Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
AVIATION DETECT AND AVOID METHOD AND SYSTEM
FIELD OF THE INVENTION
The present invention relates to tracking aerial, nautical or ground objects,
and in particular
to aviation detect and avoid method and system tracking objects in aviation
systems using a
passive secondary surveillance radar (PSSR).
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 are
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 are often supplemented with other
auxiliary systems.
Such 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) Airborne
Equipment"
from Radio Technical Commission for Aeronautics (RTCA, Inc.), an air traffic
control system
comprises an SSR main rotating antenna transmitting narrow interrogation beams
which is
assisted with an omni-directional antenna transmitting a related signal. The
air traffic control
relies on transponders located in an aircraft to reply to interrogation beams
to signal their identity
as well as their altitude. The transponder reply signal is broadcast at
another standard frequency
(for example 1090 MHz). Every interrogation message is composed by three
pulses, P1, P2 and P3
at a given standard frequency (for example 1030 MHz). P1 and P3 pulses can
only be received
when the aircraft is in the coverage of a main antenna beam (main lobe width
of 2 to 3 degrees).
Outside of the main lobe, P1 and P3 are weaker than the P2 pulse. This means
that a target object,
for example a target aircraft, can only receive valid interrogation, and then
responds when
it is in the main lobe of the main antenna beam. The P2 pulse, also referred
to as Side Lobe
Suppression (SLS) signal, is always synchronized with the P1 pulse and
transmitted by the omni-
directional antenna (hence referred to as omni signal) exactly 2 ps after the
P1 pulse. The P3 pulse
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is used to determine whether the current message is a mode A or mode C
interrogation by delaying
with different time intervals (8 ps or 21 ps) from a corresponding P1 pulse.
In a transponder, that
an aircraft is obliged to have, if a received P2 is weaker than P1 by 9 dB, a
response to the
interrogation is sent; otherwise, 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 and another antenna, is placed on the
ground or on an
airplane with known locations relative to the master SSR. The SSR
interrogation signals are
received at the PSSR station as well as at a target aircraft. The
transponder's 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.
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. This is
referenced as
"staggered pattern" or "pulse repetition frequency (PRF) pattern" in the
present application. The
staggered pattern may differ for different SSR configurations and providers.
Therefore, there is a need to develop improved methods and SSR system that
would work
reliably for new hardware designs of omni-directional antennas and staggered
interrogation
patterns.
Also, accuracy of the time measurement is important for PSSR applications.
Because a
signal travels at the speed of light, so a relatively small error in time
measurement could result in a
large distance error. This would be extremely dangerous in a crowed air space.
In this case, even a
GPS based time measurement would not be sufficiently precise or reliable for
collision avoidance.
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When the target object is not equipped with a transponder which replies to an
SSR
interrogation, a method for detecting its existence and giving an estimate of
its position need be
developed, along with the detect and avoid aviation system.
A ground based PSSR system would receive a reduced signal strength because the
SSR
antenna is not designed to cover the ground area, and because ground
structures can affect the
strength of the interrogation signals. Moreover, a pilot of an aircraft will
not be informed
immediately after the target object is detected, increasing the chance of
midair collision.
Therefore, there is a need in the industry for the development of an improved
detect and
avoid aviation system, and a method and PSSR system that would enable reliable
detection of
target objects.
SUMMARY OF THE INVENTION
It is an objective 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
covering only limited angle coverage, for example about 80 degrees (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.
In accordance with an aspect, the invention provides a method for detecting
and avoiding
a target object. The method is based on determining a position of the target
object. A Passive
Secondary Surveillance Radar (PSSR), placed at a distance from a Secondary
Surveillance Radar
(SSR) performs processes of: determining a pulse repetition frequency (PRF)
pattern for
staggered interrogation pulses (P1, P2, P3) of the Secondary Surveillance
Radar (SSR); receiving
a reply from the target object in response to an interrogation signal
comprising a P3 pulse sent
from the SSR to the target object; and estimating a transmit time of the P3
pulse of the
interrogation signal based on a reception time of the reply, and the PRF
pattern.
The position of the target object is then determined based an altitude
information h of the
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target object contained in the reply, a location of the SSR, the estimated
transmit time of the P3
pulse of the interrogation signal, and the reception time of the reply. The P2
and P3 pulses are
synchronized to respective P1 pulses with respective first and second
predefined time gaps.
Upon detecting successive P2 pulses during a time-window where the PSSR is
within
range of a wide-beam antenna of the SSR, a time-ordered sequence of intervals
separating the
successive P2 pulses is formed.
If at least two successive congruent segments of the time-ordered sequence are
identified, a
pulse repetition pattern of P2 pulses is determined as one of the segments.
subject to the constraint
that a number of intervals of the segments is within the range of an initial
length lower bound and
a predefined length upper bound. The PRF pattern is derived based on the pulse
repetition pattern
of P2 pulses and corresponding values of the first and second predefined time
gaps.
Determining the position of the target object enables controlling a specific
moving object
so as to avoid the target object.
During a time-window where the PSSR is within range of a narrow-beam antenna
of the
SSR, P1 and P3 pulses are detected and a value of the second predefined time
gap between P3 and
P1 pulses is determined, thereby an interrogation mode of the SSR is
determined.
The process of forming the time-ordered sequence comprises initializing an
array of
intervals and initializing a first pointer of the array.
The process of identifying congruent segments of the time-ordered sequence
comprises:
finding a primary string of adjoining intervals, of the time-ordered sequence,
in which a first
interval is distinct from any other interval with a last interval preceding an
interval that equals the
first interval; examining a candidate string of adjoining intervals of maximum
congruence to the
first string, following the last interval; and subject to a determination that
the candidate string is
fully congruent with the first string, determining the first string as the PRF
pattern.
Subject to a determination that the candidate string is not fully congruent
with the first
string: the candidate string is appended to the primary string and the last
interval is updated to be
the end interval of the candidate string.
The primary string and the candidate string are continually stored in the
array of intervals.
The process of determining of the PRF pattern is terminated subject to a
determination that
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either the primary string or the candidate string comprises a number of
intervals exceeding the
predefined length upper bound.
In order to verify correct identification of the PRF pattern, the pulse
repetition pattern of
P2 pulses is used as a reference string, and the number of intervals of the
reference string as a ref-
erence length. Continuing to receive P2 pulses, consecutive strings of
intervals between successive
pulses are formed, where each consecutive string comprises a number of
intervals equal to the ref-
erence length. A number of consecutive strings that are congruent with the
reference string is then
determined. Subject to a determination that the number of consecutive strings
at least equals a
predefined congruence lower bound, correctness of detected pattern is
ascertained.
If the number of consecutive strings is less than the congruence lower bound,
the initial
length lower bound is reset to a higher value not exceeding the predefined
length upper bound, and
the process of determining the PRF pattern is revisited with the increased
length lower bound.
In one implementation, determining a number of consecutive strings that are
congruent
comprises sequentially determining congruence of two successive strings,
starting with the ref er-
ence string.
The process of determining congruence of any two strings comprises determining
a re-
spective absolute value of a difference between each interval of one of the
strings and an interval
of a corresponding positions of the other string. Congruence is ascertained
subject to a determina-
tion that the respective absolute value is below a first prescribed tolerance
level.
In accordance with another aspect, the invention provides a detect-and-avoid
system for
an ownship aircraft. The system comprises a control station in communication
with an ownship
aircraft for controlling the ownship aircraft and a Passive Secondary
Surveillance Radar (PSSR)
system, at the ownship aircraft, in communication with the control station,
the PSSR.
The ownship PSSR comprises a first receiver for receiving a reply from a
target object
wherein the reply is responsive to an interrogation signal comprising P1 and
P3 pulses sent by a
narrow-beam antenna of a Secondary Surveillance Radar (SSR) to the target
object; a second
receiver for receiving a stream of P2 pulses from the SSR, the P2 pulses being
transmitted in a
staggered pattern by a wide-beam antenna of the SSR; and a first processor
coupled to the first
receiver and the second receiver.
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A second processor of the PSSR executes instructions to: determine a pulse
repetition
frequency (PRF) pattern for staggered interrogation pulses (P1, P2, P3) of the
SSR, the P2 and P3
pulses being synchronized to respective P1 pulses with respective first and
second predefined time
gaps; estimate a transmit time of the interrogation signal based on a
reception time of the reply,
and the PRF pattern; and determine the position of the target object based on
an altitude
information of the target object provided in the reply, a location of the SSR,
the transmit time of
the interrogation signal, and the reception time of the reply.
During a time-window where the PSSR is within range of the wide-beam antenna,
the
instructions cause the second processor to detect successive P2 pulses, form a
time-ordered
sequence of intervals separating the successive P2 pulses, identify at least
two successive
congruent segments of the time-ordered sequence, and determine a pulse
repetition pattern of P2
pulses as one of the segments subject to the constraint that a number of
intervals of the one of the
segments is within predefined lower and upper bounds.
During a time-window where the PSSR is within range of the narrow-beam
antenna, the
instructions cause the second processor to detect P1 and P3 pulses, determine
a value of the
second predefined time gap between P3 and P1 pulses, thereby determining an
interrogation mode
of the SSR, and derive the PRF pattern based on the pulse repetition pattern
of P2 pulses and
corresponding values of the first and second predefined time gaps.
The first processor continually determines inter-pulse intervals and stores
the intervals in a
buffer. The second processor independently reads individual inter-pulse
intervals and executes the
instructions. Thus, the pulse acquisition and inter-pulse measurement
timescale is decoupled from
processing timescale. The second processing unit is configured to ensure that
a mean execution
time per interval does not exceed a mean inter-pulse interval.
The detect-and-avoid system further comprises ground-based PSSR equipment
installed
within a ground-based surveillance system, the PSSR equipment being
communicatively coupled
to respective interface equipment within the control station.
A comparator unit, communicatively coupled to the respective interface
equipment, com-
prises a respective processor configured to: receive data relevant to safety
of the ownship gener-
ated at the PSSR system of the ownship; receive data relevant to safety of the
ownship generated
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at the ground-based PSSR equipment; and perform comparative data analysis to
enhance safety
measures.
In accordance with a further aspect, the invention provides an engine for
detecting a PRF
pattern from a stream of pulses. the engine comprising: a first processing
unit configured to: ii-
tialize an array of inter-pulse intervals, set a state to 0, and set a lower
bound of an PRF pattern as
a reference index of the array; and continually receive pulses, determine
inter-pulse intervals; and
placing the inter-pulse intervals in a buffer.
A second processing unit is configured identify a PRF pattern.
While the state is 0, the second processing unit compares each read interval
from the buffer
with a reference interval at the reference index, continues to read intervals
from the array subject
to a determination that each read interval differs from the reference
interval; and switches to state-
1 if any read interval equals the reference interval.
While the state is 1, the second processing unit compares each retrieved
interval from the
buffer with a prior interval stored at a respective designated index of the
buffer, continues to read
intervals from the array subject to a determination that each retrieved
interval equals the prior in-
terval, and switches to state-0 if a retrieved interval differs from the prior
interval.
The PRF pattern is determined as comprising the intervals read during state-1
when the
number of intervals read during uninterrupted presence in state-1 equals the
total number of previ-
ously read intervals.
The buffer is managed as a circular buffer and has a sufficient storage
capacity to hold a
number of intervals at least equal to double a predefined upper bound of the
number of intervals of
a PRF pattern.
The second processing unit is configured to realize a mean processing time per
interval not
exceeding a mean inter-pulse interval.
According to another 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-
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Date Recue/Date Received 2022-01-17
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 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 yet 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 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
pre-defined 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
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Date Recue/Date Received 2022-01-17
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, either from the
main lobe or the side lobe of the SSR antenna, and determining 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) expanding 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) expanding the staggered pattern and its corresponding
interrogation mode
pattern to the time periods when neither P2 nor 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 nor 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
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Date Recue/Date Received 2022-01-17
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.
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 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:
Date Recue/Date Received 2022-01-17
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).
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 and the PSSR is one of a stationary PSSR and a mobile
PSSR and
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) determining
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.
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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 an 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 when the PSSR is outside the beam-
width of the
wide-beam antenna.
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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, 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 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
13
Date Recue/Date Received 2022-01-17
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 reply signal and 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 baseband 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 baseband reply signal and said
baseband P2 pulse and
transmitting sampled baseband reply signal and sampled baseband 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 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-
14
Date Recue/Date Received 2022-01-17
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 having 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,
when 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 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.
Thus, an improved method and system for passive secondary surveillance radar
(PSSR)
tracking have been provided.
15
Date Recue/Date Received 2022-01-17
BRIEF DESCRIPTION OF THE DRAWINGS
The application contains at least one drawing executed in color. Copies of
this patent or
patent application publication with color drawing(s) will be provided by the
Office upon request
and payment of the necessary fee. For 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:
FIG. 1 illustrates an SSR center and ownship having a PSSR system on board for
detecting
a target object;
FIG. 2A illustrates relative positions of the SSR center, the ownship, and the
target object
of the system of FIG. 1 used for calculation of the position of the target
object;
FIG. 2B illustrates the signal received by ownship in the configuration of
FIG. 2A;
FIG. 2C illustrates relative positions of the SSR center, the ownship, and the
target object
of the system of FIG. 1 where the ownship is outside the wide-beam antenna
coverage;
FIG. 2D illustrates the signal received by ownship in the configuration of
FIG. 2C;
FIG. 3 illustrates various components of a mobile PSSR system;
FIG. 4 is a flowchart depicting a method of determining a position of the
target object, in
accordance with an embodiment of the present invention;
FIG. 5A is a flowchart depicting a method of detecting a PRF pattern from a
stream of
pulses, in accordance with an embodiment of the present invention;
FIG. 5B is a flowchart depicting a method of obtaining an interrogation
pattern;
FIG. 5C is a flowchart depicting a variation of the method of FIG. 5A for
detecting a PRF
pattern from a stream of pulses, in accordance with an embodiment of the
present invention;
FIG. 5D illustrates an engine 1100 for detecting a PRF pattern from a stream
of pulses, in
accordance with an embodiment of the present invention;
FIG. 5E illustrates a generalized method for determining a PRF pattern from a
stream of
pulses, in accordance with an embodiment of the present invention;
16
Date Recue/Date Received 2022-01-17
FIG. 5F illustrates an exemplary application of the method of FIG. 5C for
identifying a
PRF pattern, in accordance with an embodiment of the present invention;
FIG. 5G illustrates an exemplary application of the engine of FIG. 5D for
identifying a
PRF pattern from the same stream of pulses used in the illustration of FIG.
5C, but with a
specified minimum length of the PRF pattern, in accordance with an embodiment
of the present
invention;
FIG. 5H illustrates a process of decoupling a measurement timescale from a
processing
time scale, in accordance with an embodiment of the present invention;
FIG. 51 illustrates phases of determining a PRF pattern for an exemplary
sequence of inter-
pulse intervals using the method of FIG. 5C;
FIG. 5J illustrates phases of determining a PRF pattern for the sequence of
inter-pulse
intervals of FIG. 51 using the method of FIG. 5E;
FIG. 5K illustrates phases of determining a PRF pattern for another sequence
of inter-pulse
intervals of FIG. 51 using the method of FIG. 5E;
FIG. 6A and 6B illustrate implementation of a receiver unit for detecting the
P2 pulses;
FIG. 7 illustrates an alternative method for determining a position of the
target object, in
accordance with an embodiment of the present invention;
FIG. 8 is a flowchart depicting a method determining the PRF pattern using
main antenna
signals;
FIG. 9 illustrates a method 2200 of ensuring correctness of detection of the
PRF pattern.
Figures 10A to 10E illustrate application of the method of FIG. 9 to exemplary
PRF
patterns.
FIG. 11 illustrates DAA components provisioned in a target aircraft and ground
installations;
FIG. 12 illustrates communication paths between an ownship and different types
of target
aircraft;
FIG. 13 illustrates communication paths between an ownship, a target aircraft
and a UA
(Unmanned Aircraft) control station; and
17
Date Recue/Date Received 2022-01-17
FIG. 14 illustrates components of a ground-based UA control station.
DETAILED DESCRIPTION OF EMBODIMENTS
The terms "Unmanned Aerial Vehicle" (UAV) and "Unmanned Aircraft" (UA) are
used
synonymously. Although the disclosed features are described with reference to
unmanned aircraft,
the features also apply to a piloted aircraft. The term "Ownship" is used to
refer to an Unmanned
Aerial Vehicle, an Unmanned Aircraft, or a piloted aircraft.
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 practiced 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 from the "another" 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.
FIG. 1 illustrates a generic configuration 100 in which the present invention
can be
deployed showing the ownship 140, having a Passive Secondary Surveillance
Radar (PSSR)
system on board (shown in FIG. 3), in relation to the master SSR system 110
and a target object
represented as target object 160.
18
Date Recue/Date Received 2022-01-17
A major difference between the present approach of FIG 1 and the prior art
systems is that
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, the
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 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.
19
Date Recue/Date Received 2022-01-17
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.
A Detect-and-Avoid (DAA) system includes a Ground-Based Surveillance System
(GBSS)
190 and an UA Control Station 180. The GBSS has a dual communication link 185
to the UA
Control Station 180. The UA control station has a dual data link 170 to the
ownship 140.
Optionally, PSSR equipment 192 may also be installed within the GBSS 190, in
which
case the PSSR sends information relevant to the target object 160 to an
interface unit 182 installed
within the UA control station which may relay the information to the ownship
if the airborne
PSSR 142 is perceived to be malfunctioning. If both PSSR 142 and PSSR 192 are
used, there may
.. be benefits of comparing their results. A comparator unit 184 may be
installed in the UA control
station 180 for his purpose. PSSR 192 may have a propagation path 135 from SSR
110.
The comparator unit 184 is communicatively coupled to interface unit 182 and
comprises a
respective processor configured to: receive data relevant to safety of the
ownship generated at the
PSSR system of the ownship; receive data relevant to safety of the ownship
generated at the
ground-based PSSR equipment; and perform comparative data analysis to enhance
safety meas-
ures.
FIG. 2A illustrates a geometry of the above configuration in FIG. 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:
Date Recue/Date Received 2022-01-17
2b2c +.\14b4c2 - 4 (b 2 + a2 tan2 (27r - fi)(b2c2 + a2h2 - a2b2))
x =
____________________________________________________________________________
2 (b 2 + a 2 tan 2 (27r -
y = x tan (2,7 - fi )
z = - h
where a and b are defined in FIG. 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 instead of the
depicted ellipse. That is because neither of the ownship 140 and the target
object 160 is at the
same altitude of SSR 110. Other techniques that can be used to localize the
target object 160
include multilateration and triangulation techniques and are well known to
those skilled in the art.
The geometry depicted in FIG. 2A, illustrates the case where the target object
160 is within
the beam-width or coverage area of the SSR 110 MA 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 can see
neither P1 nor P3 pulses. In this geometry the ownship 140 can detect both the
P2 pulses and the
reply signals from the target object 160.
FIG. 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.
FIG. 2C shows another geometry corresponding to the case where the angle
between the
main-lobe of the SSR 110 Main Antenna and the X-axis is almost 90 degrees. The
wide-beam
21
Date Recue/Date Received 2022-01-17
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 140 are shown in FIG. 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 is 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.
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 FIG.
2D using the algorithm described below) and the reply message as shown in FIG
2D should be
calculated. Assume the stagger pattern and its interrogation mode have been
determined using the
algorithms described below, and the time between the assumed P2 pulse to the
reply is al seconds,
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 FIG.
2C. dt is actually the parameter 2a in FIG. 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
microseconds after the
P2 is transmitted, while in Mode C interrogation, the P3 pulse is sent 19
microseconds after P2.
This is why for different interrogation modes, 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 microseconds in both equations
are the fixed
transponder delay.
FIG. 3 illustrates a Passive Secondary Surveillance Radar system PSSR 300
embedded in
the ownship 140 for detecting a target object such as target object 160 and
determining its
positional information.
22
Date Recue/Date Received 2022-01-17
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
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.
The receiver unit 320 comprises a 1030 MHz receiver 325 connected to the
directional
antenna 322 or to the omni-directional antenna 324 through splitter (not
shown) for detecting 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. 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 Baseband/Intermediary Frequency (BB/IF)
processing unit
327 for receiving the signals detected by the receiver 325 and receiver 323
and converting them
into a baseband or into an intermediary frequency using a local oscillator as
will be described in
Figures 6A and 6B, respectively. The BB/IF processing unit 327 digitizes the
received signals and
pass the digitized signals along to a processor 310 for further processing.
Processor 310 may be
implemented as an assembly of multiple hardware processors arranged in
multiple processing
units.
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
23
Date Recue/Date Received 2022-01-17
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 to, -1 t, -
2 t and t3, the time-ordered
sequence of pulse intervals would be ordered as intervals II, 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 compares 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 whether 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) to 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) to 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 FIG. 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.
24
Date Recue/Date Received 2022-01-17
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.
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 FIG. 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 FIG 3 and passes the information to the
BB/IF processing
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
applies 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
Date Recue/Date Received 2022-01-17
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 mode)
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-beam antenna as depicted in FIG. 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 FIG. 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 8 and the sum of the distances
d1 and d2
described with regards to FIG. 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 flowchart at step 470, the PSSR
300 can estimate the
26
Date Recue/Date Received 2022-01-17
3D coordinates of the target object 160. the 3D coordinates can be estimated
using in particular the
spheroid equations described with regards to FIG. 2A.
FIG. 5A details the operation of step 430 for determining the PRF or stagger
pattern of the
flowchart 400. At step 510 the 1st and 2" P2 pulses are identified and a 1st
interval between the two
.. pulses is 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 FIG.3.
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 (K-1), 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 FIG. 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.
FIG. 5B 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 are well-known in the
art, so the detection of
a pulse is not limited to the one described above. For a valid interrogation
combination, each pulse
27
Date Recue/Date Received 2022-01-17
should have 2 microseconds pulse width. If only P1 and P3 pulses are detected,
they should either
be 8 microseconds apart for Mode A interrogation or 21 microseconds apart for
Mode C
interrogation. If P2 pulse is also present, it should be 2 microseconds 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 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 are 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.
FIG. 5C illustrates a method 500C of determining a PRF pattern similar to the
method of
FIG. 5A. Process 1010 receives two successive pulses and determines a value of
the (time) inter-
val between the two pulses.
Process 1020 initializes an array, denoted 443, for storing values of
successive intervals of a
PRF pattern to be detected from a series of pulses, placing the interval
between the first two pulses
28
Date Recue/Date Received 2022-01-17
in array (I) at an index, K, set to equal 0. Thus, (KO) holds the first
captured interval which is used
as a reference interval during the process of populating array 443.
Process 1030 continually receives pulses and determines inter-pulse intervals.
For each re-
ceived pulse, process 1040 increases the index, K, setting K (K+1), and stores
a respective in-
.. ter-pulse interval in array 4:13 at index K. If process 1037 determines
that K reached a predefined
upper bound K., the pattern detection process is terminated (process 1047).
Process 1050 compares a current interval value, 43(K), with the reference
interval (KO). If
43(K) is determined to be different from 43(0), process 1050 determines that
the sequence of inter-
vals corresponding to the sought PRF pattern is incomplete and returns to step
1030 to obtain an-
other inter-pulse interval. Processes 1030, 1040, and 1050, which are
recursively activated, form a
"K-Loop" of processing.
If 43(K) is determined to be equal to 43(0), process 1050 tests the
possibility that a forth-
coming succession of inter-pulse intervals may be congruent with the sequence
{(1)(0) to 43(K-1)},
in which case the sequence {(1)(0) to 43(K-1)} is considered to be the sought
PRF pattern. Process
1050 then leads to process 1055 which initializes a count, J, as zero, and
retains the respective
value of K, denoted K*, which will be needed to identify the PRF pattern
according to the array
segment {43(0) to 43(K*-1)}.
Process 1060 starts a recursive procedure to determine whether a sequence of
inter-pulse
intervals of forthcoming pulses is congruent with the sequence {43(0) to 43(K*-
1)}. Process 1060
.. continues process 1030 of receiving new pulses and determining
corresponding inter-pulse inter-
vals. With each determined new interval, index K is increased, setting K
(K+1), and a value of
new interval is placed in 43(K). The count, J, is increased, setting J
(J+1), in process 1065. If
process 1067 determines that K has exceeded the predefined upper bound K., the
pattern detec-
tion process is terminated (process 1047). Otherwise, process 1070 compares
current stored inter-
val 43(K) with previously stored interval 43(J).
If (I)(K) = (I)(J), process 1070 leads to step 1080 which concludes the PRF-
detection
process if J = (K*-1), which indicates that the sequence {43(K*) to 43(2 XK*-
1)} is congruent with
the sequence {43(0) to 43(K*-1)} which would then be considered, in process
1090, to represent
the sought PRF pattern.
29
Date Recue/Date Received 2022-01-17
If 43(K) # (KJ), process 1070 leads to step 1030 which continues to receive
new pulses
with array (I) already storing intervals of indices K* to (K*+J).
Processes 1060, 1065, 1070, and 1080, which are recursively activated, form an
"J-Loop"
of processing. The method is based on flip-flopping between the K-Loop and the
J-Loop, until
process 1090 is reached.
Consider, for example, a case of receiving a succession of pulse having inter-
pulse inter-
vals of values:
"A, B, C, D, E, A, F, A, B, G, H, P, G, Q, A, B, C, D, E, A, F, A, B, G, H, P,
G, Q",
where the individual interval values {A, B, C, D, E, F, G, H, P, Q} are
distinctly different.
Process 1010 receives two successive pulses and determines a value "A" of the
(time) in-
terval between the two pulses, which is the reference interval.
Process 1020 sets index K of an array, 443, to zero, with 43(0) A.
Process 1030 receives a new pulse after a time interval "B" from the time of
receiving the
previous pulse. Process 1040 increases the index, K, setting K (K+1) = 1, and
stores the interval
.. "B" in 43(4
Process 1050 compares 43(1) with 43(0), which are different, and returns to
process 1030 to
receive the next pulse after a time interval "C". Process 1040 increases K to
2 and stores the value
C at 43(2).
Likewise, array 4:13 stores interval D and E at 43(3) and 43(4) with the index
K increased to
K=4 in process 1040.
Process 1030 receives successor pulse after an interval "A" and process 1040
increases K
to K=5, placing the new value "A" in 43(5). Process 1050 then determines that
43(5) = 43(0), and
switches from the K-Loop to the J-Loop. Process 1055 sets a count, J, to 0 and
retains the current
value of K, denoted K*.
Process 1060 receives a subsequent pulse after a time interval "F" and
increases K to K=6,
storing the interval in 43(6). Process 1065 increases J to J=1, then process
1070 determines that
1(6)#1(1), thus the sequence segment {(1)(5), 43(6)1 cannot be part of a
replica of the captured
sequence {(1)(0), 43(4 43(2), 43(3), 43(4)1. The detection process then
switches back to the K-Loop
Date Recue/Date Received 2022-01-17
where process 1030 receives a new pulse after a time interval "A". Process
1040 increases the in-
dex K to K=7 and sets 43(7) = A. So far, the array segment {43(0) to 43(7)1
stores "A, B, C, D, E,
A, F, A".
The method then explores the possibility that the last entry "A" is a start of
a replica of ar-
ray segment "A, B, C, D, E, A, F" and switches the pattern-detection process
to the J-Loop.
Process 1055 sets the count J to 0 and retains the current value of K as K* so
that, if the last pulse
interval is a start of a replica, then the array segment {(1)(0) to 43(K*-1)}
is considered to represent
the sought PRF pattern. Process 1060 of the J-Loop receives a new pulse after
a time interval "B",
increasing the index K to K=8. Process 1065 increases the count J to J=1 and
process 1070 com-
pares 43(8) to 43(4 Since 43(8) = (DM = B, there is still the possibility that
inter-pulse intervals of
forthcoming pulses will belong to the sought PRF pattern. Process 1080
compares determines that
J#(K*-1), J being 1 and K* being 7. Thus, J-Loop processing continues with
process 1060 receiv-
ing a new pulse after a time interval "G". The index K is increased to K=9,
43(9)=G. Process 1065
increases J to J=2 and process 1070 determines that 1(9)#1(2). The pattern-
detection process
switches to the K-Loop where process 1030 receives a new pulse after a time
interval "H", Process
1040 increases K to K=10, setting 43(10) = H. Process 1050 of the K-Loop
determines that
1(10)#1(0), hence process 1030 is revisited. Upon receiving three more pulses
after time inter-
vals "P", "G", and "Q", the K-Loop increases K to K=13 with 43(11) = P, 43(12)
= G, 43(13) = Q.
Within the K-Loop, process 1030 receives a subsequent pulse after a time
interval "A".
Process 1040 increases K to 14, setting 43(14) = A. Process 1050 determines
that 43(14)
hence, the pattern-detection process switches to the J-Loop.
Following the criterion of process 1070 for remaining within the J-Loop,
process 1060 of
the J-Loop receives new pulses after time intervals: "B, C, D, E, A, F, A, B,
G, H, P, G, Q", which
meet the criterion of process 1070 for remaining within the J-Loop leading to
meeting the condi-
tion J= (K*-1)=13. Thus, the array segment {(1)(0) to 43(13)1 of 14-time
intervals represents the
sought PRF pattern.
Table-I below illustrates the above pattern-detection process.
31
Date Recue/Date Received 2022-01-17
Table I: Walkthrough of the algorithm of FIG. 5C
(1)(K) =
Pulse Interval
K cT1(K) J (DO) (MO ? K* Loop
index value
Interval
index
0
1 A 0 A _ _ _ _
2 B 1 B _ _ _ _
3 C 2 C _ _ _ _
4 D 3 D _ _ _ _
K-Loop
E 4 E _ _ _ _
6 A 5 A 0 A Y5
7 F 6 F 1 B N 5 J-Loop
8 A 7 A 0 A Y 7 K-Loop
9 B 8 B 1 B Y 7 J-Loop
G 9 G 2 C N 7
11 H 10 H 2 D N 7
12 P 11 P 2 E N 7
13 G 12 G 2 A N 7 K-Loop
14 Q 13 Q 2 F N 7
A 14 A 0 A Y 14
16 B 15 B 1 B Y 14
17 C 16 C 2 C Y 14
18 D 17 D 3 D Y 14
19 E 18 E 4 E Y 14
A 19 A 5 A Y 14
21 F 20 F 6 F Y 14
22 A 21 A 7 A Y 14 J-Loop
23 B 22 B 8 B Y 14
24 G 23 G 9 G Y 14
H 24 H 10 H Y14
26 P 25 P 11 P Y14
27 G 26 G 12 G Y14
28 Q 27 Q 13 Q Y 14*
29
*: J = (K*-1), hence array segment (KO) to (I)(J) holds the PRF cyclic pattern
FIG. 5D illustrates an engine 1100 for detecting a PRF pattern from a stream
of pulses. The
5 engine continually receives pulses and stores inter-pulse intervals in a
buffer using an appropriate
data structure, such as a simple array denoted (1). The buffer is preferably
managed as a circular
buffer having a storage capacity, in terms of a number of stored entries
(stored records), exceeding
the maximum permissible number of pulses per PRF pattern. Note that with
continuous pulse re-
32
Date Recue/Date Received 2022-01-17
ception, the number of inter-pulse intervals per PRF pattern equals the number
of pulses per PRF
pattern.
A processor continually reads the values of the stored intervals and
implements processor-
executable instructions to identify a cyclic PRF pattern. Executing the
instructions need not be co-
.. ordinated with the instants of time of receiving the pulses and measurement
of inter-pulse time in-
tervals. Thus, the measurement timescale is decoupled from the processing time
scale.
The buildup of the sequence of inter-pulse (time) intervals may start with a
segment of ar-
ray (I) comprising a number of entries equal to a specified lower bound of a
length of the sequence.
The specified lower bound is a design parameter.
Process 1110 initializes array 4:13 of inter-arrival intervals either as an
array of sufficient
number of entries each initialed as a null entry (such as a value of zero,
since an inter-pulse inter-
val cannot be equated to zero), or initializing a WRITE-index and a READ-index
of 4:13 to ensure
that any entry being read corresponds to an already inserted interval.
As in the method of FIG. 5C, the engine switches between a "K-Loop" and a "J-
Loop"
.. based on current and prior interval values placed in array 443. The process
of pattern detection is
said to be in "state 0" when the K-Loop is active, and in "state 1" when the J-
Loop is active.
Process 1120 initializes the state as 0, to start processing within the K-
Loop. An initial seg-
ment of array 4:13 including entries {(1)(0) to 43(Km,,,) is selected (setting
Km,,,) as a "seed" for
building up a sequence of records, Kõõ,, being a logical address that equals a
specified minimum
.. pattern length, in terms of a number of intervals, minus 1 (since the entry
indices of array 4:13 start
with 0).
Process 1130 receives the pulses, determines inter-pulse intervals, and stores
same into a
memory device at storage entries of 4:13 logically indexed sequentially in
steps of 1, starting with 0.
Process 1140 sequentially increases the current index K, setting K (K-F1), and
reads a
.. corresponding interval value from the (circular) buffer. If process 1142
determines that K has ex-
ceeded the predefined upper bound K., the pattern detection process is
terminated (process
1147). Otherwise, process 1150 branches to either the K-Loop or the J-Loop.
Starting with state 0
(initialized in process 1130), process 1150 leads to process 1160 which
compares (I)(K) with (KO).
Notably, in the first activation of the K-Loop, the first inspected interval
is the interval immedi-
33
Date Recue/Date Received 2022-01-17
ately following the specified initial segment {43(0) to 43(Kõ,,,,)}, which is
43(Kõõ,,+1), K being in-
creased to (Kõ,,,,+1) in process 1140. If (I)(K) # 43(0), process 1160 leads
to process 1140 which in-
creases K (setting (K+1)) and reads a corresponding interval from the
(circular) buffer. Circu-
lating the K-Loop, increasing K and executing processes {1160, 1140, 1150,
1160, ...}, continues
until a value of 43(K) equals 43(0) at which point process 1160 leads to
process 1170 to switch to
the J-Loop in order to determine whether a replica of the sequence of
intervals so far identified in
the K-Loop can be found in subsequent intervals in array (1).
Process 1170 resets the state to 1, initializes a count J to 0, and retain a
current value of K
as K*, thus setting the sequence {(1)(0) to 43(K*-1)} as a reference base. If
a replica of the ref er-
ence base is identified in the (circular) buffer, the reference base is
considered the sought PRF pat-
tern. The reference base is updated with each activation of process 1170,
i.e., with each transition
from the K-Loop to the J-Loop. Process 1170 leads to process 1140 which
increases the index K,
reads a corresponding value (I)(K) from the (circular) buffer, and proceeds to
process 1150 which
directs the process of pattern detection to process 1172 of the J-Loop since
that state is 1. Process
1172 increases the count J, setting J (J+1), and proceeds to process 1174
which compares (I)(K)
to (I)(J).
If process 1174 determines that (I)(K) # (KJ), there is no chance that a
replica of the refer-
ence base {43(0) to 43(K*-1)} will be encountered within the J-Loop. Thus,
process 1174 proceeds
to process 1175 to switch the state from 1 to 0, leading to process 1150
branching to the K-Loop.
If process 1174 determines that (I)(K) = (I)(J), there is still a chance that
a replica of the ref-
erence base {(1)(0) to 43(K*-1)} will be encountered, thus the state remains
to be 1. Process 1174
proceeds to process 1176. If process 1176 determines that J is less than (K*-
1), process 1140 is
activated to read another interval, maintaining the state as 1, hence process
1150 will continue to
lead to the J-Loop.
As new entries of array 4:13 are read in process 1140, the J-Loop either:
identifies a segment {43(K*) to 443(K*+J)} which is short of complete
congruence with the segment
{43(0) to 43(K*-1)}, thus returns control to the K-Loop in process 1175; or
determines in process 1176 that a complete congruence has been found, then
proceeds to conclude
the pattern identification process in process 1180.
34
Date Recue/Date Received 2022-01-17
FIG. 5E illustrates a generalized method 1200 for determining a PRF pattern
from a stream
of pulses. The method is implemented at a PSSR comprising a processor and
memory devices.
Process 1210 initializes a pattern string, denoted 1, for holding values of
successive inter-pulse in-
tervals, as an empty string, with a number of stored interval values set to
zero. The initial size of
string I is supplied to processes 1250 and 1260. Process 1220 receives two
pulses and measures
the interval between them which used as a reference interval. Process 1230
continually receives
pulses and determines respective inter-pulse intervals. The values of the
inter-pulse intervals are
presented to process 1240. Process 1240 identifies a candidate string of
intervals, denoted S. A
candidate string starts with an interval value determined to be equal to the
reference interval
(process 1220) and satisfies one of two conditions:
(1) where any interval of the candidate string is different from an interval
of a same index
of a current pattern string, each of the string intervals, beyond the first
interval, have a
value distinctly differentiable from the value of the reference interval; or
(2) each interval of the candidate string is determined to be equal to an
interval of a same
index of a current pattern string ¨ in which case the candidate string is
considered to be
the sought PRF pattern.
Process 1250 determines congruence, or otherwise, of a candidate string S with
pattern
string E. Initially, pattern string 1 is empty, hence process 1250 determines
that strings 1 and S
are not congruent and process 1260 is activated.
Process 1260 appends candidate string S to pattern string I to produce a
current pattern
string E. The first activation of process 1260 yields a pattern string / which
is identical to the first
candidate string S. With process 1230 continuing indefinitely to receive
pulses and determine in-
ter-pulse intervals, process 1240 is revisited to determine a current
candidate string S, starting
with an interval value deemed to be equal to the reference interval with
remaining interval differ-
ing from the reference interval. Process 1250 is revisited to compare the
current candidate string
with the current pattern string.
If the two strings are congruent, the current pattern string I is considered
to represent the
PRF pattern, and process 1270 is activated to communicate string S (or string
1) to other system
Date Recue/Date Received 2022-01-17
components. If the current pattern string and the current candidate string S
are not congruent,
process 1260 is revisited to append the current candidate string to the
current pattern string.
With process 1230 continuing indefinitely to receive pulses and determine
inter-pulse in-
tervals, activation of processes 140, 1250, and 1260 continues until a
candidate string, S, is cap-
tured and found to be congruent to the latest pattern string E.
As an example, process 1220 receives two pulses and measures the interval
between them
to equal "A". The value "A" is used as a reference interval. Process 1230
continually receives
pulses and determines respective inter-pulse intervals to be
"B, C, D, E, A, F, A, B, G, H, P, G, Q, A, B, C, D, E, A, F, A, B, G, H, P, G,
Q",
where the individual interval values {A, B, C, D, E, F, G, H, P, Q} are
distinctly different. The in-
ter-pulse intervals may be held in a circular buffer to decouple the pulse
rate from the latency of
processing circuitry.
The iterative procedure of FIG. 5E updates the contents of pattern string I
and candidate
string S until process 1270 is reached. For clarity in tracking the changes of
the two strings, the
strings are further identified as /a) and Sa), where "j" is an iteration
index, j, j being an inte-
ger.,(o) , an empty string, is a first pattern string and S (0), is a first
candidate string. Process 1260
appends Sa) to /a) to produce /(j+1).
A first visit to process 1240 identifies a first candidate string S" as {A, B,
C, D, E}. 5"
starts with interval value "A", which equals the reference interval, and
contains four other inter-
vals which individually have values differing from the reference interval.
Each of the intervals in-
cluded in S" differs from an interval of a same index in V), which, so far, is
empty. Thus, inter-
val "A" that follows received interval "E" cannot be included in S".
(0) and S" are not congru-
ent leading to a first visit to process 1260 which appends S(0) to /( ) to
produce a second pattern
string /(1) as {A, B, C, D, E}.
Subsequently, a second visit to process 1240 identifies a second candidate
string of S(1) as
{A, F}. SW starts with interval value "A", which equals the reference
interval, and contains a sec-
ond interval that differs from the reference interval. Since interval "F"
differs from the second in-
36
Date Recue/Date Received 2022-01-17
terval, "B", of E(1), interval "A" that is received following interval "F",
cannot be included in
E(1) and S(1) are not congruent. A second visit to process 1260 appends S(1)
to E(1) to produce a
third pattern string /(2) as {A, B, C, D, E, A, F}.
A third visit to process 1240 identifies a third candidate string of S(2) as
{A, B, G, H, P, G,
Q}. S(2) starts with interval value "A", which equals the reference interval,
and contains six other
intervals each of which differing from the reference interval. Interval "G" of
S(2) differs from the
corresponding interval, "C", of E(2), hence interval "A", that is received
after interval "Q", cannot
be included in S(2). /(2) and S(2) are not congruent. Hence, process 1250
leads to a third visit to
process 1260 which appends S(2) to /(2) to produce a third pattern string /(3)
as {A, B, C, D, E, A,
F, A, B, G, H, P, G, Q}.
A fourth visit to process 1240 identifies a fourth candidate string of S(3) as
{A, B, C, D, E,
A, F, A, B, G, H, P, G, Q}. S(3) starts with interval value "A", which equals
the reference interval,
and contains thirteen other intervals each of which being equal to an interval
of a same index in
E(3). E(3) and S(3) are congruent. Thus, process 1250 leads to process 1270
which communicates
S(3) to other system components as the sought PRF pattern.
The buildup of the pattern string /( ) to /(3) is summarized in the Table-II
below.
Table-II: Steps of determining the PRF pattern
Iteration
Pattern string E(j) Candidate string S(i)
Index (j)
0 Empty {A, B, C, D, El
1 {A, B, C, D, E}. {A, Fl
2 {A, B, C, D, E, A, F}. {A, B, G, H, P, G, Q}
{A, B, C, D, E, A, F, A, B, G, H, {A, B, C, D, E, A, F, A, B, G, H,
3
P, G, Q}. P, G, Q}
FIG. 5F illustrates the underlying principle of the first method, illustrated
in FIG. 5C, of
identifying the pattern of a staggered PRF stream. A stream of pulses detected
at a receiver have
inter-pulse intervals of values:
37
Date Recue/Date Received 2022-01-17
{A, B, C, D, A, B, E, F, A, G, A, B, C, D, A, B, E, F, A, G, ...},
where the values A, B, C, D. E, F, and G distinctly different.
A set of intervals identified in a first round of the K-Loop (FIG. 5C) is
identified as K" to
include {A, B, C, Dl of indices 0, 1, 2, and 3, respectively. The K-Loop
recognizes that the
.. interval "A" of index 4 as a potential beginning of a subsequent cycle of
the sought PRF pattern
then transfers the pattern-detection process to the J-Loop.
A set of intervals identified in a first round of the J-Loop is identified as
J" to include the
interval "A" of index 4, interval "B" of index 5. The J-Loop determines that
interval "E" of index
6 belongs to the sought pattern, then transfers the pattern-detection process
to the K-Loop.
A set of intervals identified in a second round of the K-Loop is identified as
Km which
includes K", intervals {A, B, El of indices 4, 5, and 6 transferred from the J-
Loop, and interval
"F" of index 7. The second round of the K-Loop also captures interval "A" of
index 8 and decides
that the interval may be a beginning of a subsequent cycle of the sought PRF
pattern. Thus, the K-
Loop transfers the pattern-search process to the J-Loop.
A set of intervals identified in a second round of the J-Loop is identified as
J(1) which
includes interval "A" of index 8, transferred from the K-Loop and interval "G"
of index 9. The J-
Loop recognizes that interval "G" cannot belong to a replica of the so-far
accumulated segment of
the PRF pattern and returns the pattern-detection process to the K-Loop. Array
4:13 now holds a
sequence of intervals {A, B, C, D, A, B, E, F, A, G}. Process 1030 of the K-
Loop (FIG. 5C)
receives a new pulse after a time interval "A" which is placed in array 4:13
at index 10 in process
1040. Process 1050 determines that the new interval "A" may be a beginning of
a replica of the
so-far accumulated sequence {A, B, C, D, A, B, E, F, A, G}.
Thus, the K-Loop transfers the pattern-detection process to the J-Loop, with
the count J set
to 0 and K*=K, which is then equal to 10. The J-Loop receives a new pulse
after a time interval
"A" which is equal to (KO), then receives a pulse after a succeeding time
interval "B", which
equals 43(4 and eight pulses after successive time intervals of "C", "D", "A",
"B", "E", "F", "A",
and "G", which are respectively equal to 43(2), 43(3), 43(4), 43(5), 43(6),
43(7), 43(8), and 43(9). At
this point, the count J in the J-Loop is 9 which equals (K*-1). Process 1080
then leads to process
1090 which identifies the sequence {43(0) to 43(K*-1)} as representing the
sought PRF pattern.
The last round of the J-Loop produces the entire pattern J(2).
38
Date Recue/Date Received 2022-01-17
It is important to note that both the K-Loop and the J-Loop place successive
intervals in a
common memory holding array 443.
FIG. 5G illustrates an exemplary application of the engine of FIG. 5D for
identifying a
PRF pattern from the same stream of pulses used in the illustration of FIG.
5C, but with a
specified minimum length of the PRF pattern.
As described above, the method of FIG. 5D starts the pattern-detection process
with any
specified value of a lower bound of the PRF pattern. The sequence of inter-
pulse intervals of the
PRF pattern are indexed in steps of 1 starting with 0. Thus, the parameter
Kõõnis the lower bound
minus 1.
With Kõõ,,=5, for example, the inter-pulse intervals {(1)(0) to 43(5)1, which
are {A, B, C, D,
A, B} are considered to be a segment of the sequence of inter-pulse intervals
of the entire PRF
pattern. A first round of the K-Loop identifies intervals "E", and "F" (of
indices 6 and 7) as
belonging to the pattern, hence K" is {A, B, C, D, A, B, E, F}, and reads
interval "A", from the
buffer (process 1120), which equals (KO). Process 1160 then transfers
execution of the pattern-
detection process to the J-Loop which identify interval "G" as belonging to
the pattern, then
transfer execution of the pattern-detection process to the K-Loop which, in
turn transfers execution
of the process to the J-Loop after reading interval "A" of index 10. The last
round of the J-Loop
produces the entire pattern J(').
FIG. 5H illustrates an arrangement 1500 for decoupling the pulse reception and
inter-pulse
measurement timescale from the processing timescale. As described above with
reference to FIG.
5C, the processes executed following determination of an inter-pulse interval
differ according to
values of prior intervals. The stream of P2 pulses comprises pulses having
different inter-pulse
periods. Thus, both the inter-pulse intervals and the requisite inter-pulse
processing effort are time
varying and generally uncorrelated. Thus, the smallest inter-pulse interval
may coincide with the
largest requisite processing effort. This suggests decoupling the processes of
pulse acquisition and
interval calculation from the processes of PRF-pattern buildup emanating from
process 1140 (FIG.
5D).
In accordance with an embodiment, process 1130 continually determines inter-
pulse inter-
vals and stores same in a (circular) buffer, specifically in array 4:13 as
described above with ref er-
ence to FIG. 5D. Process 1140 independently reads individual inter-pulse
intervals and for each in-
39
Date Recue/Date Received 2022-01-17
ter-pulse interval, relevant processes are executed before accessing the
(circular) buffer to read a
subsequent interval. Overall, the mean interval-processing rate cannot exceed
the mean pulse-ar-
rival rate.
As illustrated in FIG. 5H, a stream of pulses 1525 is acquired from a receiver
(process
1520) and inter-pulse time intervals 1530 are determined. The inter-pulse time
intervals are stored
(process 1540) in circular buffer 1550 and read one at a time after performing
respective processes
(process 1560) and supplied (process 1570) to process 1140 of the engine of
FIG. 5D. The inter-
pulse time intervals for a recurring PRF pattern, denoted Ao to Ag, are time
varying and the inter-
pulse processing durations, denoted 60 to 69, are time varying.
FIG. 51 illustrates phases of determining a PRF pattern for an exemplary
sequence of inter-
pulse intervals using the method of FIG. 5C.
FIG. 5J illustrates phases of determining a PRF pattern for the sequence of
inter-pulse
intervals of FIG. 51 using the method of FIG. 5E.
FIG. 5K illustrates phases of determining a PRF pattern for another sequence
of inter-pulse
intervals of FIG. 51 using the method of FIG. 5E.
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 highly 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.
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 processing unit 327 of FIG. 3 includes a high-speed ADC.
FIG. 6A and
6B show different implementations of the BB/IF processing unit 327. In FIG. 6A
a single channel
high-speed ADC 327-3 is used, while in FIG. 6B a dual channel high-speed ADC
327-3 is used.
Date Recue/Date Received 2022-01-17
As shown in FIG. 6A, the signals from the 1030 MHz receiver 325 and 1090 MHz
receiver
323 are mixed in the BB/IF processing 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 FIG. 3.
In Fig. 6B, each of the signals from the 1030 MHz receiver 325 and 1090 MHz
receiver
323 is mixed separately in the BB/IF processing 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 baseband
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 baseband signal. The two baseband
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 FIG. 3. In this embodiment, the two
channels in the ADC
327-9 share the same clock keeping the time between the two baseband 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 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
41
Date Recue/Date Received 2022-01-17
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 average p = (p1+p2+ ... + pn)/n.
Alternatively, the first-time interval may be determined as a mean value among
p1, p2, ...
pn time interval measurements, or as a mean square, or another function of the
time interval
measurements.
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 FIG. 7 based on the PRF pattern and the angular
rotation profile of
the SSR 110 Main Antenna (MA). The steps of the flowchart of FIG. 7 are
described below.
Step 705: profile the Main Antenna Angular or mechanical Rotation based on a
plurality of
detections 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). t241 is
42
Date Recue/Date Received 2022-01-17
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 720: 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 Fig. 2) in step 740. If "no", use the PRF and 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 140 is too far from the
SSR 110 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
43
Date Recue/Date Received 2022-01-17
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.
FIG. 8 illustrates 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 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 140 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 160. In this case, the ownship 140 will listen to the
reflection of the interrogation
signal from the target object 160. Because when the target object 160 is in
the beam of the SSR
110, the energy of the interrogation signal will be reflected from the target
object 160 and received
by the ownship 140. This receive time gives the same information as the
receive time of the reply
message, which can be used to calculate d1 + d2 in FIG. 2A together with the
staggered and
interrogation pattern. 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 receiver so that the angle of arrival
(AOA) of the reflection
44
Date Recue/Date Received 2022-01-17
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.
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 of
the mechanical rotation of the SSR antenna, the staggered pattern and the
interrogation mode
pattern are known, the time intervals between all the transmitted
interrogations in this time period
can be estimated. Therefore, 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
interrogation time intervals are ti, t2 t3
' then the samples that are ti' t2 t3 from a start
, = = = 5 5 = = =
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
coplanar; 2) whether or
not the target object 160, the ownship 140 and the SSR 110 are co-linear, but
not co-altitude; 3)
whether or not the target object 160, the ownship 140 and the SSR 110 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
Date Recue/Date Received 2022-01-17
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 speed of the target object 160 while
approaching the
ownship 140. The decision support engine during collision avoidance may use
online discrete-
event supervisory control based on a predicted TTC (time-to-collision) and a
predicted trajectory
of the target object 160 for the cases of full detectability and detection
singularity that occurs when
the ownship 140, and the SSR 110 are co-linear.
FIG. 9 illustrates a method 2200 of ensuring correctness of detection of the
PRF pattern. A
lower bound, denoted Km,,õ and an upper bound, denoted K., of the length o the
PRF pattern are
initialized in process 2210. Process 2220 detects a PRF pattern using any of
the algorithm de-
picted in FIG. 5A, 5C, 5D, or 5E. The number, N*, of intervals of the detected
PRF pattern are
stored in a circular buffer and used as a reference string of intervals
(process 2230).
Process 2240 continues to receive new pulses and determine new inter-pulse
intervals.
Process 2250 compares a number, N*, of the new intervals, forming a new string
of intervals, with
intervals stored in corresponding positions of the reference string stored in
the circular buffer.
Process 2260 determines whether the new string of N* intervals is congruent
with the ref-
erence string of N* intervals.
If congruence is ascertained, process 2270 sets the new string as the
reference string and
process 2240 is revisited. The loop of processes {2240, 2250, 2260, 2270,
2240}, referenced as the
congruence loop 2255, may continue to be activated as long as new pulses are
being received if
the reference string is the true PRF pattern. Optionally, a count, denoted x,
initiated as zero, of a
number of contiguous activations of the loop may be used as a measure of
successful acquisition
of the PRF pattern. A minimum number, xm,,õ of contiguous circulations of the
congruence loop
2255 may be specified and the latest reference string is considered to be the
true PRF pattern when
the count x reaches the value of xmin.
If process 2260 determines incongruence of the new string of N* intervals with
the refer-
ence string, process 2280 increases the value of Kmm: (N* + 1). As long as
Km,,, does not
exceed K., process 2290 leads to process 2220 which restarts computation of a
new reference
string of intervals, using any of the algorithm depicted in FIG. 5A, 5C, 5D,
or 5E, subject to a con-
46
Date Recue/Date Received 2022-01-17
straint of a minimum string size equal to the updated Kmin. If the updated
value of Km,,, in process
2290 exceeds K., process 2295 starts a process of revising operational
constraints that limit the
value of K.
Two strings of intervals, of N* intervals each, are considered to be congruent
if the
absolute value (magnitude) of a difference between intervals of corresponding
positions in the two
strings is below a first prescribed tolerance level, and the sum of N*
absolute values of the
differences is below a second prescribed tolerance level.
FIG. 10A illustrates an implementation of processes 2230, 2240, and 2250 of
FIG. 9.
Specifying a maximum permissible pattern length Km, a memory device 2310,
operated as a
circular buffer, of a storage capacity sufficient to hold a number, A, of
records of intervals, at least
equal to 2X K., is used to store a reference string of length N*, N* K.
(process 2230).
Consecutive intervals are written in successive memory divisions of the memory
2310 where each
new interval overwrites a previously stored interval in a respective memory
division.
In the example of FIG. 10A, K. = 16, but the length, N*, of the cyclic PRF
pattern is 10.
With A = 2X K., the memory divisions are indexed as 0 to 31. After receiving
the first (N*+1)
pulses and storing the corresponding N* intervals (process 2230), the time of
receiving each newly
received pulse is used to compute a value of a respective interval and store
(overwrite) the value in
a respective memory division (process 2240).
As illustrated, N* intervals (N*=10), of values denoted A, B, C, D, A, B, E,
F, A, G, are
stored in memory divisions 0 to 9 as the reference string (process 2230).
Subsequent N* interval
values forming a new string, determined in process 2240, are stored in memory
divisions 10 to 19.
A difference between a value written in a memory division of index talmodulo
A, N*ILt<(2XN*)
and a value stored in memory division (ILL-Nlmodulo A is determined (process
2250) and a sum
of absolute values of the differences is determined. If the absolute value of
each difference is
.. below a first prescribed tolerance level, and the sum of N* absolute values
of the differences is
below a second prescribed tolerance level, the reference string and the new
string are considered to
be congruent.
In the example of FIG. 10A, process 2260 determined that the new string
occupying
divisions 10 to 19 of the memory is congruent with the reference string
occupying divisions 0 to 9.
.. Then process 2270 promoted the new string in memory divisions 10 to 19 to
be the reference
47
Date Recue/Date Received 2022-01-17
string. The intervals occupying memory divisions 0 to 9 are now (logically)
discarded and may be
overwritten (the memory being operated as a circular buffer). The intervals of
memory divisions 0
to 9 may, however, be stored for further analysis.
Process 2270 leads to process 2240, to continue executing processes of the
congruence
loop 2255, with N* new interval values, forming a new string, being written in
memory divisions
20 to 29. A difference between a value written in a memory division of index
Hmodulo A,
2 XN* ILL<(3XN*) and a value stored in memory division N* [02 XN*) is
determined, and a
sum of absolute values of the differences is determined in process 2250.
Process 2260 again determined that the new string occupying memory divisions
20 to 29 is
congruent with the reference string occupying divisions 10 to 19. Then process
2270 promoted
the new string in memory divisions 20 to 29 to be the reference string. The
intervals occupying
memory divisions 10 to 19 are (logically) discarded and may be overwritten
(the memory being
operated as a circular buffer). The intervals of memory divisions 10 to 19 may
be stored for further
analysis.
FIG. 10B is a continuation of FIG. 10A where process 2270 leads to process
2240, to
continue executing processes of the congruence loop 2255, with N* new interval
values, forming a
new string, being written in memory divisions 301modulo A to 391modulo A,
which are {30, 31, 0,
1, 2, 3, 4, 5, 6, 7} since /1=2 X Kmax=32.
A difference between a value written in a memory division of index Hmodulo A,
3XN*ILt<(4XN*) and a value stored in memory division 2 XN*ILt<(3XN*) is
determined, and a
sum of absolute values of the differences is determined in process 2250.
Process 2260 again determined that the new string occupying memory divisions
{30, 31, 0,
1, 2, 3, 4, 5, 6, 7} is congruent with the reference string occupying
divisions 20 to 29. Then
process 2270 promoted the new string in memory divisions {30, 31, 0, 1, 2, 3,
4, 5, 6, 7}to be the
reference string. The intervals occupying memory divisions 20 to 29 are
(logically) discarded and
may be overwritten (the memory being operated as a circular buffer). The
intervals of memory
divisions 20 to 29 may be stored for further analysis.
At this point, the congruence loop has been consecutively traversed four
times. If the
parameter xmin is set to equal four, the congruence loop may be interrupted
and the reference
string treated as the true PRF pattern.
48
Date Recue/Date Received 2022-01-17
On the other hand, if the new string and the reference string are not
congruent, process
2260 leads to process 228- to increase the lower bound of the length of the
PRF pattern and the
entire sequence of processes starting with process 2220 are repeated as
described with reference to
FIG. 9.
FIG. 10C is a continuation of FIG. 10B, illustrating a further round of the
congruence loop
2250.
FIG. 10D illustrates two rounds of the congruence loop 2255 for a case where
N*=12,
using the same memory.
FIG. 10E illustrates two rounds of the congruence loop 2255 for a case where
N*=Kmax=16, using the same memory.
FIG. 11 illustrates DAA components provisioned in a target aircraft and ground
installations.
FIG. 12 illustrates communication paths between an ownship and different types
of target
aircraft.
FIG. 13 illustrates communication paths between an ownship, a target aircraft
and a UA
control station.
FIG. 14 illustrates components of a ground-based UA control station (or
ownship 140
control station).
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.
49
Date Recue/Date Received 2022-01-17
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. Instead,
the data is necessarily
presented in a digital form and stored in a physical data-storage computer-
readable medium, such
as an electronic memory, mass-storage device, or other physical, tangible,
data-storage device and
medium. It should also 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 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.
Thus, an improved avoid and detect method and system and a method and system
for
passive secondary surveillance radar tracking have been provided.
Date Recue/Date Received 2022-01-17
