Abstract:
A “cross-eye” type radar countermeasure in which both of the waveforms of the illuminating signal from the enemy radar received at spaced locations on the carrying vehicle is propagated through a common dual channel to balance the effect of unintentional phase distortion. Two common channels receive the radar waveform and store the same in a delay line. During the stored interval the switches at the ends of the two common channels are transitioned to the non-stable state connecting the output end of one common channel to the input end of the other, and vice versa. Both waveforms then enter the delay line of the subsequent common channel and the switches change to the stable state whereupon each waveform is retransmitted at the other spaced location on the carrying vehicle. A very accurate phase shift of 180° is introduced into a non-common waveguide and causes an out-of-phase relationship between the two retransmitted radar pulse to create the “cross-eye” type of phase front distortion of the return to the victim radar.

Description:
The Government has rights in the invention pursuant to Contract No. F33615-76-C-1288 awarded by the Department of the Air Force. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to an electronic countermeasure system which retransmits a phase shifted form of the illuminating victim radar signal thereby causing return wave front distortion. 
     2. Description of the Prior Art 
     The VanAtta array principle is known and involves the retransmission of an illuminating radar pulse back toward its source. The “cross-eye” countermeasure technique employs this principle in conjunction with the introduction of a 180° phase shift into one of the two radar signals retransmitted from spaced locations on the carrying craft to cause wave front distortion at the location of the victim radar. It is essential that the overall phase shift introduced into the retransmitted pulse be extremely accurate in that a variation of more than ±2-6° of shift causes a beacon effect which easily identifies the location of the carrying craft. The practical application of this technique within the confines of present day technology has been somewhat difficult because of the necessity of maintaining the narrow phase shift tolerance between the enemy radar pulse and retransmitted countermeasure radar pulses. 
     One prior art countermeasure system utilizing the VanAtta array principle is known as the “switching cross-eye” and involves the use of a first microwave antenna located at the extremity of one portion of the carrying craft, such as the tip of one wing, while a second and identical antenna is positioned at another extremity of the aircraft, such as the tip of the other wing. A waveguide leads from each antenna to a single time multiplexing circuit disposed about midway on the vehicle between the two antennas. The multiplexing circuits comprises a pair of switches which must be capable of operating at switching speeds in the order of two to three times the victim radar bandwidth to sample the incoming radar signals. The circuit then stores each time slice in a single delay line to mitigate the effect of reflections that occur when the high power radiated signals are applied to common antennas. After the time multiplex signal emerges from the delay line, it is supplied to a single amplifier and then to the second switch for distribution to the appropriate antenna port. Although the use of only one amplifier and delay line in a common channel causes a balancing of the phase distortion in the two retransmitted signals, the high switching speed required of the switches located at either end of the common channel is beyond the capability of present day RF switches. 
     In a commonly owned application, Ser. No. 519,465, by S. Brody, and G. Bock on Aug. 27, 1971 entitled SINGLE AMPLIFIER COUNTERMEASURE (U) there is disclosed a track radar countermeasure utilizing three channels in which a single amplifier is employed in the center channel to amplify the received energy for redirection to the enemy radar. The outside two channels store phase information and are used to recreate the correct phase. 
     The aforementioned copending application is an improvement over the invention disclosed in another commonly-owned copending application, Ser. No. 334,172, by Richard C. DiDomizio and Lester H. Kosowsky filed on Feb. 12, 1973 entitled AUTOPHASED ANGLE DECEPTION COUNTERMEASURE (U) in which the differences between the phases of signals received at three antennas from the enemy radar is utilized to generate a phase adjustment to the central antennas so that the redirected waves have a proper phase relationship even though the enemy radar is not located on a normal to the alignment of the antennas. Both of the hereabove cited patent applications, and the references cited therein, should be consulted for the purpose of putting the present invention in perspective. 
     SUMMARY OF THE INVENTION 
     The principal object of the present invention is to provide an improved track radar countermeasure system in which two common channels are utilized to reduce phase distortion inherent in a channel containing amplifier, delay line, etc. for processing radar frequency signals. 
     According to the present invention, a switching cross-eye countermeasure system utilizes two common channels between two spaced apart antennas so that any phase distortion introduced by the waveguides, amplifiers, etc. will affect the retransmitted pulses equally, rather than exceeding the phase tolerance limits acceptable for the introduction of distortion into spherical wave front redirected toward the enemy radar. 
     According to the present invention, a switching cross-eye countermeasure system is disclosed in which two common channels are provided for initially storing an enemy radar signal. Both ends of each common channel are controlled by an RF switching element to create a single common path through which a received pulse propagates from one port to the other port, and vice versa. The received pulse is delayed in the delay line of one common channel for a period at least as long as the pulse width of the received radar pulse and then the switches are transitioned to allow the pulse through to the second common channel. 
     Other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a preferred embodiment thereof, as illustrated in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE comprises a schematic block diagram of an embodiment of a dual common channel cross-track system according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to the drawing, a radar countermeasure system having dual common channels according to the present invention will now be described. Antenna  10  and antenna  12  are mounted at spaced apart locations on the carrying vehicle (not shown) such as near the wing tips. Both the antenna  10  and the antenna  12  serve the combined function of receiving microwave energy from pulses transmitted from the victim radar of an enemy vehicle and transmitting microwave energy provided thereto by the countermeasure equipment on the carrying vehicle. A waveguide  14  and a waveguide  16  lead from antenna  10  and antenna  12 , respectively, to the circuitry of the countermeasure system which would normally be positioned in a more central portion of the carrying vehicle. Of course, rather than a combined unit, each antenna  10  and antenna  12  could comprise a separate transmit antenna, including a ferrite isolator in the feed path thereof, and a separate receiving antenna with a transmit/receive (T/R) protection device but a separate transmit and receive antenna is less desirable because of possible phase discrepancies therebetween. 
     A circulator  18  is connected to waveguide  14  at the juncture with a waveguide  20  and  22  and a circulator  24  is connected to the waveguide  16  at the juncture with a waveguide  26  and a waveguide  28 . The circulator  18  is selected to have ±2° of phase tracking between the incoming and outgoing paths. As is well known to those of ordinary skill, the circulator is a coupling device often used where incoming and outgoing high frequency waveforms use a single antenna for both transmission and reception. For example, in the illustrated embodiment an outgoing radar signal is coupled from waveguide  20  to the waveguide  14  and antenna  10  while the receive path through waveguide  22  is electrically isolated to prevent overdriving of the pulse detector or other receiver type circuitry. However, incoming radar energy from a victim radar travels from the antenna  10  through waveguide  14  and is passed by the circulator  18  only to waveguide  22 . 
     A particular feature of the dual common channel countermeasure system of the present invention is that a pulse of radar energy received at the location of either antenna  10  or antenna  12  is amplified and delayed sequentially in the two common channels prior to being retransmitted so that phase distortion normally inherent in such devices as amplifiers and delay lines affects the phases of the retransmitted pulses equally. This is significant in the “cross-eye” countermeasure technique because even a small phase difference unintentionally introduced into one of the retransmitted pulses, such as 2°-6°, will not create the necessary discontinuity into the phase front and the location of the carrying vehicle can be easily identified by the victim radar. For a more complete understanding of the “cross-eye” countermeasure concept, the references cited in the prior art section of this application should be consulted. 
     Referring still to the drawing, a particular feature of the “cross-eye” type dual common channel countermeasure system of the present invention is that one of two common channels is connected to each antenna for receiving and storing the radar pulse waveform transmitted by the victim radar. The output of one channel is then fed to the input of the other channel, and vice versa, so that any phase distortion which is normally inherent in such devices as amplifiers and delay lines will affect the two waveforms equally. Hence, the waveforms propagate through the channel sequentially and the composite path is almost entirely common for both waveforms thereby eliminating phase distortion in the retransmitted radar waveforms. Referring again to the drawing, a first common channel has an input end controlled by an RF switch  30 . The RF switch  30  may be of the diode or ferrite type of microwave switch and is selected to have an equal phase shift in each of the two arms. The switching speed capability of the RF switch  30  should be less than 100 nanoseconds while maintaining phase tracking in each arm within 2°. The RF switch  30  has its input terminal  32  normally gated in the ON condition allowing electromagnetic energy to propagate into the first common channel from the waveguide  22 . An electromagnetic signal propagates through the first common channel in one direction only (bottom-to-top in the drawing) and a signal gated into the channel by the switch  30  is presented to a delay line  34  where it is stored. The delay line  34  may be an acoustic bulk wave type or merely coiled lengths of waveguides so long as it is capable of storing a radar signal for a period of time greater than the width of the radar pulse anticipated to be received from the victim radar so that the entire radar signal waveform received by antenna  10  is stored. The delay line  34  is connected to a low distortion amplifier  36  which may be one such as the well known traveling wave tube type, such as a Raytheon QKW-1710. The output end of the first common channel is controlled by an RF switch  38 , having the same switching speed and phase shift characteristics as the RF switch  30 , which has its normally closed terminal  40  coupled to the waveguide  20 . 
     In a similar manner a second common channel has its input end controlled by an RF switch  40 , selected to be identical to the RF switch  30 , which has its normally closed terminal coupled to waveguide  28 . Electromagnetic energy also propagates through the second common channel in only one direction (bottom-to-top in the drawing) and a radar signal received at antenna  12  enters the second common channel for presentation to a delay line  44  for storage. The delay line  44  may be substantially identical to the delay line  34  so that it is capable of storing the entire waveform of a radar signal for a period of time greater than the width of the pulse anticipated to be received from the enemy radar. The second common channel also includes an amplifier  46 , preferably a low distortion amplifier identical to that in the first common channel. The output end of the second common channel is controlled by an RF switch  48  which has its normally closed terminal  50  coupled to waveguide  26 . 
     A key feature of the cross-eye type countermeasure system according to the present invention is that the first and second common channels are initially coupled to receive a pulsed radar signal from a victim radar which is illuminating the carrying vehicle and storing that precise waveform within the respective channel. Next, all of the switches are transitioned so that the output from one feeds the input of the other and the stored waveform propagates through the other channel. To accomplish this, the switch  38  at the output end of the first common channel has its normally open terminal  52  connected to a waveguide  54  which is in turn coupled to the normally open terminal  56  of RF switch  40  at the input end of the second common channel. Similarly, the normally open terminal  58  of the RF switch  48  is connected to a waveguide  60  which is in turn coupled to the normally open terminal  62  of the RF switch  30  at the input end of the first common channel. It will be appreciated by those of ordinary skill that the RF switches are not required to sample the incoming radar pulse since the switches are transitioned after the pulse is stored. This is significant because each switch must only operate at microsecond cycle times rather than nanosecond cycle times which would be required if a pulse sampling technique as described in the prior art section were employed. 
     As more fully described in the prior art section of this application and the references cited therein, a radar countermeasure apparatus of the “cross-eye” type requires the introduction of a 180° phase shift into one of the two waveforms transmitted back along the azimuth from which the illuminating pulses were received. Accordingly, a phase shift device  66 , is disposed in the waveguide  26 , one of the non-common portions of the channel. The phase shift device itself could be a waveguide which is twisted 180° or one half turn or it could be a ferrite phase shifter or a diode phase shifter so long as it were able to introduce a precise 180° phase shift into the signal waveform propagating therethrough. 
     The control circuit for the RF switches includes a RF pulse detector  64  which is coupled to the waveguides  22  and  28  for sensing a radar pulse from the victim radar that strikes either the antenna  10  or the antenna  12 . The output from the pulse detector  64  triggers a one-shot multivibrator  70  and in turn presents a suitable gate signal to transition the RF switches  38  and  48  simultaneously to their non-stable states. The output of the pulse detector  64  is delayed in the delay unit  72  before being presented to the one-shot  74  and finally to the switches at the input end of the common channel, RF switches  30  and  40 . Both the one-shot  70  and  74  have a cycle time of slightly greater than the delay time of the delay  34  or the delay  44  plus the width of the received radar pulse. The delay  72  is selected so that it has a delay time comparable to the time period of either the delay unit  34  or the delay unit  44 , the reason therefore being explained in greater detail hereinafter. 
     In operation, in order to act as a “cross-eye” type countermeasure it is important to control the relative amplitude and phase of the two signals retransmitted toward the enemy radar, the relative phase control being the most critical. For maximum destructive interference, so as to provide a non-spherical wave front, it is essential that the signal retransmitted by the two antennas be substantially out of phase with one another. Because of the frequency range in which microwave radar operates, it has been previously considered practically impossible to maintain the 180° phase difference required between the pulses to make the probability of confusing the enemy radar sufficiently high to consider the “cross-eye” technique viable. The present invention has solved this problem within the capability of existing technology by propagating the illuminating enemy radar signal through a common path so that any distortion inherent in the waveguides, amplifiers, delay lines, etc. will affect the retransmitted pulses equally and the only phase shift in the system is that intentionally introduced by the phase shift  68  and the unintentional phase shift associated with the non-common signal path. Accordingly, as an illuminating pulse from a victim aircraft strikes the carrying aircraft it is received by both the antenna  10  and the antenna  12  and is presented through the input end of the first common channel at switch  30  and also to the input of the second common channel at switch  40 . Each of the input switches are in their normally closed position, identified as NC in the drawing, so that microwave energy received at the input end passes immediately into the respective delay units where it is stored. 
     Simultaneously, the RF pulse detector  64  senses the received radar pulse as it propagates through either the waveguide  22  or the waveguide  28  and presents an output signal to the one-shot  70 . With the received radar pulse still in the delay units of the respected channels, the one-shot  70  causes the switch  38  and the switch  48  at the output end of the two channels to transition to the non-stable state, and the normally open legs are now closed. The delay output from the RF pulse detector  64  via the one-shot  74  now causes the RF switch  30  and  40  at the input ends of the common channel to transition to the non-stable state closing the normally open legs at the input end of each channel. With the switching elements at the output ends of the common channels in also their non-stable states, the once stored radar pulse leaves the first common channel by the terminal  52  and passes via the waveguide  54  to the input end of the second common channel. In a similar manner, the once stored radar pulse in the second common channel leaves the output end via the terminal  58  and propagates through the waveguide  60  and enters the second common channel via the terminal  62 . While each pulse is stored in the second delay unit, the one-shot  70  reverts to its normal state causing the RF switches  38  and  40  at the output end of the two channels to revert back to the normal condition so that each twice delayed pulse can leave the output end of the respective channel through the normally closed leg. Of course, the phase shift  68  disposed in the waveguide  26  introduces a phase shift in the signal propagating therethrough so that the retransmitted waveform from the antennas  10  and  12  will be precisely out of phase. 
     As will be appreciated by those of ordinary skill, in this system if both antennas receive an illuminating pulse at the same instant, the retransmission of the pulse will also occur at the same relative instant. If the illuminating radar is at an angle with respect to the common front of the two antennas  10  and  12 , then one of the antennas will receive the pulse sooner than the other by a time corresponding to the difference in the received instant but for the 180° phase shift. 
     Thus, although the invention has been shown and described with respect to the preferred embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes and omissions in the form and detail thereof may be made without departing from the spirit and scope of the invention.