Time delay passive ranging technique

A method of passive ranging and geolocation of multiple emitters by a single detection platform. Two independent emission sequences support formulation of two independent algebraic equations involving a triangular arrangement of platform and emitters. One sequence constitutes an interrogation signal by one emitter and a transponded or reflected signal from another. A second emission sequence constitutes the reversed order of emitters from those of the first emission sequence. The method utilizes the steps of measuring the time difference of arrival at the platform of signals having travelled the direct path and the transponded or reflected paths, and measuring the angles of arrival of received signals for each independent emission sequence. A series of steps computing ranges and angles based on prior measurements provide a set of desired ranges and angles identifying the relative positions of the emitters relative to the platform. The invention may be employed in bistatic or transponded mode depending on the kind of signal emissions that are to be exploited. In the bistatic mode, the energy from an emitter is reflected from the other emitter. In the transponder mode the emitters communicate in an interrogation-transpond format with signals with known and small internal time delays. In the transponder mode, both signals are direct path signals.

BACKGROUND 
The present invention relates to target location, and more particularly, to 
a passive method of locating targets. 
In electronic warfare, the enemy typically employs high-power radars, 
navigation beacons and identification friend or foe (IFF) equipment to 
detect penetrating aircraft and to aid and recognize its own aircraft. 
When the radar detects a penetrating aircraft, a radar operator 
interrogates it with an IFF signal to determine its identity. If the 
aircraft is friendly, it responds to the IFF interrogation with a 
transponded signal, usually on a different frequency than that of the 
interrogation. During this short time of response to interrogation, the 
friendly aircraft is an active emitter. The command and response sequence 
can be accurately timed to determine the range between the interrogator 
and the aircraft. If an unfriendly aircraft is detected, the radar 
operator vectors a fighter aircraft to intercept the penetrating aircraft. 
In the setting described above, it is very desirable that the penetrating 
aircraft passively locate the enemy radar and interceptors in range and in 
azimuth without using the active sensors. To this end, the penetrating 
aircraft carries direction finding equipment such as an electronic support 
measure (ESM) system, for example. However, the direction finding 
equipment may be any type of SONAR or electromagnetic system which 
includes detection equipment suitable for detecting target emissions. The 
detection equipment may be located on any type of vehicle, including an 
aircraft, submarine or surface ship. Consequently, due to the generic 
applicability of the present invention, the vehicle carrying the detection 
system is hereinafter referred to as a platform. 
Conventional electronic ranging and geolocation techniques do not fully 
utilize available information, and rely on signals originating from active 
search radars or bistatic returns provided by echoes from other targets in 
the vicinity. The techniques fall into roughly three categories. 
The first category includes those that rely on long baselines and use 
either crossing of bearings or hyperbolic lines of position derived from 
time difference of arrival. These techniques require either multiple 
platforms or special tactics. An example of multiple platforms is a system 
with two receivers located in each wing tip of an aircraft, for example. 
This approach acquires theoretical information with which to compute the 
range to the emitter. An example of special tactics requires that a single 
platform fly along a baseline while taking measurements over a long time. 
Multiple platforms result in a more costly and complex system. Flying 
along a baseline requires that the emitter be stationary and preferably 
off to the side of the platform thus rendering the system less effective 
for emitters directly ahead of the platform. Furthermore, the process 
requires a relatively long time, and is inconsistent with adequate 
reactive maneuvers by the platform while under enemy attack. These 
disadvantages make it impractical for single platform penetration 
missions. 
The second category includes techniques that employ bistatic ranging with 
prior knowledge of one or more of the sides of the triangle formed by the 
emitters and a receiver. This information, however, may not be available 
or current immediately prior to a mission. Likewise, it may not be 
sufficiently accurate in a dynamic engagement mission. This work is 
described in U.S. Pat. No. 4,370,656 to Frazer and Lewis. 
The third category includes techniques that employ bistatic ranging along 
with determination of one or more of the angles of the triangle formed by 
the platform, an emitter and a secondary reflector. One such technique 
determines the angle between the range vectors from the platform to the 
emitter and from the platform to a reflecting target by directional 
measurements relative to the platform. A second approach determines the 
angle between the range vectors from the emitter to the platform and from 
the emitter to a secondary target. This is done by measuring the time 
delay as the main radar beam sweeps through the platform and the secondary 
target. The measurement is taken as the time delay between the passing of 
the main beam and the measurement of the main beam reflection from the 
secondary target. This technique requires knowledge of the scan time of a 
search radar. This technique is described in U.S. Pat. No. 4,670,757 to 
Munich and Schecker. Either technique has limited areas of application 
since modern radars may not scan in a regular pattern or with a constant 
scan speed. 
Accordingly, it is an objective of the present invention to provide a 
method of range and azimuth determination that utilizes all available 
active emissions from other sources. Another objective of this invention 
is to provide a method for measuring range and heading information that 
utilizes only momentary emission from secondary sources such as enemy IFF 
transmissions. It is a further objective of the present invention to 
provide a method for measuring range and heading which utilizes the 
responsive nature of the IFF transmissions and utilizes known internal 
delays of IFF transmissions responsive to radar interrogation. Yet another 
objective of this invention is to provide a method for measuring range and 
heading which utilizes existing, fielded and operational, direction 
finding equipment and systems. A still further objective of the present 
invention is the provision of a method for measuring range and heading 
which operates both in a bistatic mode or transponded mode, depending on 
available signals to be exploited. Another objective of this invention is 
to provide a method for measuring range and heading which obtain solutions 
of target locations that is substantially instantaneous in time. 
SUMMARY OF THE INVENTION 
In view of the foregoing and other objectives and features, the present 
invention provides a method for location of multiple emitters that is 
entirely passive and that obtains location solutions that are 
substantially instantaneous in time. The method of the present invention 
utilizes the time difference of arrival of radar signals between direct 
paths and transponded or reflected paths and also the angles of arrival of 
the radar signals to determine range and azimuth. The method provides 
measurement of the angle at the platform between the path to a secondary 
target and the path to the emitter and the time delays of radar signals 
arriving via the different paths, and uses these values to calculate the 
location (range and azimuth) of both the radar and the target. 
The measurements are taken twice. A first set of measurements considers 
signals originating at the radar emitter and measures the time difference 
of arrival of signals via a direct path from the emitter, and those 
arriving via the transponded or reflected path from a secondary target. A 
second set of measurements is concerned with the signals originating at 
the secondary target, such as IFF transmissions, and measures the time 
difference of arrival of signals via a direct path from the secondary 
target and those arriving after having been transponded by, or reflected 
from, the emitter. These measurements are then used to determine ranges 
and directional angles of the emitter and the secondary target relative to 
the platform. 
The method of the present invention employs existing direction finding 
equipment, such as is found in electronic support measures ESM and radar 
warning in receivers (RWR) to measure the bearings of the targets and 
accurate timing of the time differences between the direct path signal 
from an emitting platform and that reflected or transponded by other 
platforms. The invention may be employed in bistatic or transponded mode, 
depending on the kind of signal emissions to be exploited. In the bistatic 
mode, the energy from an emitter is reflected from the other targets. In 
the transponder mode the targets communicate in an interrogation-transpond 
format with signals with known or small internal time delays. In the 
transponder mode both signals are direct path signals. 
The method of the present invention may be applied to passively locate 
targets such as aircraft, missiles, helicopters, ships, spacecraft, or 
submarines from a single platform, and in a time interval comprising only 
a few pulses. The technique may be applied to electronic support measures 
(ESM), radar warning receivers, SONAR and navigation equipment. Passive 
ranging techniques have a major advantage in defense systems since the 
platform performing the ranging may remain completely covert. Passive 
ranging techniques may be employed for early warning, for cueing other 
sensors on the platform, including IR, optical, LASER and radar so that 
emissions may be eliminated or reduced to the minimum required to target 
weapons systems.

DETAILED DESCRIPTION 
Referring now to FIG. 1 there is shown a representation of the geometric 
relationship 10 that forms the basis for the passive ranging and 
geolocation method that is the subject of the present invention. In FIG. 1 
there is shown a platform 11, an emitter 12 and a secondary target 13 
which form three corners of a triangle representing the geometric 
relationship 10 formed by the three objects. The platform 11 may be 
located on any type of vehicle, including an aircraft, satellite, 
submarine or surface ship, for example. The distances between the three 
corners represent relative ranges between the three objects, thus a 
platform-to-target range 14 is labeled R, a platform-to-emitter range 15 
is labeled D and an emitter-to-target range 16 is labeled R.sub.T. The 
platform-to-emitter range 15 and the platform-to-target range 14 subtend a 
platform angle 17 that is labeled .alpha.. Similarly, the 
platform-to-target range 14 and the emitter-to-target range 16 subtend a 
target angle 18 that is labeled .beta., and the platform-to-emitter range 
15 and the emitter-to-target range 16 subtend an emitter angle 19 that is 
labeled .gamma.. A platform reference 20 denotes the general orientation 
of the platform 11 and serves as the reference for the measurements of an 
emitter heading 21, labelled .theta..sub.1, and a target heading 22, 
labelled .theta..sub.2. The labels are symbolic representations of the 
variables for use in subsequently described algorithms. 
In operation, the platform 11 may be any craft, such as an aircraft, ship, 
spacecraft, or land vehicle that is free to generally move relative to 
other objects, or it may be a fixed intercept station. Typically, however, 
the platform 11 is mounted on an intruding aircraft trying to penetrate 
enemy defensive systems. The platform 11 may carry with it any type of 
receivers which can detect electromagnetic or SONAR emissions from other 
platforms. The receivers may also include direction finding equipment 
typically, but not necessarily, of the type associated with an electronic 
support measure (ESM) system. 
The emitter 12 may be the emitter element of a surveillance system, and may 
be mobile or fixed, and may be radiating electromechanical energy or 
acoustical energy. The emitter 12 typically illuminates intruding objects 
to determine the position and speed of the object. Additionally, the 
emitter 12 interrogates the object with IFF signals to determine whether 
the intruding object is a friend or an undesirable intruder. 
The secondary target 13, in order to be useful to the present system and 
method, is generally a friendly object relative to the emitter 12. As 
such, it will generally respond to the interrogations with IFF 
transmissions. During this response, the secondary target 13 emits an 
active signal in response to the interrogation of the emitter 12 and will 
itself, for a short time, be an active emitter whose emanations may be 
intercepted by the intruder, or platform 11. 
The totality of emissions, echoes, interrogations and responses emanating 
from emitter 12 and secondary target 13 constitute two distinct emission 
sequences. A first emission sequence is initiated by the interrogation by 
the emitter 12 and is followed by an answering active emission by, or 
passive reflection from, the secondary target 13. A second emission 
sequence is initiated by the answering active emission by the secondary 
target 13. An example is provided by a response to the emitter query with 
a signal for the purpose of ranging or IFF back to the emitter 12. The 
second emission sequence is completed by an active response by, or passive 
return from, the emitter 12. 
The receiver on platform 11 generally monitors these emissions. However, 
according to this invention, the platform 11 focuses on the emission 
sequences and specifically measures the time difference in arrival of two 
signals constituting one emission sequence. The first emission sequence 
starts with a first interrogation signal by the emitter 12 that arrives at 
the platform 11 as a first received signal. The first received signal 
arrives after having travelled the direct path D shown in FIG. 1. A timer 
is started upon detection of this signal. The second received signal 
arriving at the platform 11 also originated at the emitter 12 and is the 
first interrogation signal by the emitter 12 reflected off the primary 
target 13 or its transponder direct signal with its internal delay. This 
signal then has traveled the indirect path, (R.sub.T +R), as shown in FIG. 
1. The timer is now stopped, thus marking a time difference in arrival of 
the two signals. This time difference, labeled .tau..sub.1, when combined 
with a known velocity of propagation of the signals, labeled c, form the 
basis for one algebraic equation relating the path differences shown in 
FIG. 1. Similarly, the second emission sequence starts with an active 
signal originated by the primary target 13 as an interrogation of the 
secondary target 13 or as an independent IFF interrogation. The first 
received signal of the second emission sequence arrives at the platform 11 
after having traveled the distance R as shown in FIG. 1. A timer is 
generally started upon detection of this signal. The second received 
signal of the second emission sequence arriving at the platform 11 also 
originated at the secondary target 13. However, this signal have traveled 
the indirect path, (R.sub.T +D), as shown in FIG. 1. The timer is now 
generally stopped, thus marking a time difference in arrival of the two 
signals. This time difference, labeled .tau..sub.2, when combined with a 
known velocity of propagation of the signals, labeled c, form the basis 
for a second algebraic equation relating the path differences shown in 
FIG. 1. 
The second received signal, or echo, of the two emission sequences may also 
have originated from active emissions provided that they are sent in 
response to an interrogating signal and that the internal time for the 
response is known prior to the engagement. Thus, for the first emission 
sequence, an answering active response by the secondary target 13 may be 
used to measure the time difference in travel time. In this case, however, 
the true difference in path travel time is the time difference of the 
received signals minus the known response time of the secondary target 13. 
In the case of the second emission sequence the second received signal may 
be the active response of the emitter 13 to IFF interrogations by the 
primary target 12. 
Similarly, the composition of the signals in any given emission sequence 
may be any combination of identified signals provided that their place in 
a emission sequence are identified. The time differences .tau..sub.1 and 
.tau..sub.2 of the received signals are linked symbolically by the 
formulas: 
EQU c.tau..sub.1 =R+R.sub.T -D (1) 
EQU c.tau..sub.2 =R+R.sub.T +D (2) 
where c is the velocity of propagation of the signal. 
The platform angle 17, .alpha., is readily determined from the bearing 
measurements with respect to some arbitrary reference direction, such as 
north, magnetic north or platform relative. Thus 
EQU .alpha.=.theta..sub.1 -.theta..sub.2 (3) 
From the measurements of the differential path lengths one can immediately 
determine the emitter-to-target range 16 by: 
EQU R.sub.T =c(.tau..sub.1 +.tau..sub.2).div.2 (4) 
Then from the law of cosines we can calculate the platform-to-target range 
14 by: 
##EQU1## 
where the positive root of the radical is used. Then it is evident from 
the first equations that: 
##EQU2## 
The angle 18 and the angle 19, symbolically .beta. and .gamma., are then 
readily computed via the law of sines: 
##EQU3## 
to complete the set of equations for the unknown parameters. A solution of 
available for the symbolic angle, .alpha., and the ranges of the triangle 
of FIG. 1 after measuring the two time differences, .tau..sub.1 and 
.tau..sub.2, and the symbolic angles .theta..sub.1 and .theta..sub.2. 
Referring now to FIG. 2 there is shown a graphic representation of an 
events sequence 30 illustrating the sequential steps of the method of the 
present invention. A start event 31 represents the beginning of a search 
sequence. An interrogation detection event 32, a first measuring event 33 
and a timer event 34 follows in the event sequence. A first emission event 
35, comprising a search event 36 and a detection event 37, follows and is 
in turn followed by a second measuring event 38. A second emission event 
39, which is a duplicate of the first emission event 35, follows from the 
second emission sequence. The last event in the sequence of FIG. 2 is a 
computation event 40 for calculating desired variables based on previously 
measured variables. 
In operation, the events sequence 30 is initiated by an interrogation 
signal from the emitter 12 of FIG. 1. This marks a series of steps taken 
during the first emission sequence. As soon as the interrogation signal is 
detected by the interrogation detection event 32, the first measuring 
event 33 obtains a measure of the symbolic bearing angle .theta..sub.1 of 
the emitter 12 relative to the platform 11 as shown in FIG. 1. 
Simultaneously the timer event 34 starts a timer for measurement of the 
time difference .tau..sub.1 and also initiates the first emission event 
35. 
The first emission event 35 maintains the search event 36 which initiates a 
receiver search for a response, or an echo, of the interrogation signal. 
The arrival of the interrogation signal initiates the detection event 37 
which interrupts the first emission event 35 and initiates the second 
measuring event 38. The event 38 marks the measuring of the symbolic 
bearing angle .theta..sub.2 of the secondary target 13 relative to the 
platform 11 as shown in FIG. 1. The second measuring event 38 also marks 
the stopping of the timer to obtain the time difference .tau..sub.1 
representing the different arrival time of signals traveling the direct 
and indirect paths. The second emission event 39 repeats the steps taken 
throughout the first emission sequence. 
The steps thus provide for restarting the timer for the second emission 
sequence and thus preparing to measure the time difference .tau..sub.2 
denoting the difference in arrival time by signals having traveled a 
direct and indirect path. The steps continue by searching, waiting for and 
then detecting the answering signal and finally obtain a measurement of 
the time difference .tau..sub.2. The computation event 40 represents 
computer algorithms which solve for selected variables according to 
equations (1) through (8) above. Thus symbolic representations of angles 
and ranges as shown in FIG. 1 are obtained. 
Referring to equations (1) through (8) above, to the events sequence 30 of 
FIG. 2 and to the representations of angles and ranges shown in FIG. 1, 
there is shown sufficient equations relating the variables depicted in 
FIG. 1, as well as measurements of pertinent variables, to solve for the 
unmeasured, and hence unknown, variables. The measurements and computer 
algebraic computations are an inherent part of a computer program. The 
angles and ranges depicted in FIG. 1 are therefor solved substantially 
instantaneously. There is thus no need for any fly out maneuver, base leg 
establishments or of prior knowledge of emitter location before accurate 
estimates are available for ranges to enemy crafts or installations. This 
is of utmost importance in a combat environment. 
Thus there has been described a new and improved method for single platform 
passive ranging and geolocation of multiple emitters by employing time 
difference of arrival between the direct paths and the transponded or 
reflected paths and the angles of arrival to determine range and azimuth. 
The method may be used on any platform having direction finding equipment. 
The platform may be installed in a moving object, such as an aircraft, on 
stationary objects or on a combination of moving and stationary objects. 
The method may be used to locate a number of emitting targets in either a 
bistatic or interrogator/transponder mode. 
It is to be understood that the above-described steps of the overall method 
of this invention are merely illustrative of some of the many specific 
steps which represent applications of the principles of the present 
invention. Clearly, numerous and other arrangements can be readily devised 
by those skilled in the art without departing from the scope of the 
invention.