False return signal apparatus

An optical acoustic charge transport (ACT) device provides a variable and continuous delay for RF signals. Received RF signals are converted to light and focused onto an optical ACT device. The continous delay is utilized to generate a false radar return.

BACKGROUND OF THE INVENTION 
This invention relates to signal processing; and more particularly, to 
generating false radar return signals. When radar is employed in an 
electronic counter measure (ECM) system, it is desirable to transmit false 
radar return signals. By delaying these signals in time, the target can 
appear to an unfriendly radar system, to be at a location other than its 
actual location. This technique is known as Range Gate Pull-Off (RGPO), 
and utilizes the automatic range tracking teacher of an unfriendly radar 
system. Conventionally, in RGPO systems, the unfriendly radar signal is 
captured, delayed and retransmitted. The delay is increased, which 
affectively walks the unfriendly radar off the appropriate range. 
Typically, the false radar return signal is then turned off leaving the 
unfriendly radar range gate with no signal; thus, affectively breaking the 
unfriendly radar's range tracking. Typically, this causes the unfriendly 
radar to revert to a reacquisition mode and to begin a range search. 
There are currently several approaches to false radar return signal 
generation. For example, a first approach, snapshots of a received radar 
signals are frozen (i.e., briefly stored) and then retransmitted after a 
delay. With this approach, there is a limit on the signal length that can 
be captured and retransmitted. The limit depends upon the length of the 
device that captures the signal. For example, in an acoustic charge 
transport (ACT) device, the device channel length limits the signal length 
that can be captured. Currently, ACT devices can capture, at best, 
approximately 10.mu.s of a signal; and can hold the signal for 
approximately 20 .mu.sec. 
Another ACT device approach is an ACT tapped delay line device. In this 
device, a set of electrodes (taps) are positioned at various points along 
the ACT channel. Switching between the various taps provides a variable 
delay. However, there is a discrete change between the different delays. 
This discrete change could be detected and used to disregard the delayed 
or false radar return. 
Presently, a digital radio-frequency memory (DRFM) is commonly used to 
provide delayed signals in radar systems that generate false returns using 
the delayed signals. Such DRFM systems, however, require hundreds of watts 
of power. The systems also used A/D and D/A converters which, by their 
very nature, introduce distortion in the system that causes unwanted 
sidebands in a delayed/false return signal. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a low power false 
return signal apparatus having a simple and light weight structure. 
It is another object of the present invention to provide a false return 
signal apparatus that can generate a continuously variable delay in a 
false radar return signal. 
It is a further object of the present invention to provide a false return 
signal apparatus with no limit on the signal length that can be delayed 
and returned. 
It is still another object of the present invention to provide a false 
return signal apparatus that eliminates or minimizes unwanted sidebands in 
the returned signal. 
It is still another object of the present invention to provide a false 
return signal apparatus capable of providing multiple false return 
signals. 
To achieve the above and other objects, the present invention provides a 
false return signal apparatus including a receiver means for receiving 
radar signals and for generating light responsive to the received radar 
signals; a detector means for variably deflecting the light across a 
predetermined area; and an acoustic charge transport (ACT) device 
positioned in the predetermined area and having an output connected to 
provide a return signal responsive to the light received from the 
deflector means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
I General System 
FIG. 1 is a block diagram of a system embodying the present invention. In 
FIG. 1, reference numeral 10 identifies an optical acoustic charge 
transport (ACT) device. This device comprises, for example, an n-type 
channel layer 15 and a light sensitive channel plate 20. The light 
sensitive channel plate 20 can comprise, for example, a P.sup.+ doped 
GaAs layer. Reference numeral 25 in FIG. 1 identifies an output terminal 
of the optic ACT device. The operation and structure of an optical ACT 
structure is well known and is described in, for example, Miller et al., 
"Direct Optical Signal Injection In An ACT Channel," IEEE Ultrasonics 
Symposium Proceedings, IEEE, Cat. No. 87 - CH 2492-7, 1987. Briefly, the 
P.sup.+ /n junction comprising the channel plate 20 and n layer 15 is 
reverse biased. When a light beam 30 illuminates the channel plate 20, 
electron-hole pairs are generated in the layer 15. As is known, a surface 
acoustic wave travels in the layer 15. This surface acoustic wave groups 
the generated electrons and carries them towards the output terminal 25. 
The length L determines the amount of delay between the time that the 
light beam 30 illuminates the channel plate 20 and the grouped electrons 
reach the output terminal 25. More particularly, the time is expressed by 
T=L/v, where v is the speed of the acoustic wave travelling in the layer 
15. 
In the FIG. 1 system, an antenna 35 receives an r.f. signal which is 
translated down to an intermediate frequency by way of a local oscillator 
40. The intermediate frequency signal passes through a biasing circuit 45 
that biases a laser diode 50. The intermediate frequency signal amplitude 
modulates the laser diode 50 to generate an amplitude modulated optical 
beam 55. As shown in FIG. 1, the optical beam 55 passes through a 
deflector 60. The output of the deflector 60 passes through a lens 65 to 
form the light beam 30. 
The deflector 60 moves the light beam 30 across a predetermined area on the 
channel plate 20 in accordance with a position control signal. As the 
light beam 30 moves in a direction of the arrow A, the amount of delay is 
increased; and a Doppler shift is automatically created in the signal 
provided at the output terminal 25. In general, the amount of doppler 
shift is expressed by .DELTA.f=kv, where .DELTA.f is the amount of doppler 
shift, k is a propagation constant and v is a velocity of a target. In the 
FIG. 1 system, the doppler shift can be expressed as follows 
.DELTA.f=.omega.dT/dt=(.omega./v)dL/dt, where .omega. is the frequency of 
the i.f. signal in radians. 
As shown in FIG. 1, the optical beam 55 is deflected by a deflector 60. 
However, the deflector 60 could actually move the laser diode 50, rather 
than simply moving the optical beam 55. This could be implemented with a 
simple tilt mechanism. To minimize the number of moving parts, the 
deflector 60 can be an acoust optical device. Acoust optical deflectors 
can be purchased commercially, such as the acoust optic deflector model 
AOD-150 manufactured by IntraAction Corp., 3719 Warren Avenue, Bellwood, 
Ill. 
The delayed signal provided by output terminal 25 is amplified by an 
amplifier 70 and translated back to the frequency of the received r.f. 
signal by a local oscillator 75. The delayed signal is then retransmitted 
by antenna 80. The retransmitted signal is stronger than the real radar 
reflection, and therefore, is locked onto by the originating radar. As the 
light beam 30 is moved in the direction A, the retransmitted or false 
return signal causes the originating radar to indicate that the target's 
location and velocity are different than they actually are. More 
particularly, the range (delay) and the doppler shift (frequency) are 
modified so that the originating radar can no longer track the target. 
Referring to FIG. 1, it is possible to have two independently movable light 
beams such as 30 and 175. The ability to have two independently 
controllable light beams allows the delay provided by a ACT device to be 
extended. For example, referring to FIG. 1, when an initial signal is 
received and generates a light beam at a point 21, the position control 
signal gradually sweeps the light beam in the direction of arrow A. When 
the light beam reaches the position indicated by 22 in FIG. 1, a switch 
180 is moved from the position B to the position C. The second light beam 
is switched on at the point 21 so that two lights spots are generated at 
the positions 21 and 22. The unfriendly or victim radar range gate is 
still locked on the delayed signal corresponding to the spot at location 
22. The switch 185 is then switched from the position B to the position C. 
The spot at position 21 is then within the range gate of the unfriendly or 
victim radar. The spot at location 22 is ignored by the victum radar. The 
spot at location 22 can then be moved in the direction of A to further 
increase the range delay to a maximum twice the delay of the ACT device 
10. When the spot reaches the location 22, the process can be repeated and 
another (not shown) fixed delay can be switched in if desired. Thus, the 
return signal generated by the light beam 175 is delayed by a fixed delay 
provided by fixed delay 185. The amount of delay provided by the fixed 
delay 185 corresponds to the total amount of delay provided by the ACT 
device 10. Thus, to the originating radar, the signal 175 appears to be 
delayed by the same amount of time that the original delayed signal 30 was 
delayed by. By moving the light beam 175 in the direction of A, the amount 
of delay is thus doubled. 
FIG. 2 schematically illustrates the optics of the FIG. 1 system. The laser 
diode 50 emits a fan-shaped beam. A spherical lens 95 and cylindrical lens 
100 astigmatically focus the light beam into a ribbon of light which is 
applied to the deflector 60. In a preferred embodiment, the laser diode 50 
emits light having a wavelength of 820 nm and a power of approximately 30 
mW. The spherical mirror 95 has a focal length of 25 mm and a 12 mm 
diameter, while the cylindrical lens 100 has a 25 mm focal length. 
A quarter-wave plate 105 provides circularly-polarized light to the 
deflector 60. The deflector 60 includes a TeO.sub.2 Bragg cell; and 
therefore requires circularly-polarized light. The remainder of the 
optical path shown in FIG. 2 is insensitive to polarization. In one 
embodiment of the present invention, the deflector 60 has a center 
frequency of approximately 60 MHz with a 40 MHz bandwidth. Because optical 
acoustic charge transport devices require only a small amount of optical 
input power, the deflector 60 can be driven with modest RF power, such as 
100 mW of RF drive power. 
In order to achieve a small spot size, the light exiting the deflector 60 
passes through a cylindrical lens 110 and a spherical lens 115. In an 
embodiment of the present invention, the cylindrical lens 110 has a 25 mm 
focal length, and the spherical lens 115 has a 180 mm focal length. 
Mirrors 120 and 125 redirect the beam in order to minimize the space 
occupied by the optical system. The resulting spot illuminates a point on 
the light sensitive channel plate of an ACT. In an embodiment of the 
present invention, the spot can be deflected over 9 mm and has a size of 
approximately 20 .mu.m. The deflector 60 can provide, for example, 450 
resolvable spots. To achieve a 3 .mu.sec delay, a 9 mm light sensitive 
channel plate is needed. As a result, the spot should be no larger than 20 
.mu.m. Larger delays can be accommodated by using a longer focal-length 
lens in the position of lens 115, although the spot size will increase 
proportionally. For example, to obtain a 10 .mu.sec delay, a 600 mm focal 
length lens could be used as the lens 115, which would produce a 60.mu.m 
spot diameter. 
FIG. 3 schematically illustrates an example of a circuit for providing the 
position control signal in the FIG. 1 system. In FIG. 3, digital position 
control inputs 130 are applied to a D/A converter 135. The output of the 
D/A converter 135 is applied to a DC amplifier 140. A voltage controlled 
oscillator (VCO) provides the needed frequency range to drive the 
deflector 60. The output of a crystal oscillator 150 is mixed in a mixer 
155 with the output of the VCO 145. A low pass filter 160 selects the 
difference frequency provided by the mixer 155. The output of the low pass 
160 is amplified by a RF amplifier 165 to provide the control signal for 
the deflector 60. 
In general, the change in deflection angle of a light beam (in radians) to 
a change .DELTA.f of the RF position control signal is 
EQU .DELTA..theta.=[.lambda./nv ].DELTA.f (1) 
where: 
.lambda.=optical wavelength in air 
n=index of refraction of Bragg cell material 
f=Bragg cell RF drive frequency 
v=sound velocity in Bragg cell 
.theta.=deflection 
The deflection angle determines the deflection position of the optical spot 
on the light sensitive channel plate 20, and the position of the optical 
spot is given by 
EQU .DELTA.X=.apprxeq.F.DELTA..theta. (2) 
where: F=focal length of lens 115. 
From the above, it is seen that the output of the VCO 145 can smoothly vary 
the RF position control signal as applied to the deflector 60. As a 
result, the range delay provided by the system is smoothly varied. To 
achieve the desired smoothness in the range delay variations, a high 
precision D/A converter 135 can be employed, for example, 12 bits. The 
system of the present invention can use a high precision D/A converter 
because the speed of the response of the system does not need to be very 
great. For example, the range sweep can occur over a period of 5-10 
seconds. Thus, the speed of a D/A converter is not critical to the present 
invention. 
II. Realistic False Target Generation 
FIG. 4 is a second embodiment of the present invention. The object of the 
second embodiment is to allow the present invention to realize a desired 
impulse response h(t) which simulates the impulse response of a real radar 
return such as shown in FIG. 5B. The radar return shown in FIG. 5B 
represents an approximation of a return from a target shown in FIG. 5A 
illuminated with a radar beam 82. FIG. 4, illustrates one approach to 
realizing this object. In FIG. 4 an amplitude shaping aperture 85 is 
positioned between the deflector 60 and the lens 65. To do this, the 
amplitude shaping aperture 85 smears the light beam 30 to approximate a 
reflection from an actual target as well as ground bounce and other radar 
propagation phenomenon. 
The amplitude shaping aperture 85 generates an intensity distribution I(x) 
of light on the light sensitive channel plate 20. This distribution I(x), 
is directly related to the desired impulse response The position x on the 
light sensitive channel plate 20 is related to the time t that the 
injected pulse takes to reach the output, and the acoustic velocity in the 
device by the following 
EQU t=x/v.sub.sound 
Using this, the desired impulse response is expressed as 
EQU h(t)=I(x/v.sub.sound) 
Since the light sensitive channel plate 20 is in the focal plane of the 
lens 65, in order to achieve the distribution I(x), the amplitude shaping 
aperture 85 has an optical transmission that varies across its aperture 
and described by the function A(x'), where x' is the coordinate in the 
plane of the amplitude shaping aperture, and the function A is a scaled 
version of the function I(x) in accordance with the geometry 65. 
The approximation of a radar reflection from an actual target can also be 
achieved by replacing the amplitude shaping aperture 85 with a circuit to 
appropriately vary the microwave position control signal shown in FIG. 4. 
In accordance with the present invention, the position of a laser beam, 
such as the light beam 30, on an optical acoustic charge transport device 
10 is controlled by the acousto-optic deflector 60. The spatial behavior 
of the light beam 30 (e.g., spot size and position) are a function of the 
microwave position control signal applied to the deflector 60. For a 
narrow band signal, the spot will generally have a Gaussian spatial 
intensity pattern. The diameter of the spot will be a function of the 
aperture of the deflector 60 and the focusing of the lens 65. By inputting 
a broadband microwave position control signal to the deflector 60, the 
spot size may be enlarged and its spatial intensity profile varied. It is 
important, however to ensure that the generated false return signal 
provided by antenna 80 is a constant energy signal. 
As shown in FIG. 5A, a real target has radar-reflecting surfaces at 
different ranges from an illuminating radar. A typical 30MHz radar has 
approximately a three foot resolution. Thus, a return such as shown in 
FIG. 5B would be generated by conventional aircraft. One way of generating 
different returns at different ranges, such as shown in FIG. 5B, is 
inputting a microwave position control signal to the deflector 60 that 
includes multiple frequencies. This would generate multiple spots at 
different positions on the light sensitive channel plate 20 shown in FIG. 
4. If two such spots do not overlap, then there is no interaction between 
the spots on the light sensitive channel plate 20; and thus no interaction 
in the two false radar returns. If, however, two of the spots overlap on 
the light sensitive channel plate 20, then a beat signal will be generated 
that may interfere with generating a realistic false radar return signal. 
This is because no frequency shift and therefore no beat signal is present 
in a real radar return signal. In addition, it is important to ensure that 
a constant energy position control signal be generated 
Referring to FIG. 4, if the laser diode 50 is modulated with a signal 
corresponding to (A=A.sub.1 COS.omega..sub.IF t), the optical intensity of 
the light provided by the laser diode 50 will be (P=P.sub.1 
COS.omega..sub.IF t), where w.sub.IF represents the intermediate frequency 
of the signal modulating the laser diode 50. The optical amplitude 
therefore becomes (P.sub.1 COS.omega..sub.IF t).sup.1/2 COS(.omega.t), 
where .omega. is the optical frequency. Due to the frequency shifting 
properties of the deflector 60, the optical amplitude of the light beam 30 
will satisfy the following 
EQU A.sub.0 =(P.sub.1 COS .omega..sub.IF t).sup.1/2 COS(.omega.+.omega..sub.1)t 
where .omega..sub.1 is the frequency required by the deflector 60 to 
deflect light to the desired location on the light sensitive channel plate 
20. If the desired spots to not overlap, carriers will be generated in the 
light sensitive channel plate 20 at two separate locations. The number of 
carriers is proportion to the optical intensity in accordance with the 
following 
EQU P.sub.0 =A.sub.0.sup.2 =P.sub.1 COS.omega..sub.IF t[COS.sup.2 
(.omega.+.omega..sub.1)t]. 
The COS.sup.2 term contains a DC component and a component at twice the 
frequency that carriers can be generated in the light sensitive channel 
plate 20. Thus, practically speaking, the carriers generated in the light 
sensitive channel plate 20 are proportional to P.sub.1 COS .omega..sub.IF 
t. 
If, however, two spots generated on the light sensitive channel plate 20 
overlap the optical amplitude at the overlap satisfies the following 
EQU A.sub.0 =(P.sub.1 COS.omega..sub.IF t).sup.1/2 
[COS(.omega.+.omega..sub.1)t+COS(w+.omega..sub.2) t] 
and the optical intensity corresponds to 
EQU P.sub.0 =A.sub.0.sup.2 =(P.sub.1 COS.omega..sub.IF 
t)[COS(.omega.+.omega..sub.1)t+COS(.omega.+.sub.2)t].sup.2. 
Because the cross-product term [COS (.omega.+.omega..sub.1)t+COS 
(.omega.+.omega..sub.2)t has sum and difference frequency terms, the 
output of the optical acoustic charge transport device 10 will, in effect, 
be proportional to (P.sub.1 COS .omega..sub.IF t)(D+COS (.omega..sub.1 
-.omega..sub.2)t), where D corresponds to the sum of DC terms of the 
equation for P.sub.0. Thus, without appropriate care, the generated signal 
will be undesirably amplitude modulated at a beat frequency corresponding 
to .omega..sub.1 -.omega..sub.2. Thus, overlapping spots created by 
multiple frequencies in the microwave position control signal applied to 
the deflector 60 should be avoided. 
One way of generating the appropriate radio frequency (RF) position control 
signal to avoid this problem is described below. If, for example, a false 
radar return signal such as that shown in FIG. 5B is desired, then this 
return is used as a basis for designing an appropriate RF position control 
signal to be applied to the deflector 60. The range illustrated in FIG. 5B 
is directly related to the time delay which must be experienced by 
carriers generated by the light beam 30 in the optical ACT device. Thus, 
the range shown in FIG. 5B is directly related to a frequency component of 
the RF position control signal applied to deflector 60. If this signal 
contains a multiplicity of frequency components, then a multiplicity of 
optical beams perhaps overlapping, f will be directed to the light 
sensitive channel plate 20. The optical intensity of each beam is directly 
proportional to the electrical intensity or power of the corresponding 
frequency component of the RF position control signal. 
The optical intensity distribution thus created on the channel plate 20 is 
directly proportional to the impulse response of the false radar return. 
The shape of this impulse response h(t) is, in turn, the same as the shape 
of the power spectrum X.sub.p (.omega.) of the RF position control signal 
x(t), because it does not specify phase. Depending on the relative phases 
of the various frequency components, x(t) may have an amplitude which is 
relatively constant in time for one set of phases, or highly pulse-like 
for different set. An x(t) whose amplitude is relatively constant is 
needed to create realistic false radar return. 
One straight forward way to design an RF position control signal x(t) is to 
form an amplitude spectrum X.sub.a (.omega.) whose magnitude is the square 
root of the desired power spectrum X.sub.p (.omega.) and whose phase 
.phi.(.omega.) is a random, noise-like function of frequency: 
EQU X.sub.a (.omega.)-.vertline.X.sub.p (.omega.).sup.1/2 
e-.sup.j.phi.(.omega.) 
The inverse Fourier transform of this spectrum will be a control signal 
x(t) with the desired properties: a relatively constant amplitude signal 
having the required power spectrum. 
The results of this computation could be stored in a Read Only Memory (ROM) 
in the form of a sequence of samples {x.sub.n }. The number of frequency 
samples depends on the target size and the radar resolution. A typical 30 
MHz radar has approximately a three foot resolution. Thus, 64 samples 
would provide 192 feet of target size, the number of samples can be varied 
in accordance with the desired size of the false target. During operation, 
these numerical samples could be sequentially D/A converted to provide an 
analog RF control signal suitable for input to defector 60. 
By mixing this signal x(t) with the output of a VCO, as exemplified by 
replacing the fixed oscillator 150 FIG. 3 by the signal x(t), a realistic 
false radar return which is movable can be realized. FIG. 6 shows a 
preferred embodiment of this scheme. Samples of x(t) are converted to an 
analog signal by D/A converter 190, producing analog signal x(t) having 
the correct power spectrum to make a realistic radar return impulse 
response h(t). Digital position control signal 130 is converted to an 
analog signal by D/A converter 135. After suitable scaling by DC amplifier 
140, this analog signal forms the control signal of VCO 145. Fixed 
oscillator 150, mixer 155, and low pass filter 160 are used to offset the 
variable frequency RF to a lower frequency range, to provide a capability 
of sweeping over a large fractional bandwidth. The output of LPF 160 is 
mixed with analog signal x(t) in mixer 170, filtered by 180, and amplified 
by 165 to provide 165 to provide the RF input signal for deflector 60. 
III. Multiple False Targets 
FIG. 7 is a third embodiment of the present invention. FIG. 7 adds a 
diffraction grating 90 between the deflector 60 and the lens 65. This has 
the effect of optically generating a number of false return signals by 
generating multiple light beams exemplified by beams 31 and 32. This is 
similar to electrically generating multiple beams as noted above. Each of 
the beams 31 and 32 inject a signal in the ACT device 10. The various 
signals experience different delays, and automatically are imparted with a 
doppler shift as discussed above. If the diffraction grating 90 has a 
uniform pitch, then the multiple light beams 31 and 32 move in synchronism 
across the channel plate 20. If, however, the diffraction grating 90 has a 
nonuniform pitch, then the multiple light beams 31 and 32 can be made to 
move in a fashion. In this case, the diffraction grating 90 is moved 
transversely to this axis of the light received by the diffraction grating 
90. This produces false return signals which simulate relative motion 
among targets. Each of the false return signals automatically includes a 
doppler shift. Since the multiple light beams 31 and 32 do not move in 
synchronism, they can have different doppler shifts. This simulates 
movement of multiple realistic targets. 
It is also possible to combine the amplitude shaping aperture 85 and 
diffraction grating 90 to simulate multiple targets which have smeared or 
realistic returns. 
The foregoing is considered as illustrative of the principles of the 
invention. Further, since numerous modifications and changes will readily 
occur to those skilled in the art, it is not desired to limit the 
invention to the exact construction and application shown and described, 
and accordingly, all suitable modifications and equivalents may be 
resorted to, falling within the scope of the invention and the appended 
claims and their equivalents.