Abstract:
Spatial-temporal waveform diversity methods vary or modulate the far-field radiated waveform of a radar and/or communication antenna as a function of look direction (sidelobe structure). Radar detection performance and angle of arrival estimation are enhanced to deny a coherent reference to non-cooperative bistatic radars and coherent repeater jammer systems. In the most general case, the spatial-temporal modulated waveform varies as a function of angle, based upon the principles of multi-dimensional Fourier synthesis. Spatial-temporal denial is achieved with as few as two auxiliary antennas bracketing a main antenna. The same methods for spatial-temporal waveform diversity can also embed communications signals into the transmitted radar waveform for one-way simulcast of both waveform types. These same directionally dependent simulcast waveforms can incorporate navigation signals for enhanced precision engagement.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to radar applications, and, in particular, to waveform diversity methods that enhance radar performance and/or prevent unauthorized utilization of emissions that result from the application of bistatic technology to radar and communication systems. 
     Applying bistatic technology to radar can lend itself to impermissible use of sidelobe emissions through unfriendly or unauthorized detection and tracking of targets. Prior efforts at controlling radar signatures have focused on developing ultra-low sidelobe antennas. Prior-art methods attempt to defeat signal acquisition by affecting the temporal aspect of sidelobe energy. Although sidelobe masking has shown some success in defeating non-cooperative bistatic operation, sophisticated systems can still use sidelobe energy to decode and eventually detect radar transmission signals. 
     What is needed is to increase traffic in multiple access schemes (e.g., a greater opportunity to reuse frequencies in space-division multiple access) while maintaining the security aspects of a transmitted radar waveform. In communications systems the benefits of waveform diversity are similar to those in simple radar systems. Preventing the acquisition and manipulation of signals is key to maintaining communication security. 
     SUMMARY OF THE INVENTION 
     Spatial waveform manipulation is the alteration of a signal&#39;s sidelobe waveform so that it differs from that of the mainlobe signal. Unlike temporal manipulation, where a sidelobe&#39;s signature is merely obscured, spatial manipulation can change the information contained within a signal with respect to the main beam. Although spatial manipulation alone of a waveform would make it more difficult to decipher than temporal manipulation, a prominent radar sidelobe is still problematic, as its signature can still be acquired. The present invention eliminates this possibility by joint spatial-temporal modulation. 
     The present invention diversifies radar and/or communication signal signatures by modulating the spatial and temporal attributes of a transmitted waveform in the main beam differently from its sidelobes. This diversification accomplishes three objectives. First, a non-cooperative receiver intercepting sidelobe energy observes a signal that is devoid of information contained in the main beam response. Second, since multipath signals no longer cohere to the main beam signal, channel fading and scintillation are mitigated (this is also true for the radar application). Third, sidelobe suppression is enhanced, since sidelobe signals can be more readily recognized and separated from the response from the main beam. Further, angle of arrival estimates are no longer subject to extreme error from multiple paths. This result also makes the present invention useful for precision navigation (i.e., an improved global positioning system (“GPS”)). 
     The present invention allows monostatic radars and communications systems to operate while it eliminates the interception of sidelobe energy and its exploitation for non-cooperative applications, including bistatic radar and wireless communications transmission. Modem radar systems typically require 40-60 dB of sub-clutter visibility (“SCV”) for operation (Skolnik). The present invention limits the adversary&#39;s SCV on conventional targets to less than 10 dB and on weak targets to well under 10 dB. Specific calculations show that SCV can be limited, in fact, to 1.58 dB, where SCV is defined as the ratio of the signal plus interference before filtering to the signal plus interference after filtering. 
     Additionally, embedded communications (a new type of multiple access) and precision navigation signals may now be impressed upon the radiated waveform of an otherwise classically designed radar or communications system. 
     In communications systems the present invention can mitigate multi-path transmission problems, suppress sidelobes, and provide for highly accurate estimates of angle of arrival. The present invention thus enhances existing multiple access systems (e.g., Space-Division Multiple Access) by allowing greater reuse of frequencies, thereby increasing capacity for traffic. 
     Briefly stated, the present invention uses spatial-temporal waveform diversity methods to vary or modulate the far-field radiated waveform of a radar and/or communication antenna as a function of look direction (sidelobe structure). Radar detection performance and angle of arrival estimation are enhanced to deny a coherent reference to non-cooperative bistatic radars and coherent repeater jammer systems. In the most general case, the spatial-temporal modulated waveform varies as a function of angle, based upon the principles of multi-dimensional Fourier synthesis. Spatial-temporal denial is achieved with as few as two auxiliary antennas bracketing a main antenna. The same methods for spatial-temporal waveform diversity can also embed communications signals into the transmitted radar waveform for one-way simulcast of both waveform types. These same directionally dependent simulcast waveforms can incorporate navigation signals for enhanced precision engagement. 
     According to an embodiment of the invention, a method of substantially preventing the interception of information associated with radar or communication waveforms that comprise a main signal and a plurality of sidelobe signals comprises: (a) manipulating a spatial waveform to cause sidelobe signals to differ from a mainlobe signal; and (b) manipulating a temporal waveform through sidelobe signal time-domain encoding to reduce sidelobe signal signature. 
     According to a feature of the invention, a method of substantially preventing the acquisition and use of radar or communication waveforms that comprise a main signal and a plurality of sidelobe signals comprises: (a) manipulating signatures of the plurality of sidelobe signals through temporal manipulation of waveforms of the plurality of sidelobe signals, thereby causing substantial difficulty in signal acquisition; and (b) manipulating information contained within the plurality of sidelobe signals with respect to the main signal through spatial waveform manipulation, thereby causing substantial difficulty in signal deciphering. 
     According to another feature of the invention, apparatus for denying the acquisition and use of radar or communication waveforms that comprise a main signal and a plurality of sidelobe signals comprises a multichannel transmit antenna system that further comprises at least two sidelobe antenna elements carrying spatial-temporal sidelobe information to accompany the transmission of a main antenna carrying said main signal. 
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the pertinent art from the following detailed description of a preferred embodiment of the invention and the related drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a geometry within which the present invention can be implemented. 
     FIG. 2 shows a geometry for which the present invention prevents a non-cooperative receiver from intercepting sidelobe emissions from transceivers. 
     FIG. 3 shows a baseline antenna architecture for spatial-temporal denial. 
     FIG. 4 a  shows an antenna pattern when two elements are fed in phase. 
     FIG. 4 b  shows the response of the main antenna of FIG.  3 . 
     FIG. 4 c  shows the results of a phase difference (between FIGS. 4 a  and  4   c ) applied between a two-element interferometer formed by the two elements shown in FIG.  3 . 
     FIG. 4 d  shows the resultant response using waveform diversity methods. 
     FIG. 5 a  shows the transmit waveform diversity for an interferometric spatial-temporal denial antenna pair. 
     FIG. 5 b  shows the resultant time response for each element when phase modulation occurs between pulses and also within the pulse period. 
     FIG. 6 shows the result of waveform diversity for spatial-temporal denial. 
     FIG. 7 shows an alternative design that uses a multi-channel phased array with separate multi-channel waveform generation timing and control units for forming the main beam and the spatial-temporal denial sidelobe modulation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a geometry the present invention was designed to enhance. The radar geometry has a wide angular separation between a monostatic direct path  115  to a target  120  and a path  155  to a non-cooperative bistatic receiver  140 . For non-cooperative bistatic radar  140  to detect returns from target  120  in the presence of clutter, a coherent reference signal  150  is required. If reference signal  150  is modulated to be different from a main beam signal  110  transmitted from a radar transceiver  100 , then coherent signal processing, including filtering to reject clutter, cannot be achieved. Thus the large clutter residue masks all but the strongest target returns. 
     FIG. 2 illustrates a geometry for which the present invention denies non-cooperative bistatic receiver  140  the capacity to intercept sidelobe emissions from radar transceiver  100  and second radar transceiver  230 . Non-cooperative receiver  140  has a main lobe  240 . 
     The baseline antenna architecture for spatial-temporal denial is shown in FIG.  3 . In this case, a main antenna  350  is an N-element phased array bracketed by an interferometric spatial-temporal denial pair  330  and  340 . Main antenna  350  may take on characteristics other than an array. A classical waveform generation, timing, and control unit  320  drives main antenna  350 . Interferometric spatial-temporal denial pair  330 ,  340  is driven by a separate waveform generation, timing, and control unit  360 , which provides for both intra- (i.e., within the pulse) and inter-pulse phase modulation. 
     The change in antenna pattern between FIG. 4 a  and FIG. 4 c  shows what happens when a phase difference is applied between the two-element interferometer formed by interferometric spatial-temporal denial pair  330 ,  340  shown in FIG.  3 . FIG. 4 a  illustrates an antenna pattern  410  when both elements are fed in phase (e.g., on the first pulse). Note that a beam  415  is formed along and broadside to the antenna. Referring to FIG. 4 c , when interferometric spatial-temporal denial pair  330 ,  340  is fed out of phase (e.g., on the second pulse), the antenna pattern rotates and beam  415  becomes a beam  430 . When combined with a classical low-sidelobe main antenna with a pattern  420 , as shown in FIG. 4 b , there results an antenna pattern  440  as shown in FIG. 4 d . Referring again to FIG. 3, note that the gain in directivity is determined by main antenna  350  while the sidelobes are dominated by the pattern generated from interferometric spatial-temporal denial pair  330 ,  340 . The interferometric antenna pattern varies according to the phase difference between the two elements of interferometric spatial-temporal denial pair  330 ,  340 , as noted above and shown by antenna pattern  410  and beam  430  in FIGS. 4 a  and  4   b.    
     Radiating two signals from auxiliary elements at the array&#39;s endpoints generates the interferometer pattern. The time waveforms are given by: 
     
       
           s   1 ( t )= a   1 ( t )cos(ω 1   t+φ   1 ( t )) 
       
     
     
       
           s   2 ( t )= a   2 ( t )cos(ω 2   t+φ   2 ( t )) 
       
     
     In FIG. 5 a , a 1 (t)=a 2 (t), ω 1 =ω 2 , and φ 1 (t)=φ 2 (t), which produces the in-phase pattern of FIG. 4 a . When φ 1 (t)=φ 2 (t)+π, the out-of-phase pattern of FIG. 4 c  is generated. In FIG. 5 b , random modulation is illustrated for the general case where φ 1 (t)≠φ 2 (t). 
     The angular response pattern as a function of azimuth angle θ, wavelength λ, and element distance of separation d, as illustrated in FIG. 4 c , is given by: 
     
       
           R (θ)=| a   1 ( t )cos(ω 1   t+φ   1 ( t )−π d  cos(θ)/λ)+ a   2 ( t )cos(ω 2   t+φ   2 ( t )+π d  cos(θ)/λ)| 
       
     
     In the simplest design, the radiated pulse sequence of only one member of interferometric spatial-temporal denial pair  330 ,  340  (see FIG. 3) needs to be phase-modulated, with respect to the phase coding of main antenna  350 , to achieve spatial-temporal denial of radar and communications systems. This is the baseline approach. 
     When the radiated waveformns from both members of interferometric spatial-temporal denial pair  330 ,  340  are phase-modulated with respect to main antenna  350 , a further enhancement takes place. FIG. 5 a  shows the transmit waveform diversity for interferometric spatial-temporal denial antenna pair  330 ,  340  when both its elements are phase-modulated with uniform random-phase variation, taken ±30 degrees between pulses with respect to the signal reference of main antenna  350 . In FIG. 5 a  the time response of the left element ( 330 ) of interferometric spatial-temporal denial pair  330 ,  340  is shown in a top panel  500 ; the similar response from the right element ( 340 ), in a bottom panel  525 . Note that the pulse sequence in top panel  500  that drives the left element exhibits pulse-to-pulse random phase modulation. So does the pulse sequence in bottom panel  525  that drives the right element. 
     When phase modulation occurs between pulses and also within the pulse period, the resulting time response for each element is shown in FIG. 5 b . Again the time response of the left element ( 330 ) is shown in a top panel  550 ; the similar response for the right element ( 340 ), in a bottom panel  575 . Note that the pulse sequence in top panel  550  drives the left element ( 330 ), and the pulse sequence in bottom panel  575  drives the right element ( 340 ). Referring again to FIG. 3, the pulse sequences shown in FIG. 5 b  exhibit both intra- and inter-pulse modulation on a pulse-to-pulse basis with reference to main antenna  350 . 
     The result of this waveform diversity for spatial-temporal denial is shown in FIG.  6 . For illustration a 10-element phased-array main antenna is assumed. The element spacing is taken to be a half wavelength. A distance of six (6) wavelengths bracketing the main antenna separates the elements of interferometric spatial-temporal denial pair  330 ,  340 . A top panel  600  shows the normalized amplitude vs. pulse number. A solid line  610  is the time response observed at main beam signal  110  (broadside) when the two elements of interferometric spatial-temporal denial pair  330 ,  340  are driven with uniform random inter-pulse phase modulation taken between ±30 degrees. A dotted line  620  in top panel  600  is the subsequent time response of the antenna sidelobe at 30 degrees off boresight. Note that the main beam amplitude stays within ±5 percent of the maximum amplitude. A bottom panel  625  shows the phase in degrees against the pulse number. A solid line  630  in bottom panel  625  is the time response of the main beam when the two elements of interferometric spatial-temporal denial pair  330 ,  340  are driven with uniform random inter-pulse phase modulation between ±30 degrees. A dotted line  640  in bottom panel  625  shows the subsequent time response of the antenna sidelobe at 30 degrees off boresight. Note that the main beam response of solid line  630  shows negligible phase variation while the phase variation in the sidelobe response of dotted line  640  is extreme. Phase mismatch of the latter varies between −40 and +20 degrees. 
     Referring again to FIG. 1, this phase mismatch between pulses determines a non-coherent signal behavior, to be observed in the sidelobe of monostatic radar transceiver  100  compared to main beam signal  110 . Since non-cooperative bistatic receiver  140  in FIG. 1 must coherently process returns from target  120 , a reference signal identical to monostatic main beam signal  110  is required. However, the operation of the present invention has essentially destroyed the ability of non-cooperative bistatic receiver  140  to perform. The present invention has limited sub-clutter visibility to less than 5 dB, where typically a sub-clutter visibility (“SCV”) of 40-60 dB is required to detect a target. SCV is a function of the cancellation ratio (“CR”), which determines the degree to which unwanted radar clutter returns can be suppressed. In terms of phase and amplitude error, this function is: 
     
       
           CR= (δπ/180) 2 +(10 A/20 −1) 2   
       
     
     where δ=phase error in degrees and A=amplitude error in dB. CR is expressed in dB by CR dB =10 log 10 (CR). A phase error of 32.5 degrees, as shown in FIG. 6, introduced by the present invention, limits non-cooperative bistatic receiver  140  to less than 5 dB SCV. The bistatic receiver will achieve 40 dB SCV only with δ&lt;0.57 deg. and A&lt;0.086 dB and 60 dB SCV only with δ&lt;0.057 deg. and A&lt;0.0086 dB. To achieve high SCV levels for target detection in the presence of strong radar clutter requires either knowing the varying antenna pattern or knowing a priori the coherent reference function. 
     FIG. 7 shows an alternative design that employs a multi-channel phased array  700  with separate multi-channel waveform generation timing and control units to form a main beam  730  and a spatial-temporal denial sidelobe modulation  720 . These two signals are summed into a signal  710  and radiated for radar, communications, or precision navigation applications. Additionally, the duty cycle of the spatial-temporal denial waveform may be selected to be higher (up to 100%) in order to prevent non-cooperative bistatic receiver  140  (see FIG. 1) from cohering to dominant main beam  110  scatterers in place of a strong coherent reference signal  150 . 
     Clearly many modifications and variations of the present invention are possible in light of the above teachings. It should therefore be understood that, within the scope of the inventive concept, the invention may be practiced otherwise than as specifically claimed.