Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 PASSIVE RANGING OF AN AIRBORNE EMITTER BY A SINGLE
NON-MANEUVERING OR STATIONARY SENSOR
The present invention relates to measuring the
range of moving targets, and more particularly, to a method
of passively measuring the ranqe of moving targets from
a single test platform which is either stationary, or
moving at a constant velocity along a linear trajectory. ~
Various passive ranging systems and methods .
have been previously developed. In a passive ranging
system the position of a target, such as an aircraft
or ship, is typically determined from radiant signals
emanating from the target itself~ rather than echo
signals developed from radiant energy being transmitted
towards the target, as in active ranging systems.
Generally, the systems and methods previously developed
involve the use of a variety of optical, electromagnetic
and acoustic sensors, as well as the use of cooperating
ground based receiving and transmitting stations.
Many such systems also require a plurality of cooperating !,,
20 sensor units, and cross-correlation equipment. Such `
syst~ms are generally complex, and often include operating ~-~
limitations such as the ability to sense only continuously
emitted signals, or the inability to be effective against
targets moving at a velocity greater than the velocity ~
25 of the observing vehicle itself. -~:
Many of the problems associated with prior
passive ranging systems and methods have been overcome
by the passive ranging methods described in U.S. Patent
No. a,l79,697 (the '697 patent), issued December 18,
1979 in the name of the present inventor, Martin Golinsky,
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1 and in U.S. Patent ~l,558,323 (the '323 patent)
also in the
name of the present inventor. Both the '697 patent
and the '323 patent disclose passive ranging methods
which permit the range to an emitter target to be deter-
mined from a single moving vehicle, commonly referred to
as a test platform.
In the method disclosed in the '697 patent,
a single measuring aircraft is constrained to fly along
a curved trajectory at a constant speed, while simulta-
neously performing a sequence of passive bearing measure-
ments of the target aircraft relative to itself. Such
measurements produce a geometric pattern of intersecting
rays which provides the data for mathematical calculations
used to determine the target's range.
In the method disclosed in the '323 patent
a single measuring aircraft is again used, however, it
is constrained to fly along a linear trajectory at a
non-constant speed (i.e., under acceleration or decelera-
tion), while also simultaneously performing a sequenceof passive bearing measurements of the target aircraft
relative to itself. Again such measurements are
used to produce a geometric pattern of intersecting
rays which also provides the data for a mathematical
solution to the target's range.
Although both of these methods are valid
and useful for certain applications, there are other
applications where it would be desirable to passively
measure the range of a target emitter without a test
3 platform having to meet the travel constraints imposed
25~ 2
1 by these prior methods. For example, it is desirable
for any aircraft traveling in a normal manner, i.e.,
traveling along a linear trajectory at a constant
velocity, to be able to readily measure the range
to other aircraft without going off its assigned
course or changing its normal air speed. Similarly,
it is also desirable for a stationary test platform,
such as an air traffic control tower, or a hovering
helicopter, which are incapable of satisfying the
travel constraints of the prior methods, to also be
able to measure the range to other aircrafts.
The foregoing problem is overcome, and other
advantages are provided by a ranging method utilizing
a succession of bearing and frequency measurements .
obtained at discrete periods of time from a radiant
signal emitted by a target vehicle. In accordance
with the invention, only a single measuring
vehicle or test platform is required to accomplish
the ranging process. The test platform carries a
sensor of radiation emitted by a target vehicle. The
emitted radiation may be pulsed or continuous. The
sensor, which may include an array antenna, beam
forming circuitry provides bearing and frequency
information about the source of radiation which is ~.
25 utilized to determine the range to the target aircraft. ~`
The velocity and direction of travel of the emitter
may then also be calculated from this information.
The method of the present invention provides
for the linear movement of a radiant energy sensor
~30 at a constant velocity. At discrete times during its
,. ~
292
1 displacement electrical circuitry connected to the sensor
provides for a sampling of the radiant energy signal
transmitted by the emitter target. As a result of the
combined movement of the target and sensor there is
produced a geometric pattern of rays of received
radiation. The bearing angles and frequencies of these
rays of radiation are measured and then arithmetically
combined to provide the range to the target vehicle
relative to the sensor.
10Although the present invention is described
herein with reference to the tracking of aircraft,
it should be understood that the method is equally
applicable to ships employing sonic sensors and
to satellites employing optical sensors. It should
also be noted that certain assumptions utilized
in the aforementioned Golinsky patent and application `
are also valid here. Specifically, the target whose
range is to be determined is not maneuvering, and the ;
test aircraft is at a long range from the target so
20 tnat they may be considered to be in the same horizontal -
plane.
In the accompanying drawings, Fiqure 1 is an
illustration of a moving vehicle, shown as a test air-
craft, carrying a sensor for receiving a radiant ^
energy signal emitted by a target vehicle, also shownas an aircraft.
Figure 2 is a block diagram of a passive
ranging system for use with the passive ranging method
of the present invention.
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1 Figure 3 is a geometric diagram which illu-
strates the relationship between the target aircraft and
the moving test aircraft at discrete point of detection.
Figure 4 is a geometric diagram which illu-
strates the relationship between the target aircraft and
a stationary test platform a~ a single point of detection. .
Figure 1 illustrates a typical scenario involvingthe method of the present inven~ion. A moving target
20, depicted as an aircraft which emits electromagnetic
lQ radiation, is shown traveling along a flight path 22.
The path 22 is assumed to be a ~traight line, while the
velocity of the target is assumed to be constant.
A test platform 24, also depicted as an aircraft,
carries a sensor 26 of the electromagnetic radiation
15 emitted by target 20. The test platform, which is '~
also shown to be moving along a straight path 28, ~;-
is constrained to operate at a constant velocity. ~
Figure 2 illustrates an apparatus which -`
could be used in implementing the present invention, and
20 indicates how this apparatus is to be connected. .`
When a target, such as aircraft 20, emits an --
energy signal, such as a radar pulse, the test aircraft
24 receives the signal through its antenna 30, after `
which it is channeled into a passive detection system `,7
32 measuring bearing and received frequency of the
emitted signal. Systems of this type are available
from amnufacturers such as Litton Industries, Amecom
Division. Litton Model No. ALR-73, is an example of
such a passive detection system.
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1 The test platform 24 also includes a navigation
system 34, a tracking system 36 and a display 38. The
navigation system 34 and the tracking system 36 employ well p
known circuitry and are available from manufacturers such
5 as Litton Industries. Examples of these systems include
Litton's navigation system ASN-92 and Litton's tracking
computer OL-77/ASQ. The tracking system is capable of
accomplishing the steps of the mathematical calculations
required to carry out the method of the present invention as -
10 hereinafter described. In the preferred embodiment of the
invention this system is used, although it should be understood
that a dedicated system may also be employed. Thus, where
the calculations are performed by the tracking system, the
present invention only requires systems already aboard many
15 test aircrafts, and therefore, it offers distinct advantages
over systems which require additional bulky and expensive
equipment.
Figure 3 illustrates the fundamental concept of
the present invention. Vector AC shown therein represents the
20 trajectory of test platform 24, while Vector DF represents
the trajectory of target emitter 20. The test platform
travels along vector AC at a constant velocity, Vp, so as to
cross points A, B and C at times Tl, T2 and T3, respectively.
Similarly, target 20 is assumed to travel along Vector DF
25 at a constant velocity, Vt, so as to cross points D, E and
at times Tl, T2 and T3, respectively. In the diagram shown
in Figure 3, the time intervals between Tl and T2, and T2
and T3 are taken as being equal. Thus, the distances AB and
BC traveled by the test platform during these time periods
30 are equal, while the distances DE and EF traveled by the -~
emitting target during these same periods, are also equal.
.
92
1 The assumption of equal time intervals, and therefore, equal
distances, is made only for the convenience of explaining the
concept of the present invention. In actuality, however, the
time between T1 and T2, and T2 and T3 need not be equal, and
5 the distances traveled during such time periods by the
platform and taxget also need not be equal.
Referring now to Figure 3 in combination with
Fi~ure 2, the sensor 26 of test platform 24 samples the
radiant signal emitted by target 20 as the test platform
lO crosses point A, B and C. At each of these points, the
passive detection system 32 aboard the platform utilizes
such samples to measure the bearing to the target aircraft
20, and the frequency of the emitter signal upon its arrival
at the platform. Once these samples have been obtained, and
15 their bearings and frequencies have been measured, tracking
system 36 utilizes this data to mathematically calculate the
range to target 20, as hereinafter described.
In contrast to the passive ranging methods described
in the aforementioned patents 4,179,697 and 4,553,323 in
20 which measurements were required at four points along the
platform's trajectory, it should be noted that the method of
the present invention only requires three measurements to be
made along the trajectory of the test platform. However,
in the present invention since frequency measurements are
25 required in addition to the bearing measurements utilized
in the previous methods, a total of six observables are used
in determining a target's range, in contrast to the four
observables used in the previous methods.
For purposes of describing the method of the
3O present invention, the values of the bearing and frequency
measurements taken at times T1, T2 and T3 are designated as
l' fl; 2' f2; and 03, f3, respectively. Generally, the
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1 three measure~ bearing angles will differ in value because
of the geoinetry of the flight paths o~ the test platform
and the target emitter. Similarly, the measured frequencies
will also vary, even if the frequency transmitted by the ~'
5 emitter is constant, because of changing doppler shifts.
At point A, the measured frequency ~1 can be
defined as follows:
-.
fl fo + ~ [Vt cos ~ + Vp sin ~1] (1) -
where
fQ equals the transmission frequency of the emitter ;~
~presently unknown); -
~ equals the transmitted wavelength;
15 --equals the angle between a bearing line, AD, to the emitter
and the target velocity Vector DF;
81 equals the measured bearing;
Vp equals the platform velocity (which is known by virtue of
onboard navigation system 34); and
20 Vt equals the velocity of the target aircraft (presently
unknown). :
The right side of equation (1) represents the
one-way doppler shift of the transmission frequency fO. ~
Similar equations can be developed to define frequencies ~-
25 f2 and f3. Thus t~hen test platform 24 is at points B and C,
the respective measured frequencies, f2 and f3, are defined
by the following equations
f2 = fo + 1 [Vt cos (~ + ~1 2) + V sin ~2] (2)
f3 = fo + ,` [Vt cos ~. + l 3) + Vp sin 03]
(3)
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1 where
2 equals the measured bearing at point B
03 equals the measured bearing at point C
l 2 equals ~ 2
5 ~1 3 equals l ~ 3 ,~
and the other parameters have been previously defined. All `
angles are measured positive in a counter--clockwise direction. :~-
Thus, in Figure 3, 03 would have a negative value.
Previous equations 1-3, set forth above, contain ~.
10 three unknowns, ~ , Vt and fO. These equations can, therefore, --
be solved for these unknowns, yielding the following equations:
fO= f1Sin 32,3~f2Sin ~1 3+f3sin ~1 2
1,2 ~1~3+5in ~2,3 + Vp (sin 03sin 4
c (4)
sin 02sin ~1 3+sin ~lsin ~2,3)
where -~-
2,3 ~2 ~ ~3, and
20 c equals the velocity of propagation of electromagnetic waves :
`
cos ~1 2 \ f -f - V f sin o2
= tan~l , ~ c ,. ''
i ~1,2 (5)
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1 The value of ~ can be readily solved because every variable on ~;
the riqht side of equation 5 is ]cnown by reason of the fact
that fO has been calculated using equation 4. It should be
noted that by retaining the signs of the numerator and
5 denominator of the argument, the usual angular ambiguity
associated with the arc tangent is resolved.
The last unknown Vt is determined from the following `~
equation: ~`
-
lG ^~-
" ~ c -Vp sin ~1 r~
cos (6)
Having calculated the values of the unknowns Vt and ~ , and
knowing the time interval T between measurements (which equals
the time periods between Tl and T2, and T2 and T3), it is '`
then possible to solve for the unknown ranges from the test
20 platform 24 to the target aircraft 20, i.e., AD, BE and CF
shown in Figure 3. Thus,
DE = EF = VtT (7)
while,
AB = BC = VpT (8 )
AB 2 (9)
sln ~1 2
3 GD = DE sin (r+ 1,2 (10)
sin ~1,2
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32
l Therefore, the range to targe-t 20 a-t point A is AD which is
defined as follows:
1 t.
AD = AG + GD = sin l 2 [AB cos ~2 + DE sin (' + ~1,2)] (11~ --
where all quantities on the right side of equation 11 are known.
Similarly, the range to target 20 at points B and C is BE and ~
CF, respectively. They are defined as follows: ;
BE = EG + GB = sin ~ [AB cos 1 ( 12)
CF = CJ + JF = sin O [AB cos 1 (13)
Thus, the solutions to equations 11, 12 and 13 represent the
15 ranges at any sample points A, B and C from a test platform to
a moving emitter where the test platform is traversing a linear
course at a constant velocity. Once this range information has
been calculated, it can be used to determine the velocity and
direction of travel of the emitter target using the standard
20 on-board systems of the test platform. i-
Figure 4 illustrates the fundamental concept of the ~-
present invention where the test platform is stationary.
E~amples of this type of application include an air traffic
control tower, a hovering helicopter, or an anchored ship. :
25 Here, since the velocity of the test platform is zero, Vp is
set equal zero in equations 1, 2 and 3. Notwithstanding this,
the method of determining the range to the emitter target at
points D, E and F is still the same. AD, BE and CF then
represent ranges from a common point A to the moving emitter,
3 since points B and C are now co-located with point A.
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1 To improve the accuracy of the calculated range ,~
in the presence of measurement errors, the measurements made
at times T2 and T3 may be used with a fourth measurement to
update the range calculation. This updating may be continued
5 for as long as desired.
It is obvious that the present invention can be
used in a number of military applications. It should be noted,
however, that in addition to such military applications, the
present invention may also be used in commercial applications
10 such as, for example, an aid to air traffic control. In such
applications, the transmission frequency, fO, would be
known. As such, only two sample points, e.g., A and B, rather
than three would then be needed, resulting in a reduced time
interval required to determine the applicable range to the
15 target emitter.
The calculations used to carry out the inventicn can
be performed in essentially real time on different sets of -
three measurements, where each set is determined by the passive
detection system to have resulted from a distinct emitter. --~
2G Therefore, the number of emitters that may be positioned by the
technique is limited only by the number of allocated track
files in the tracking system.
It is to be understood that a number of variations
may be made in the invention without departing from its
25 spirit and scope. The terms and expressions which have been
employed are used in a descriptive and not a limiting sense, ~;
and no intention of excluding equivalents of the invention i-
described and claimed is made.
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