Abstract:
An uncorrelated clutter noise cancellation method and apparatus employing a measured ambiguity function sample for each randomly-modulated transmission pulse in a randomly-modulated pulsed Doppler radar system. The ambiguity function samples are calculated from a stored copy of the randomly-modulated transmission signal. Estimates of the uncorrelated clutter backscatter are first developed by calculating the amplitude and phase of the radar returns detected in target range and velocity cells corresponding to stationary scatterers. The stationary scatterer contribution to each target cell, computed according to the sample ambiguity function, is then subtracted to eliminate the uncorrelated noise component in the return signal for the target cell. This clutter cancellation technique does not rely on correlations between the randomly-modulated transmission signal and the clutter return signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to radar signal processing methods for enhancing target detection in clutter and, in particular, to an uncorrelated clutter return cancellation technique using sample ambiguity function measurements. 
     2. Discussion of the Related Art 
     Wideband randomly-modulated radar signal waveforms offer a number of advantages for many radar applications. A radar waveform having random modulation of a known bandwidth centered about a carrier frequency has application to conventional pulsed Doppler radar receiver structures such as pulse compressors in combination with cross-correlation circuits for correlating a reconstructed copy of the transmitted modulation with the radar return signal. 
     The radar ambiguity function is a mathematical concept that provides a measure of the “ambiguity” inherent in a radar modulation waveform with respect to the radar return signal time delay intervals (target range “cells”) and Doppler frequency shifts (target velocity “cells”) of interest. In the context of a specific target and interference environment that includes clutter interference, the “goodness criteria” of a radar modulation waveform are generally based on the radar system&#39;s capability to distinguish the differences superimposed by two or more targets on their respective reflected radar signal components. One such criterion is the “mean square departure” of the modulated waveform from its shifted self in time and frequency. This criterion can be expressed as a squared magnitude of the two-dimensional autocorrelation function that is well-known in the art as the radar ambiguity function. 
     The radar ambiguity function can be appreciated as a surface above the target range-velocity (time delay-Doppler shift) plane. The height of such a surface is a measure of the ambiguity (or interference) generated by the radar modulation waveform at a point displaced from a target&#39;s true position and true Doppler shift by an amount equal to the corresponding coordinates oh the target range-velocity plane. Conversely, the radar ambiguity function predicts the interference created at the range-velocity cell location of a desired signal by an undesired target located at a range and velocity offset equivalent to the corresponding coordinates on the target range-velocity plane. 
     The ambiguity function is commonly used by modulation waveform designers to describe the resolution and interference problems anticipated for a particular modulation waveform. The ideal ambiguity function is known in the art as the “thumbtack”, which simultaneously provides both good Doppler and good range target resolution (see, e.g. FIG.  4 A). The ambiguity function for a randomly-modulated waveform is a random function whose mean value represents the spike at the center of the “thumbtack” ambiguity surface known in the art. The mean squared value of the radar ambiguity function determines the degree to which distributed scattering surfaces (clutter) will cause interference in potential target range-velocity cells over the range-velocity plane. Refer to, for example, Barton et al., “Radar Evaluation Handbook”, Artech House, Norwood, Mass., 1991, for additional teachings of the related art. 
     The randomly-modulated radar signal waveform has attractive features for radar operation in hostile environments but introduces new and unique clutter problems. Clutter analysis has shown that the baseline random modulation waveform is severely clutter limited. Such signal clutter can be described as two components. First, the correlated clutter component results from the radar return signal at the range corresponding to the reference range gate. Secondly, the signal-induced clutter component arises because of modulation that occurs at ranges other than the correlated range gate. This second component is uncorrelated with respect to the randomly-modulated radar reference signal used in the cross-correlation detection process. 
     The first correlated clutter component is similar to the clutter signal found in range-gated airborne radars using non-random modulation and has a clutter spectrum shaped by the antenna pattern and Doppler distribution. The second uncorrelated component exhibits a flat power spectral density and appears as an increase in total detection noise level. This noise level increase appears as a pedestal on the ambiguity surface (see, e.g. FIG. 4B) at an elevation corresponding to the variance of the random signal-induced clutter noise. Thus, clutter, which can include deception and jamming signals, at Doppler frequencies separated from the target Doppler can introduce an uncorrelated noise level in the target cell much larger than the correlated signal received from targets of interest. 
     This signal-induced uncorrelated clutter noise effect does not occur in conventional non-random modulation radar designs and is a direct result of the randomly-modulated signal reflections from scatterers at ranges other than the target range. The typical application of a randomly-modulated signal radar employs bandwidths of 500 MHz or more to obtain correspondingly refined range resolution. Thus, the uncorrelated clutter noise power is seen to be much greater than the power of the first correlated clutter component, which is equivalent to the clutter levels known for non-random modulated radar systems. 
     Most conventional radar clutter rejection methods rely on the correlated properties of the received clutter signals. Correlated clutter can be suppressed by conventional Doppler-processing techniques known in the art, which exploit the correlation (power spectral density function) of the received signal. Such techniques include moving target indicator (MTI), pre-whitening and/or bandpass filters. Such filters can be made adaptive to the shape of the received signal spectrum by including means for estimating characteristics of the clutter environment and dynamically adjusting the filter characteristics accordingly. However, correlated clutter suppression techniques known in the art are ineffective in reducing the uncorrelated clutter components arising from randomly-modulated signal-induced noise. Since the uncorrelated noise has a uniformly flat Doppler spectrum (equivalent to the variance of the random modulation function), known filters that exploit the correlation or shape of the frequency response spectrum cannot reduce the level of this random signal-induced noise term. 
     The matched filter receiver has been known in the art since the 1940&#39;s. All matched filter receivers correlate the received signal with a reference copy of the transmitted signal. For instance, in U.S. Pat. No. 4,891,649, Labaar et al. disclose a method for achieving a “coherent-on-receive” RF receiver by storing a reference copy of the transmitted signal for use in a correlation receiver. Similarly, in U.S. Pat. No. 4,882,668, Hans-Peter Schmid et al. disclose an adaptive matched filter for extracting known signals from intense noise fields using a form of correlation. Neither of these basic ideas address the problem of canceling uncorrelated clutter signal because such filtering is ineffective for the reasons discussed above. 
     In U.S. Pat. No. 4,742,353, D&#39;Addio et al. disclose an adaptive filter approach to clutter cancellation. Their method estimates parameters for an assumed signal and clutter model that adjusts filter response characteristics to increase signal-to-noise ratio (SNR). The disclosed filter discriminates signals on the basis of Doppler frequency shift. Because signal-induced clutter has a uniformly flat power spectral density, Doppler filtering approaches such as this are ineffective for reducing such uncorrelated noise. 
     In U.S. Pat. No. 4,931,800, Ward discloses a compensation method for a moving target detector that removes the effects of pulse stagger during transmission. Other practitioners in the art have addressed problems related to clutter cancellation for non-random and random modulation radar systems but none have proposed a method for canceling the uncorrelated random signal-induced clutter components that are unique to randomly-modulated signal radars. Refer to U.S. Pat. Nos. 3,668,702, 3,710,387 and 3,938,145 for additional teachings representative of efforts in the art to improve clutter cancellation in pulsed Doppler radar systems. 
     There has not previously been any viable means for reducing the randomly-modulated signal-induced clutter components arising from clutter at ranges other than the correlated gate range, despite the very high levels of this noise in randomly-modulated signal radars. This unresolved problem is clearly felt in the art and is solved by this invention in the manner described below. 
     SUMMARY OF THE INVENTION 
     The present method for canceling the signal-induced clutter noise requires an estimate of the autocorrelation function of the transmitted randomly-modulated signal. The sample ambiguity function is measured for each transmission pulse by calculating the autocorrelation function for a stored copy of the transmitted randomly-modulated signal. This measured autocorrelation function is then processed using a Doppler processor (such as a Fast-Fourier-Transform (FFT) Processor) matched to the radar Doppler processor used in the radar receiver. The corresponding ambiguity function created by the Doppler processor is the exact power spectral density of the transmitted randomly-modulated signal over a selected region of the range-velocity plane. 
     The next step in this cancellation method is to estimate clutter backscatter sources by calculating the amplitude and phase of the radar signal returns occurring in detection cells that correspond to stationary scatterers. By looking for large signal peaks in the received signal Doppler filter bank, the system of this invention can identify the location of the stationary scatterers whose clutter contribution is to be canceled. The large signal peaks from the received signal Doppler filter bank are then combined with the measured modulation ambiguity function to obtain an estimate of the stationary clutter contribution to the noise in each target resolution cell. 
     Finally, the estimated clutter noise power in each cell is subtracted to produce a clutter-canceled power level in each. The clutter source signal and modulation spectral density (ambiguity function) can be combined according to a weighting algorithm based on the Maximum Likelihood Principle using a priori knowledge of antenna parameters if desired. Also, the combination can be made adaptive through the use of a suitable gradient search algorithm to minimize the residual noise level in the target cell. Both the Maximum Likelihood Principal and suitable examples of gradient search algorithms are known in the art. 
    
    
     Thus, this noise cancellation method employs a unique approach to weighting of the measured modulation matched filter response by the estimated scatterer reflectivity to form an estimate of noise interference arising from the modulation of reflecting surfaces by the randomly-modulated radar waveform. The foregoing, together with other features and advantages of this invention, will become more apparent when referring to the following specification, claims and the accompanying drawing. 
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the invention, reference is made to the following detailed description of the embodiments illustrated in the accompanying drawing, wherein: 
     FIG. 1 shows an illustrative random signal waveform from the prior art; 
     FIG. 2 shows an illustrative random signal radar receiver from the prior art; 
     FIGS. 3A and 3B present Eqns. 1-14 describing the mathematical relationships embodied in the receiver of FIG. 2; 
     FIGS. 4A-4C illustrate the radar ambiguity function on the range-velocity plane for several signal waveforms; 
     FIG. 5 illustrates the theoretical relationship between pseudo-random noise (PRN), radar sampling loss and the sampling window-noise bandwidth product; 
     FIG. 6 shows an illustrative random signal radar clutter distribution over the range-velocity plane; 
     FIG. 7 illustrates random signal radar return spectra for a variety of sinal and noise sources; 
     FIG. 8 is an illustrative embodiment of the random signal radar receiver of this invention; 
     FIG. 9 is a detailed illustrative embodiment of the random signal-induced clutter interference canceler apparatus of this invention from FIG. 8; 
     FIG. 10 presents Eqns. 15-18 describing the mathematical relationships embodied in the interference canceler apparatus of FIG. 9; 
     FIG. 11 presents Eqns. 19-20 describing the theoretical residual power and associated signal-to-noise ratio improvement for the interference canceler apparatus of FIG. 9; and 
     FIG. 12 shows the random signal-induced noise cancellation results expressed in terms of the reduction in ambiguity surface pedestal height over the noise floor as a function of the antenna beamwidth in degrees. 
     FIG. 13 illustrates the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is made to the following Table 1 for a summary of the symbol nomenclature used herein and in the drawing. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 SYMBOL NOMENCLATURE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A 
                 = 
                 Scalar Amplitude Offset Factor 
               
               
                   
                 a nm (Δ) 
                 = 
                 Sampled Reference Signal (Complex) 
               
               
                   
                 {tilde over (c)} 
                 = 
                 Clutter Scattering Function 
               
               
                   
                 c 
                 = 
                 Propagation Velocity (m/sec) 
               
               
                   
                 E{} 
                 = 
                 Expected Value 
               
               
                   
                 f n   
                 = 
                 n th  Doppler Frequency (Hz) 
               
               
                   
                 f D   
                 = 
                 Doppler Frequency Bin Width (Hz) 
               
               
                   
                 G(θ,φ) 
                 = 
                 One-way Antenna Power Gain 
               
               
                   
                 i 
                 = 
                 Scatterer Index (1,N S ) 
               
               
                   
                 j 
                 = 
                 Scatterer Index (1,N S ) 
               
               
                   
                 L 
                 = 
                 Number of Subdwell Periods Per 
               
               
                   
                   
                   
                 Coherent Processing Interval 
               
               
                   
                 m 
                 = 
                 Sample Window Index 
               
               
                   
                 m(t) 
                 = 
                 Random Modulation Function 
               
               
                   
                   
                   
                 (Complex Reference Signal Envelope) 
               
               
                   
                 M 
                 = 
                 Number of Chips Per Pulse Dwell Period 
               
               
                   
                 n 
                 = 
                 Doppler Frequency Index 
               
               
                   
                 N 
                 = 
                 Number of Pulse Per Subdwell Period 
               
               
                   
                 N S   
                 = 
                 Number of Discrete Scatterers 
               
               
                   
                 p(t) 
                 = 
                 Pulse Gate Function (0,1) 
               
               
                   
                 P C   
                 = 
                 Received Power Before Clutter Cancellation 
               
               
                   
                   
                   
                 (Watts) 
               
               
                   
                 ΔP C   
                 = 
                 Residual Clutter Power Received (Watts) 
               
               
                   
                 r nm (Δ) 
                 = 
                 Demodulated/Sampled Reference Signal 
               
               
                   
                   
                   
                 (Complex) 
               
               
                   
                 R(τ) 
                 = 
                 Range As A Function of Time Delay 
               
               
                   
                 {tilde over (s)}(τ) 
                 = 
                 Complex Scattering Function 
               
               
                   
                 s n (t) 
                 = 
                 Random Transmit Signal 
               
               
                   
                 ŝ(τ ci ) 
                 = 
                 Estimated Scattering Function 
               
               
                   
                   
                   
                 (i th  Scatterer) 
               
               
                   
                 T S   
                 = 
                 Sample Window (Seconds) 
               
               
                   
                 T 
                 = 
                 Pulse Repetition Time (PRT) (Seconds) 
               
               
                   
                 t 
                 = 
                 Time (Seconds) 
               
               
                   
                 v c   
                 = 
                 Clutter Velocity (Doppler) 
               
               
                   
                 w n   
                 = 
                 nth Weighting Factor 
               
               
                   
                 x(t) 
                 = 
                 Received Signal (Volts) 
               
               
                   
                 x n (t) 
                 = 
                 Sampled Received Signal (Volts) 
               
               
                   
                 x nm   
                 = 
                 Demodulated/Sampled Received Signal (Volts) 
               
               
                   
                 y n (Δ) 
                 = 
                 Correlation Output Signal (Volts) 
               
               
                   
                 z(Δ,F n ) 
                 = 
                 Doppler Filter Bank Response Function 
               
               
                   
                 {circumflex over (z)}(Δ t ,f t ) 
                 = 
                 Uncorrelated Clutter Estimate in Target Cell 
               
               
                   
                   
                   
                 (Δ T ,f T ) 
               
               
                   
                 y 
                 = 
                 Transmission Phase Delay (Radians) 
               
               
                   
                 δ 
                 = 
                 Demodulator Phase Delay (Radians) 
               
               
                   
                 Δ 
                 = 
                 Range Time Delay (Seconds) 
               
               
                   
                 λ 
                 = 
                 Wavelength (Meters) 
               
               
                   
                 θ 
                 = 
                 Spherical Coordinate Basis (Radians) 
               
               
                   
                 θ a   
                 = 
                 Antenna Pointing Azimuth Angle (Radians) 
               
               
                   
                 φ 
                 = 
                 Spherical Coordinate Basis (Radians) 
               
               
                   
                 σ SL   
                 = 
                 Standard Deviation of Random Signal Function 
               
               
                   
                   
                   
                 Sidelobes 
               
               
                   
                 σ c   
                 = 
                 Clutter Backscatter Coefficient 
               
               
                   
                 X(τ,f d ) 
                 = 
                 Complex Matched Filter Response Function 
               
               
                   
                 X r (Δ,f) 
                 = 
                 Stored Reference Signal Response Function 
               
               
                   
                 X R (0,0) 
                 = 
                 Reference Signal Autocorrelation Function 
               
               
                   
                 τ 
                 = 
                 Correlation Time Delay (Seconds) 
               
               
                   
                 ω c   
                 = 
                 Carrier Frequency (Radians/Second) 
               
               
                   
                   
               
             
          
         
       
     
     The Clutter-Induced Noise Problem 
     It is an object of this invention to cancel the uncorrelated clutter noise arising from the clutter modulation of a randomly-modulated radar signal by a stationary scatterer. It is another object of this invention to provide a method and apparatus for adaptively minimizing such clutter modulation noise in an adaptive pulsed Doppler radar system. It is yet another purpose of this invention to obtain large improvements in signal-to-noise ratio (SNR) through such clutter modulation noise cancellation. 
     A basic pseudorandom noise (PRN) type of random signal waveform is illustrated in FIG.  1 . This waveform is suitable for use with this invention. Within a single coherent processing interval (CPI)  13 , the random signal includes a plurality (L in number) of subdwell intervals typified by subdwell interval  14  embracing subdwell- 1 . A plurality of pulses (N in number), typified by pulse  15 , are dispersed over subdwell interval  14 . A plurality of chips (M in number), typified by chip  16 , are distributed throughout each pulse repetition interval (PRI). The M chips are shown in FIG. 1 as collected together into a single pulse  15 . However, the chips are uncorrelated (pseudorandom) and may be dispersed over each of the pulse repetition intervals (PRIs) typified by PRI  18 . 
     The illustrated waveform contains N pulses  15  distributed within each subdwell interval  14  according to the length of PRI  18 . PRI  18  may be fixed in length throughout the subdwell interval  14  or may vary according to any suitable stagger scheme known in the art. Each pulse  15  consists of M chips  16 , where the chip width is approximately equal to the reciprocal of the signal bandwidth and where M is equal to the pulse compression ratio (PCR). 
     The peak pulse power of pulse  15  is limited to a constant value over CPI  13 . Selection of pulse repetition interval  18  is governed by a trade-off between processor speed and acceptable Doppler ambiguity. The processing gain for PRN signal waveform radar systems is generally known to be equal substantially to the number of chips per pulse multiplied by the number of pulses per subdwell interval multiplied by the number of subdwell intervals per coherent processing interval, written as L*M*N. Accordingly, increased processing gain is obtained by increasing the length of CPI  13  and by increasing the number M of chips through a decrease in the chip width resulting from an increase in signal bandwidth. 
     An increase in either bandwidth or CPI duration causes an increase in signal time-bandwidth product and a corresponding increase in target resolution. For target maneuvers at 2 g&#39;s, CPI  13  is limited practically to less than 15 milliseconds (to avoid range smearing). Moreover, within such an interval, range motion compensation is necessary for time-bandwidth products in excess of, say, 125,000, because of the relationship between signal propagation velocity and relative target velocity. That is, with a 15 millisecond coherent processing interval, 2 g target motion compensation is generally required for bandwidths greater than 8 MHz. Because the apparatus of this invention is intended for operation at bandwidths above 500 MHz, the methods of this invention require some suitable motion compensation technique. 
     FIG. 2 presents an illustrative embodiment of a PRN signal receiver used in conventional pulsed Doppler radar systems. The operation and function of such a receiver can be readily understood with reference to, for example, D. K. Barton, et al., “Radar Evaluation Handbook”, Artech House, Norwood, Mass. 1991. 
     A signal is received at antenna  20  and amplified in an RF amplifier  22  for distribution to the integration and dumping circuit described below. The PRN reference signal  24  is amplified in an RF amplifier  26  and similarly presented to another such integration and dumping circuit. The amplified RF signal  28  and the amplified reference signal  30  are each presented to a pair of multipliers labelled I and Q in the manner shown. Two signals  34  and  36  from the local oscillator  32  are also presented to the two multiplier pairs in the manner shown. 
     The product of signal  28  and local oscillator output  34  is proportional to the in-phase (I) component of signal  28 . The product of signal  28  and the quadrature (90 degrees delayed) local oscillator signal  36  is proportional to the quadrature (Q) component of signal  28 . Similarly, the in-phase (I) and quadrature (Q) components of signal  30  are separated by the other multiplier pairs as shown. 
     The I and Q components of receiver signal  28  are each presented to an integrator  37 , wherein they are integrated over the sample period T S  before being dumped to a pair of analog-to-digital (A/D) converters  38  and  40 . Similarly, the I and Q components of reference signal  30  are presented to integrators  42  where they are similarly integrated and dumped to the A/D converters  44  and  46 , respectively. 
     The digital output signal  48  from A/D converter  44  is presented to a first shift register  50  and the output signal  52  from A/D converter  46  is presented to a second shift register  54 . Shift registers  50  and  54  merely introduce time delay increments to the digital in-phase and quadrature reference signals  48  and  52 , respectively. 
     The schematic representation of A/D converters  38  and  40  shows a similar delay scheme for generation of a series of I and Q received signal components delayed in time in a manner that simulates a series of range cell delays. For simplicity, the pulse compression network/ 56  is illustrated as an array of multipliers  64  and summers  68  organized according to range cell time delay. Thus, the output signals from A/D  38  include the undelayed digital in-phase component  57  and a plurality of delayed components, each delayed behind the previous one by a single range cell increment, ending with the final maximally delayed in-phase component  58 . 
     Similarly, the quadrature component of the received signal is digitized and spread from the undelayed Q component  60  through a number of incrementally delayed components, ending with the maximally delayed quadrature component  62 . 
     As can be appreciated from FIG. 2, the plurality of multipliers  64  serves to generate a series of products of the in-phase and quadrature components of the received and reference signals, representing the elements of a cross-correlation function. Thus, the undelayed reference signal in-phase component  48  is multiplied by the undelayed in-phase received signal component  57  to form the signal product  66  for the first or closest range cell. Signal  66  is presented to a summer  68 , whose output represents the in-phase correlated received signal component accumulated over the pulse repetition interval. 
     Similar multiplication and summing procedures are effected for the undelayed quadrature component  60  of the received signal and for the I and Q components of each received signal component for each of the several range cells illustrated. The resulting array of I and Q (complex) received signals are presented to an array of Doppler processors  70 . Each Doppler processor  70  performs a Fast Fourier Transform (FFT) operation on an incoming complex digital signal representing the correlated echo in a single range cell, thereby spreading the range cell over a second interval of Doppler frequency shift cells. This frequency shift or target velocity spreading operation is performed for each of the several time-delayed receiver signals (target range cells) presented by pulse compression network  56 . The final result is a two-dimensional array of complex signals representing the target echo power in each of a number of incremental target range and target velocity cells. 
     Range shift registers  50  and  54  process the complex PRN reference signal  30  to make it compatible with the complex received signal  28  at A/D converters  38  and  40 . The effect of multipliers  64  is to correlate the incoming receiver signal with the PRN reference signal to retrieve the information that was initially spread over the spectrum as pseudorandom noise. This operation is coordinated through the use of the timing control circuit  72  and a range motion compensation circuit  74 . Circuit  74  is necessary because of the time-bandwidth product of the illustrative apparatus in FIG. 2 relative to the rate of change of target range expected for the target types of interest (missiles maneuvering at accelerations up to 2 g&#39;s). Without range motion compensation  74 , the receiver in FIG. 2 would lose performance because of signal smearing over several range cells during each pulse repetition interval. 
     FIGS. 3A and 3B provide a summary of the mathematical relationships from the pulsed Doppler radar receiver art that are implemented in the receiver of FIG.  2 . Eqns. 1-14 can be appreciated with reference to the Barton et al. reference and to the Schmidt et al. and Ward patents cited hereinabove. 
     Eqns. 1-14 are related to FIG. 2 as follows. Eqn. 1 defines the random reference signal  24  in FIG.  2 . Eqns. 2-4 define the complex demodulated and sampled reference signal shown as in-phase signal  48  and quadrature signal  52 . Eqn. 5 defines the composite received signal at antenna  20  in FIG.  2 . Eqn. 6 defines the n th  clutter component of the received signal of. Eqn. 5, represented as a volume integral. Eqns. 7 and 8 define the complex demodulated and sampled received signal represented as in-phase component  57  and quadrature component  60 . Eqn. 9 describes the correlation output signal input to each Doppler processor  70  in FIG. 2, and Eqns. 10-14 describe the complex output, signals from Doppler processors  70 . 
     FIGS. 4A-4C provide three illustrative graphical views of the radar ambiguity function over the range-velocity plane for random signal radars. In FIG. 4A, the “thumbtack” ambiguity surface is illustrated. This surface is interpreted as an ideal ambiguity function, where none of the energy at the center range-velocity cell is dispersed over other cells in the range-velocity plane. The thumbtack surface represents the statistical mean of an ideal random signal waveform ambiguity function. 
     FIG. 4B illustrates the same thumbtack  76  sitting on a new pedestal  78 , which may be thought of as a smearing of the energy at thumbtack  76  over a portion of the range-velocity plane. As described above, pedestal  78  represents the variance of the random signal-induced clutter noise that is uncorrelated with the thumbtack  76  target cell. 
     FIG. 4C illustrates the effects of increasing the Pulse Repetition Frequency (PRF) on the ambiguity function. Increasing the PRF squeezes together the pedestals in the range dimension as shown. Ambiguous thumbtack peaks occur along the main range pedestal spaced at the PRF in the velocity dimension. 
     FIG. 5 presents a theoretical analysis of the decrease in Signal-to-Noise Ratio (SNR) as a function of noise bandwidth product for the PRN radar receiver of FIG.  2 . This graphical result demonstrates the limitations of performance that require Analog-to-Digital (A/D) sampling rates on the order of the reciprocal of signal bandwidth. 
     FIG. 6 provides another view of the range-velocity plane. The main beam correlated clutter pedestal  80  is shown spread over a number of range cells at velocity, v C . A thermal noise field  82  is superimposed at the top of pedestal  80 . Everything in the range-velocity plane is shown referred to an uncorrelated noise floor  84 , including the energy in the range-velocity cell labelled as target  86 . The ambiguity surface in FIG. 6 demonstrates the difficulties presented to the radar receiver designer in attempting to distinguish target  86  from uncorrelated noise floor  84  and from other random noise energies over the range-velocity plane. 
     FIG. 7 illustrates another view of the random signal radar return spectra. The main beam clutter component  88  is located at v C  and extends well above the main beam clutter-induced noise level  90 . The actual target echo component  92  is distinguished from v C  on the Doppler frequency axis but the signal energy at  92  is washed out by noise level  90 . The thermal noise level  94  and A/D quantization noise level  96  are included to demonstrate that target signal  92  can be easily detected if main beam clutter-induced noise level  90  is sufficiently reduced. 
     Interference Canceler Invention 
     The fundamental clutter-induced noise problem discussed above in connection with FIGS. 1-7 is solved by the method and apparatus of this invention. FIG. 8 shows the PRN radar receiver embodiment of FIG. 2 with an additional clutter interference canceler apparatus representing an illustrative embodiment of this invention. This canceler apparatus includes an autocorrelation processor  95  that receives inputs from each of the two range shift registers  50  and  54 . These inputs are the same in-phase and quadrature signal components  48  and  52  shown in FIG.  2 . The outputs from autocorrelation processor- 95  are distributed to a bank of Doppler processors  96  that are substantially identical in function to Doppler processors  70  in FIG. 2. A plurality of signals representing the autocorrelation function of the PRN reference signal  30  are shown as signals  98  from Doppler processors  96 . Signals  98  represent the measured signal ambiguity function and are presented to a target cell interference estimator circuit  100 . The plurality of range-velocity cell signals from Doppler processors  70  represents estimates of the radar return signal power in each of the target range and velocity cells, which are shown in FIG. 8 as the cell signals  102 . Cell signals  102  are presented to interference estimator  100  to permit the initial estimation of clutter locations in the range-velocity plane by combination with signals  98 . This location of the clutter source is achieved by the interference estimator  100  searching for large signal peaks to select at least one uncorrelated clutter source whose contribution is to be canceled. 
     Target cell interference estimator  100  provides a group of estimate signals  104  to a subtraction array  106 . Subtraction array  106  removes the estimated interference from the received signal and produces a group of suppressed interference target cell output signals  108 . Each of the signal groups  102 ,  104  and  108  represent an array of target cells arranged over the range- velocity plane. 
     FIG. 9 provides an illustrative embodiment of the clutter interference canceler apparatus of FIG.  8 . Autocorrelation processor  95  is shown having a plurality of delay lines  110 , which present in-phase and quadrature signals  48  and  52  to a plurality of multipliers  112  and therefrom to a plurality of summers  114 . Each summer output is labelled as b i  (i=1,m), representing an autocorrelation estimate for a particular range cell. Each such autocorrelation estimate is presented to an FFT network (Doppler processor  96 ), which produces a plurality of output signals spread over the Doppler velocity region of the range-velocity plane. 
     The outputs from FFTs  96  represent the measured ambiguity function surface  118 . Ambiguity function  118  is presented to a weighting and cancellation network  116 , which combines the target cell interference estimator  100  and subtraction array  106  functions discussed above in connection with FIG.  8 . Network  116  presents the suppressed interference target cell output signals  108  to the next stage of the radar system, which could be a system detection processor (not shown) for cataloging, identifying and tracking targets in any suitable manner known in the art. 
     The clutter interference canceler shown in FIG. 9 can be better appreciated with reference to Eqns. 15-18 in FIG.  10  and Eqns. 19-20 in FIG.  11 . Eqn. 15 defines the scattering noise model assumed for a plurality of clutterers, c i  (i=1,N S ). The clutter scatterers are assumed to be N S  in number. Eqn. 16 represents the Doppler processor response signals  102  (FIG. 8) to the accumulated scattering noise from Eqn. 15. Signals  102  are used to select the locations of suspected scatterers in the range-velocity plane. 
     Eqn. 17 describes the scattering noise estimate for the i th  scatterer expressed as a summation of contributions from all suspected scatterers over the index j. Eqn. 18 describes the uncorrelated clutter estimate signals  104  (FIG. 8) for the target cell (Δ T ,F t ) 
     FIG. 11 presents an approximate expression for residual clutter power following cancellation of clutter-induced noise by the methods and apparatus of this invention. Eqn. 19 shows that the estimated residual clutter power is inversely proportional to the chips per pulse and pulses per subdwell for the random signal in FIG.  1 . Eqn. 20 is estimated pre-cancellation input power expressed in terms of the clutter statistics. Eqn. 21 shows the uncorrelated clutter cancellation SNR improvement to be substantially equivalent to the main beam illumination dwell divided by the coherent processing gain. This result is illustrated graphically in FIG. 12 for a range of 2-way antenna beamwidths. 
     FIG. 12 shows that the reduction in the level of pedestal  78  (FIG. 4B) ranges from 10 dB at 30° beamwidth to over 30 dB at 2° beamwidth. The improvement is limited by the Doppler processing gain limit for the carrier frequency (f C =35 GHz) and Doppler cell width (f D =70 Hz) used in the model. 
     FIG. 13 illustrates one embodiment of the method of the invention. An estimate of the radar return signal power in each of a plurality of range and velocity cells is generated in step  200 . At least one uncorrelated clutter source is selected at step  202  by identifying a peak value of the radar return signal power. In step  204 , the radar modulation signal ambiguity function magnitude for the radar system is measured for each of a corresponding plurality of range and velocity cells. Since the relative location of the selected clutter source (step  202 ) with respect to any given range and velocity cell is know, an estimate of the signal-induced clutter modulation noise power in each of the plurality of cells can be made by combining the peak radar return signal power with the modulation signal ambiguity function magnitude in step  206 . The estimated noise power (from step  206 ) is then subtracted in step  208  from the estimate of radar return signal power (from step  200 ) thereby providing cancellation 
     It is a feature of this invention that the measured modulation power spectral density is used to estimate interference levels arising from stationary scatterers. Conventional radar waveforms are deterministic with a known frequency response. Randomly-modulated signal waveforms are not deterministic and their frequency response must be continuously measured on a dwell-to-dwell basis to effect cancellation of signal-induced interference. 
     It is an advantage of this invention that improving the SNR does not require higher radiated power or more expensive components but can be accomplished merely by additional processing procedures. It is a further advantage of this invention that it can be applied to a variety of applications requiring highly refined target range resolution and to both surface and airborne radar systems. 
     Further, in this invention, inverse filtering can be used to widen the effective bandwidth or range resolution with which stationary scatterers are identified for cancellation. This approach is applicable to large reflectors such as signal repeaters or other airborne and stationary point scatterers. 
     Obviously, other embodiments and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such obvious embodiments and modifications when viewed in conjunction with the above specification and accompanying drawing.