Covert electronic battlefield identification system

An electronic identification system for use by vehicles on a battlefield comprises an interrogator circuit in an attack vehicle and a transponder circuit in several other vehicles which are friendly but could be mistaken for the enemy. In operation, the interrogator circuit transmits a time shifted code which is very difficult for an actual enemy to detect and/or jam, and which causes the transponder circuits to send a response from only a selected one of the other friendly vehicles that is being examined by the attack vehicle as a target.

BACKGROUND OF THE INVENTION 
Casualties due to "friendly fire" have been an unfortunate element of 
warfare throughout history. Often, such casualties occur because the 
technology to identify a potential target as friendly under the fog of war 
has not always kept pace with the technology to acquire that target and 
destroy it. 
In Operation DESERT STORM, a simple apparatus was used to identify friendly 
vehicles. This apparatus, installed in each friendly vehicle, merely 
emitted an infra-red beacon. Unfortunately, such a beacon will also serve 
to positively identify the vehicle to a moderately sophisticated enemy. 
What is needed is an electronic identification system which determines 
whether or not a "target" is friendly without bringing either the 
targeting vehicle or the target vehicle to the attention of the enemy. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, an electronic identification 
system for use on a battlefield, comprises an interrogator circuit in a 
targeting vehicle and a respective transponder circuit in each of several 
friendly target vehicles. Each of the interrogator and transponder 
circuits includes a code generator which generates the same sequence of 
chip signals, and they include a timing circuit which generates 
synchronized reference timing signals. Also, the interrogator circuit 
further includes--1) a range finding circuit which determines the 
propagation time for a radio signal to travel from the interrogator 
circuit to a selectable one of the friendly target vehicles, and, 2) a 
transmitting circuit which transmits via radio the sequence of chip 
signals advanced in time relative to the reference timing signals by the 
propagation time. In addition, the transponder circuit includes--1) a 
correlator circuit for receiving the transmitted chip signals in 
synchronization with the reference timing signals and for correlating same 
with the sequence of chip signals from its own code generator, and 2) a 
response circuit for transmitting via radio a response only if the result 
of the correlating operation exceeds a certain threshold.

DETAILED DESCRIPTION 
Referring now to FIG. 1, it shows three battlefield vehicles V.sub.1, 
V.sub.2, and V.sub.3 which incorporate an electronic identification system 
in accordance with the present invention. This electronic identification 
system includes an interrogator circuit 10 in vehicle 1 and a transponder 
circuit 20 in each of the vehicles V.sub.2 and V.sub.3. All of the 
structural details of the interrogator circuit 10 and the transponder 
circuit 20 are shown in FIG. 2, and the signals and operational details of 
those circuits is shown in FIG. 3. 
In operation, the interrogator circuit 10 and the transponder circuits 20 
enable an operator of vehicle V.sub.1 to select any one of the vehicles 
V.sub.2 or V.sub.3 and electronically determine if the selected vehicle is 
friendly or belongs to an enemy. Such a determination is needed in order 
to prevent the vehicle V.sub.1 from firing upon and destroying a friendly 
vehicle by mistake. With the identification system of FIG. 1, the 
distinguishing of a friendly vehicle from an enemy vehicle is achieved by 
the circuits 10 and 20 as follows. 
First the operator of vehicle V.sub.1 selects one of the vehicles V.sub.2 
or V.sub.3 as a potential target. Assume for example that vehicle V.sub.2 
is selected. Then, the transponder circuit 10 transmits a radar type 
signal and measures the propagation time for the signal to travel from the 
interrogator circuit 10 to the selected vehicle V.sub.2. This propagation 
time in FIG. 1 is shown as .DELTA.T.sub.2. Vehicles V.sub.2 and V.sub.3 
are at different distances from vehicle V.sub.1, and thus the propagation 
time for the signal to travel to vehicle V.sub.3 will be different. That 
propagation time is shown in FIG. 1 as .DELTA.T.sub.3. 
Next, the interrogator circuit 10 transmits a sequence of chip signals that 
are advanced in time, relative to an interrogate timing instant t.sub.I, 
by the propagation time .DELTA.T2. This chip sequence at vehicle V.sub.1 
is shown as signal S.sub.1 (t.sub.I -.DELTA.T.sub.2). Then, the chip 
sequence travels to both of the vehicles V.sub.2 and V.sub.3 ; however, 
due to their different distances from vehicle V.sub.1, the chip sequence 
reaches the vehicles V.sub.2 and V.sub.3 at different times. 
More specifically, the transmitted chip sequence starts to reach vehicle 
V.sub.2 at the interrogate time instant t.sub.I, whereas it starts to 
reach vehicle V.sub.3 at some other time. Then, starting at the 
interrogate time instant t.sub.I, the transponder circuit 20 in both of 
the vehicles V.sub.2 and V.sub.3 performs a correlation between the signal 
which it receives and the same chip sequence that was transmitted. 
If the result of the above correlation exceeds a certain threshold in one 
of the transponder circuits 20, then that circuit transmits a response 
signal S.sub.2 (t.sub.R) which begins at a response timing instant 
t.sub.R. This response signal is shown in FIG. 1 as being transmitted from 
vehicle V.sub.2 since in the above example, the transmitted chip sequence 
S.sub.1 was shifted in time by the signal propagation delay between 
vehicles V.sub.1 and V.sub.2. 
Next, beginning at time instant t.sub.R +.DELTA.T.sub.2, the interrogator 
circuit 10 performs a correlation operation on whatever radio signal it 
receives and the expected response signal S.sub.2. If that correlation 
exceeds a certain threshold, the interrogator circuit 10 generates a 
signal indicating the potential target vehicle is friendly. 
Turning now to FIG. 2, the internal structural details of a preferred 
embodiment of the interrogator circuit 10 and the transponder circuit 20 
which operates in the above fashion will be described. This particular 
embodiment of the interrogator circuit 10 includes an antenna 10a, a range 
finding circuit 10b, an interrogate timing slot generator (I-slot 
generator) 10c, a timing slot advance circuit 10d, a code generator 10e, a 
transmitter circuit 10f, a response timing slot generator (R-slot 
generator) 10g, a time slot delay circuit 10h, and a correlator circuit 
10i. All of these circuits are interconnected as shown. 
Initially, the interrogator circuit 10 is started by a synchronizing signal 
SYNC that is received via the antenna 10a from an external source. 
Suitably, the synchronizing signal comes from a satellite such as the 
global positioning satellite GPS. That synchronizing signal SYNC is then 
sent to the I-slot generator 10c and the R slot generator 10g, whereupon 
they respectively begin generating a series of interrogate timing signals 
I and response timing signals R. These timing signals I and R are shown in 
FIG. 3 as voltage waveform 31. 
Waveform 31 is made up of a series of consecutive time periods T.sub.P1, 
T.sub.P2, etc.; and, within each time period, a single pulse of the I 
signal and a single pulse of the R signal occurs. Also, within each of the 
time periods T.sub.P, the I pulse and the R pulse begin at different time 
instants which vary in a quasi random fashion from one time period to 
another. These time instants for the I pulse are labeled in FIG. 3 as 
T.sub.I1, T.sub.I2, etc.; and for the R pulse they are labeled as 
T.sub.R1, T.sub.R2, etc. 
Each I pulse from circuit 10c is sent to the time slot advance circuit 10d, 
and each R pulse from circuit 10g is sent to the time slot delay circuit 
10h. Those circuits 10d and 10h also receive a signal .DELTA.T from the 
range finder 10b which indicates the propagation delay of a radio signal 
from the interrogator vehicle V.sub.1 to the selected target V.sub.2 or 
V.sub.3. In response to the signals I and .DELTA.T, circuit 10d generates 
a timing signal I.sub.A which is the same as the signal I but which is 
advanced in time by the propagation delay .DELTA.T. This is shown in FIG. 
3 by waveforms 32 and 33. Likewise, in response to the signals R and 
.DELTA.T, circuit 10h generates an output signal R.sub.D which is the same 
as the signal R but which is delayed in time by .DELTA.T. 
Signal I.sub.A is sent to the code generator 10e and to the transmit 
circuit 10f. Upon receiving the signal I.sub.A, the code generator 10e 
sends a sequence of chip signals C.sub.x, C.sub.x+1, .... to the 
transmitter circuit 10f. At the same time, the circuit 10f responds to the 
signal I.sub.A by transmitting the chip signal C.sub.x, C.sub.x+1, via the 
antenna 10a to the transponder circuits 20. This is shown in FIG. 3 by 
waveform 34. 
In the preferred embodiment of FIG. 2 each of the transponder circuits 20 
includes six components which are labeled 20a, 20c, 20e, 20f, 20g, and 
20i; and, those components are similar to the components of the 
interrogator 10 which have the same reference letter. For example, 
component 20a is an antenna similar to antenna 10a; component 20c is an 
I-slot generator circuit which is similar to the I-slot generator 10c; 
component 20e is a code generator that is similar to the code generator 
10e; etc. These components 20a, 20c, 20e, 20g, 20i, and 20f are 
interconnected as shown in FIG. 2. 
All of the I-slot generators 20c and the R-slot generators 20g are 
initialized via a synchronizing signal SYNC that is received via the 
antenna 20a at the same instant that the interrogator circuit 10 is 
initialized. Thereafter, the I-slot generator 20c and the R slot generator 
20g in both of the transponders 20 generate respective I and R timing 
signals in synchronization with the I and R signals of the interrogator 
circuit 10. This is shown in FIG. 3 by voltage waveforms 35 and 36. 
Each time the I slot generator 20c generates an I pulse, that pulse is sent 
to the code generator 20e and to the correlator 20i. In response, the code 
generator 20e generates the same chip sequence C.sub.x, C.sub.x+1 that was 
previously transmitted by the interrogator circuit 10; and, the correlator 
circuit 20i performs a correlation operation between the chip sequence 
from the code generator 20e and the chip sequence which it receives from 
the antenna 20a. 
If those two chip sequences are in time synchronization with each other, 
this is detected by the correlator 20i which in turn sends a MATCH signal 
to the transmitter circuit 20f. Otherwise, no MATCH signal is generated. 
In the above example where the interrogator circuit 10 transmits chip 
sequences that are advanced by .DELTA.T.sub.2 from the timing instants 
t.sub.I1, t.sub.I2, etc., those chip sequences will be received in time 
synchronization in vehicle V.sub.2 with the internally generated chip 
sequences from code generator 20e. Conversely, in vehicle V.sub.3, the 
received chip sequences will be out of time synchronization with the 
internally generated chip sequences. This is shown in FIG. 3 by voltage 
waveforms 37 and 38. 
Preferably in the correlator circuit 20i, the received chip sequence from 
the antenna 20a is correlated with the chip sequence from the code 
generator 20e during several consecutive I-pulses (e.g.--five to fifteen) 
before a determination is made to generate the MATCH signal. This 
correlation, can, for example be performed by circuit 20i by logically 
ANDing the received chip sequence and the generated chip sequence with 
each other and integrating the result over several of the I-pulses. Then, 
when the result of that integration exceeds a certain threshold level, the 
MATCH signal is generated. 
When the match signal is generated, it is sent to the transmitter circuit 
20f. That transmitter circuit also receives the R timing signals from 
circuit 20g along with a sequence of chips from the code generator 20e. 
Then, in response to the MATCH signal, the transmit circuit 20f transmits 
via the antenna 20a, the sequence of chip signals from the code generator 
20e in time synchronization with the timing signals R. 
As the R timing signals are being generated by circuit 20g in the 
transponder 20, the same R timing signals are being generated by circuit 
10g in the interrogator 10. From circuit 10g, the R timing signals are 
sent to the time delay circuit 10h wherein they are delayed by the time 
interval .DELTA.T (i.e. the propagation time for a signal to travel 
between the vehicles V.sub.1 and V.sub.2 ). This delayed timing signal is 
indicated in FIG. 2 as signal R.sub.D. 
In time synchronization with signal R.sub.D, the code generator 10e 
generates the same sequence of chips that was previously transmitted by 
the transponder 20; and, that code sequence is correlated with whatever 
signals are being received on the antenna 10a. If a MATCH signal was 
previously generated by a transponder 20, then during the timing signal 
R.sub.D, the signals which are received on the antenna 10a should match 
the chip sequence from the code generator 10e. In that case, the 
correlator 10i generates an output signal "FRIEND" indicating that the 
potential target is a friendly vehicle. 
A primary feature of the above described electronic identification system 
is that its operation is covert and difficult for an enemy to detect. In 
particular, in order to detect the disclosed system, the enemy must know: 
1) the chip sequence C.sub.x, C.sub.x+1... that is being generated by the 
code generators, 2) the interrogate time instants T.sub.I1, T.sub.I2, ... 
at which the interrogate chip sequences begin, 3) and the response time 
instants T.sub.R1, T.sub.R2, at which the response chip sequences begin. 
Also, the disclosed system is made even more difficult to detect by 
constraining the width of the I and R pulses to be very small relative to 
the time periods T.sub.P. Preferably, each I pulse and each R pulse is 
less than 10% of the time period T.sub.P. As one specific example, the 
time period T.sub.P can be two milliseconds; the I pulse can be 125 
microseconds and include a chip sequence of 4,000 chips; and the R pulse 
can be 100 microseconds and includes chip sequence of 16,000 chips. 
Still another feature of the disclosed system is that its operation 
requires the use of just a single frequency band. This feature arises 
because the I pulse and the R pulse start at different time instants; and 
do not overlap. 
Yet another feature of the disclosed system is that it precludes spurious 
replies to interrogations from all but the one targeted vehicle. This 
feature results from the fact that the potential target vehicle is 
identified as being friendly not just on the basis of the code but also on 
its distance from the interrogator. 
Still one additional feature of the disclosed system is that the time which 
it takes to identify a vehicle as a friend or a foe is quite short. This 
result occurs because in the vehicle which is the potential target, the 
chip sequence from the interrogator is timed to reach that vehicle in time 
synchronization with the vehicle's internal code generator and thus a 
correlation can be made quickly. Likewise, the interrogator synchronizes 
the timing of its correlation with the receipt of the response from the 
potential target. If, for example, each time period T.sub.P is two 
milliseconds and a correlation is made in ten periods, then the total 
identification time is only forty milliseconds plus two transmission times 
(2.DELTA.T). 
Throughout the above description of FIGS. 1-3, it was assumed that the 
signal propagation time .DELTA.T.sub.2 from vehicle V.sub.1 to vehicle 
V.sub.2 as measured by the range finder circuit 10b had no inaccuracies, 
and it was assumed that the interrogate timing pulses I as generated by 
the I-slot generators 10c and 20c are in perfect synchronization. If, 
however, some inaccuracies are introduced by the range finder 10b and/or 
the I-slot generators 10c and 20c, then those inaccuracies will be 
compensated for by the preferred embodiment of the correlator 20i which is 
shown in FIG. 4. 
This FIG. 4 correlator includes a sequential control circuit 40, a storage 
circuit 41, and an AND-INTEGRATE-COME circuit 42. All of these circuits 
are interconnected as shown. In operation, the storage circuit 41 stores 
whatever signal is received via the antenna 20a during an extended I 
timing period I.sub.E. Signal I.sub.E starts a few chip times before the 
normal I pulse and ends a few chip times after the normal I pulse. In FIG. 
4, signal I is shown by voltage waveform 51, and signal I.sub.E is shown 
by waveform 52 as extending beyond signal I by two chip times. 
By storing the received interrogator signals in the storage circuit 41 
during the extended timing period I.sub.E, the chip sequence C.sub.X, 
C.sub.X+11 ... will be captured even though it is received slightly out of 
sync with the I pulse. For example in FIG. 4, waveform 53 shows the case 
where the chip sequence C.sub.X, C.sub.X+1, ...is received one chip time 
early relative to the I pulse; and waveform 54 shows the case where the 
chip sequence C.sub.X, C.sub.X+1... is received 1/2 chip time late. 
After the extended timing period Iz, the sequential control circuit 40 
sequentially generates five control signals R+1, R+1/2, R, R-1/2, and R-1. 
In response to signal R, storage circuit 41 reads out the set of signals 
that it stored during the period I; in response to signal R+1/2, storage 
circuit 41 reads out the set of signals that it stored during the period I 
delayed by 1/2 of a chip time period; in response to the signal R-1/2, the 
storage circuit 41 reads out the set of signals that it stored during the 
period I advanced by 1/2 of a chip period; etc. 
Each set of signals which are read from the storage circuit 41 is sent to 
the AND-INTEGRATE-COME circuit 42, and there they are correlated with 
the chip sequence C.sub.x, C.sub.x+1, ... from the code generator 20e. If 
any one of those correlations produces a MATCH signal, then that signal is 
sent to the transmitter 20f which sends a response to the interrogator as 
previously described. 
One preferred embodiment of an electronic identification system that is 
constructed according to the invention has now been described in detail. 
In addition however, many changes and modifications can be made to these 
details without departing from the nature and spirit of the invention. For 
example, if less security can be tolerated, then the interrogator circuit 
10 can continuously transmit the chip sequence C.sub.x, C.sub.x+1 delayed 
by .DELTA.T.sub.2 (or .DELTA.T.sub.3) in one frequency band, and the 
transponder 20 can transmit the response chip sequence in a different 
frequency band. Accordingly, it is to be understood that the invention is 
not limited to the details of the illustrated and described preferred 
embodiment but is defined by the appended claims.