Radar scene simulator

The device includes an array of antenna systems which send to a target seeking radar scene simulation signals synchronized with target seeking signals from the target seeking radar. Each of the antenna systems receives control signals which it uses to adjust the scene simulation signals. For FM/CW target seekers, the antenna systems mix the control signals with the target seeking signals. For pulse target seekers, the antenna systems mix the control signals with an illumination signal provided from an illumination radar.

BACKGROUND OF THE INVENTION 
This invention relates to the field of scene simulators for target seekers 
and has special applicability to millimeter wave radar target seekers. 
One purpose of radar scene simulators is to help test target seeking 
radars, especially those used in missiles. Due to the complexity and high 
cost of missiles, it has become too expensive to test-fire missiles to 
determine whether their target seekers are operating properly. Besides the 
great cost of such testing, test-firing makes it difficult to analyze a 
missle target seeker which failed. 
Radar scene simulators were designed to test whether radars respond as 
designed to specific scenes. Early scene simulators used only a corner 
reflector situated in the radar field of view. The corner reflector 
reflected transmitted signals back to the target seeker and was helpful in 
determining only whether the target seeking radar was receiving 
reflections of the signals it transmitted. 
Another method of testing target seeking radars uses a radar transponder in 
the target seeker's field-of-view which transmits to the missile signals 
designed to simulate the signals which would be reflected from a known 
target. Because such transmitting radars are not synchronized with the 
target seeker, a small difference in frequency between the transmitting 
radar and the target seeker signals could cause a correctly functioning 
radar to respond incorrectly. 
FIG. 1 shows a more elaborate system for radar scene simulation. A target 
seeker (not shown) transmits its radar signal at frequency f.sub.s to an 
array 100 of RF transceiver antennae. Antenna array 100 includes a 
plurality of antenna triads. The use of an array of antenna triads is 
explained in U.S. Pat. No. 4,467,327, filed on Sept. 22, 1981 and issued 
on Aug. 21, 1984, which is incorporated herein by reference. 
Each transceiver antenna receives the target seeker radar signal. The 
signals from an antenna triad are placed on whichever of lines 110.sub.1 
-110.sub.z that correspond to that triad and sent through switch matrix 
110 into signal generator 120. Switch matrix 110 switches the signals from 
a selected antenna triad onto lines 121-123 and to signal generator 120. 
Signal generator 120 forms signals to create the desired radar scenes for 
the target seekers by mixing the received signals, R.sub.s, with target 
signals, T.sub.s. Signal generator 120 gives this mixed signal, denoted as 
R.sub.s +T.sub.s, the proper amplitude and sends it to a selected antenna 
triad via switch matrix 110 and the lines 110.sub.1 -110.sub.z which 
correspond to that triad. 
Data processor 130 controls signal generator 120 to create the target 
signals Ts and controls switch matrix 110 to ensure the correct routing of 
signals. 
The system in FIG. 1 is adequate for CW target seekers operating in the 
radio frequency range because transmission lines 110.sub.1 -110.sub.z can 
be coaxial cables. For target seekers operating at millimeter wave 
frequencies, however, transmission lines 110.sub.1 -1l0.sub.z must be 
mechanical waveguides. Such waveguides are bulky, difficult to maintain, 
and very expensive. 
An object of this invention to create low cost scene simulator systems for 
millimeter wave target seekers. 
A further object of this invention is to create scene simulators which have 
a great deal of flexibility and will allow complete testing of both CW and 
pulse target seeker radars. 
Additional objects and advantages of this invention will be set forth in 
part in the description which follows and in part will be obvious from 
that description or may be learned by practice of the invention. The 
objects and advantages of this invention may be realized and obtained by 
the methods and apparatus particularly pointed out in the appended claims. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problems of the prior art and achieves 
the objects listed above by antenna systems which scene simulation signals 
that are synchronized and phase coherent with target seeking signals 
transmitted by that target seeker. To test an FM/CW target seeker, the 
antenna systems mix control signals with the target seeking signals at the 
selected array antennae, and to test a pulse target seeker, the antenna 
systems mix at the selected array antennae range delayed intermediate 
frequency control signals with external illumination signals which are 
offset in frequency by that intermediate frequency. 
To achieve the objects in accordance with the purpose of this invention, as 
embodied and as broadly described herein, the device of this invention for 
simulating radar scenes to be detected by a target seeker radar emitting 
target seeking signals, comprises an array of antenna systems sending to 
the target seeking radar scene simulation signals synchronized and 
coherent with the target seeking signals, each of the antenna systems 
including means for adjusting the scene simulation signals sent by that 
system, and control means connected to each of the antenna systems in the 
array for sending different control signals to each system thereby to 
control the scene simulation signals. 
The method of simulating scenes of this invention for a target seeker radar 
emitting target seeking signals comprises the steps of: generating control 
signals; generating, from the control signals, scene simulation signals in 
synchronism with the target seeking signals; and transmitting the scene 
simulation signals to the target seeker. 
The accompanying drawings, which are incorporated in and which constitute a 
part of the specification, illustrate embodiments of this invention, and, 
together with the description, explain the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to presently preferred embodiments of 
the invention, examples of which are illustrated in the accompanying 
drawings. Although the inventive concept has applicability to many 
different types of target seekers, the specific examples shown in the 
accompanying drawings relate to FM/CW target seeker radars and coherent 
and noncoherent pulse target seeker radars. 
FIG. 2 shows an FM/CW target seeker 200 transmitting millimeter FM/CW 
target seeking signals to scene simulator 205. Scene simulator 205 
includes antenna system array 210 and control signal generator 220. 
Antenna system array 210 comprises several identical antenna systems which 
are arranged in a matrix. The size of antenna array 210 and the 
arrangement of antenna systems in that array depend on the target seekers 
to be tested. 
In FIG. 2, for example, antenna system array 210 is divided into sets of 
antenna system quads. One antenna system quad is shown comprising antennas 
210.sub.a -210.sub.d. The purpose of the antenna quads is similar to the 
purpose of the antenna triads discussed above. By sequentially energizing 
the antennae in an antenna system quad with signals sequencing more 
quickly than the scene rate of the target seeker and which bear 
predetermined relative weighting with each other, the antenna system quads 
can simulate the reflections from an object in an area defined by the 
boundary of the antenna system quad. 
An example of an antenna system which can be used in array 210 is shown in 
FIG. 3. Antenna system 300 comprises a millimeter wave antenna 310 capable 
of receiving millimeter wave target seeking signals and transmitting 
millimeter wave scene simulation signals. 
The antenna system shown in FIG. 3, as used in this invention, forms the 
scene simulation signals at the antenna system itself and eliminates the 
need of routing the target-seeking signal over great distances through 
cumbersome waveguides. 
In accordance with the present invention, antenna system 300 includes means 
for adjusting the scene simulation signals sent by that system. For an 
FM/CW target seeker, antenna system 300 includes means for mixing the 
target seeking signals with a different one of a plurality of control 
signals to form the scene simulation signals. 
There are two basic mixer types which can be used with the antenna system 
of this invention. An example of the simpler type is shown in FIG. 3 in 
which double sideband mixer 320 includes a single diode 330 matched to and 
spanning a waveguide horn antenna 325. Mixer 320 adjusts and forms the 
scene simulation signals to be transmitted by antenna system 300 to the 
target seeker. 
Diode 330 and waveguide 325 produce both a signal whose frequency is the 
sum of the frequencies of the control signal and the target seeking signal 
and another signal whose frequency is the difference between those 
signals' frequency (the reflected signal at the target seeking signal 
frequency is ignored by the target seeker). These signals appear to FM/CW 
target seeker 200 as two targets separated by a range equal to twice the 
distance between the target seeker 200 and antenna array 210. 
If the additional signal from the double sideband mixer would cause 
interference with the primary target output, then the second mixer type, a 
single sideband mixer, should be used instead. 
Another mixer type is the single sideband mixer which can be fabricated 
from two mixers interconnected by phase shift networks. The combination at 
the antenna system of the phase-shifted and balanced outputs from the two 
mixers enhances the primary target output while cancelling the second 
target output. To accomplish this, each mixer must provide equal amplitude 
conversion, and the correct phase relationships must exist. 
One simple example of a single sideband mixer that could be used with this 
invention is shown in FIG. 3a. Antenna system 350 comprises antenna 360 
coupled to waveguide sections 370, 371 and 372 which are connected to form 
a "T." Diodes 381 and 382 span wave guide sections 371 and 372, 
respectively, and are spaced from the "T" junction by distances which 
differ by 1/8 wavelength. Diodes 381 and 382 are fed with control signals 
fi and fi', respectively, which are 90.degree. out of phase with respect 
to each other. This 90.degree. phase shift can be accomplished by a phase 
shift network. The transmitted signals are mixed with the control signals 
in waveguide sections 371 and 372 and the resulting signals are combined 
in waveguide 370. The unwanted sidebands cancel and the desired sidebands 
add. 
In accordance with the present invention, the scene simulator also includes 
control means connected to each of the antenna systems in the array for 
sending different control signals to each system. 
In the embodiment shown in FIG. 2, control generator 220 sends control 
signals f.sub.i to the antennae in antenna system 210 to control the scene 
simulation signals in the manner described above. 
Control signal generator 220 includes computer system and control interface 
221. Once operators have entered parameters of the desired scene into 
computer system and control interface 221, it translates those parameters 
into activation signals which generate the control signals necessary to 
provide the desired scene. 
In the embodiment shown in FIG. 2, the computer system and control 
interface 221 sends an activation signal to voltage controlled oscillator 
(VCO) 223 to set the proper frequency for the control signals. The 
information about range and length of the simulated scene is contained in 
the frequency content of the control signals. 
Interface 221 sends another activation signal to amplitude control 224, 
which can be a conventional attenuator, to set the amplitude of the 
control signals generated by VCO 223. Amplitude control 224's purpose is 
to simulate range-related signal propagation losses, target area size and 
fluctuations in apparent size. 
Switch 226, which is also under control of computer system and control 
interface 221, routes the control signals to the proper antenna systems or 
antenna quads in FIG. 2. Proper setting of switch 226 allows control of 
both the position and motion of the scene to be simulated. 
The scene simulator in FIG. 2 works since the reflection signals received 
by an FM/CW target seeker are essentially the sum of the target seeking 
signal striking the antenna system plus an offset signal containing target 
information, e.g., size, shape, range, etc. Control signals f.sub.i in 
FIG. 2 are formed to match those offset signals of desired targets. When 
the mixers in the antenna systems sum the control signals with the target 
seeking signals, they then create the proper scene simulation signals. 
This invention allows great flexibility because, through the control means, 
the scenes can be easily changed as can their position and movement. This 
invention also provides an efficient and low cost method for testing 
millimeter wave target seekers since the millimeter wave target seeking 
signals do not have to be transmitted back to the controller and there is 
no need for the expensive waveguides that would otherwise be required in 
systems like that in FIG. 1. 
FIG. 4 shows a scene simulator for a millimeter wave pulse target seeker. 
Target seeker 400 is a pulse target seeker which transmits a pulse target 
seeking signal at, for example, 90 GHz. 
Illumination radar 420 transmits a millimeter wave illumination signal. The 
illumination signal covers the entire antenna array 410. For this example, 
the frequency of the illumination signal is 80 GHz. The reflections of the 
illumination signals from the array structure and surrounding areas will 
not interfere with the target seeker as long as the frequency of the 
illumination signals differs sufficiently from that of the target seeking 
signal. 
The scene simulator of this invention when used for a pulse target seeker 
comprises control means coupled to the illumination radar and to the pulse 
target seeker radar for generating a plurality of control signals and for 
sending a different control signal to each antenna system. In the 
embodiment shown in FIG. 4, mixer 430, computer system and control 
interface 440, and switch 450 generate and send control signals to the 
antenna systems in antenna array 410. 
Mixer 430 is coupled to target seeker 400 and illumination radar 420 and 
mixes the illumination signal and the target seeking signal to form an 
intermediate signal having a frequency equal to the difference of the 
illumination signal and the target seeking signal frequencies. The sum 
frequency formed by a double sideband mixer is so different from the 
difference frequency that the sum frequency sideband signal can be easily 
filtered out. For an illumination signal at 80 GHz and a target seeking 
signal of about 90 GHz, the intermediate signal's frequency would be about 
10 GHz. The intermediate signal is sent to computer system and control 
interface 440. 
Computer system and control interface 440 creates control signals using the 
intermediate signal. The control signals have approximately the same 
frequency as the intermediate signals. The control signals are routed to 
the antenna systems in antenna array 410 via switch 450. Switch 450 in 
FIG. 4 operates analagously to switch 226 in FIG. 2. 
The antenna systems in array 410 can be similar to those shown in FIG. 3. 
In accordance with this invention each antenna system includes means for 
mixing the illumination signal with a different control signal to form the 
scene simulation signals transmitted to the target seeker. When the 
antenna systems in FIG. 3 are used in the embodiment in FIG. 4, they mix 
the control signal coupled to diode 330 and the illumination signal. For 
pulse radar seekers, the "unwanted" target from the additional signal 
created by the diode mixer can be positioned out of the target seeker 
frequency band. Alternatively, a single sideband mixer can be used. 
The control signal having a frequency of 10 GHz is added to a illumination 
signal of approximately 80 GHz to form a scene simulation signal at 
approximately 90 GHz, which is the frequency of the target seeking signal 
and the frequency of the reflection signals which the target seeker 
expects to receive. The scene simulation signals are formed from the 
control signals which in turn are formed from the intermediate signals. 
Since the intermediate signals are formed from the target seeking signals, 
the scene simulation signals are thus coherent with the target seeking 
signals. 
FIG. 5 shows computer system and control interface 440 in greater detail to 
illustrate how the intermediate signal is used to form the control signals 
in this embodiment. 
Gate 510 is a variable delay line which delays the intermediate signal a 
predetermined amount to reflect the intended range of the scene to be 
simulated. 
Target extender line 520 is connected to the output of variable delay line 
520 in FIG. 5 and widens the pulses of the intermediate signal to adjust 
the lengths of the targets. This line can include of several tap delay 
lines whose outputs are combined. 
To simulate target movement, mixer 540 mixes the output of target extender 
520 with a doppler signal generated by doppler frequency generator 530. 
Preferably, mixer 540 is a single sideband mixer which can be adjusted to 
add (or subtract) the frequency generated by doppler frequency generator 
530 to the intermediate signal. 
The output of mixer 540 feeds amplitude control 550 which sets the proper 
amplitude of the control signals. In FIG. 5, amplitude control 550 is a 
standard attenuator. 
Computer 500 contains the parameters of the scene desired to be simulated 
and translates those parameters into activation signals. Elements 510-550 
are all under the control of activation signals from computer 500. 
The scene simulator in FIG. 4 can also be configured to add a second scene 
or to add background clutter. For such a configuration, computer system 
and control interface 440 comprises computer 500 and another set of 
elements similar to those elements 510-550. One set of elements creates 
the primary control signals and an identical set of elements generates 
background control signals. Both the background control signals and the 
primary control signals are switched to antenna array 410 through switch 
450 and combined with the illumination signal at the antenna systems in 
array 410 to form the scene simulation signals. 
There are two principal ways of combining the primary and background 
control signals. One is to time division multiplex the control signals to 
the antenna system at a rate much greater than the target seeker senses. 
The target seeker then combines the different signals into one "scene." An 
alternative way of combining the signals is to add them in a transmission 
line combiner. 
FIG. 6 shows a slightly different embodiment of this invention of a scene 
simulator for a pulse millimeter wave target seeker 600. 
Signal generator 620 generates a signal f.sub.il which feeds mixer 630 
along with the target-seeking signal from target seeker 600. The output of 
mixer 630 is an intermediate signal which feeds delay generator 640. Delay 
generator 640 delays the intermediate signal by an amount appropriate for 
imparting the desired range of the scene. 
The output of delay generator 640 feeds target extender gate 650 whose 
output is connected to mixer 660 (single or double sideband) along with 
the illumination signal f.sub.il. Target extender gate 650 controls the 
pulse width of the intermediate signal. 
Mixer 660 mixes f.sub.il and the output of range extent 650 to form a 
signal which illumination radar 610 broadcasts to antenna array 690 as the 
illumination signal. 
For example, if the target seeker seeking signal is 94 GHz and f.sub.il is 
90 GHz, then the intermediate signal is 4 GHz. That 4 GHz intermediate 
signal, after passing through delay generator 640 and range gate 650, is 
then subtracted from the 90 GHz f.sub.il by single sideband mixer 660 to 
form an 86 GHz signal which then feeds illumination radar 610. 
The intermediate signal is also sent through frequency doubler 670 to 
compensate for the fact that, after passing through mixers 630 and 660, 
the illumination signal now differs from the target seeking signal by an 
amount equal to twice the intermediate signal. Frequency doubler 670 can 
be, for example, a nonlinear element that produces harmonics of the 
intermediate signal and a filter tuned to the intermediate signal's second 
harmonic. 
The output of frequency doubler 670 is frequency locked by element 672 
which can be a standard phase or frequency lock circuit. 
Doppler shift and amplitude control is added by elements 674, 676 and 678 
in the manner described above. The control signals appear at the output of 
amplitude control 678 and are switched to an antenna array 690 via switch 
680 in the manner previously described. Elements 670-680 operate under the 
control of computer 675 by activation signals. 
The operation of the radar scene simulator in FIG. 6 differs from the radar 
scene simulator in FIG. 4 in part because delay and target length 
information is provided by th illumination signal. Antenna array 690 
provides an active scene simulation signal at the proper frequency only 
when the illumination signal is received by the antenna systems in array 
690. 
FIG. 7 shows a scene simulator that can test both FM/CW target seeking 
radars and pulse target seeking radars. The functions of target seeking 
radar 710, the illumination radar 720, mixer 725, array 780 are as 
described above with regard to FIGS. 2-4. 
Computer system and control interface 730 controls elements 740-775 to 
simulate the desired scene in the manner explained in the discussion 
accompanying FIGS. 2, 4 and 5 above. Briefly, range control 740 imparts 
delay and target length information. VCO 750 operates in a manner similar 
to VCO 530 to control the target doppler. 
Amplitude control 760, which is an attenuator in FIG. 7, adjusts the 
amplitude of the control signal from VCO 750 and feeds that control signal 
to multi-throw switch 770 for transfer to antenna array 780. Multi-throw 
switch 770 is controlled by computer system control interface 730 via 
position control driver 775 which translates position commands into 
control signals for different switch positions. 
It will be apparent to those skilled in the art that modifications and 
variations can be made in the radar signal processing methods and 
apparatus of this invention. For example, use of the invention need not be 
limited to millimeter wave target seekers. The invention in its broader 
aspects is not limited to the specific details, representative methods and 
apparatus, and illustrated examples shown and described. Departure may be 
made from such details without departing from the spirit or scope of the 
general inventive concept.