Abstract:
A process for evaluating the radar pattern of a phased array antenna subsystem by utilizing transmissions from a global positioning satellite. This process utilizes a delayed radar target on a moving platform, a remote differential global positioning receiver on the moving platform, a fixed global positioning receiver on the phased array antenna, and a communication data link between the remote global positioning receiver and the fixed global positioning receiver. In the first step of the process a first signal is radiated from the radar source towards the delayed radar target, then a first radar reply is received from the delayed radar target, then a second signal is received from the global positioning satellite at the remote global positioning receiver from the global positioning satellite, and then a third signal is received at the fixed global positioning receiver from the remote global positioning receiver, via the GPS data link, and then a fourth signal is transmitted from the said global positioning satellite to the fixed global positioning receiver.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     Priority for this patent application is based upon applicant&#39;s provisional patent application No. 60/262,259, filed on Jan. 16, 2001. 
     This application is a continuation-in-part of applicant&#39;s copending patent application U.S. Ser. No. 09/982,331, filed on Oct. 18, 2001 now allowed. 
    
    
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,164,734 of Duane G. Fredericks et al. disclosed and claimed an apparatus for producing an electromagnetic signal which contained: (a) means for receiving a radar signal which has frequency of from about 10 to about 94 gigacycles, (b) means for decreasing the frequency of said radar signal by at least about 65 percent, thereby producing a reduced frequency electromagnetic signal with a center frequency of from about 700 to about 1,300 megacycles; (c) delay means for delaying said reduced frequency electromagnetic signal by at least about three radar pulse widths and from about 5 to about 100 microseconds, thereby producing a delayed electromagnetic signal wherein the delay means is comprised of a first delay circuit and a second delay circuit, each of which is substantially non-attenuating and non-dispersive, (d) means for increasing the frequency of said delayed electromagnetic signal to produce a delayed electromagnetic wave with an increased frequency which is substantially identical to the frequency of said radar signal, thereby producing a delayed radar signal of from about 1 to about 94 gigagcycles; (e) means for modulating said delayed radar signal with a frequency of from about 1 to about 94 gigacycles, thereby producing a modulated, delayed radar signal; and (f) means for transmitting said modulated, delayed radar signal. The entire disclosure of this United States patent is hereby incorporated by reference into this specification. 
     However, the device claimed in U.S. Pat. No. 5,164,732 was operable only over a relatively narrow bandwidth and, because it utilized down conversion, produced spurious emissions. 
     It is an object of this invention to produce a radar target with delayed reply means that overcomes these limitations. 
     It is another object of this invention to provide a process evaluating the radiation pattern of a phased array antenna using the radar target of this invention. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, there is a process for evaluating the radar pattern of a phased array antenna utilizing transmissions from a global positioning satellite, comprising the steps: (a) providing a delayed radar target on a moving platform, a remote differential global positioning receiver on said moving platform, a fixed global positioning receiver on said phased array antenna, and a data link between said remote differential global positioning receiver and said fixed global positioning receiver, (b) radiating a first signal from said radar towards a said delayed radar target, (c) receiving a first radar reply from said delayed radar target, (d) receiving a second signal from said global positioning satellite at said remote global positioning receiver from said global positioning satellite, (e) receiving a third signal at said fixed global positioning receiver from said remote global positioning receiver, and (f) transmitting a fourth signal from said global positioning satellite to said fixed global positioning receiver, and (g) processing said first signal, said second signal, said third signal, and said fourth signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which: 
     FIG. 1 is a block diagram of one preferred optical delay device; 
     FIG. 2 is a block diagram of one preferred device of this invention and 
     FIG. 3 is a schematic diagram of one preferred process of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In this specification, one aspect of the invention will be described by reference to FIGS. 1 and 2, and a second aspect of the invention will be described by reference to FIG.  3 . 
     FIG. 2 is a diagram of one preferred device  10  of applicant&#39;s invention. In the first step of the process, a radio frequency signal with a frequency of from about 0.5 to about 94 gigahertz is received by receiver  12  equipped with microwave antenna  14 . One may use any of the microwave antennas known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 6,268,774, 6,254,601, 6,253,871, 6,252,558, 6,252,556, and the like. The entire disclosure of this United States patent is hereby incorporated by reference into this specification. 
     In one embodiment, microwave antenna  14  is a three-foot parabolic antenna with a gain of about 31 decibels. In another embodiment, microwave antenna  14  is a microwave horn antenna with a gain of from about 5 to about 15 decibels. 
     One may use any of the receivers known to those skilled in the art for receiving radio frequency energy over the range of from about 0.5 to about 94 gigahertz. Thus, by way of illustration and not limitation, and referring to FIG. 2, one may utilize the receiver  12  depicted by dotted line  16 . This receiver  12  is comprised of radio frequency duplexer  18 , radio frequency protective limiter  20 , a 10 decibel radio frequency pad  22 , directional coupler and pad  24 , a detected log video amplifier  26 , variable radio frequency attenuator  28 , and various other radio frequency components. As will be apparent to those skilled in the art, the receiver depicted in FIG. 2 receiving the radar signals without substantially changing any property of the signals. 
     Referring again to FIG. 1, the signal which is received in receiver  12  is fed via line  30  to a preamplifier  32 . This preamplifier anticipates a certain amount of attenuation which will occur in the rest of the circuitry, including the fiber optical portions thereof, and compensates for the anticipated losses. Referring to FIG. 2, the preamplifier  32  is incorporated into the fiber optic delay unit  34 . 
     Referring again to FIG. 1, the amplified signal from preamplifier  32  is passed via line  34  to isolator  36 . The function of isolator  36  is to maximize power transfer to the external mach-Zender modulator  38  by adjusting the impedance of the signals. In the embodiment depicted in FIG. 2, the isolator  36  is incorporated into the fiber optic delay unit  34 . 
     One may use many of the isolators known to those skilled in the art as isolator  36 . Thus, e.g., one may use ferrite isolators. Reference may be had, e.g., to U.S. Pat. Nos. 6,100,841, 6,011,449, 5,886,534, 5,882,310, 5,760,667, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     The signal from isolator  36  is fed via line  37  to electro-optic modulator  38  which, in the preferred embodiment depicted, is an external Mach-Zender modulator. These modulators are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 6,259,544, 6,246,498, 6,222,658, 6,181,450, 6,163,394, 6,151,438, 6,141,086, 6,046,838, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     As is disclosed in the aforementioned U.S. Pat. No. 6,046,838, optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. 
     A typical optical communications system includes a transmitter, an optical channel (e.g., optical fiber), and a receiver. Within the transmitter, an electro-optic modulator is often used to modulate an optical carrier with the information to be transmitted. The modulated carrier is transmitted across the optical channel to the receiver, which receives/retrieves the information from the carrier. 
     A common electro-optic modulator used in such systems is the Mach-Zender modulator (MZM), which operates on the principle of phase interferometry. The MZM, however, is an inherently non-linear device while many communications systems would benefit from linear operation. As a result, the MZM is often operated in a mode which minimizes the non-linear effects of its transfer function, such as second and higher order harmonics, by applying a bias signal to the MZM to establish an operating point, or bias point, in the most linear region of the MZM&#39;s transfer function and then operating the MZM over a limited range about this bias point. The bias point, however, may vary due to temperature variations, signal fluctuations, manufacturing tolerances, aging, and other factors. In fact, since the MZM is based on phase interferometry, small changes in the environment or operation of the MZM may lead to significant changes in optical path lengths within the MZM which, in turn, will cause the bias point to drift significantly. If the proper bias point is not maintained, the MZM will exhibit stronger non-linearity, including the generation of even-order harmonics and the reduction of the signal strength. This, in turn, will decrease the maximum dynamic range of the optical communications link and will otherwise degrade the performance of the overall system. 
     Therefore, it is important to control the bias signal applied to the modulator to ensure operation at the correct bias point. 
     In one approach, amplitude modulation is applied to the information signal being transmitted by the modulator. As a result, the optical output of the modulator is also amplitude modulated. This amplitude modulation at the output is detected and used to control the bias point. Amplitude modulation, however, introduces upper and lower sidebands. Since the information signal typically has a broad bandwidth, these upper and lower sidebands can also be quite wide. As a result, the amplitude modulation approach results in distortion of the information signal. An envelope detector is also required to detect the amplitude-modulated component. 
     Furthermore, many of these approaches rely on non-coherent detection techniques. For example, an approach based on the sum component typically relies on detecting only the amplitude, and not the phase, of the resulting sum component. The amplitude modulation approach typically relies on an envelope detector which also loses any phase information. Hence, in both approaches, control of the bias point can only be based on the amplitude and not the phase of the feedback signal, thus limiting the types and effectiveness of suitable control algorithms. 
     Referring again to FIGS. 1 and 2, and in the preferred embodiment depicted therein, applicants solution to these problems is presented. Laser  50  provides light via line  52  to electrooptic modulator  38 . The light produced by laser  50  is a continous wave light emission. Reference may be had, e.g., to U.S. Pat. Nos. 4,853,534, 6,143,138, 6,121,984, 6,002,452, 5,844,588, 5,808,200, 5,774,196, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     It is preferred that the light so provided be stabilized light from a stabilized light source. As used herein, the term stabilized light refers to amplitude stabilized light in which the amplitude of the light is substantially constant and does not vary by more than about 2 percent. Reference may be had, e.g., to U.S. Pat. Nos. 6,134,011, 5,953,122, 5,898,181, 5,777,737, 5,726,455, 5,694,m114, 5,519,530, 5,485,274, 5,350,913, 5,311,592, 5,218,426, 5,063,568, 4,927,266, 4,856,899, 4,492,859, 4,271,123, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     The stabilized light preferably has a power stability of about 0.1 percent over a period of one hour at a constant temperature. Such stabilized light is produced, e.g., by high performance diode laser assemblies that include thermal electric cooling device  54  for temperature stability. See, for example, pages 45.8 to 45.9 of the Melles Griot catalog which describe, e.g., “LAZERGUIDE” Industrial Alignment Diode Laser Assemblies. 
     In the preferred embodiment depicted in FIG. 1, a thermal electric cooling device  54  is used to regulate the temperature of the laser  50  and to insure that its light output has substantially constant output. In one embodiment, the cooling device  54  operates on the well-known Peltier effect via thermoelectric cooling. As is known to those skilled in the art, thermoelectric cooling is a refrigeration method. When an electric current passes through a thermocouple of two dissimilar metals joined in two places, heat is absorbed at the cold junction and dissipated at the hot junction. The cold junction can be mounted in a chamber to be cooled. 
     Referring again to FIG. 1, degenerative feedback from the delay circuit  56  (defined by dotted line  58 ) is fed back to the modulator  38  by means of line  60 . Referring to FIG. 2, the feedback circuit  56  is preferably incorporated within delay unit  34 . 
     In the modulator  38 , with its associated circuitry, means are provided for phase coherently amplitude modulating a continuous wave light with said radar signal with said frequency of from about 1 to about 94 gigahertz, thereby producing a phase coherent amplitude modulated light pulse. 
     The modulation which occurs in modulator  38  is phase coherent, i.e., the optical pulse produced by the modulator  38  has substantially the same characteristics as the radio frequency input pulse fed to the modulator via line  37 . Phase coherent amplitude modulation is well known. Reference may be had, e.g., to U.S. Pat. Nos. 5,920,551, 5,301,167, 4,694,257, 4,475,247, 4,163,233, and the like. Reference also may be had to U.S. Pat. Nos. 6,029,116, 6,016,990, 5,999,561, 5,928,152, 5,726,657, 5,619,212, 5,529,068, 5,428,361, 5,347,283, 5,159,406, 5,019,826, and the like. The entire disclosure of these United Sates patent is hereby incorporated by reference into this specification. 
     Referring again to FIG. 1, and in the preferred embodiment depicted therein, there are also provided fiber optic delay means for delaying said amplitude modulated light pulse for at least about one radar pulse width, thereby producing a delayed amplitude modulated light pulse. Thus, e.g., one may use fiber optic delay unit  34  (see FIG. 2) for this purpose. In one embodiment, the delay is greater than one radar pulse width. 
     One may purchase fiber optic delay units which provide the desired functionality. Thus, by way of illustration and not limitation, assembly  34  may be purchased as an optical delay assembly with a 16 microsecond delay which is sold by the Custom Microelectronic Systems, Inc. of 2903 Robinwood Lane, Northport, Ala., as part number L1516us. Alternatively, one may purchase fiber optic delay unit FW/CMSDL15116us from Fredwal Inc. of Lockport, N.Y. Alternatively, one may use any of the fiber optical delay assemblies known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 6,269,198, 6,255,992, 6,249,622, 6,225,619, 6,101,454, 6,084,233, 6,040,571, 6,028,300, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In general, the delay effected by fiber optic delay unit  34  (see FIG. 2) does not change the phase of the signal delayed. Referring to FIG. 1, the light pulse exiting modulator  38  at point  61  has a phase which is substantially identical to the phase of the delayed pulse appearing at point  62 . As will be apparent to those skilled in the art, because the delay is not affected by means of inductances and/or capacitances, there is substantially no phase change, the phase being within about 5 degrees of the original. 
     Referring again to FIG. 1, device  34  is comprised of means for detecting the delayed modulated amplitude modulated light pulse, thereby producing a detected amplitude modulated light pulse. In the embodiment depicted, this function is performed by photodiode  70  (see FIG.  1 ), and/or by other suitable photodiode detectors. Reference may be had, e.g., to U.S. Pat. Nos. 6,298,914, 6,267,913, 6,267,722, 6,266,142, 6,262,407, 6,259,545, 6,255,118, 6,252,689, 6,251,679, 6,248,078, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     As will be apparent, and referring again to FIG. 1, the output of photo diode  70  is a series of electrical signals corresponding to the input light pulses, and correlating to the envelope of the input radio frequency pulses. Each train of light pulses is a component of a radio frequency output envelope that corresponds to the radio frequency input envelope to antenna  14  (see FIG.  2 ). 
     The radio frequency pulses fed from photodiode  70  are fed to a splitter  72  which, by feeding back a portion of the input via line  74  to noise reduction bias circuit  76 , reduces the noise in the modulator  38 . Another portion of the input to splitter  72  is fed out via line  80  to pad  35  and then to amplifier  81  (see FIG.  2 ). 
     The device  10  of this invention also is comprised of means for modulating said converted radio frequency pulse, thereby producing a delayed modulated radio frequency pulse. Thus, and referring to FIG. 2, and in the preferred embodiment depicted therein, the output from delay unit  34  is preferably amplified by amplifier  81  and thereafter modulated in modulator  82 . 
     One may use any means of modulation such as, e.g., frequency modulation, pulse modulation, phase modulation, etc. Thus, e.g., one may use one or more of the means of modulation disclosed in U.S. Pat. No. 5,351,054, the entire disclosure of which is hereby incorporated by reference into this specification. 
     Thus, e.g., and is disclosed in U.S. Pat. No. 5,351,054, the signal may be modulated in a modulator  114 , in which a Doppler velocity is added or subtracted from the carrier. Alternatively the signal may be modified in a device in which the incoming pulse repetition frequency is generally divided by 2 and the carrier is bi-phase modulated by a 180 degrees on alternate received PRF pulses. 
     In another embodiment, discussed in U.S. Pat. No. 5,351,054, in a specified modulator  106 , certain frequency characteristics of the signal wave are varied or selected. Thus, by way of illustration, modulator  106  may be a Serrodyne Doppler frequency modulator, and the like. These Serrodyne modulators are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 4,654,608, 6,060,917, 5,880,689, 5,878,335, 5,384,637, 5,303,412, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In one embodiment, the target  10  of such patent may be used to calibrate pulse Doppler radar signals. As is known to those in the art, when a target is moving toward or away from a transmitter, the frequency of the returned echo is shifted from the original frequency by an amount dependent on the radial velocity of the target. The change in frequency is called the Doppler frequency. Alternatively, or additionally, the target  10  of such patent may be used as a beacon transponder. 
     In one embodiment, depicted in FIG. 6 of U.S. Pat. No. 5,351,054, one may use a radar Bi-phase modulator  112  which can be used as in the target device of the invention. Referring to such FIG. 6, it will be seen that modulator  112  is comprised of input connector  166 , into which is fed the output from the up converter mixer  72 . The signal from input connector  166  is then fed via line  168  to directional coupler  170 , in which the signal is split into two directions. One portion of the signal is passed via line  172  to Bi-phase modulator  174 ; and the other portion of the signal is passed via line  176  to radio-frequency crystal detector  178 . The detected radio-frequency signal is converted to a positive video pulse in detector  178  and then passed via line  180  to video amplifier  182 . The output from video amplifier  182  is fed via line  184  to a flip/flop, divide-by-two multivibrator  186 , where the PRF (pulse repetition frequency) is divided by  2 . The output from the multivibrator  186  is then fed via line  188  to Bi-phase modulator  174 , which controls the modulation of the radio-frequency input signal applied to the modulator via line  172 , thus producing, on alternate pulses, zero phase shift or 180 degrees phase shift. 
     The radar Bi-phase modulator  112  is a commercially available unit which may be purchased from Fredwal Inc. of Lockport, N.Y. The modulator used in this embodiment is well known to those skilled in the art. It has been used for many years in a device identified as “Model 8701-0100-2 ‘X’ Band Pulse Doppler Radar Target,” which is sold by the Fredwal Inc. of 160 Washburn Street, Lockport, N.Y. 14094. The device is described in a publication made in April, 1991 and entitled “Operations and Maintenance Manual for Model: 8701-0100-2 . . . “Tactical Radar Training System.” 
     In one embodiment, depicted in FIG. 7 of such U.S. Pat. No. 5,315,054, there is illustrated a tactical radar training system  200  which is adapted to simulate moving objects such as, for example, low altitude threat aircraft. This system  200  provides a cost effective simulation system which is adapted to train radar operators without the need to fly aircraft against the radar system. In one embodiment of system  200 , the radar is adapted to simulate aircraft ranges of from about 100 microseconds (8 miles) down to about 5 microseconds (less than 0.5 mile) beyond the target location. The range of velocities that can be developed is from about 5 knots to about 4,000 knots, or more. The signals can be modulated to emulate such simulation signals. The training system  200  may be used, for example, to simulate the approach of a single highspeed, low-altitude threat aircraft. Alternatively, training system  200  may be used to simulate the approach of several aircraft approaching simultaneously. Alternatively, the system  200  can be used to simulate a slower moving helicopter. 
     Referring to FIG. 7 of such U.S. Pat. No. 5,351,054, it will be seen that radar training system  200  is comprised of a high performance radar system  202  (such as, e.g., a tactical radar system), a multiplicity of the delayed radar targets  204 ,  206 ,  208 , and  210 , a multiplicity of antenna assemblies  13 , and a remote data link  212 , which allows one to select the parameters of system  200  and to alter its performance. 
     Data link  212  is operatively connected to targets  204 ,  206 ,  208 , and  210 . This data link  212  can initiate the radar targets to simulate multiple approaching threat aircraft, singly or simultaneously. Several programmed combinations of Doppler frequency and rates that the range delay will incrementally reduce are available. The range to the targets  204  and/or  206  and/or  208  and/or  210  can be incrementally changed at a rate to coincide with the Doppler frequency (velocity) imparted to the radar carrier. The amplitude of the return can be attenuated to simulate a small aircraft, or the return can be amplified to simulate a larger aircraft. 
     In one embodiment, discussed in U.S. Pat. No. 5,351,054, each of targets  204 ,  206 ,  208 , and  210  is mounted on a high tower which is high enough to appear to the radar as an approaching low altitude helicopter or a high performance aircraft. Alternatively, the targets  204 ,  206 ,  208 , and  210  may be mounted on vehicles to relocate these targets to another site for maximum training authenticity. Each of the targets  204 ,  206 ,  208 , and  210 , after having been actuated by the data link  212  and illuminated by the host radar  202 , will approach on a radial path, i.e., the simulated aircraft (or other moving objects) will approach the host radar at a constant azimuth angle and a constant elevation angle. 
     In another embodiment, depicted in FIG. 8 of U.S. Pat. No. 5,351,054, there is illustrated a process for using applicants&#39; delayed radar target  10 , illustrating how the use of applicants&#39; delayed radar return system provides a radar return which is free of clutter and skin return. Referring to FIG. 8, it will seen that, in one preferred process, the target  10  is mounted on a tall object  219 , such as a tower, building, or mountain top. This is done to elevate the radar beam above the radar horizon. Host radar  214  will be adjusted so that radar beam  216  has an elevation angle  218  several degrees of elevation above the radar horizon and the beam is above the ground at the delayed range. Referring to FIG. 8 of such patent, it will be seen that beam  216  is comprised of the beam center (antenna axis)  220 , azimuth beamwidth  222 , and elevation beamwidth  224 . Referring again to FIG. 8, it will be seen that, the radar pulse  226  received by the target  10  with a pulse width  228  is delayed by a distance  230 . Delayed output  232  has a pulse width  234  which is substantially the same as pulse width  228 . 
     In another embodiment, illustrated in FIG. 9 of U.S. Pat. No. 5,351,054, there is disclosed a system  240  for assisting an aircraft (not shown) in locating the centerline  242  of an unattended airport runway  244 . System  240  is preferably comprised of a pair of Doppler radar targets  246  and  248  placed at opposite ends  250  and  252  of runway  244 . Each of Doppler targets  246  and  248  is comprised of two horn antennas pointing in opposite directions along the centerline  242  of runway  244 . Thus, target  246  is comprised of horn antennas  254  and  256 ; and target  248  is comprised of horn antennas  258  and  260 . Targets  246  and  248  are located distances  262  and  264  from ends  250  and  252  of runway  244 . It is preferred that horn antennas  254 ,  256 ,  258 , and  260  each have a wide azimuth beamwidth and a narrower elevation beamwidth. In this embodiment, it is also preferred that these horn antennas be pointed up by approximately one-half of a beamwidth. Thus, by way of illustration, the azimuth beamwidth can be about 20 degrees and the elevation beamwidth can be about 10 degrees. The aircraft (not shown) may approach from either direction. As the aircraft approaches the area within the beamwidth, two replies would appear on its radar scope. When both replies appear lined up with the zero degree cursor, the aircraft would then be “on” the centerline. The range to the runway could be observed as the aircraft approached the runway. 
     In another embodiment, disclosed in U.S. Pat. No. 5,351,054, the signal is modulated to simulate a helicopter. As is known to those skilled in the art, a helicopter is one type of vertical takeoff and landing (VTOL) devices which is lifted vertically and may be moved horizontally in any direction, or kept hovering, by large, motor-driven rotary blades which are mounted horizontally. By way of illustration and not limitation, the CH-47 Chinook helicopter is a medium lift helicopter which is sold by the Boeing Vertol Company of Philadelphia, Pa.; it is described, e.g., on page II/267 of Volume II of the “International Defense Equipment Catalog (IDEC),” which was published by Jules Perel&#39;s Publishing Company, Amstelveen, NL 1180, Netherlands. 
     By way of further illustration, the EH-101 is a triple-turbine helicopter sold by Gruppo Agusta, Via Caldera, 21, 1-20153 Milano, Italy. It is described, e.g., on page II-268 of the aforementioned “International Defence Equipment Catalog.” 
     Gruppo Agusta also manufactures the A109A MkII/A 109K high cruise speed helicopter and the A-129 multimission helicopter; see pages II-269 and II-270 of said “International Defence Equipment Catalog.” 
     By way of further illustration, Hughes Helicopters, Inc. (of Centinela and Teale Streets, Building 1, Mailing Station T137, Culver City, Calif.), manufactures the AH-64A Apache attack helicopter. Hughes also manufactures the 500E and 530F business helicopters, and the MG defender light combat helicopter; see, e.g., page II/271 of said “International Defense Equipment Catalog.” 
     By way of further illustration, the PAH-1 twin-engined anti-tank helicopter is manufactured by Messserschmitt-Bolkow-Blohm GmbH, Helicopter and Military Aircraft Group, D-8000 Munchen 80, Germany. 
     Many other helicopters are known to those skilled in the art. Thus, for example, and is disclosed in Charles Messenger et al.&#39;s “Armed Forces of the World” (Gallery Books, 112 Madison Avenue, New York, N.Y., 1982), American companies manufacture the AH-1 Huey Cobra, the UH-1, the SH-2 Seasprite, the H-3 Sea King series, the OH-6A, the HH-52, the H-53 Stallion, the Super Stallion, the CH-54 Tarhe, the OH-58 Kiowa, the H-60 Black Hawk, the Sea Hawk, and the AH-64 Apache helicopters. British and French companies manufacture the Gazelle, the Puma, the Lynx, the Scout, and the Alouette helicopters. The Bo helicopter is manufactured by a German Company. 
     Attack helicopters, with their ability to sense enemy tanks by electronic means, and to ascend into view of a tank just long enough to fire a missile and thereafter to descend, are an especially fearsome instrument of war. In order to combat these attack helicopters, it is important that radar systems be able to readily sense the presence and location of such helicopters and, equally important, be able to distinguish whether any particular helicopter so sensed is an enemy helicopter or a friendly helicopter. 
     The instant embodiment of applicants&#39; invention is designed to simulate the radar signature of various helicopters, among other things. It can be used to verify whether any particular radar system is capable of discriminating between various types of helicopters. 
     As will be apparent to those skilled in the art, any of the modulated information used in EW simulators and/or described such U.S. Pat. No. 5,351,054 may be imparted to the radio frequency carrier in modulator  82 . Thereafter, the modulated signal is fed to radio-frequency switch  18  and thence to antenna  14 , wherein it is transmitted back to the interrogating radar (not shown) 
     The device depicted in FIGS. 1 and 2 is capable of being used with narrow bandwidth or wide bandwidth applications, unlike the device depicted in U.S. Pat. No. 5,351,054. Thus, this device can be used with, e.g., a bandwidth of from about 0.5 to about 94 gigahertz. In one embodiment, it is used with a bandwidth of from about 5.4 gigahertz to about 5.9 gigahertz (the C band). In another embodiment, it can be used with a bandwidth of from about 0.5 to 10 gigahertz. 
     The device of this invention is capable of utilizing wide pulse width radar inputs, with pulse widths of from about 0.1 microseconds to up to 20 microseconds or greater. 
     In one preferred embodiment, the device illustrated in FIG. 12 of U.S. Pat. No. 5,351,054 is modified by connecting the fiber optic delay unit  34  (see FIG. 2) between elements  40  and  42  of the device of such FIG. 12 (replacing device  522  of such FIG. 12) and, additionally, connecting the output delay of unit  34  to counter  520  via diode  538 . 
     In another embodiment, the device  10  of this invention can be used to measure the phase coherency of a radar system. In particular, the system can be used as a beacon transponder to verify the phase coherency of the transmitted and received pulses of a radar system. 
     A Preferred Process of the Invention 
     In the remainder of this specification, reference will be made to the process depicted in FIG.  3 . 
     Modem radar systems utilize sophisticated phased array antennas to provide unique capabilities that will very accurately track multiple targets and can instantly scan the whole field of view of the antenna. 
     These antenna systems are usually completely tested at the radar manufacturer prior to delivery and installation at a military agency. Unfortunately, many of these systems cannot be easily field-tested to verify their continued accuracy and performance. Sometimes these antenna systems can experience a degradation of performance as the many phase shifters age and deteriorate. This can contribute to “tracking errors” and cause “rf holes” in the antenna pattern. 
     The process described hereinbelow, in FIG. 3, provides a means for evaluating the performance of phased array antenna radar system assemblies. These assemblies, which generally include a radar source and a phased array antenna, are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 4,912,477, 4,872,016, 6,121,917, 5,955,991, 6,184,825, 5,467,072, 4,675,678, 3,680,100, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     The process of this invention preferably involves the step of tracking a delayed radar target installed on a moving object (such as a helicopter) with a differential global positioning system receiver. 
     In one embodiment of the process, the radar angular output and the DGPS angular output is recorded (with the relative radar signal strength). A comparison is preferably made of the “angular output” (azimuth and elevation) of the radar system with the DGPS generated azimuth/elevation of the helicopter from the axis of the radar antenna. Subtracting these angles will determine any radar error that is present in the radar tracking data. 
     The radar signal strength preferably is also recorded and is plotted across the field of view of the phased array antenna. This will locate any “holes” in the radio frequency pattern of the antenna. 
     Referring to FIG. 3, and in the preferred embodiment depicted system  100  is one preferred embodiment of the invention. In the preferred embodiment depicted, one or more global positioning satellite sends out a multiplicity of signals. These global positioning satellites are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 6,121,922, 6,075,481, 5,857,433, 5,364,093, 6,182,011, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Referring again to FIG. 3, global positioning satellite  102  sends a signal to a movable object  104 , such as a helicopter preferably moving in the pattern depicted as pattern  106 . The pattern  106  is often referred to as a multi-bar tracking pattern. This multi-bar tracking pattern is at a constant elevation angle through the field of view of the phased array antenna  108 , and it preferably should be at distance from about 1 to about 2 miles from the radar antenna  108 . 
     The movable object (helicopter)  104  is preferably comprised of a delayed radar target  110 . One may use any of the delayed radar targets known to those skilled in the art. In one embodiment, delayed radar target so used is described elsewhere in this specification. This delayed radar target is a radar target with delayed reply means, wherein said radar target comprises: (a) means for receiving a radar signal which has a frequency of from about 0.5 to about 94 gigahertz, (b) means for phase coherently amplitude modulating a continuous wave light emission with said radar signal with said frequency of from about 1 to about 94 gigahertz, thereby producing an phase coherent amplitude modulated light pulse, (c) fiber optic delay means for delaying said amplitude modulated light pulse for at least about one radar pulse width, thereby producing a delayed amplitude modulated light pulse, (d) means for detecting said delayed modulated amplitude modulated light pulse, thereby producing a detected amplitude modulated light pulse,(e) means for converting said detected amplitude modulated light pulse into a converted radio frequency pulse, (f) means for modulating said converted radio frequency pulse, thereby producing a delayed modulated radio frequency pulse, and (g) means for transmitting said delayed modulated radio frequency pulse. 
     Referring again to FIG. 3, and in the preferred embodiment depicted therein, the phased radar antenna array  108  provides a radar signal  112  that impacts delayed radar target  110  and is returned as signal  114 . 
     Simultaneously, the global positioning satellite  102  sends a signal  116  to a remote global positioning satellite receiver  118 . Global positioning satellite receivers, also known as “GPS receivers,” are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 6,619,212, 5,805,210, 6,031,488, 6,121,921, 5,999,878, 5,969,670 and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Referring again to FIG. 3, the GPS satellite  102  also sends a signal  120  to a fixed global positioning satellite receiver  122 . In the embodiment depicted in FIG. 3, for the sake of simplicity of representation, the fixed GPS receiver  122  is shown as being physically separate from the radar phased array antenna assembly  108 . It is preferred, however, that such fixed GPS receiver  122  be physically located near or on the phased array antenna assembly  108 , preferably being located on its pedestal. The physical proximity or contiguity is indicated by dotted line  124 . 
     Referring again to FIG. 3, the movable object  104  is also comprised of a global positioning satellite data link  126 . These GPS data links are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 6,271,798, 6,204,807, 6,278,396, 6,160,858, 5,929,783, 5,691,723, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     The gps data link  126  communicates with GPS receiver  118 . Thereafter, it sends a signal  128  to DGPS (differential GPS) data link receiver  130 . These differential GPS receivers are well known. Reference may be had, e.g., to U.S. Pat. No. 6,324,473, Reissue U.S. Pat. No. 37,256, U.S. Pat. Nos. 6,249,245, 6,236,940, 6,236,359, 6,212,475, 6,211,817, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Information from DGPS receiver  130  is fed via line  132  to DGPS processor  134 . These DGPS processors are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 6,249,245, 6,236,359, 5,982,324, 6,326,916, 6,307,505, 6,061,632, 5,491,636, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Referring again to FIG. 3, information from fixed GPS receiver  122  is also fed via line  134  to GPS processor  134 . Thereafter, in step  136 , the DGPS processor  134  can determine the azimuth of the helicopter  104  from the radar pedestal  108 . Additionally, in step  136 , the DGPS processor determines the elevation angle of the helicopter  104  with respect to the radar pedestal  108 , and it also determines the slant range of the helicopter  104  with respect to the radar pedestal  108 . This information specifies the exact location of the helicopter  104  at a particular point in time. 
     The information derived from processing step  136  is fed via line  138  to computer  140 . Simultaneously, information from the phased array radar antenna system  108  is fed via line  142  to computer  140 . The computer  140  then compares this data and determines to what extent, if any, there is any discrepancy between them. The existence of any such discrepancy indicates an error in the antenna system  108  being evaluated. 
     As will be apparent to those skilled in the art, the information conveyed to the computer  140  via line  138  relates to the true azimuth angle, the elevation angle, and the slant range (distance) of the helicopter  104 . The information conveyed to the computer  140  via line  142  includes radar azimuth, radar elevation angle, radar range, and radar signal strength. 
     The computer  140 , in effects, compares corresponding data from lines  138  and  142  to determine whether they are substantially identical. Any differences there between are shown in display  144  as error which should be investigated. 
     In the embodiment depicted in FIG. 3, the information from computer  140  also may be printed in printer/plotter  146  and/or permanently stored in storage device  148 . 
     With the process of this invention, periodic tests of any phased array antenna radar assembly will identify any problem or degradation of the system and alert maintenance personnel to such problem. 
     It is to be understood that the aforementioned description is illustrative only and that changes can be made in the apparatus, in the ingredients and their proportions, and in the sequence of combinations and process steps, as well as in other aspects of the invention discussed herein, without departing from the scope of the invention as defined in the following claims.