned to
1090 MHz frequency band for receiving and filtering signals around 1090 MHz
through the
omni-directional antenna 324. The 1090 MHz receiver 323 detects reply signals
from
target object 160 which are transmitted at the 1090 MHz frequency. Both the
1030 MHz
receiver 325 and 1090 MHz receiver 323 are connected to a Base-
band/Intermediary
Frequency (BB/IF) sampling unit 327 for receiving the signals detected by the
receiver 325
and receiver 323 and converting them into a base band or into an intermediary
frequency
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using a local oscillator as will be described in Figures 6A and 6B,
respectively. The BB/IF
sampling unit 327 digitizes the received signals and passes the digitized
signals along to a
processor 310 for further processing.
In one embodiment processor 310 provides the processing power for performing
the operations of the present invention. The processor 310 can be a micro-
controller or a
microprocessor or any processor device capable of executing the operations of
the
present invention, such processor devices are well known to those skilled in
the art. The
processor 310 receives digital signals from the receiver unit 320 and executes
operations
dictated by operating modules embedded or connected to the processor 310. In
this
embodiment a P2 intervals processing unit 380, along with the processor 310,
process the
signals corresponding to the P2 pulses for determining the time intervals
between P2
Pulses received at the PSSR 300. The P2 intervals processing unit 380 creates
a time-
ordered sequence of P2 Pulse intervals that are stored in a memory device 340.
The time-
ordered sequence of P2 Pulse intervals is a sequence of intervals formed from
the
received P2 pulses and ordered according to the reception time of the P2
pulses. As an
example for 4 pulses received respectively at times t0, t t and t3, the time-
ordered
-1, -2
sequence of pulse intervals would be ordered as intervals 11, 12 and 13 with
In formed from
P2 pulses received at time n and at time n-1. The P2 intervals processing unit
380 adds as
well any new interval determined from a new P2 pulse and the last received P2
pulse to
the time-ordered sequence of pulse intervals, and compare the new interval to
the
previously stored pulse intervals in the time-ordered sequence of pulse
intervals. The PRF
Identifier 370 based on the result of that comparison applies a procedure to
identify a
repeating sequence of intervals and determine the PRF pattern. The procedures
applied
by the P2 intervals processing unit 380 and the PRF identifier 370 would be
described in
detail with regard to FIG 5A.
In another embodiment processor 310 communicates with the SSR main antenna
(MA) signal processing unit 390. The MA signal processing unit 390 identifies
and
decodes the Mode A/C messages that includes P1 and P3 pulses, no matter if P2
is
stronger or weaker than P1. These messages could come from the main lobe or
side lobe
of the MA. The main functions of the MA signal processing unit 390 include two
parts: i)
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determine the pattern of the interlaced Mode A/C interrogation, which is the
interrogation
pattern. This pattern could be ACACAC or AACAAC, etc. With the MA
interrogation
sequence and the interval between successive interrogations, a match of the MA
pattern
inside the whole stagger pattern can be found, and further to determine the
type for every
interrogation in the stagger pattern; ii) determine the mechanical rotation of
the MA. The
procedures applied by the MA signal processing unit 390 will be described in
detail with
regard to FIG 5B.
As illustrated in Figure 3, the PSSR 300 relies on a data storage system 330
and a
memory 340 both connected to the processor 310 to store data and information
necessary
to its operation. Permanent or long term data such as SSR location, PRF
pattern once
identified can be stored in the data storage 330 while short term data such as
time-
ordered sequence of pulse intervals, cached data or other program instructions
can be
stored in the memory 340.
The PSSR system 300, in a preferred embodiment, comprises a Global
Positioning System (GPS) unit 350 for determining the location of the ownship
140. All the
information related to the position and trajectory of the ownship 140 as well
as the target
object 160 is displayed on a display for advising the pilot of the ownship
140. In one
embodiment, the display is part of a tracking system 360 that monitors the
relative
distance between the two objects (target object 160 and ownship 140). The
tracking of the
position and trajectory of the ownship 140 and target object 160 on the
display provides a
visual cue to the pilot of the ownship 140 to know the relative spacing
between the
ownship 140 and target object 160 and to take appropriate measures to mitigate
any
potential problem. More importantly, this allows the prediction of the target
object
movement based on the previous detection results and provide a confident
estimation of
the position of the target object even when the detection of the target object
is missed in
several detections. Additionally an audio alarm system may be provided as part
of the
tracking system 360 to alert the pilot as well. Alternatively, the display may
be standalone
or shared with other components such as a computing device within the ownship
140 and/
or the GPS unit 350 and the tracking system 360.
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A general operation of the PSSR 300 for finding location information of a
target
object such as target object 160 will now be described using an exemplary
method
depicted in the flowchart 400 of Figure 4. At step 410 the PSSR receives
signals
transmitted by the SSR 110 at the 1030 MHz frequency band. The signals are
received
through the 1030 MHz receiver 325 which processes the signals as described in
Figure 3
and passes the information to the BB/IF sampling unit 327 at step 420 for
detecting the P2
pulses from the signals received. The reception times of the P2 pulses are as
well
recorded for the computation of the P2 pulses intervals. After detecting the
P2 pulses and
the interrogation sequence a PRF determining step 430 apply a PRF
identification
procedure to identify a repetition pattern and corresponding interrogation
type of the P2
pulses based on time intervals of the detected P2 pulses and the
interrogations inside the
MA. FIG 5 will detail the procedure used by step 430 to determine the PRF (or
stagger)
and interrogation pattern of the P2 pulses.
As stated above one objective of the PSSR is to determine a position of a
target
object such as a target object 160 and display its positional information on a
display of the
ownship 140. For that purpose the PSSR 300 onboard the ownship 140 receives
reply
signals at step 440 from the target object 160 and determines the reception
time of the
reply signal. The target object 160 transmits the reply signal in response to
receiving from
the SSR 110 an interrogation signal comprising P1 and P3 pulses transmitted
through the
main lobe of the narrow-beam antenna of the SSR 110. The reply signal contains
the
target object 160 identification information as well as its current altitude
information. At
step 450 the PSSR 300 uses the reception time of the reply signal from the
target object
160 and the estimated interrogation signal from SSR 110 to determine the
ellipse shown in
FIG 2A and FIG 2C. As stated above, the P2 pulse is synchronized with the P3
pulse with
a predefined time interval equal to 6 microseconds for Mode A interrogation
and 19
microseconds for Mode C interrogation. Therefore the PRF of the P2 pulses
mimics the
PRF of the P3 pulses albeit with a 6 or 19 microseconds time shift. The PRF of
the P2
pulses also mimics the PRF of the P1 pulses with a 2 microseconds time shift.
The critical
point in measuring the position of the target object 160 is to estimate or
predict when and
what mode of the interrogation signal is transmitted from the SSR 110. In the
case the
reply signal is received while the PSSR 300 is within the coverage area of the
SSR wide-
CA 3072396 2020-02-13

beam antenna as depicted in Figure 2A, the P2 pulse is then readily detectable
from the
wide-beam antenna and the PSSR 300 can directly estimate the transmit time of
the
interrogation signal P1 through the detection of P2 pulse and the estimation
of the
corresponding mode of this P2 pulse.
Alternatively, for the time/angles when the P2 pulses are not observed or too
weak
to be identified, which corresponds to the scenario depicted in Figure 2C, the
transmit time
of the interrogation signal is not known directly, and hence need to be
predicted in real
time based on the stagger pattern and interrogation pattern determined. In
this scenario,
the PSSR 300 predicts a transmit time of the interrogation signal P3 based on
the stagger
pattern and corresponding interrogation mode identified at step 430. Because
the
interrogation signal P3 is always synchronized with the P2 pulse, when the
transmit time
and corresponding interrogation mode of a P2 pulse is known, the end of the
transmit time
of the interrogation signal associated with this given P2 is known. The
transmit time of the
P1 pulse can as well be derived from the PRF pattern based on the known time
delay
between the 2 pulses.
At step 460, the PSSR 300 estimates the angle f3 and the sum of the distances
d1
and d2 described with regards to Figure 2A based on the interrogation mode,
transmit time
and reply signal reception time. Using the mechanical pointing direction of
the SSR MA
and the ellipse determined by the sum of the distance d1 and d2 in the art at
step 470, the
PSSR 300 can estimate the 3D coordinates of the target object 160. the 3D
coordinates
can be estimated using in particular the spheroid equations described with
regards to
Figure 2A.
Figure 5A details the operation of step 430 for determining the PRF or stagger
pattern of the flowchart 400. At step 510 the 1st and 2nd P2 pulses are
identified and a 1st
interval between the two pulses are determined at step 520. The 1st interval
is used as the
initial interval of the time ordered-sequence of pulse intervals. When a new
pulse is
received, a new interval is computed at step 530, in the present invention,
computing a
new interval is based on a new pulse and the last valid received pulse, as
stated
previously in the description of Figure 3.
31
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The identification of the PRF pattern is based on an identification of a
repeating
sequence of intervals within the time-ordered sequence of pulse intervals as
defined
previously. The PSSR 300 at step 540 compares the new interval with the first
interval and
if there is no match the new interval is added to the time-ordered sequence of
pulse
intervals at step 550 and the flowchart loops back to step 530 to receive a
new P2 pulse
and determine a new interval.
If at step 540 a new interval matches the 1st interval, the procedure for
identifying
the repeating sequence starts at step 560 with said new interval identified as
the Kth
interval. The ith (i from 1) interval after the Kth interval will be examined
one by one to see if
it matches the 1 + ith interval until a) if i reaches N, for example N = 6,
then the intervals
before Kth are the stagger or PRF pattern (1 to k-1th ); or b) if the ith
interval after Kth does
not match 1 + ith interval, then all the intervals between Kth (include Kth)
and K + ith (include
K + ith) will be added to the end of the stagger pattern and the algorithm
goes back to 530
to continue to examine new arrived P2 pulses.
Although the flowchart of Figure 5A compares at step 540 the new interval to
the
1st interval, the comparison could be performed between the new interval and a
previous
mth interval and therefore the PRF pattern would be the intervals between the
Mth and the
Kth interval.
Figure 58 details the operation of step 430 for determining the interrogation
pattern and mechanical rotation. Step 511 reads the PRF pattern determined and
stored
from the procedure shown by FIG 5A. Step 512 identifies the valid P1-P3 or P1-
P2-P3
pulse combinations. The confirmation of the pulse is based on the evaluation
of its
adjacent samples. If a sample passed a threshold set based on the average of
the
samples close to it, it will be considered to belong to a pulse. Other similar
techniques to
determine a pulse is a well-known art, so the detection of a pulse is not
limited to the one
described above. For a valid interrogation combination, each pulse should have
2 us pulse
width. If only P1 and P3 pulses are detected, they should either be 8 us apart
for Mode A
interrogation or 21 us apart for Mode C interrogation. If P2 pulse is also
present, it should
be 2 us away from the P1 pulse. Step 513 determines the interrogation mode
based on
the time interval between P1 and P3 pulses. For interrogation pattern
determination, the
32
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interrogation mode sequence received from MA is passed to step 514, in which
the
interrogation repetition pattern is determined. For example, the MA
interrogation sequence
could be ACACACA if 7 valid interrogation combinations are received. Because
the SSR
normally does not change the interrogation pattern during operation, the
algorithm will
identify one Mode A after one Mode C as the repetition pattern of the SSR.
Then the step
515 will search through the stored PRF or stagger pattern for a match of the
intervals
among the received MA interrogations and mark the matched section with the
corresponding interrogation mode. After that, step 516 will mark the rest of
the
interrogations in the stagger pattern with the interrogation sequence
identified in 514, so
that the interrogation mode of all the interrogations inside the stagger
pattern is known.
The staggered pattern together with the interrogation pattern are then output
to the
processor so that the transmit time and mode of any predicted interrogation is
determined.
In step 518, the time center of the valid interrogations can be calculated,
which
represents the time when the center of the MA points to the ownship. With two
of this time
information, the rotation period can be calculated. Because the SSR rotates at
a constant
speed, the pointing angle of the SSR MA can be estimated for any given time
instance.
This information is also passed to processor to estimate the angle 13 in FIG
2A or FIG 2C.
The accuracy of the positioning of the target object 160 is very sensitive to
the
accuracy of the time measurement because the distance used in the algorithm is
calculated by the product of the time and the speed of light.
One embodiment is to increase the accuracy of the time measurement of the
leading edge of each pulse. One traditional way of accurate time measurement
is to use
the GPS time, which generally gives an error of above 50 ns. Even the high
accurate GPS
device has an error of about 10 ns, which corresponds to a distance error of 3
m. In some
singular cases when the algorithm is very sensitive to the distance
measurement, even
this 3 m of error can cause a large error in the position calculation. Instead
of using GPS,
an Analog-to-Digital Converter (ADC) can be used to measure relative time. For
example,
with a high speed ADC such as a 1 GS/s ADC, the time accuracy is 1 ns, which
is ten
times better than a good GPS receiver.
33
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The present invention proposes an improved time measurement strategy that uses
a high-speed ADC in the acquisition of the interrogation signals and the reply
signals. In
one embodiment, the BB/IF sampling unit 327 of Figure 3 includes a high-speed
ADC.
Figure 6A and 6B show different implementations of the BB/IF sampling unit
327. In
Figure 6A a single channel high-speed ADC 327-3 is used, while in Figure 6B a
dual
channel high-speed ADC 327-3 is used.
As shown in Figure 6A, the signals from the 1030 MHz receiver 325 and 1090
MHz receiver 323 are mixed in the BB/IF sampling unit 327 with a 1060 MHz
Local
Oscillator 327-1 using a single mixer to generate one channel of intermediate
frequency
(IF) signal. This signal is then sent to a single channel high-speed ADC 327-3
for A-to-D
conversion and the digitized output signal is sent to the processor 310 for
further
processing as described in Figure 3.
In Figure 6B, each of the signals from the 1030 MHz receiver 325 and 1090 MHz
receiver 323 is mixed separately in the BB/IF sampling unit 327 with a
corresponding local
oscillator before being fed to a dual channel high-speed ADC 327-9. The signal
from the
1030 MHz receiver 325 is mixed with a 1030 MHz Local Oscillator 327-7 in a
mixer to
generate a first base band signal. The signal from the 1090 MHz receiver 323
is mixed
with a 1090 MHz Local Oscillator 327-5 in a mixer to generate a second base
band signal.
The two base band signals are then sent to a dual channel high-speed ADC 327-9
to
generate a digital output signal sent to the processor 310 for further
processing as
described in Figure 3. In this embodiment, the two channels in the ADC 327-9
share the
same clock keeping the time between the two base band signals still accurate.
In the embodiments of the present invention, especially when the ownship needs
to predict the time instance of a P2 pulse or interrogation when they are not
received,
depends heavily on the stability of the time of SSR transmission. If the SSR
interrogation
time changes slowly during time, due to time drift in the electronics of the
PSSR, an error
will accumulate and propagate so that the predicted/estimated P2 pulse or
interrogation
time no longer equals the real transmit time of the same P2
pulse/interrogation. In this
case, the position calculation of the target object when none of the P1, P2,
or P3 pulses is
received may be incorrect. Therefore, it is necessary to calibrate the time
instance of each
34
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of the interrogations in the stagger pattern frequently. The present invention
discloses a
method for calibrating the time-base using the P2 transmitted from the wide
beam antenna
of the SSR, which can be done once every several rotations or for every
rotation.
The calibration procedure takes several successive P2 pulses or successive
interrogations or successive combination of both, to match within the stagger
pattern.
Once a match is found, the method will compare the predicted time and the real
ADC time
that those pulses are received, and adjust the predicted time to the real
time. The
predicted time for other interrogations will also be adjusted by a same
amount. To reduce
the error of the match and calibration, averaging the real receiving time can
be done.
Because there are a lot more P2 pulses received in each rotation of the SSR
than the P1-
P3 pulses, using P2 pulses to calibrate the time drift is more accurate
because a statistical
process can be done more accurately using more samples, though P1-P3 pulses
may be
also used if required.
Generally, for faster positioning of the target object 160 after the PSSR 300
is
turned on, the first PRF pattern determined according to the method described
above will
be used for predicting the interrogation transmit time. However, for the time
measurement
of the P2 pulses, there could be an error compared to the real P2 time. There
are two
causes for this error. Firstly, the time measurement of the leading edge of
the P2 pulse
could have several samples deviation. Secondly, the sampling time may not
align with the
real leading edge of the transmitted P2 pulses.
Therefore, as more P2 pulses are observed, the original PRF pattern calculated
is
updated statistically. In one embodiment, an exponential filter for better
measuring the P2
pulse time is used. As an example, assuming the first time interval in the
first determined
PRF pattern is pi, the first time interval in the second determined PRF
pattern is 132, ..., the
first time interval in the nth determined PRF pattern is pn, then the first
time interval of the
updated PRF pattern used in the algorithm can be calculated as as average
p= p 1+ p 2 +. . . + pn . Alternatively, the first time interval may be
determined as a mean value
n
among p1, p2, ... pn time interval measurements, or as a mean square, or
another
function of the time interval measurements.
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For other time intervals between the adjacent interrogations in the PRF
pattern,
the same process is performed. This process keeps running at the background as
more
P2 observed (and hence the same PRF pattern can be determined more times). As
the
number of observed P2 increases, the filtered PRF pattern will approach the
real PRF
pattern used by the SSR 110, and hence increase the accuracy of the estimated
position
of the target object 160. Using P2 for this process can be much easier than
only using the
MA transmission.
In one exemplary embodiment, a method for finding the position of a target
object
such as target object 160 is shown in Figure 7 based on the PRF pattern and
the angular
rotation profile of the SSR 110 Main Antenna (MA). The steps of the flowchart
of Figure 7
are described below.
Step 705: profile the Main Antenna Angular or mechanical Rotation based on a
plurality of detection of SSR Main antenna signals at the ownship 140 by
recording the
time t1, t2, t3, ..., every time the ownship 140 is in the MA beam (ownship
140 receives
valid interrogation). 12-t1 is the time that MA of SSR 110 rotates 360 degrees
with a
constant speed. Knowing t1 and angular rotation speed va=360/(t241)
degrees/sec, the
pointing direction of MA can be calculated at any given time t. Additionally,
the angular
position may be also calibrated every time the MA illuminates the ownship 110
to prevent
rotation drift error. This step also decodes the mode of the successive
interrogation
messages and determines the interrogation pattern sequence using the P1-P3
pulses or
valid interrogation receive in MA.
Step 710: Use signal from wide-beam antenna to determine the PRF pattern of
P2. The algorithm for determining the PRF pattern is executed by the processor
310 as
stated earlier. After the staggered pattern is determined, the interrogation
pattern for all the
interrogations in the stagger pattern can be determined using the procedure in
FIG 5B.
Step 730: When a reply message from the target object 160 is received, we
first
check if it is within between two P2 pulses. Alternatively the check can be
performed
based on P1-P3 combination or P1-P2-P3 combination from the MA of the SSR 110.
If
yes, calculate d1+d2 (as shown in Figure 2) in step 740. If not, use the PRF
and
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interrogation pattern to predict interrogation time in Step 730. If the
prediction is correct,
the reply message will be in between two estimated interrogation times. Then
d1+d2 can
still be calculated.
Step 750: Decode the reply message to get the aircraft ID and altitude.
Step 760: At the same time, record the receiving time of the reply message.
Because the angular rotation of the MA is profiled, the angle 13 at which the
target object is
in the main lobe of MA (main antenna) beam is calculated.
Step 770: Solve the spheroidal equations to obtain the x,y,z coordinates of
the
target object in local coordinates system.
Step 780: Calculate the GPS position of the target object using local x, y and
z
coordinates.
Step 790: Input the GPS information into the display of the tracking system
360
and provide alarm to the ownship 140 when needed.
When P2 cannot be received, for example, the ownship is too far from the SSR
so
that only the main lobe interrogation signal can be received, it is still
possible to only use
the main lobe interrogation signal to determine the staggered pattern.
However, this could
take longer time because only part of (normally 5 to 10 interrogations
depending on the
signal strength) the staggered pattern can be received for each rotation of
the SSR MA.
Figure 8 shows a procedure for determining the staggered pattern using only
the
SSR MA signal. All the received successive MA pulses are treated and stored as
a group
as shown in block 801. For any new group, the algorithm first checks if any
part of the new
group, which should be at least 2 successive intervals, matches any part of
the previous
group (step 802). If there is no match, the algorithm will add the new group
to the
unresolved groups in step 803 and wait until new group is received. If there
is a match, the
algorithm will first stitch the new group with the matched group and then go
through all the
unresolved groups to see if there is any new match in 804 because the new
group could
bridge two existing unresolved groups. If there are still unresolved groups,
all the stitched
group will be stored as a longer new unresolved group, and the algorithm goes
back to
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801 to read new group. This process continues until all the unresolved groups
are stitched
together, which forms a temporary staggered pattern, and the algorithm goes to
step 807.
This step reads a new group and tries to match it in the temporary staggered
pattern. In
step 808, there is a timer that controls how many new matches are considered
to be
enough. For example, if the algorithm takes time T to form the current
temporary
staggered pattern, then it could be another nT (n can be 1, 2, 3, ...) time in
step 808 to be
considered. If all the new groups received in this nT interval match the
temporary pattern,
the algorithm will propose the temporary pattern to be a final staggered
pattern (step 809).
Once the ownship can receive the wide-beam SLS signal, the algorithm will
automatically
verify the staggered pattern determined using the P2 pulses train to see if
both staggered
patterns match.
In another aspect of the invention, when the target object does not have a
transponder, the principle of the embodiments of the present invention can
still be used to
determine a position of the target object. In this case, the ownship will
listen to the
reflection of the interrogation signal from the target object. Because when
the target object
is in the beam of the SSR, the energy of the interrogation signal will be
reflected from the
target object and received by the ownship. This receive time gives the same
information
as the receive time of the reply message, which can be used to calculate d1 +
d2 in Figure
2A together with the staggered and interrogation pattern, yet by using a
different equation
given above. The information that is still missing is the altitude of the
target object, without
which only a 2D positioning is possible. The lack of the altitude information
can be
compensated by using a phased array antenna or mechanically scanned antenna so
that
the angle of arrival (AOA) of the reflection is determined. With the AOA
information, the 3D
position of the target object can be now determined. The accuracy of the
position depends
on the accuracy of the AOA measurement, which means a larger array will give
better
position accuracy. After the AOA is measured, the intruder's altitude can be
determined by
AOA and the altitude of the ownship.
In another aspect of the invention, a coherent or non-coherent processing can
be
performed when multiple reflected interrogation signals are received. At any
given time
period, because the mechanical rotation of the SSR antenna, the staggered
pattern and
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the interrogation mode pattern are known, the time intervals between all the
transmitted
interrogations in this time period can be estimated. Therefore, an expected
time intervals
between the reflections of these interrogations are also known. Hence a
coherent
processing can be done by adding samples separated with these time intervals
to improve
the signal to noise ratio (SNR). For example, if the expected time intervals
between
several interrogations are t1, t2 t3, ..., then the samples that are t1, t2,
t3, ... from a start
point of the received signal will be added together to compete with noise.
This start point
of the coherent process can be sliding within a reasonable window inside which
the first
reflected interrogation can arrive. By doing this, the reflected
interrogations that are
submerged within the noise floor can be enhanced and detected, and so will be
the target
object.
In a further embodiment, past measurements may be used to make the position of
the target object 160 more precise, for example the target object 160 is
interrogated every
n seconds if SSR 110 rotates at delta rpm, where both the target object 160
and the
ownship 140 (observer) are moving.
The teachings of the present disclosure can be applied in various scenarios
including 1) whether or not the target object 160, the ownship 140 and the SSR
110 are
co-linear, but not co-altitude; 2) whether or not the target object 160, the
ownship 140 are
co-linear and co-altitude (singularity scenario).
The present PSSR system for target object detection can be used as part of an
advisory system to support a decision making during potential collision of a
UAV or a
manned aircraft.
The present invention can as well be used to predict the target object future
trajectory for a certain time look-ahead, and graphical display of current and
predicted
trajectory in 4D on the display of the ownship 140 and/or a Ground Control
Station (GCS)
computer. It may further comprise a decision support engine in the situation
of high
probability of potential collision and use the tracking system 360 for
graphical and audio
warnings to the pilot. The target object 160 trajectory prediction may be made
with a
certain time lookahead, where the lookahead time depends on the estimated
heading and
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speed of the target object while approaching the ownship 140. The decision
support
engine during collision avoidance may use online discrete-event supervisory
control based
on a predicted TIC (time-to-collision) and a predicted trajectory of the
target object for the
cases of full detectability and detection singularity that occurs when the
ownship 140, and
= the SSR 110 are co-linear.
In yet another embodiment, Figure 1 illustrates a generic configuration 100 in
which the present invention can be deployed showing the ownship 140, in
relation to the
SSR system 110 and a non-cooperative object, represented as an intruder 160.
In yet another embodiment of the invention, the SSR 110 transmits
interrogation
signals P1, P2 and P3 that can be received at the target object 160, this
transmission path
is represented as path 120. P1 and P3 pulses are transmitted through a narrow
beam
antenna of the SSR 110. P2 pulse is transmitted by a wide-beam antenna. P1, P2
and P3
pulses are received by the ownship 140. This is represented as path 130. The
reflected
signals from the intruder 160 are received at the ownship 140 through
transmission path
150 for further processing to derive information necessary to locate and
identify the target
object 160 as will be described hereinafter.
If the intruder does not have an onboard transponder, the interrogation signal
will
not be responded and the air traffic control (ATC) tower will not know the
existence of the
aircraft. However, this interrogation signal will still be reflected and can
be received by a
receive device on ownship 140. The advantage of detecting the reflected signal
from
ownship 140 is that the attenuation to the reflected signal is much less if
the ownship is
close to the intruder 160. The ownship can also receive the interrogation
signal directly
from SSR for profiling its transmission.
To use the received 1030 MHz reflection signal for calculating the intruder's
position, the transmission time of the reflected signal must be known. Modern
SSR 110
uses a staggered transmission interval to avoid interference from the other
SSRs.
Therefore, the profile of the interrogation time needs to be established first
using either P2
or P1/P3 of interrogation messages.
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Figure 9 illustrates a schematic 900 of the calculation of the intruder's
position
after the interrogation time of the SSR 110 is profiled. At any time ti, the
reflected
interrogation signal is received by the ownship, and if we know the
transmission time tO of
this interrogation signal, then ti - to is a known value. This means the sum
of the distance
(Al + A4 in Figure 9) from SSR to intruder and from intruder to ownship is a
known
constant. In a 3-dimensional (3D) space, the surface composed by the points
with this
constant sum distance is a spheroid. Therefore, the intruder must be on the
spheroid
plotted in Figure 9. The two focal points in this case are the SSR and the
ownship. By
using the fixed value calculated by (ti ¨ to), the spheroid can be determined.
The azimuth
position of the intruder can be measured by using the mechanical rotation of
the SSR
main antenna (MA).
In Figure 9, Al and A2 are the top boundary and bottom boundary of the SSR MA
fan beam. The antenna rotates about the Z-axis clockwise. When it points to
the intruder
160, the interrogation signal is transmitted and reflected by the intruder and
is received by
the ownship 140. Because the intruder 160 can only be on the spheroid, the
intersection
between the fan beam with the spheroid, which is shown by the dashed curve
111, is all
the possible positions where the intruder could be. The 3D coordinates of the
intruder can
be calculated if the altitude of the intruder is known. This is normally not
the case for a
non-cooperative target. Therefore, only an estimated position can be obtained.
In Figure 2A, the center of the SSR MA is a line after projected to X-Y plane
and is
marked by dl 113. It has an interception with the ellipse which is the 2D
projection of the
spheroid shown in Figure 9 onto the X-Y plane. The SSR MA rotates clockwise on
the X-Y
plane. The angle between the X axis and the MA when it points to the intruder
is p. The
angle 13 can be calculated using the receipt time of the reflected signal if
the rotation of the
SSR MA is known. Because the time it takes for the microwave to travel the
distance dl +
d2 is very short, a few microseconds, the assumption that the SSR does not
rotate in this
time interval is adopted. Therefore, the time the ownship receives the
reflection is the time
when the SSR MA points to the intruder.
For reducing the ambiguity caused by the lack of the altitude information of
the
intruder 160, a phased array antenna (not shown) or mechanically scanned
directional
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antenna (MSDA) 322 to decide the angle of the arrived reflection signals.
Because the
position of the dashed curve 111 in Figure 9 is known after the pointing angle
of the SSR
antenna is profiled, the phased array antenna or the MSDA 322 can scan along
the
dashed curve 111 to decide the 3D position of the intruder 160. During the
scan, the
received signal strength will vary with the change of the scan angle. The
angle where the
strongest signal is received indicates the altitude of the intruder. For
example, if the
phased array or MSDA 322 on the ownship 140 scan along the dashed curve 111 in
Figure 9, at position shown by A3, the received signal will be maximized
because the
reflected signal comes from this direction. Assume the angle between A3 and
the X-Y
plane is a, as shown in Figure 9. The expression for aircraft altitude h is
h -=-(2a - d) tan a where a, b is defined by the spheroid.
In another embodiment of the invention, if a phased array antenna or an MSDA
322 is not available, another way to minimize the effect of the unknown
altitude is to
assume the intruder 160 is at the same altitude as the ownship 140 (co-
altitude). This
determines an avoidance cylinder 401, centred at the intruder 160, that the
ownship 140
should avoid, shown in the schematic diagram 1000 of Figures 10. Figure 10
shows the
avoidance cylinder 401 centred at the intruder 160, and the ownship 140 in
relation to
each other. The intruder 160 and the ownship 140 are assumed to be co-
altitude, which is
indicated by the co-altitude line 407 in Figure 10. For a different ownship
140 in a different
area, the size and definition of the avoidance cylinder 401 is different.
For example, for a ownship at a terminal area, the protection area is an
avoidance
cylinder 401 centered at the intruder 160 with height of 450x2 feet and
diameter of 1500
feet. Besides the standard, the calculation error of the algorithm should be
considered.
This error depends on the accuracy of time measurement and SSR MA rotation
measurement. Assume the diameter error is El and altitude error is E2, then
the volume
of the avoidance cylinder 401 should be adjusted accordingly. The adjusted
volume for this
example is shown in Figure 10, where the avoidance cylinder height 403, !lac,
and the
avoidance cylinder diameter 405, dac, is calculated as follows:
hõ = 2*450 feet + 2*E2 = 900 feet + 2*E2
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dõ = 1500 feet + 2*E1
The size of the avoidance cylinder 401 depends on the standard at different
scenarios and is not limited to the example given above. The algorithm will
switch to
different avoidance cylinder 401 definitions, according to different
situations and different
measurement errors. The avoidance area depends on the aviation industry
standards
which are well defined in public documents, such as: DO-365 "Minimum
Operational
Performance Standards (MOPS) for Detect and Avoid (DAA) System", Appendix C,
RTCA,
May 31, 2017.
The ownship 140 is not suggested to change the flying altitude in this
situation
because the altitude of the intruder 160 is not known. The best approach is to
avoid the
avoidance cylinder 401 without changing the flying altitude.
Generally, the avoidance cylinder 401 (a few hundreds of meters, see the
example
above) that should be avoided by the ownship 140 is small, meaning that the
time it takes
an ownship to avoid the intruder 160 (avoidance time), is a few seconds. The
avoidance
time depends on the size of the ownship 140 and the intruder 160 and their
speeds. For
example, it may only take 10 seconds for the ownship to fly around the
avoidance volume,
which does not affect the total flying path of the ownship. As another
example, the
avoidance time may be in a range from about 1 second to about 10 seconds, or
alternatively from about 2 seconds to about 5 seconds, or yet alternatively
from about 3
seconds to about 6 seconds, etc. Therefore, this will not significantly affect
the planned
path of the ownship 140. A good tracker can also help to resolve the altitude
of the intruder
160. In the case that the intruder 160 is not at co-altitude with the ownship
140, it is not
possible for the intruder 160 and the ownship 140 to collide, and therefore
this case may
be disregarded.
The receivers on different ownships 140, or ground stations, can be networked
to
provide better measurement accuracy or to solve the altitude ambiguity. For
example, if
the ownship 140 is a UAV, it might be operated nearby the ground station. If
we install
receivers on both UAV and the ground station, they may receive reflections
from the same
intruder 160. In this situation, we have two sets of measurements for the same
intruder
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160. In another example, if there are multiple receiver-equipped ownships 140
in the same
area, all their detections can be used together at a centre processing unit
for better
detection and measurements.
The first use of the multiple networked receivers is to, resolve ambiguity, in
which
case there would be multiple spheroids in Figure 9. Each spheroid has a dashed
curve 111
and 111 (not shown) indicating all the possible positions of the intruder
measured from its
own onboard receiver. Two spheroids will solve the altitude ambiguity because
two curves
(on the two separated spheroid) can only have one intersection.
The second use of multiple receivers is to improve measurement accuracy. For
example, the position calculated by each receiver can be averaged to generate
a more
accurate measurement.
Figure 11A and 11B illustrate a Secondary Surveillance Radar (SSR) 110 system
1100 and 1150 embedded for detecting a non-cooperative object, such as an
intruder 160
and determining its positional information.
The SSR 110 system 1100 of Figure 11A comprises a receiver unit 320 for
receiving, through an antenna system. In a preferred embodiment, the receiver
unit 320
comprises an omni-directional antenna 324, for example a dipole. In this case
ownship
140 can always receive reflected interrogations from the target object 160.
The receiver
unit 320 may further comprise a directional antenna 322 (shown in Figure 11B),
such as
an electronically scanned antenna array or mechanically scanned antenna to
estimate the
angle of arrival (AOA) of the reflected interrogation signal from intruder,
This system is
illustrated in Figure 11B.
In Figure 11A, the receiver unit 320 comprises a receiver 325 (1030 MHz)
connected to the omni-directional antenna 324. The directional antenna 322 and
the omni-
directional antenna 324 can also be connected separately to the receiver a
receiver 325A
(1030 MHz) and a receiver 325B (1030 MHz) through a beam steering unit 326,
respectively, shown in Figure 11B. The directional antenna 322 and the omni-
directional
antenna 324 may also be connected through a splitter 700, shown in Figure 11C.
The
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receiver 325 is tuned to the 1030 MHz frequency band for receiving and
filtering P2 as
well as P1 and P3 signals in that frequency band.
The 1030 MHz receiver 325 is connected to a Base-band/Intermediary Frequency
(BB/IF) sampling unit 327 for receiving the signals detected by the receiver
325 and
converting them into a base band or into an intermediary frequency using a
local oscillator.
The BB/IF sampling unit 327 also digitizes the received analog signals by an
analog to
digital converter (ADC), and passes the digitized signals along to a processor
310 for
intruder 160 position calculation.
The receiver 325 which is tuned to 1030 MHz of Figure 11A is used to receive
the
signals. The receiver 325 has two functions: one function is to receive the
interrogation
signal directly from the SSR 110 so that the interrogation time, type and SSR
antenna
rotation can be profiled; the second function is to receive the reflected
interrogation signal
from the intruder 160. There can be a single or multiple receive channels on
the device
depending on the configuration of the receiver 325. For example, in one
configuration, the
receiver 325 only has one channel that is connected to an omni-directional
antenna 324.
In this case, both the direct signal from the SSR 110 and the reflected signal
from intruder
160 are received and analyzed by the receiver 325. The omni-directional
antenna 324
makes sure that signal from all directions can be received so that full
awareness of the
nearby intruders 160 is provided. The system architecture for this case is
shown in Figure
11A.
An alternate system architecture 1150 is shown in Figure 11B. The receiver 325
can have two channels. One channel 325A is connected to an omni-directional
antenna
324 (or a directional antenna which is not shown for channel 325A) that
receives both a
direct signal from the SSR 110 and a reflected signal from intruder 160. The
other channel
325B is connected to a phased array antenna (not shown) or MSDA 322, which is
used to
determine the altitude of the intruder 160. The steering angle of the phased
array antenna
or MSDA 322 is controlled by the signal processing unit through a beam
steering unit 326
for searching along the dashed curve 111 of Figure 9 once it is known. The
beam steering
unit could be a mechanical motor that drives a directional antenna or a
controller for the
phased array antenna that controls the electronic scan of the phased array
antenna. The
CA 3072396 2020-02-13

directional antenna 322 or the phased array antenna is indicated on top of the
beam
steering unit. After the processing unit determines the dashed curve 111 in
Figure 9, the
beam steering unit points the antenna beam mechanically or electronically to
the curve
and then scan along the curve. When the beam of the phased array antenna or
MSDA
322 point to the direction of the intruder 160, the strength of the received
reflection
reaches maximum. The current pointing angle is then reported to the processor
for
calculating the altitude of the intruder 160. In this case, the phased array
antenna or
MSDA 322 can also be used to point to the moving direction of the ownship 140
when not
scanning on the dashed curve to detect a reflected signal so that the SNR and
detection
range can be improved.
The processor 310 is connected to a GPS unit 350 to measure the position of
the
ownship 140. The processor 310 also stores the location of the SSR 110 so that
the
distance between the ownship 140 and the SSR 110 can be calculated.
In one embodiment processor 310 provides the processing power for performing
the operations of the present invention. The processor 310 can be a micro-
controller or a
microprocessor or any processor device capable of executing the operations of
the
present invention, such processor devices are well known to those skilled in
the art. The
processor 310 receives digital signals from the receiver unit 320 and executes
operations
dictated by operating modules embedded or connected to the processor 310.
The SSR interrogation profile unit 371 predicts the transmission time and mode
for
any given SSR interrogation based on the interrogations received directly from
the SSR at
the ownship. This process is described by Figure 5A or Figure 8. The SSR MA
rotation
profile unit 381 predicts the pointing direction of the SSR MA so that when a
time instance
is given, the angle 13 in Figure 2A or Figure 9 can be determined. This
process is
described in Figure 5B. The reflection process unit 391 is used to process the
received
reflected signal so that the position of the intruder can be determined. The
system and
process are described in Figures 15A-D below.
The system architectures 1100 and 1150 of Figure 11A and 11B rely on a storage
unit 330 and a memory 340, both connected to the processor 310 to store data
and
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information necessary to its operation. Permanent or long term data such as
SSR location,
PRF pattern once identified can be stored in the storage unit 330, while short
term data
such as time-ordered sequence of pulse intervals, cached data or other program
instructions can be stored in the memory 340.
Furthermore, there is a Global Positioning System (GPS) unit 350 for
determining
the location of the ownship 140. All the information related to the position
and trajectory of
the ownship 140 as well as the target object 160 is displayed on a display for
advising the
pilot of the ownship 140. In one embodiment, the display is part of a tracking
system 360
that monitors the relative distance between the intruder 160 and the ownship
140. The
tracking of the position and trajectory of the ownship 140 and intruder 160 on
the display
provides a visual cue to the pilot of the ownship 140 to know the relative
spacing between
the ownship 140 and target object 160 and to take appropriate measures to
mitigate any
potential problem. More importantly, this allows the prediction of the
intruder movement
based on the previous detection results and provide a confident estimation of
the position
of the target object even when the detection of the target object is missed in
several
detections. Additionally an audio alarm system may be provided as part of the
tracking
system 360 to alert the pilot as well. Alternatively, The display may be
standalone or
shared with other components such as a computing device within the ownship 140
and/or
the GPS unit 350 and the tracking system 360.
Figure 11D shows a schematic block diagram 1170 for detecting, tracking and
avoiding non-cooperative objects, or target 160, by an ownship 140, which
employs the
systems described above and in Figures 11A, 11B and 11C above, as well as a
processor
310.
The first step 1170a is to detect a reflected interrogation signal from the
non-
cooperative object (target 160), having been sent from a secondary
surveillance system
(SSR). This method step is performed by the receiver 320, which is shown in
Figures 11A
and 11B.
The next step 1170b is to process the reflected interrogation signal, yielding
a
processed reflected interrogation signal. This step is performed by the BB/IF
sampling unit
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327, which digitizes the signal, and by the reflection process unit 391, in
conjunction with
the processor 310.
The final step 1170c is to determine a position of the non-cooperative object
(target 160) from the processed reflected interrogation signal, thereby
allowing the
ownship 140 to track and avoid the target 160. The step 1170c is performed by
the
reflection process unit 391, which additionally constructs a spheroid of
possible locations
of the intruder 160. The tracking information is monitored in the tracking
system 360,
which displays tracking information.
Figure 11E illustrates an expanded schematic block diagram 1170b for
processing
the reflected interrogation signal, and expands on the method step 1170b from
Figure 11D
above. The first step 1170b-1 of processing the reflected interrogation signal
is to
determine a range of durations for time windows, during which the reflected
interrogation
signal arrives at the ownship, the durations being comparable to an
interrogation time of
travel from a secondary surveillance radar, SSR, to the ownship. The next step
1170b-2 is
to integrate the reflected interrogation signal across the time windows
determined in the
step 1170b-1. The final step 1170b-3 is to identify and classify peaks in the
integrated
reflected interrogation signal integrated in the step 1170b-2. These steps are
performed
within the integration unit 949 of Figure 15D.
Figure 11F illustrates an expanded schematic block diagram 1170c for
determining
the position of the target 160 from the processed reflected interrogation
signal, and
expands on the method step 1170c from Figure 11D above. The first step 1170c-1
is to
calculate a range of possible positions of the non-cooperative object from the
processed
reflected interrogation signal. The next step 1170c-2 is to scan the range of
possible
positions of the target 160, with a phased array antenna or MSDA 322, if it
exists. The final
step 1170c-3 is to detect the position of the target 160 based on the
scanning. These
steps are performed within the position calculation unit 952 of Figure 15D.
Figure 12 shows a schematic diagram 1200 displaying a working example of using
a single reflected interrogation signal to detect intruders 160. The signal
shown in Figure
12 is part of a signal received at the ownship 140. The pulse starting from
sample 17 is the
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received P1 pulse of a interrogation at ownship, and the pulse starting from
sample 177 is
the P3 pulse of the same interrogation received at ownship 140. The pulses
starting from
samples 49 and 210 are the reflections of the P1 pulse and P3 pulse received
at ownship
140, separately. Because the P1 and P3 pulses comprising the interrogation
signal are
strong, their reflections can be observed directly if the intruder is a good
reflector and is
close to the ownship. From Figure 12, we know that the reflections come from
the same
intruder 160 because they are all 32 samples from their original signal. If we
know the
sampling rate of the ADC is 20 MHz, the distance dl + d2 in Figure 2A is
32/20e6*3e8/2 +
dõ-= (240 + dõ) m, where dõ is the distance between the ownship 140 and the
SSR 110.
One advantage of the SSR 110 over PSR is that it transmits less power to
detect
the transponder equipped aircraft. For this reason, the reflection of the 1030
MHz SSR
signal is generally weak due to the small power transmitted by SSR 110 and can
be easily
submerged in the noise. However, the SSR 110 transmits at predictable
intervals, which
makes the coherent/non-coherent integration (simply referred to as integration
if not
specified) of the reflections possible.
Figure 13A shows a schematic diagram 1300 of a standard Mode A/C interrogation
message transmitted by the SSR 110. The message is composed by 3 separate
pulses,
P1, P2 and P3. Each pulse is 0.8 us long. P2 is always 2 us from P1, and P3 is
8 us from
P1 if it is a Mode A interrogation or 21 us from P1 if it is a Mode C
interrogation.
Figure 13B shows a schematic diagram 1350 of how the SSR interrogations and
their corresponding reflections are placed. B1, B2, B3, ... Bn are the
interrogation signals
from SSR that is directly receive at ownship and Bri, Br2, Br3, ... Brn are
the
corresponding reflections from an intruder received at ownship. Bri and Bi
(i=1,2,3,4,...,n)
are all composed by one or all of the pulses in Figure 13A. In other words,
Bri and Bi
(i=1,2,3,4,...,n) can either be an interrogation message which includes P1, P2
and P3
shown in Figure 13A, or they can be only the side-lobe suppression pulse P2
from Figure
13A. Modern SSRs interrogate with a pulse repetition frequency (PRF) around
100 Hz. If
the assumption is adopted that the intruder 160 does not move in a short
period of time,
then the distance between Bri and Bi (i=1,2,3,4,...,n) is constant. If all Bri
(i=1,2,3,4,...,n)
can be observed, then the integration is easy because the position of Bri
(i=1,2,3,4,...,n)
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can be read directly from the data. If some of the Bri (i=1,2,3,4,...,n) is
lost, then their
position must be estimated. For example, if the position of Bi (i=1,2,3,...,n)
and Bri on
the time axis of Figure 13B are known, the positions of other Bri
(i=2,3,...,n) can be
predicted, as long as the stationary intruder assumption holds. If all the Bri
(i=1, 2,3,...,n)
are added coherently (using phase) or non-coherently (using only amplitude or
square of
amplitude or so on), the signal to noise ratio (SNR) will be enhanced. This
can make the
submerged 1030 MHz reflections stand out of the noise and detectable.
Moreover, the separated pulses of Figure 13A comprising each of the Bri
(i=1,2,3,4,...,n) can be coherently/non-coherently integrated too. For
example, in one
case, the leading edge of Bri could be the leading edge of P1 pulse in Br1. If
the
interrogation type of B1 is known, the position of the P2 and P3 pulses in Bri
are known.
Then, P2 and P3 pulses can be added to the P1 pulse to further improve the
SNR.
There are two problems in practice when applying the coherent/non-coherent
integration.
Firstly, the Bi (i=1,2,3,4,...,n) in Figure 13B may not be received when Bri
(i=1,2,3,4,...,n) is received (ownship is outside of the SSR coverage but
intruder is within
the SSR coverage). Because the start of the integration needs to be aligned to
the start of
transmission time Bi (i=1,2,3,4,...,n), their position on the time axis have
to be
predicted/estimated. After knowing the positions of Bi (i=1,2,3,4,...,n), the
position of the
intruder can be calculated. This can be done by profiling the SSR and
predicting the time
of Bi.
Secondly, all the Bri (i=1,2,3,4,...,n) in Figure 13B may be submerged in the
noise
so reflections are not seen at all. This causes the problem that it is not
known to which
samples the coherent/non-coherent integration should be applied. To solve this
problem, a
window Wi can be taken after each Bi (1=1,2,3,4,.. .,M) under consideration. M
is the
number of reflections that are integrated. All the Wi (i=1,2,3,4,... ,M) have
the same size
and have the same time delay after Bi (i=1,2,3,4,...,M). Assuming the intruder
does not
move during the period of the integration, the same time delay in each of WI
corresponds
to a possible reflection from the same intruder.
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Coherently/non-coherently adding the corresponding samples among the windows
will increase the SNR of the reflected signal if there is any. If all the
samples in each
window are noise, the integration result is still noise. After this
integration, the integration
within an interrogation (or between P1, P2 and P3) can be performed. For
example, in
Figure 12, the reflection of P3 pulse can be integrated to the reflection of
P1 pulse for
each sample separately in the window to further improve the SNR.
The position and size of the window depends on the range in which the
submerged reflection needs to be searched. It can be the whole time between
successive
interrogations so that all possible reflections from intruder are considered.
Figure 14A shows a signal collection diagram 1400 where the signal is
collected
during 1 s interval. The peaks shown in Figure 14A are P2 pulses from the SSR
omni-
directional antenna 324 for sidelobe control purposes.
Figure 14B is an expanded view of the signal collection diagram 1400, showing
a
zoomed view of one of the P2 pulses in Figure 14A. The peak at the 501th
sample is the
leading edge of the original P2 pulse, and no reflections from this P2 pulse
can be
observed. This is because the P2 pulse is much weaker than P1 and P3 pulses
shown in
Figure 12, so its reflections are submerged in the noise.
The window size is chosen for integration to be within 500 samples centered
at
each P2 pulses. Then con-coherent integration is performed for the samples
between the
windows as described above. Basically, all the first samples in each window
are added,
and all the second samples in each window are added, and so on. The result has
the
same length as the window size.
Figure 14C shows the results after the non-coherent integration is applied to
the
signal collection diagram 1400, taken from Figure 14A. Two major reflections
are obtained,
which are located at 532ed samples and 589th samples after the original P2
pulse at
500th samples. Compared to Figure 12, which originates from the same test
configuration
but using single P1 and P3 pulse for measuring reflection, the peaks in Figure
14C are
better defined and separated, especially the second major reflection which is
not obvious
in Figure 12. Note that the relative position of the first reflections is the
same in both
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Figure 12 and Figure 14C, which is all 32 samples from the original signal.
This is an
indication that the reflection comes from the same intruder.
The example of Figure 14A, 14B and 14C demonstrates the non-coherent
integration of the P2 reflections. Because P1 and P3 pulses are not available
in this
example, we did not perform the integration within interrogations as described
above. The
integration method can be performed for P1 or P3 pulses as well, or can be
further
performed within the interrogation, in which case it would be required to
integrate the P3
pulse to P1 pulse (or P1 to P3) for further SNR improvement.
Figure 15A shows a schematic block diagram for processing the reflected
interrogation signal. The parameters are calculated in the block 901,
including the SSR
interrogation time and type, the SSR MA rotation, the number of integrated
reflections M,
and the window size and delay from Bi (i=1,2,3,4,...,M). The following
processing steps of
the algorithm are summarized as follows:
1. Profile the interrogation time and type (whether it is a Mode A or C
interrogation) of
the SSR 110.
2. Profile rotation of the SSR antenna (where is the antenna of SSR pointed to
at any
time).
3. Determine the time Tr it takes for the interrogation signal to reach the
ownship 140
based on the distance between the SSR 110 and the ownship 140. Because the
SSR 110 and the ownship 140 position is known, this distance can be calculated
using a standard method. After Tr is known, the transmission time of the
interrogations can be back calculated based on the time when the ownship 140
receives these interrogations.
4. Determine a time window (both the size and position) to process after the
predicted
interrogation time. The window should not be too close or too far from the
interrogation time. It is better that Tr from step 3 is in the middle of this
window. In
this case, if there is any aircraft that is close to the ownship 140 and from
whom the
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reflected signal is submerged by the noise, the integration can make the
intruder
160 detectable. Durations of time windows may be defined by a user
depending on a monitoring distance. For example, the ownship 140 may
monitor a target 160 within 2 to 20 km from the ownship 140, then the
predetermined monitoring distance will be defined to be from about 2 km to
about 20 km. It is understood that other monitoring distances are also
possible.
5. Determine the number of windows that should be integrated based on the ADC
sampling rate and the expected speed of the intruder (assume this number is
M).
The intruder 160 is assumed to be stationary within the reception time of
those
reflections. For example, if the goal is to non-coherently integrate the P1 of
Bri, then
the tolerance for the movement of the aircraft is 0.8 us x 3e8 m/s = 240 m. As
long
as the aircraft moves less than 240 m, it can be seen as stationary because
the
peak of the reflected P1, which lasts 0.8 us, will still be integrated. This
can
normally give 100 - 200 of reflections to be effectively integrated.
6. Take a window with the same size and time delay after each of Bi
(i=1,2,3,4,... ,M),
and define these windows as VVi (i=1,2,3,4,...,M). Coherently/non-coherently
adding
the corresponding samples among the windows. The result is a data vector Si
with
the same length of the window size. These steps are represented by blocks 903
and 905.
7. Detect peaks in the vector Si. By knowing the type of the transmitted
signal from
SSR, the characteristic of reflected signal can be determined. For example, if
the
transmitted signal is a full interrogation with P1 and P3 pulses, the
reflected signal
is the combination of P1 and P3 pulses like in Figure 12. This combination of
P1
and P3 pulses should be classified as one reflection from the same target. If
the
transmitted signal is only the P2 control pulse, then the reflected signal is
also a
single pulse with the same duration as P2. This step is represented by block
907.
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8. Integrate P3 (or P1) pulse into a P1 (or P3) pulse for each sample in the
result data
vector Si to further improve the SNR, taking the first sample in Si for
example. If
we assume this sample is from the P1 pulse, then the position of the P3 pulse
can
be calculated in each WI (i=1,2,3,4,... ,M), and be added to the first sample
of Si.
Assume the result is S2. The reason to find the P3 pulse in WI is that the
position of
P3 is different depending on the type of the interrogation. In each Wi, the
reflection
type of the signal expected in this window is known because the SSR
interrogation
type for this is known. Then the position of P3 in each WI can be determined.
This
step is represented by block 909. If the sample is not the last sample (block
911),
move on to block 913 and repeat the process of block 909.
9. Detect new peaks that appeared in S2. These are the ones that even
submerged in
the noise of Si after the first integration. This step is represented by block
915.
10. If there is any reflection either in Si or S2 after classification,
calculate the time
interval between the reflection and the interrogation time, which gives the
sum
distance of Al + A4 in Figure 9. Then use the principle illustrated in Figure
9 and
Figure 2A to calculate the spheroid on which the intruder is located. In the
reflection
case, 3 us transponder response time should not be considered like in the
cooperative case because the reflection of the interrogation signal is
immediate.
This step is represented by block 917.
11. After the spheroid is determined, determine whether there exists a phased
array
antenna 10 or MSDA 322. This step is represented by block 919.
12. If there is phased array antenna 10 or MSDA 322, scan the dashed curve 111
of
Figure 9 for the accurate position of the intruder 160. This step is
represented by
block 923. Go to step 14.
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13. If the phased array antenna 10 or MSDA 322 is not available, assume the
intruder
160 is co-altitude with the ownship 140 and calculate the avoidance cylinder
401.
This step is represented by block 921. Go to step 16.
14. Use the altitude information from the previous steps (represented by
blocks 923)
to detect the position of the target 160 (represented by block 934).
15. The position of the intruder 160 is then used to perform tracking and
avoidance of
the intruder 160 by the ownship 140, which is represented by block 935.
16. Set the current B2 as B1 for the next run and go to step 6.
Figure 15B is the schematic block diagram for processing the reflected
interrogation signal from Figure 15A above, showing the more general method
steps
1170a, 1170b and 1170c from Figure 11D above.
Figure 15C is the schematic block diagram for processing the reflected
interrogation signal from Figure 15A above, showing the method steps 1170b-1,
1170b-2
and 1170b-3 from Figure 11E, and the steps 1170c-1, 1170c-2 and 1170c-3 from
11F.
Figure 15D shows a schematic diagram of the reflection process unit 391. The
reflection process unit 391 is connected to the processor 310 to store data
and information
necessary to its operation, and is connected to the tracking system 360 which
monitors
the position of the intruder 160. The reflection process unit 391 comprises a
data
processing unit 940, for processing the reflected interrogation signals.
Calculation of
parameters, such as the SSR interrogation time and type, the SSR MA rotation,
the
number of integrated reflections M, and the window size and delay from Bi
(i=1,2,3,4,...,M)
is performed by the parameter calculation unit 943. Steps 1-5 in the
processing steps of
the algorithm above (Figure 15A) are performed in the parameter calculation
unit 943.
The parameters are sent to the integration unit 949 for the first integration.
This is
the step 6.
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The resulting data vector Si is sent to the detection unit 946 where the
characteristics of the reflected signals are determined, and the signals are
distinguished
and classified into reflected signals from the target 160. The step 7 (Figure
15A) is
performed in the detection unit 946.
The peak detected signals are again sent to the integration unit 949, where
the the
inner interrogation integration is performed to further improve the SNR. The
step 8 (Figure
15A) is performed in the integration unit 949. If additional peaks occur,
representing the
reflected signals, after integration is performed in the integration unit 949,
this data is sent
back to the detection unit 946 before returning to the integration unit 949.
Step 9 (Figure
15A) is performed in the detection unit 946.
The improved signals are sent to the position calculation unit 952, where the
position of the reflected pulse is determined. This process first constructs a
spheroid of
possible locations of the intruder 160. The position calculation unit 952
instructs a phased
array antenna or MSDA 322, if it exists, to scan for the accurate position of
the target 160.
The position of the target 160 is detected based on the scanning. The position
information
is then sent to the tracking system 360. If the phased array antenna or MSDA
322 does
not exists, the position calculation unit 952 takes on the assumption that the
target 160 is
at co-altitude with the ownship 140, and calculated the avoidance cylinder 401
of Figure
10. The steps 10-15 (Figure 15A) in the Figure 15A above are performed in the
position
calculation unit 952.
In yet another embodiment of the invention, shown in schematic diagram 1600 of
Figure 16, the reflected SSR signal can be masked by the direct SSR signal. P1
and P3
are the pulses received directly from the SSR and P1' and P3' are the
reflected pulses
from the intruder 160. In this case, the reflected P1 pulse is overlapped with
the direct P3
pulse. Because the direct signal is normally much stronger than the reflected
signal, the
P1' pulse is masked by P3 and will not be detected. The reflected
interrogation will then
have a different characteristic from the transmitted one. For example, the
transmitted
interrogation is composed by two pulses, but only one reflected pulse is
observed.
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To solve the masking problem, the integration can be performed on different
type
of interrogations. The reflections can be integrated corresponding to Mode A
interrogation
and Mode C interrogation separately. Then, the mask issue may happen in one
type of
interrogation and will not happen in the other, because the P3 pulse has a
different
distance from the P1 pulse for a different interrogation type.
A summary of the present invention is reproduced below for convenience. There
is
provided a method for tracking and avoiding a non-cooperative object by an
ownship,
comprising employing at least one hardware processor for: detecting a
reflected
interrogation signal from the non-cooperative object, the reflected
interrogation signal
being an interrogation signal sent from a secondary surveillance radar and
reflected off the
non-cooperative object, processing the reflected interrogation signal,
yielding a processed
reflected interrogation signal, and determining a position of the non-
cooperative object
from the processed reflected interrogation signal, thereby allowing the
ownship to track
and avoid the non-cooperative object. The method further comprises tracking
and avoiding
the non-cooperative object.
The detecting step of the method comprises capturing the reflected
interrogation
signal by an antenna to generate a captured reflected interrogation signal,
and forwarding
the captured reflected interrogation signal to a 1030 MHz receiver. The
capturing the
reflected interrogation signal comprises one of the following capturing the
reflected
interrogation signal by a directional antenna, capturing the reflected
interrogation signal by
an omni-directional antenna, capturing the reflected interrogation signal by a
directional
antenna and an omni-directional antenna, which are connected by a splitter.
The processing step of the method comprises (i) determining a range of
durations
for time windows, during which the reflected interrogation signal arrives at
the ownship, for
example the durations being comparable to an interrogation time of travel from
a
secondary surveillance radar, SSR, to the ownship, (ii) integrating the
reflected
interrogation signal across the time windows determined in the step (i), and
(iii) identifying
and classifying peaks in the integrated reflected interrogation signal
integrated in the step
(ii).
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The integrating the reflected interrogation signal across the time windows
further
comprises determining a plurality of sequences of time windows, within which
respective
reflected interrogation signals arrive at the ownship, each time window Wi in
a sequence
Wi having a same duration and a same time delay from a respective start point
for said
each time window, and for each sequence VVi', processing corresponding samples
of the
reflected interrogation signal. The processing corresponding samples further
comprises
one of the following processing the corresponding samples coherently,
processing the
corresponding samples non-coherently. Additionally, the determining a range of
durations
for time windows comprises choosing durations to cover a predetermined
monitoring
distance, for example from about 2km to about 20km from the ownship.
The integrating the reflected interrogation signal across the time windows
further
comprises determining a number of time windows to be integrated, based on at
least one
of the following: the non-cooperative object being considered stationary for
said number of
time windows to be integrated, an analog-to-digital (ADC) sampling rate, an
expected
speed of the non-cooperative object. The identifying and classifying peaks
comprises
comparing the reflected interrogation signal and/or the integrated reflected
interrogation
signal with an interrogation pattern of P1, P2 and P3 pulses generated by the
SSR.
The determining step of the method comprises calculating a range of possible
positions of the non-cooperative object from the processed reflected
interrogation signal,
scanning the range of possible positions of the non-cooperative object, and
detecting the
position of the non-cooperative object, based on results of the scanning.
The calculating the range of possible positions of the non-cooperative object
comprises calculating a spheroid, wherein the secondary surveillance system is
at a first
focal point of the spheroid, and the ownship is at a second focal point of the
spheroid, and
the non-cooperative object is on the spheroid. The scanning the range of
possible
positions comprises one of the following scanning with a phased array antenna,
scanning
with a mechanically scanned directional antenna (MSDA). Furthermore, the
scanning the
range of possible positions comprises changing a scan angle along the range of
possible
positions of the non-cooperative object, detecting a strongest signal strength
along the
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range of possible positions of the non-cooperative object, determining a
strongest scan
angle, corresponding to the strongest signal strength, and calculating an
altitude of the
non-cooperative object from the strongest scan angle.
The determining step of the method comprises applying a co-altitude assumption
between the non-cooperative object and the ownship, determining an avoidance
area
around the non-cooperative object, by using the processed reflected
interrogation signal
and the co-altitude assumption, and assuming the position of the non-
cooperative object is
within the avoidance area. The determining the avoidance area further
comprises one of
the following choosing a size of the avoidance area so that an avoidance time
for avoiding
the non-cooperative object by the ownship is in a range from about 1 second to
about 10
seconds, choosing a size of the avoidance area in accordance with aviation
standards.
The avoidance area may be a cylinder.
A system for tracking and avoiding an non-cooperative object by an ownship is
provided, comprising a memory device for storing computer readable
instructions thereon
for execution by at least one processor, causing the at least one processor to
detect a
reflected interrogation signal from the non-cooperative object, the reflected
interrogation
signal being an interrogation signal sent from a secondary surveillance radar
and reflected
off the non-cooperative object, process the reflected interrogation signal,
yielding a
processed reflected interrogation signal, and determine a position of the non-
cooperative
object from the processed reflected interrogation signal, thereby allowing the
ownship to
track and avoid the non-cooperative object.
The computer readable instructions further cause the at least one processor to
track and avoid the non-cooperative object. The computer readable
instructions, causing
to detect, further cause the at least one processor to capture the reflected
interrogation
signal by an antenna to generate a captured reflected interrogation signal,
and forward the
captured reflected interrogation signal to a 1030 MHz receiver. The computer
readable
instructions, causing to capture the reflected interrogation signal, further
cause the at least
one processor to perform one of the following capture the reflected
interrogation signal by
a directional antenna, capture the reflected interrogation signal by an omni-
directional
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antenna, capture the reflected interrogation signal by a directional antenna
and an omni-
directional antenna, which are connected by a splitter.
The computer readable instructions, causing to process, further cause the at
least
one processor to a memory device for storing computer readable instructions
thereon for
execution by at least one processor, causing the at least one processor to:
detect a
reflected interrogation signal from the non-cooperative object, the reflected
interrogation
signal being an interrogation signal sent from a secondary surveillance radar
and reflected
off the non-cooperative object, process the reflected interrogation signal,
yielding a
processed reflected interrogation signal, and determine a position of the non-
cooperative
object from the processed reflected interrogation signal, thereby allowing to
track and
avoid the non-cooperative object. The computer readable instructions, causing
to integrate
the reflected interrogation signal, further cause the at least one processor
to determine a
plurality of sequences of time windows, each time window WE in a sequence Wi'
having a
same size and a same time delay within which respective reflected
interrogation signals
arrive at the ownship, and for each sequence VVi', process corresponding
samples of the
reflected interrogation signal. The computer readable instructions, causing to
process
corresponding samples, further cause the at least one processor to perform one
of the
following process the corresponding samples coherently, process the
corresponding
samples non-coherently. The computer readable instructions, causing to
determine a
range of durations for time windows, further cause the at least one processor
to choose
durations to cover a predetermined monitoring distance, that is to receive the
reflected
interrogation signal within the monitoring distance. The computer readable
instructions,
causing to integrate the reflected interrogation signal across the time
windows, further
cause the at least one processor to determine a number of time windows to be
integrated,
based on at least one of the following: the non-cooperative object being
considered
stationary for the number of time windows to be integrated, an analog-to-
digital (ADC)
sampling rate, an expected speed of the non-cooperative object.
The computer readable instructions, causing to identify and classify peaks,
further
cause the at least one processor to compare the the reflected interrogation
signal and/or
the integrated reflected interrogation signal with an interrogation pattern of
P1, P2 and P3
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pulses generated by the SSR. The computer readable instructions, causing to
determine,
further cause the at least one processor to calculate a range of possible
positions of the
non-cooperative object from the processed reflected interrogation signal, scan
the range
of possible positions of the non-cooperative object, and detect the position
of the non-
cooperative object, based on results of the scanning.
The computer readable instructions, causing to calculate a range of possible
positions, further cause the at least one processor to calculate a spheroid,
wherein the
secondary surveillance system is at a first focal point of the spheroid, and
the ownship is
at a second focal point of the spheroid, and the non-cooperative object is on
the spheroid.
The computer readable instructions, causing to scan the range of possible
positions,
further cause the at least one processor to perform one of the following scan
with a
phased array antenna, scan with a mechanically scanned directional antenna
(MSDA).
The computer readable instructions, causing to scan the range of possible
positions, further cause the at least one processor to change a scan angle
along the range
of possible positions of the non-cooperative object, detect a strongest signal
strength
along the range of possible positions of the non-cooperative object, determine
a strongest
scan angle, corresponding to the strongest signal strength, and calculate an
altitude of the
non-cooperative object from the strongest scan angle. The computer readable
instructions, causing to determine, further cause the at least one processor
to apply a co-
altitude assumption between the non-cooperative object and the ownship,
determine an
avoidance area around the non-cooperative object, by using the processed
reflected
interrogation signal and the co-altitude assumption, and assume the position
of the non-
cooperative object is within the avoidance area. The computer readable
instructions,
causing to determine the avoidance area, further cause the at least one
processor to
perform one of the following choose a size of the avoidance area so that an
avoidance
time for avoiding the non-cooperative object by the ownship is in a range from
about 1
second to about 10 seconds, choose a size of the avoidance area in accordance
with
aviation standards.
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In a system for tracking and avoiding a non-cooperative object, having a means
for
detecting a reflected interrogation signal from the non-cooperative object,
the reflected
interrogation signal being an interrogation signal sent from a secondary
surveillance radar
and reflected off the non-cooperative object, to provide an apparatus,
comprising a
memory device for storing computer readable instructions thereon for execution
by at least
one processor, causing the at least one processor to process the reflected
interrogation
signal, yielding a processed reflected interrogation signal, and determine a
position of the
non-cooperative object from the processed reflected interrogation signal,
thereby allowing
the ownship to track and avoid the non-cooperative object. The computer
readable
instructions further cause the at least one processor to track and avoid the
non-
cooperative object.
There is provided an apparatus for tracking and avoiding a non-cooperative
object,
comprising a memory device for storing computer readable instructions thereon
for
execution by at least one processor, causing the at least one processor to
process a
reflected interrogation signal, yielding a processed reflected interrogation
signal, and
determine a position of the non-cooperative object from the processed
reflected
interrogation signal, thereby allowing the ownship to track and avoid the non-
cooperative
object.
The computer readable instructions, causing to process, further cause the at
least
one processor to (i) determine a range of durations for time windows, during
which the
reflected interrogation signal arrives at the ownship, the durations being
comparable to an
interrogation time of travel from a secondary surveillance radar to the
ownship, (ii)
integrate the reflected interrogation signal across the time windows
determined in the step
(i), and (iii) identify and classifying peaks in the integrated reflected
interrogation signal
integrated in the step (ii). The computer readable instructions, causing to
determine,
further cause the at least one processor to calculate a range of possible
positions of the
non-cooperative object from the processed reflected interrogation signal, and
scan the
range of possible positions of the non-cooperative object, and detect the
position of the
non-cooperative object, based on results of the scanning.
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A method for tracking and avoiding a non-cooperative object is provided,
comprising employing at least one hardware processor for processing a
reflected
interrogation signal, the reflected interrogation signal being an
interrogation signal sent
from a secondary surveillance radar and reflected off the non-cooperative
object, yielding
a processed reflected interrogation signal, and determining a position of the
non-
cooperative object from the processed reflected interrogation signal, thereby
allowing the
ownship to track and avoid the non-cooperative object.
Thus, an improved method and system for secondary surveillance radar (SSR) for
tracking non-cooperative objects without a transponder have been provided.
The methods and systems described with regards to Figures 1-8 for positioning
cooperative target with a transponder may be applicable for tracking and
avoiding a non-
cooperative target without a transponder, as described in Figures 9-16 herein.
Although specific embodiments of the invention have been described in detail,
it
should be understood that the described embodiments are intended to be
illustrative and
not restrictive. Various changes and modifications of the embodiments shown in
the
drawings and described in the specification may be made within the scope of
the following
claims without departing from the scope of the invention in its broader
aspect. For
example, the principles of the invention can be applied to other contexts such
as marine or
nautical and terrestrial context.
The processes described above, as applied to a social graph of a vast
population,
are computationally intensive requiring the use of multiple hardware
processors. A variety
of processors, such as microprocessors, digital signal processors, and gate
arrays, may
be employed. Generally, processor-readable media are needed and may include
floppy
disks, hard disks, optical disks, Flash ROMS, non-volatile ROM, and RAM.
It should be noted that methods and systems of the embodiments of the
invention
and data sets described above are not, in any sense, abstract or intangible.
It should be
noted that the currently described data-processing and data-storage methods
cannot be
carried out manually by a human analyst, because of the complexity and vast
numbers of
intermediate results generated for processing and analysis of even quite
modest amounts
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of data. Instead, the methods described herein are necessarily carried out by
electronic
computing systems having processors on electronically or magnetically stored
data, with
the results of the data processing and data analysis digitally stored in one
or more
tangible, physical, data-storage devices and media.
Methods and systems of the present invention have tangible and practical
advantages, providing more expedient and more reliable processing of vast
amounts of
data.
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Representative Drawing