Abstract:
Apparatus for detecting the difference in phase between received signals at two spaced antennas and for then retransmitting equal amplitude antiphase signals from the two spaced antennas is disclosed.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains generally to electronic counter-measure (ECM) systems and more particularly to an adaptive interferometer for causing errors to be introduced into a tracking radar. 
     The cost of modern fighter aircraft coupled with the increased effectiveness of ground-to-air missile defense systems has resulted in the development of ECM systems designed to mask such aircraft from ground-based fire control radars. One known type of ECM system designed to protect a penetrating aircraft against a surface-to-air missile which is being guided by a command guidance system is the so-called “Cross-Eye” system. With such a system a portion of the signal transmitted by a control radar, say a ground-based fire control radar, is received at the penetrating aircraft and then is processed to be retransmitted as a pair of equal amplitude, but 180° phase-opposed, signals (referred to hereinafter as the “jamming signals”) in the direction of the ground based fire control radar. The magnitude of the jamming signals is sufficient to mask the skin return from a penetrating aircraft so that the ground-based fire control radar is caused to attempt to track on the jamming signals with the final result that unacceptably large tracking errors are engendered and a guided missile in flight toward the penetrating aircraft is misguided. 
     Generally, an aircraft employing a “Cross-Eye” system has an appropriate transmitting and receiving antenna located on each of its wings. Thus, a receiving antenna located on a first one of the wings is connected, via an amplifier and requisite transmission lines, to a transmitting antenna disposed on the second wing. In like manner, a receiving antenna located on the second wing is connected, via a 180° phase shifter, an amplifier, and requisite transmission lines, to a transmitting antenna located on the first wing. 
     It will be appreciated by those of skill in the art that the effectiveness of such “Cross-Eye” systems is dependent upon how well the amplitude and phase of the retransmitted signals are controlled. Unfortunately, however, with transmitting and receiving antennas located on opposite wing tips, it is virtually impossible, even in the best of conditions, to provide the required accuracy in phase and amplitude of the retransmitted, or jamming signals. The problem is even more difficult in the severe vibration environment and over temperature extremes often experienced by any aircraft. 
     SUMMARY OF THE INVENTION 
     With this background of the invention in mind, it is an object of this invention to provide an adaptive interferometer which will automatically compensate for differential phase shifts between a pair of receiving and transmitting antennas. 
     The foregoing and other objects of the invention are attained generally by providing an adaptive interferometer comprising a pair of antennas which may operate as either receiving or transmitting antennas and signal processing means to maintain a desired relationship in phase and amplitude between jamming signals. In the receive mode of operation received signals from a first one of the antennas is split with a first portion thereof being applied, via a quadrature hybrid, to a pair of mixers and a second portion being down-converted to suitable video signals and subsequently applied, via a quadrature hybrid, to a pair of quadrature phase detectors to which reference signals derived from signals received by the second antenna are also applied. After suitable processing, the output signals from the quadrature phase detectors are applied as reference signals to the pair of mixers. The vector sum of the output signals from the mixers, which are proportional-to the product of the input signals, is formed and combined with the signal received by the second antenna, ultimately to cancel that signal. When cancellation of the received signals is achieved, a transmitter is activated. The output signal from the transmitter is passed via a variable attenuator to the second antenna and via a phase shifter, controlled by the microprocessor, to the first antenna, such that the signal radiated by the first antenna is the complex conjugate of the signal that,when combined with the signal received by the second antenna, cancelled that received signal. The variable attenuator, which is also controlled by the microprocessor, is provided in the path between the transmitter and the second antenna to compensate for the insertion loss of the phase shifter to ensure that equal amplitude signals are fed to both antennas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following detailed description read together with the accompanying drawing in which the single FIGURE is a simplified block diagram of an adaptive interferometer according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the single FIGURE, an adaptive interferometer according to the herein contemplated invention is shown to include a pair of antennas  11 ,  13  which are here assumed to have identical gain characteristics and which are used for both receiving signals from a fire control radar (not shown) and for transmitting jamming signals back to that radar. The signals received by antenna  11  are passed, via a bandpass filter  15  and a high power circulator  17 , to a power divider  19 . The latter is operative to split the received signal into two portions, with a first portion being applied (via an amplifier  21 ) to a mixer  2 . 3  to be heterodyned with a signal from a local oscillator  25  to produce a corresponding video signal. Such video signal is passed to a quadrature hybrid  27  operative to split the video signal into two parts, a so-called in-phase (I) and quadrature phase (Q) output signal which are applied, respectively, to phase detectors  29 I,  29 Q. The reference signals for the latter are the signals received by the antenna  13  which have been passed, via a bandpass filter  31 , a circulator  33 , a 180° hybrid combiner  35 , and an amplifier  37 , to a mixer  39 . The reference signal to the mixer  39  is the same as that applied to the mixer  23  so that video signals are produced which serve, after being split by a power divider  41 , as reference signals to the phase detectors  29 I,  29 Q. 
     It will now be appreciated by those of skill in the art that, because the input signals to the phase detectors  29 I,  29 Q are at the same frequency, the output signal from each of such detectors is a DC voltage whose magnitude is proportional to the phase difference between the signals received by antennas  11 ,  13 . The output signals from the phase detectors  29 I,  29 Q are filtered by low pass filters (not numbered) comprising, respectively, resistor R 1  and capacitor C 1  and resistor R 2  and capacitor C 2 . The output signals from the low pass filters (not numbered) are passed, via shaping amplifiers  43 I,  43 Q, as reference signals to a pair of mixers  45 I,  45 Q. It is noted here in passing that a portion of the output signals from each one of the shaping amplifiers  43 I,  43 Q is also passed, via analog-to-digital (A/D) converters  47 I,  47 Q, to a microprocessor  49  for reasons which will be explained in detail hereinafter. 
     The mixers  45 I,  45 Q are shown to receive quadrature-phased input signals from a quadrature hybrid  51  corresponding to a portion of the signals received by the antenna  11 . The vector sum of the output signals from the mixers  45 I,  45 Q is formed in a power combiner  53  and passed via an amplifier  54  to the 180° hybrid combiner  35  for combination with the signals received by the antenna  13 . 
     Digressing here now for a moment, it will be shown that if the mixers  45 I,  45 Q are operated as square law devices, a component of the output signals from such mixers will be proportional to the product of the input signals. The well known power series representation of a mixer is useful in predicting the various output products,. Thus, the current, i, flowing in a nonlinear resistance may be represented by a power series of the voltage, V, across the resistor terminals as follows: 
     
       
         i=a 0 +a 1 V+a 2 V 2 +a 3 V 3 + . . . +a n V n ,  Eq. (1) 
       
     
     where a 0  . . . a n  are constants. 
     The voltage V IN , applied to the mixers  45 I,  45 Q, is equal to the sum of a local oscillator voltage and the signal voltage and may be expressed as: 
     
       
         V IN =R(t)+sin ωt  Eq. (2) 
       
     
     where R(t) is the local oscillator voltage applied to the mixers  45 I,  45 Q and f is the radio frequency (RF) of the applied signal. If the mixers  45 I,  45 Q are operated in their square law regions, then all but the third term of Eq. (1) may be neglected with the result that Eq. (2) applies, meaning that if (2) is inserted in the retained portion of Eq. (1) and the indicated expansion is carried out, there will be a component proportional to the product of the input signals to the mixers  45 I,  45 Q. It is here immaterial that the “local oscillator” signal into the mixers  45 I,  45 Q is a D.C. signal. 
     It should now be appreciated by those of skill in the art. that the just described elements are effective to form a closed loop which will force the signals being combined in the 180° hybrid combiner  35  to cancel each other. It will also be appreciated that when cancellation is obtained, the phase of the signals received by the antenna  11 , as seen at the input of the 180° hybrid combiner  35 , will have been rotated to be coincident in phase with the signals received by the antenna  13 , also as seen at the input of the 180° hybrid combiner  35 . The output signals of the phase detectors  29 I,  29 Q are D.C. signals, which, taken together, represent the phase difference between the signals received by the antennas  11 ,  13 . 
     As previously mentioned, the output signals from the phase detectors  29 I,  29 Q are passed, via the low pass filters (not numbered), the shaping amplifiers  43 I,  43 Q, and the A/D converters  47 I,  47 Q, to the microprocessor  49 . The latter, which is here a Model 9900 manufactured by Texas Instruments, Dallas, Tex., is effective to, inter alia, provide a control signal to a transmitter  51 , which is here of conventional design and may, for example, comprise a high power traveling wave tube (TWT) amplifier. That control signal is effective to gate “ON” the transmitter  51  when the microprocessor  49  senses that the phase-lock loop (not numbered) has reached a lock condition as determined by the signals from the phase detectors  29 I,  29 Q. It is felt that the requisite control program to enable the microprocessor  49  to monitor the input signals from the phase detectors  29 I,  29 Q ultimately to produce the transmitter control signal is a matter involving ordinary skill in the art and it will there fore not be recounted here. 
     The output signal from the transmitter  51  is split in a power divider  55 D, with a first portion being passed, via a variable attenuator  57 , which is here of conventional design, the circulator  33  and the bandpass filter  31  to the antenna  1 .S. The second output signal from the power divider SSD is passed, via a high power phase shifter  59 , the circulator  17  and the bandpass filter  15  to the antenna  11 . The high power phase shifter  59  may, for example, be a digitally controlled phase shifter Series 84-32-114 manufactured by Microwave Associates, Burlington, Mass. Both the variable attenuator  57  and the high power phase shifter  59  are shown to receive control signals from the microprocessor  49  to balance the phase and amplitude of the signals transmitted from the antennas  11 ,  13 . 
     It may be shown that the jamming signals retransmitted to the fire control radar (not shown) are most effective when the antenna  11  transmits a signal which is 180° out-of-phase with the signal from the power combiner  53  when cancellation within the 180° hybrid power combiner  35  is achieved. It should be noted here in passing that the signals from the antenna  11 , as seen at the input of the 180° hybrid power combiner, experience an additional phase delay vis-a-vis the signals from antenna  13  to the same point by virtue of the fact that the signals from the former traverse the power divider  19 , the quadrature hybrid  51 , the mixers  45 I,  45 Q and the power combiner  53  before reaching the 180° hybrid power combiner  35 . It will be appreciated by those of skill in the art that the phase delays through the just-recited devices may be stored within the microprocessor  49  and used by the latter in conjunction with the input signals from the phase detectors  29 I,  29 Q to provide the requisite control signals for the high power phase shifter  59 . It should also be noted that the variable attenuator  57  is provided in the path between the power divider  55 D and the circulator  33  to compensate for the insertion loss of the high power phase shifter  59  in the path between the power divider  55 D and the circulator  17  to ensure that equal amplitude signals will be provided to both the antennas  11  and  13 . 
     Having described a preferred embodiment of this invention, it will be clear to one of skill in the art that changes may be made without departing from my inventive concepts. For example, if the antenna  11  were higher in gain than antenna  13  the variable attenuator  57  would be provided in series with the high power phase shifter  59 . It is felt, therefore, that this invention should not be restricted to its disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.