Abstract:
A radar system including a main antenna and a smaller auxiliary antenna, the main antenna having a radiation pattern with a main lobe and relatively small sidelobes, and the auxiliary antenna having an omnidirectional radiation pattern. A received signal in the sidelobes of the main antenna and the corresponding received signal of the auxiliary antenna are spectrally divided by sets of bandpass filters with the signals of corresponding ones of the bandpass filters being processed to null out a signal such as a jamming signal received via a sidelobe of the main antenna pattern. The processing includes inphase and quadrature detection of a partially cancelled signal with the resulting complex error signal being applied as a modulation of the signal of the auxiliary antenna to be subtracted from the signal of the main antenna to produce the cancelled sidelobe signal.

Description:
BACKGROUND OF THE INVENTION 
     In military applications involving a radar system such as a missile tracking radar which tracks a friendly signal from a missile, an enemy jamming signal may be present outside the main lobe of the radiation pattern of the radar antenna to be received via a sidelobe of the radiation pattern which has a very much lower gain than the gain of the main lobe. An auxiliary antenna having an omnidirectional radiation pattern may be utilized to produce a second received jamming signal which can be subtracted from that received by the sidelobe of the aforementioned tracking antenna, or main antenna, to remove the effect of the jamming signal as a source of error in the tracking operation. A problem arises in that the electrical phase centers of the two antennas must necessarily be displaced with the result that, for the typical broad band jamming signal, a temporal delay exits between the propagations from the jamming source to each of the antennas, this producing a decorrelation between the waveforms and a difference in the phases of the received jamming signals. The phase differences are frequency dependent with the result that when the jamming signal from the auxiliary antenna is combined with the jamming signal received by the sidelobe of the main antenna, good cancellation of the jamming signal is obtained only over a relatively narrow spectrum of the passband of the radar system, other frequency components of the jamming signal lying within the passband of the radar system being attenuated, or cancelled, to a lesser extent. Thereby, an undesireably large amount of jamming signal power is found with the friendly signal from the missile resulting in a diminution of precision in tracking the friendly signal. 
     SUMMARY OF THE INVENTION 
     The aforementioned problem is overcome and other advantages are provided by a sidelobe canceller system for a radar employing a main radar antenna and an auxiliary radar antenna wherein, in accordance with the invention, two identical sets of bandpass filters are utilized for receiving the signal of the auxiliary antenna and the signal of the main antenna. The main antenna may have the aforementioned radiation pattern of a main lobe and relatively small sidelobes, while the auxiliary antenna has the aforementioned omnidirectional radiation pattern. The jamming component of signal appearing in each bandpass filter of the main antenna is separately cancelled by use of the auxiliary antenna signal appearing in the corresponding bandpass filter of the auxiliary antenna. Thereafter, each of the signals, including a residue of the jammer cancellation, appearing in the respective ones of the bandpass filters of the main antenna are summed together to regenerate the complete received signal of the main antenna, the received signal comprising the aforementioned friendly signal with a residual amount of uncancelled jamming signal. 
     The widths of the bandpass filters are sufficiently small such that the temporal delays between the propagation times of the jamming signals to each of the radar antennas is small compared to the response time of an individual one of the bandpass filters. Thereby, the amount of decorrelation between the waveforms of the portions of the jamming signal appearing within a bandpass filter of the main antenna and the corresponding filter of the auxiliary antenna are sufficiently small such that adequate cancelling can be accomplished with respect to that portion of the jamming signal. 
     Upon the aforementioned summing together of the signals with the residues in each of the bandpass filters of the main antenna, it is found that the resulting uncancelled residue is very much smaller than that of the friendly signal appearing in the summation of the signals; thus, the jamming signal has been adequately cancelled. 
     With respect to the cancellation of the portion of the jamming signal appearing in an individual one of the bandpass filters of the main antenna, a replica jamming signal, obtained with the aid of a reference signal from the corresponding bandpass filter of the auxiliary antenna, is subtracted from the aforementioned signal appearing in the individual one of the bandpass filters of the main antenna. The subtraction produces a cancelled signal, the adequacy of the cancelling of the cancelled signal depending on the amplitude and phase of the replica signal. The cancelled signal is detected by inphase and quadrature synchronous detectors utilizing the reference signal of the auxiliary antenna, the synchronous detection producing orthogonal vector representations of the cancelled signal relative to the reference signal which facilitate a vector rotation of the reference signal to bring it into phase with the aforementioned signal of the corresponding bandpass filter of the main antenna. The synchronously detected signals are filtered and then applied as multiplying factors to a pair of multipliers for a complex multiplication times the reference signal. The multiplication may be accomplished by amplitude modulators and alters the amplitude of the vector components to produce the vector rotation and magnitude scaling of the reference signal. The products of the pair of multipliers are summed together to produce the replica signal. The foregoing replica generation circuitry operates in the form of a feedback loop wherein the loop gain minimizes differences between the amplitude and phase of the replica and that of the signal appearing in the main antenna. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned aspects and other features of the invention are explained in the following description taken in connection with the accompanying drawings wherein: 
     FIG. 1 is a stylized pictorial representation of a tracking antenna and an auxiliary antenna including a graphical presentation of wide band and narrow band signals received by these antennas in accordance with the invention; 
     FIG. 2 is a block diagram of a system, coupled to the antennas of FIG. 1, for cancelling a jamming signal in accordance with the invention; 
     FIG. 3 is a block diagram of an alternative embodiment of the system of FIG. 2 for use with a plurality of auxiliary antennas. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is seen a main antenna  20  and an auxiliary antenna  22  positioned for receiving a friendly signal from a missile  24 . The main antenna  20 , by way of example, is shown as a radar tracking antenna which is mechanically or electronically steered to direct the main lobe  26  of its radiation pattern in the direction of the missile  24  to receive the radiation of the friendly signal therefrom. The auxiliary antenna  22  has an omnidirectional radiation pattern  27 . Also, shown is a source  28 , such as an airborne transmitter, of a jamming signal which is radiated toward both the main antenna  20  and the auxiliary antenna  22 . A set of four graphs  31 - 34  are shown in registration with each other and appended to the antennas  20  and  22  for portraying the signals received by the antennas  20  and  22 , the graphs  31  and  32  being the portrayal of a signal as viewed by a wide band filter, while the graphs  33  and  34  portray the signals as seen by a narrow band filter. 
     The side lobes  36  of the radiation pattern of the main antenna  20  produce a signal gain which is substantially smaller, for example by a factor of 40 dB (decibels), than the gain by the main lobe  26 . Thereby, the jamming signal received via the side lobe  36  is comparable in amplitude to the friendly signal received by the main lobe  26  even though the jamming signal is many times larger, for example, possibly as much as 40 dB. In contrast, the jamming signal received by the auxiliary antenna  22  is very much greater in amplitude than the friendly signal because of the uniformity of the radiation pattern. Thus, even though the friendly signal and the jamming signal propagate toward the auxiliary antenna  22  from different directions, the antenna gain from both directions is substantially the same with the result that the relative magnitudes of the jamming and friendly signals is retained. 
     The foregoing relationship between the relatively large jamming signal and the relatively small friendly signal as received by the auxiliary antenna  22  permits an accurate jamming reference signal to be extracted from the sum of the friendly and jamming signals at the output of the auxiliary antenna  22 . This reference can then be utilized for the generation of a signal which is an accurate replica of the jamming component of the summation of the friendly and jamming signals produced by the main antenna  20 . Circuitry for producing the reference and replica signals will be disclosed hereinafter with reference to FIG.  2 . An important factor in the degree of similarity between the replica signal and the jamming signal actually received by the main antenna  20  is the amount of coherence between the jamming signal received at the two antennas  20  and  22 . 
     When the jamming signals received at the antennas  20  and  22  are coherent with each other, this being the case when the times of arrival of the jamming signals are substantially equal at the two antennas  20  and  22 , a maximum correlation is obtained between the reference signal and the jamming signal received at the main antenna  20 . A decorrelation and lack of coherence result when jamming signals reach the antennas and  22  at differing times, FIG. 1 showing the situation wherein the main antenna  20  receives the jamming signal after the auxiliary antenna  20  receives the jamming signal, the time delay, τ, being shown on the graphs  31 - 34 . 
     A feature of the invention is based on the recognition that the loss in correlation depends both on the magnitude of the aforementioned time delay as well as on the bandwidth of the jamming signal. In the case of a relatively narrow bandwidth jamming signal, the amplitude of the signal varies slowly with time so that, even in the case of a maximum time delay which occurs when the source  28  is colinear with the auxiliary antenna  22  and the main antenna  20 , the amount of decorrelation is sufficiently small such that a minimal residue results from the cancellation of the jamming signal. However, for a relatively wide bandwidth jamming signal, wherein the amplitude of the signal varies rapidly with time, the jamming signal received at the auxiliary antenna  22  is substantially decorrelated from the jamming signal received through the sidelobes  36  of the main antenna  20 . With reference to the graphs  31 - 34 , the jamming component of the sum of the jamming and friendly signals received by the main antenna is presented in graph  31  and is presented in graph  32  for the auxiliary antenna  22 . The graphs  33  and  34  correspond respectively to the graphs  31  and  32  and depict the situation wherein a filter, having approximately one-tenth the bandwidth of the filter utilized for the graphs  31  and  32 , is employed for coupling a relatively narrow portion of the bandwidth of the signals passed by the antennas  20  and  22 . 
     Referring now to FIG. 2, there is seen a block diagram of a system  38  coupled between the auxiliary antenna  22  and the main antenna  20  for cancelling the jamming component present in the signal received by the main antenna  20 . Amplifiers  40  and  42  are coupled respectively to the antennas  20  and  22  for amplifying the signals received therefrom to a sufficient amplitude for operation of the system  38 . The amplifiers  40  and  42  include bandpass filters having a pass band equal to the bandwidth of the friendly signal of FIG. 2 for receiving that signal and excluding the spectral components of noise and jamming signal lying outside the bandwidth of the friendly signal. 
     In accordance with the invention, the system  38  further includes a pair of filter banks  44  and  46  coupled via the amplifiers  40  and  42  respectively to the main antenna  20  and the auxiliary antenna  22 , a set of modulators  48  with individual ones thereof being further identified by the legends A, B and C when it is desired to refer to a specific one of the modulators  48 , a set of subtractors  50  with individual ones thereof being further identified by the legends A-C, and a summer  52 . The output signal of the summer  52  on line  54  comprises the regenerated friendly signal with a minimal amount of residue of the cancelled jamming signal. By way of example, the signal on line  54  is shown being applied to a signal processor  56  for futher filtering and signature identification as is well known in the radar art, an output signal of the signal processor  56  being displayed on a display  58  for presenting data of the missile  24  of FIG.  1 . 
     The amplifiers  40  and  42  have the same pass band to enable reception of the friendly signal by both the antennas  20  and  22 . Similarly the frequency characteristics of the two filter banks  44  and  46  are the same for deriving the corresponding spectral components of the signals received by the antennas  20  and  22 . The overall pass band of the filter bank  44  is equal to the pass band of the amplifier  40  and, similarly, the overall pass band of the filter bank  46  is equal to the pass band of the amplifier  42 . The filter banks  44  and  46  comprise a set of bandpass filters  60  each of which is tuned to a specific band of frequencies, the pass band of one of the filters  60  in the filter bank  44  being contiguous in the frequency spectrum to the pass band of the adajacent filter  60 , the pass bands of each of the filters  60  in the filter bank  46  being equal to that of the corresponding filter  60  in the filter bank  44 . By way of example, ten filters  60  may be employed in the filter bank  44  with an equal number in the filter bank  46 , the pass bands of the filters  60  of the filter bank  44  being approximately equal for dividing the pass band of the amplifier  40  into ten contiguous spectral portions. For example, considering the amplifier  40  to have a pass band of one MHz (megahertz), each of the filters  60  of the filter bank  44  have pass bands of approximately 100 kHz (kilohertz). It is noted that the pass bands of the filter  60  may each be equal to 100 kHz or, alternatively, the pass bands of the filter  60  may vary slightly in geometeric fashion such that the ratio of pass band to center frequency of an individual one of the filters  60  is equal to the ratio of pass band to center frequency of each of the other filters  60 . 
     To facilitate the description, only three of the bandpass filters  60  are shown in each of the filter banks  44  and  46 . The output signals of the three filters  60  of the filter bank  44  are individually processed by signal channels identified by the legends A, B and C. Similarly, the signals from the corresponding bandpass filters  60  of the comb filter  46  are utilized as reference signals respectively for the channels A, B and C. The modulator  48 A and the subtractor  50 A form a part of channel A and similarly, the modulators  48 B and  48 C and the subtractors  50 B and  50 C respectively form parts of the channels B and C. 
     In accordance with the invention, the signals in each of the channels A, B and C are separately processed for cancelling the jamming component of the jamming signal present in the individual channels. Thus, for example, with reference to channel A, a replica signal produced by the modulator  48 A is subtracted from the bandpass filter  60  of the filter bank  44  by the subtractor  50 A to produce a cancelled signal, the cancelled signal comprising the component of the friendly signal passed by the bandpass filter  60  of the filter bank  44  plus the residue of the jammer cancellation operation. Similar comments apply to the subtraction operation of the subtractors  50 B and  50 C. Thereafter, the cancelled signals from each of the subtractors  50  are summed together by the summer  52  to produce the regenerated friendly signal on line  54 , the signal on line  54  also including residues from the jammer cancellation operation. 
     Referring also to the graphs  31 - 34  of FIG. 1, a signal of graph  31  corresponds to the signal appearing at the output of the amplifier  40  while the signal of the graph  32  appears at the output of the amplifier  42 . The signal of graph  33  appears at the output of one of the bandpass filters  60  of the filter bank  44 , for example, the filter  60  of channel A. Similarly, the signal of graph  34  appears at the output of the filter  60  of the filter bank  46  for channel A. Thus, it is seen that each of the channels of the system  38  operates with signals which are slowly varying in time because of the relatively narrow bandwidths of the filters  60 , these bandwidths being only one-tenth the pass band of the amplifiers  40  and  42  in the present example. Again, with reference to the graphs  33 - 34 , it is seen that the time delay between the times of reception of the jamming signals at the antennas  20  and  22  is sufficiently small relative to the overall waveform of the signal being processed by channel A such that the modulator  48 , by suitably scaling the vector of the reference signal of channel A and by suitably adjusting the phase angle of the vector, can produce a replica closely approximating the instantaneous amplitude and phase of the vector representing the jamming component provided by the filter bank  44  for channel A. 
     Each modulator  48  comprises a 90° phase shifter  62 , synchronous detectors  64  and  65 , low pass filters  68  and  69  each of which includes an amplifier (not shown), multipliers  72  and  73  which comprise amplitude modulators and a summer  76 . With reference to the operation of the modulator  48 A, the operation of the modulators  48 B-C being identical thereto, the synchronous detector  64  receives at one input terminal a reference signal from the filter bank  46  and at its second input terminal a cancelled signal from the subtractor  50 A. Recalling that the amplitude of the jamming signal received at the auxiliary antenna  22  in very much larger than the amplitude of the friendly signal received at the antenna  22 , the reference signal of channel A is almost wholly composed of the jamming signal and therefore enables the detector  64  to synchronously detect the inphase component of the jamming signal in the cancelled signal of the subtractor  50 A. In a similar fashion, the reference signal is shifted 90° by the phase shifter  62  and applied to the detector  65  for synchronously detecting the quadrature component of the jamming signal in the cancelled signal of the subtractor  50 A. The production of inphase and quadrature components of the vector representing the jamming signal from the subtractor  50 A enables the rotation of the vector and the scaling of the vector by simply multiplying the inphase and quadrature components by suitable multiplying factors. These mutliplying factors represent the loop gain provided by the amplifier of the filter  68  for the inphase component and the loop gain of the amplifier of the filter  69  for the quadrature component. Thus, the amplitude of the reference signal is scaled in the multiplier  72  by a factor proportional to the detected inphase component of the jamming signal provided by the detector  64 . Similarly, the amplitude of the quadrature reference signal is scaled by the multiplier  73  with a factor proportional to the amplitude of the quadrature jamming component is detected by the detector  65 . The bandwidth of low pass filters  68  and  69  is selected to produce an overall feedback bandwidth commensurate with the bandwidth of filters  60  of the filter bank  44 . 
     A graph  78  adjacent the modulator  48 A demonstrates the rotation and scaling of the reference signal vector to produce the replica signal vector by the aforementioned process of scaling the inphase and quadrature components of the reference vector. Thus, for example, considering the X axis to represent the inphase component and the Y axis to represent the quadrature component, it is seen that a scaling of the inphase value X2 to produce X2 and a scaling of the quadrature value Y2 to produce the value Y1 results in a replica which is rotated from the reference vector and also has a different magnitude from the reference vector. The inphase and quadrature components as scaled by the multiplier  72  and  73  are summed together by the summer  76  to produce the replica signal for the subtractor  50 A, the summation of the scaled components of the reference signal being in accordance with the vector summation of the graph  78 . The construction of feedback loops which are similar in operation to that of the modulator  48  may be seen in FIG. 2 of the U.S. Pat. No. 3,939,407 which issued in the name of W. J. Bickford on Feb. 17, 1976, and also in FIG. 5 of the U.S. Pat. No. 3,794,921 which issued in the name of M. G. Unkauf on Feb. 26, 1974. The use of such feedback loops in adaptive antennas is disclosed in articles by Howells and Appelbaum in the IEEE Transactions on Antennas and Propagation, September 1976 at pages 575 through 598. 
     By way of alternative embodiments it is noted that a filter bank such as the filter bank  44  having many bandpass filters  60 , for example, twenty or thirty of the bandpass filters  60  may be regarded as a spectrum analyzer wherein each filter of the spectrum analyzer produces a narrow bandwidth signal about the spectral line to which the filter is tuned. Spectrum analyzers may be synthesized by means of a digital fast Fourier transformer wherein the signals produced by an antenna and its amplifier, such as the antenna  20  and its amplifier  40 , are first converted by an analog-to-digital converter (not shown in the figures) to digital numbers representing a succession of samples of the signal taken over a predetermined interval of time, the succession of samples being applied to the aforementioned digital fast Fourier transformer to produce an equal set of numbers each of which represents the amplitude of a signal at the corresponding Fourier spectral line. The analog operation of the modulator  48  would then be replaced by the analogous digital operation to produce a cancellation of the jammer component in a manner analogous to that which has been described with reference to FIG.  2 . 
     Referring now to FIG. 3, there is shown a block diagram of a system  80  which employs the modulators  48  to produce a replica signal for cancellation of the jamming signal as was disclosed in FIGS. 1 and 2, the system  80  further incorporating two additional auxiliary antennas similar to the auxiliary antenna  22  of FIG. 1, the three auxiliary antennas each being identified in FIG. 3 by the legend  22 , to provide improved cancellation of the jamming signal component present in the signal received by the main antenna  20  of FIG.  1 . The system  80  comprises a set of filter banks such as the filter bank  46  of FIG. 2, the filter bank being identified in FIG. 3 by the legends  46 A-C, coupled respectively to the auxiliary antennas  22  by amplifiers (not shown) such as the amplifier  42  of FIG.  2 . The tracking antenna  20  is coupled by an amplifier (not shown) such as the amplifier  40  of FIG. 2 to the filter bank  44  as was previously disclosed in FIG.  2 . The signals appearing at the output terminals of the filter bank  44  are each individually processed by separate channels, three such channels A, B and C being shown in FIG. 3, in a manner similar to that disclosed previously with reference to FIG. 2 wherein the subtractors  50 A-C are utilized both in FIGS. 2 and 3 for subtracting a replica signal in each channel from the signal of the corresponding output terminal of the filter bank  44 . The summer  52 , the signal processor  56  and the display  58  operate in FIG. 3 in the same manner as disclosed previously in FIG.  2 . 
     The system  80  of FIG. 3 further incorporates a set of summers  82 , each of which is further identified by the legends A-C when it is desired to refer to a specific one of the summers  82 , which are coupled to the replica input terminal respectively of the subtractors  50 A-C in each of the three channels. Comparing FIGS. 2 and 3, it is seen that in FIG. 2 each modulator  48  is coupled directly to the subtractor  50  of a signal processing channel such as the coupling of the modulator  48 A to the subtractor  50 A in channel A. In FIG. 3, since there is an equal set of modulators  48  for each of the auxiliary antennas  22 , the signals of the corresponding modulators  48  from each of the sets of modulators  48  are first summed together by a summer  82 , and the cancelled signals produced by the corresponding subtractor  50  is coupled back to input terminals of these modulators. In order to identify the input terminals of the various modulators  48  to which the cancelled signals of the subtractors  50  are coupled, the terminals of the modulators  48  are identified by the legends  1 A-C,  2 A-C and  3 A-C wherein the numerals  1 ,  2  and  3  identify the first, second or third auxiliary antenna  22  while the letters A, B and C identify the signal processing channel for which the output signal of each of the modulators  48  is utilized in producing the replica signal. Thus, it is seen that the modulators having terminals  1 A,  2 A and  3 A are coupled to the summer  82 A, the modulators having the terminals  1 B,  2 B and  3 B are coupled to the summer  82 B, and the modulators having terminals  1 C,  2 C and  3 C are coupled to the summer  82 C. 
     In a typical installation for the system  80  of FIG. 3, the three auxiliary antennas  22  would be positioned about the main antenna  20  of FIG. 1, for example, at the locations corresponding to the vertices of an equilateral triangle centered on the main antenna  20 . It is readily appreciated that for a given location of the source  28  of FIG. 1, the delay time between the times of arrival of the jamming signal at the first, the second and the third auxiliary antenna  22  as compared to the time of arrival of the jamming signal at the main antenna  20  would differ such that a zero time delay with respect to one of the auxiliary antennas  22  would correspond to a near maximum time delay relative to another of the auxiliary antennas  22 . As a result of the combination of the signals of the three auxiliary antennas  22 , the replica signal produced in each of the channels is based on an average value of the time delay of the graphs  31 - 34  of FIG. 1 as that time delay pertains to each of the individual ones of the auxiliary antennas  22  of FIG.  3 . Accordingly, it is seen that the generation of the replica signal in FIG. 3 is based upon a correlation, such as that disclosed with reference to the graphs  31 - 34 , which is better than the correlation obtained with the maximum time delay. 
     It is understood that the above-described embodiments of the invention are illustrative only and that modifications thereof may occur to those skilled in the art. Accordingly, it is desired that this invention is not to be limited to the embodiments disclosed herein but is to be limited only as defined by the appended claims.