CW Radar Range Measuring System With Improved Range Resolution

This invention relates generally to continuous wave (CW) radar range measuring systems and more particularly to systems of such type wherein a transmitted CW signal is modulated in accordance with a known code and target radar returns are correlated with differently time delayed replica of the code to determine target range.

As is known in the art, CW radars have been used to accurately, and unambiguously measure the velocity of a target by detecting a Doppler frequency shift in radar returns from the target. In order to enable such CW radar system to determine the range to the target, the transmitted carrier signal frequency, or phase is modulated in some known fashion to determine target range. This technique is described on pages 86 through 111 of a book entitled "Introduction to Radar Systems" by Merrill I. Skolnik, published 1962 by McGraw-Hill Book Company, Inc.

One typical application for such CW radar range measuring system is in missile fuzing systems. In one such fuzing system, the phase of the transmitted CW signal is changed by 180 degrees, or not changed, selectively in accordance with a binary, pseudo-random noise code. Thus, a code of for example, N bits, is stored in a recirculating shift register memory. The bits are read out at a predetermined rate, f.sub.s, and fed to a bi-phase modulator along with the CW signal. The phase of the CW signal changes 180 degrees in response to a logic 1 bit and remains unchanged in response to a logic 0 bit, for example. The coded CW signal is transmitted through a transmit antenna. Radar returns from a target are received by a receive antenna. The received CW returns will have the same code as the transmitted code; however, the received code pattern will be time delayed with respect to the transmitted code an amount related to the target's range. Target radar returns are correlated with differently time delayed replica of the code to determine target range. Such technique is described on pages 10.19-10.25 in "Radar Handbook-Second Edition", edited by Merrill I. Skolnik, published by McGraw-Hill Publishing Company, 1990. Further, the frequency of the returns will be shifted by the targets' velocity (i.e., Doppler frequency of the target).

One system used to determine the target's range separates the received signals into a plurality of channels. The signal in each channel is fed to a correlator where the signal is multiplied with the code (albeit that the code fed to each channel has a different predetermined time delay relative to the transmitted code) and integrated. Thus, each channel corresponds to a range channel. In the absence of noise and/or clutter, the one of the channels having the greatest received power indicates the range to the target. (That is, the received code will "correlate" with the transmitted code in the range channel corresponding to the target's range.) Further, the frequency component in the frequency spectrum for such range channel having the greatest power indicates the target Doppler velocity (i.e., Doppler frequency, f.sub.D).

While such system may provide adequate target range measurement accuracy in some application, amplitude and phase mismatches among the channels may adversely effect such system's performance in other applications.

As is also known in the art, with such system, the bits of the code is transmitted at a bit rate, f.sub.s, and therefore the target range is resolved to a range resolution cell of 1/f.sub.s. While such range resolution may be adequate in some applications, in other applications greater range cell resolutions or accuracies are required.

In accordance with the present invention, a continuous wave (CW) radar range measuring system is provided. The system includes a transmitter for transmitting a continuous wave radar signal phase encoded in accordance with a transmit code at a rate, f.sub.s, and a receiver adapted to receive a radar return from the transmitted radar signal. The receiver includes a receive code generator for storing a replica of the transmit code and for producing differently time delayed replica of such transmit code, such differently time delayed replica being produced a predetermined time after the phased encoded radar signal is transmitted. A digital correlator/integrator is fed by the radar return and the receive code produced by the receive code generator, for providing an indication of radar return energy as a function of range from the transmitter.

With such an arrangement, target range may be determined to an accuracy less than that corresponding to a time delay of 1/f.sub.s, the rate the at which the phase of the transmitted signal is phase encoded.

In a preferred embodiment of the invention, the transmitter transmitting a continuous wave radar signal phase encoded at a predetermined rate, f.sub.s. The rate f.sub.s is produced by providing a clock pulse source adapted to generate clock pulses at a rate Pf.sub.s and dividing the rate produced by the clock pulse source to a rate f.sub.s where P is an integer greater than 1. The receiver includes a time delay circuit, fed by a digital word representative of a desired time delay, .DELTA., and the clock pulses produced by the clock pulse source, for producing at an output thereof the clock pulses produced by the clock pulse generator delayed in time by the desired time delay, .DELTA.. The desired time delay, .DELTA., is equal to S/(Pf.sub.s), where S is an integer less than (P-1). The receiver includes a receive code generator for storing a replica of the transmit code and for producing differently time delayed replica of such transmit code in response to the clock pulses produced at the output of the time delay circuit. An analog to digital converter section is provided, responsive to the clock pulses produced at the output of the time delay circuit, for converting a sequence of samples of the radar return into a corresponding sequence of digital words at the rate f.sub.s. A digital correlator/integrator is fed by the digital words and the receive code produced by the receive code generator, for providing an indication of radar return energy as a function of range from the transmitter. The system includes a transmitter adapted to transmit a continuous wave radar signal phase encoded in accordance with a code; and, a receiver adapted to receive a target reflected radar return from the transmitted radar signal.

Other embodiments of the invention, as well as the invention itself, will become more readily apparent with reference to the following detailed description taken together with the following drawings, in which:

FIG. 1 is a block diagram of a CW radar range measuring system according to the invention; and,

FIGS. 2A-2D show correlation function as a function of range for the system of FIG. 1, FIG. 2A showing such function in the case of a radar return from an ideal target and without any accompanying clutter, FIG. 2B showing such function in the case of a radar return from clutter and without any target return, FIG. 2C showing such function in the case of a radar return from a target return and with accompanying clutter, and FIG. 2D showing such function in the case of a radar return from a target return and with accompanying clutter after correcting for determined clutter residual effects on the correlation process;

FIG. 3 is a block diagram of a CW radar range measuring system according to another embodiment of the invention;

FIGS. 4A and 4B are TABLES useful in understanding the operation of a time multiplexor used in the system of FIG. 3, FIG. 4A showing data at input ports of the time multiplexor as a function of time and FIG. 4B showing data at an output port of such time multiplexor as a function of time;

FIG. 5 is a TABLE useful in understanding the operation of multipliers used in an correlation/integration section of the system of FIG. 3; and

FIG. 6 is a block diagram of the CW radar range measuring system of FIG. 1 or FIG. 2 modified according to another embodiment of the invention to provide increased range resolution.

Referring now to FIG. 1, a CW radar range measuring system 10 is shown to include a transmitter 12 and a receiver 14. The transmitter 12 is adapted to transmit a continuous wave radar signal phase encoded at a rate, f.sub.s, in accordance with a code. The code is here a sequentially repeating N-bit binary, pseudo-random noise code stored in a recirculating shift register 16, as shown.

The receiver 14 is adapted to receive a target reflected radar return from the transmitted radar signal and includes: (a) a heterodyning section 18 for translating the radar returns to a baseband frequency; (b) an analog to digital converter section 20 for converting samples of the radar return into a series of corresponding digital words at the rate, f.sub.s ; (c) a digital correlator/integrator and frequency spectrum analyzer section 22 fed by the series of digital words and the differently time delayed replica of the successively repeating N-bit binary code for producing an output representative of energy in the radar return as function of range from the transmitter 12 for each of a plurality of frequencies; and (d) a processor 24 fed by the output of the digital correlator/integrator and frequency spectrum analyzer section 22 for indicating target range and Doppler frequency.

Thus, referring in more detail to FIG. 1, a radio frequency (RF) carrier source 30 producing a signal having a frequency, f.sub.RF, and an ultra-high frequency (UHF) reference source 32 producing a signal having a frequency, f.sub.UHF, are fed via 2:1 power dividers 34, 36, respectively, to mixer 38 to thereby produce a CW signal on line 40 having a carrier frequency f.sub.RF +f.sub.UHF. The CW signal on line 40 is then bi-phase modulated in bi-phase modulator 42 with the pseudo-random noise code stored in the recirculating shift register 16. The bi-phase modulated signal is amplified in amplifier 44 and transmitted via transmit antenna 46. Radar returns are fed via receive antenna 48 and filter/limiter-amplifier 50 to the heterodyning section 18. More particularly, the signal produced at the output of filter/limiter-amplifier 50 is fed to a mixer 52 along with an amplified portion of the RF carrier produced by source 30 after passing through amplifier 31, as indicated. The output of mixer 52 is fed to an anti-aliasing filter 54, here a single, Gaussian bandpass Surface Acoustic Wave (SAW) filter. The output of the anti-aliasing filter 54 is fed to quadrature mixer section 56 along with in-phase and quadrature components of the UHF carrier source 32 produced signal produced by quadrature modulator 57, thereby providing in-phase (I) and quadrature (Q) signals on lines 58, 59, respectively, as shown.

The in-phase and quadrature signals on line 58, 59 are fed to an analog to digital converter section 20, as shown. Thus, the in-phase signal on line 58 is fed to analog to digital converter (A/D) 60(I) and the quadrature signal is fed to A/D converter 60(Q). The A/D converters 60(I), 60(Q) convert samples of the baseband signals fed thereto on lines 58, 59, respectively, into corresponding digital words at the rate, f.sub.s. It is here noted that the anti-aliasing function of filter 54 is required in a sampled data system as the sampling function (like a sub-harmonic mixer) makes frequencies at multiples of the sample rate, f.sub.a, ambiguous. Furthermore, frequencies above one-half the sample frequency, f.sub.s, are ambiguous by the Nyquist criteria. Therefore, the anti-alias filter 54 rejects signal content above one-half the sample frequency (i.e., the frequency, f.sub.s /2) sufficiently to result in unambiguous resolution of the desired target Doppler frequency band signal content at the correlator/integrator and frequency spectrum analyzer section 22. The filter 54 is realized in SAW technology at UHF to avoid a requirement to implement two video low-pass filters, one in each of the I and Q mixer 61, 63 outputs, whose gain and phase mismatches would contribute to time sidelobes in the correlation function. In addition, the SAW filter 54 is smaller than its discrete component, video counterpart and provides more easily repeatable, controlled performance. The filter parameters, especially the impulse response, are chosen to provide a close match to the Gaussian filter, because the filter function modifies the realized auto-correlation function--essentially smearing the auto-correlation function according to the rise and decay of the impulse response. Therefore, it is very undesirable to have an impulse response which has slow or un-symmetric rise and fall times, or have ripple (decaying oscillation). The Gaussian filter 54 is chosen because of its quick decay and nearly symmetric impulse response, which maintains the symmetry of the auto-correlation response of the system 10, and minimizes the smearing of the response so that acceptable sidelobe levels and out-of-range rejection are achieved at ranges two range channels or more beyond the range channel of interest. Here, as will be described in connection with FIGS. 2A though 2D, here one out-of-range rejection channel is used, i.e., the maximum expected target range channel corresponding to a maximum expected target range R.sub.MAX.

The A/D converters 60(I), 60(Q) in the in-phase and quadrature channels 61(I), 61(Q), respectively, sample synchronously at the bit rate, f.sub.s, of the pseudo-random noise code sequence in order to correlate the received signal digitally. More particularly, a correlator/integrator section 63 is provided having a plurality of, here N, correlator/integrator 62.sub.1 -62.sub.N for the in-phase channel 61(I) and a plurality of, here N, correlator/integrator 66.sub.1 -66.sub.N for the quadrature channel 61(Q). Each one of the correlator/integrators 62.sub.1 -62.sub.N in the in-phase channel 61(I) and each one of the correlator/integrators 66.sub.1 -66.sub.N in the quadrature channel 61(Q) are fed by A/D converters 60(I), 60(Q), respectively, and a different time delayed replica of the N-bit binary code on line 71.sub.1 -71.sub.N, respectively. Each time delay corresponds to a different range channel from the transmitter 12 thus providing, for the in-phase channel 61(I) and the quadrature channel 61(Q), a pluarilty of, here N, range channels 67.sub.1 -67.sub.N. Each range channel output producing an output representative of the amount of energy in the radar returns having encoded with the transmitted code.

More particularly, with N-stage recirculating shift resister 16 storing in each one of the N stages C.sub.1 -C.sub.N thereof a corresponding one of the N-bit binary code, it follows that each one of the N stages C.sub.1 -C.sub.N will produce at its output a replica of the code; the nth stage, C.sub.n, producing its output replica delayed in time by 1/f.sub.s from the preceding (n-1)th stage, Cn-1, where n is an integer 2 through N+1. The differently time delayed replica codes produced at stages C.sub.1 -C.sub.N of the recirculating shift register 18 are fed, via a delay adjustment circuit 69 to in-phase and quadrature channel multiplier pairs 64.sub.1, 66.sub.1 through 64.sub.N, 66.sub.N, respectively, as shown, on lines 70.sub.1 through 70.sub.N, respectively. A time delay adjustment circuit 69 introduces a fixed time delay to each of the time delated code replica passing thereto from the outputs of stages C.sub.1 -C.sub.N to account for any signal delays in the receiver 14 prior to the multipliers. Thus, the binary code on line 71.sub.n is delayed in time 1/f.sub.s from the binary code on line 71.sub.n-1, where, as noted above, n is an integer from 2 through N+1. It follows then that the digital words (i.e, the phase code in target returns) are multiplied by (i.e., correlated with) differently time delayed replica of the transmitted code.

Thus, as noted above, each A/D converter 60(I) output is then digitally passed to digital multipliers 64.sub.1 -64.sub.N ; the digital multipliers 64.sub.1 -64.sub.N being disposed in range channels 67.sub.1 -67.sub.N, respectively, of the in-phase channel 61(I). Likewise, for the quaradrature signals in quadrature channel 61(Q), each A/D converter 60(Q) output is digitally passed to digital multipliers 66.sub.1 -66.sub.N ; digital multipliers 66.sub.1 -66.sub.N being disposed in range channels 71.sub.1 -71.sub.N, respectively. Here, there are N range channels, i.e., one for each of the N bits of the pseudo-random noise code. However, fewer than N range channels may be required. The multiplication of the A/D converters 60(I), 60(Q) sample (i.e., digital word) sequence by the reference pseudo-random code in each channel is simply a process of "multiplying" the sign bit of each digital word produced by the A/D converters 60(I), 60(Q) by the appropriate pseudo-random noise code sequence of binary 1 (for a plus one bit) or 0 (for a -1 bit). Here, the multiplication is realized by passing the sign bit of the A/D converter 60(I), 60(Q) produced digital words and code on lines 71.sub.1 -71.sub.N produced by the recirculating shift register memory 16 through an exclusive-OR gate. That is, each digital multiplier 64.sub.1 -64.sub.N, 66.sub.1 -66.sub.N is here an exclusive-OR gate and the TABLE below illustrates the equivalence of this process to analog multiplication:

The output of each pair of in-phase and quadrature multiplier 64.sub.1, 66.sub.1 through 64.sub.N, 66.sub.N is fed to a corresponding pair of digital low-pass filters 70.sub.1, 72.sub.1 through 70.sub.N, 72.sub.N, respectively, as shown. Here, each of the digital filters 70.sub.1, 72.sub.1 through 70.sub.N, 72.sub.N includes an accumulator (ACCUM) and register (REG). The accumulators (ACCUM) are reset after the N-bit binary code has recirculated through the recirculating shift register 16. Thus, the accumulators (ACCUM) are all are reset after Nf.sub.s. Further, just prior to being reset, the accumulator (ACCUM) contents are stored, or loaded (LD), in the register (REG). The reset and load (LD) signals are produced on line 73 by feeding the shift register 16 clock signal of frequency f.sub.s, through divide by N frequency divider 75. Thus, pulses are produced on line 73 at a rate, f.sub.s /N. The filters 70.sub.1, 72.sub.1, through 70.sub.N, 72.sub.N perform the initial coherent integration and lower the data bandwidth input to a frequency spectrum analyzer section 76 here having a plurality of M point, here M is 128, Fast Fourier Transformers (FFTs) 80.sub.1 -80.sub.N, one for each one of the here, N, range channels 67.sub.1 -67.sub.N. This step is taken to reduce the FFT size required to achieve the final desired target Doppler frequency resolution. (The FFT processing time is directly proportional to the number of additions and multiples required, and the maximum clock rate is limited. Therefore, if continuous processing is to be achieved, the FFT must be performed within the time required to take the number of signal samples required to perform the FFT--otherwise, the amount of FFT hardware must be proliferated, or time gaps in target observation occur. In fact, it is desired to process all range channel FFTs in a single FFT application specific integrated circuit (ASIC).) FFTs are performed for each range channel 67.sub.1 -67.sub.N.

The output of the correlator/integrator section 63 thus provides an auto-correlation between the target radar return and each of a different time delayed replica of the pseudo-random code stored in recirculating shift register 16. Thus, the target radar return has been correlated for each of N ranges from the transmitter 12. If the radar return were only for a target at range R.sub.n (i.e., a radar return having no clutter or noise), the resultant auto-correlation would appear as shown in FIG. 2A. It is noted that range channel 67.sub.n would have a peak at +N and all other range channels 67.sub.1 -67.sub.n-1, 67.sub.n+1 -67.sub.N would have an auto-correlation residue of -1. It is also noted that the maximum range expected for a target is at a range R.sub.MAX, which is a range less that corresponding to the greatest range channel 67.sub.N. If, on the other hand, no target is within the expected range (R.sub.1 -R.sub.N-1 <R.sub.MAX =R.sub.N) but there is clutter, or noise, in ranges outside of the expected target range (i.e., ranges >R.sub.MAX), the out-of-range channels, here channel 67.sub.N, will produce auto-correlation function such as that shown in FIG. 2B. That is, the peak auto-correlation function will be KN and will have a residue -K, as shown, where K is the ratio of the clutter radar return voltage to the target radar return voltage and may be a complex number. In a more realistic condition, the radar return will include both target return and clutter return, as shown in FIG. 2C. It is noted in FIG. 2C that the clutter return greatly suppresses the target auto-correlation function peak (now (N-K)) at range Rn. It is also noted that the auto-correlation residue will be (-K-1)=-(K+1) at a range outside the expected target range, here at the maximum target expected range, R.sub.MAX, i.e., here a range corresponding to range channel 67.sub.N. Thus, here the Nth range channel, 67.sub.N, R.sub.MAX =R.sub.N, is used to provide a measure of the residue (-K-1). As will be described this Out-Of-Range residue (-K-1) will be subtracted from the auto-correlation functions produced in range channels 67.sub.1 -67.sub.N, thereby resulting in the auto-correlation function shown in FIG. 2D. In FIG. 2D it is noted that within the expected range of the target, i.e., at ranges less than R.sub.MAX, the peak target auto-correlation function will be (N-K)+(K+1)=N+1, as indicated, and therefore at a level more readily detectable than that shown in FIG. 2C.

Referring again to FIG. 1, the output of the correlator/integrator section 63 is fed to the frequency spectrum analyzer section 76. The frequency spectrum analyzer section 76 includes, as noted above, a plurality of, here N, frequency spectrum analyzers, here FFTs 80.sub.1 -80.sub.N. Each one of the FFTs 80.sub.1 -80.sub.N is fed by a corresponding pair of the correlator/integrator 62.sub.1, 66.sub.1 through 62.sub.N, 66.sub.N in the in-phase and quadrature channels 61(I), 61(Q), respectively, to determine the frequency spectrum of the radar return in a corresponding one of the plurality of range channels 67.sub.1 -67.sub.N, respectively.

More particularly, the outputs of each in-phase and quadrature phase filter pair 70.sub.1, 72.sub.1 through 70.sub.N, 72.sub.N are fed to a corresponding one of N, M-point FFT 80.sub.1 -80.sub.N, respectively, as shown. Thus, as noted above, the frequency spectrum section 76 includes a plurality of FFTs 80.sub.1 -80.sub.N ; one for each range channel 67.sub.1 -67.sub.N. Further, as noted above, the pair of filters 70.sub.1, 72.sub.1 through 70.sub.N, 72.sub.N are fed to a corresponding one of the FFTs 80.sub.1 -80.sub.N, respectively, as shown. Here, as also noted above, each FFT 80.sub.1 -80.sub.N is an M point FFT, where here M is 128. Thus, each one of the FFTs 80.sub.1 -80.sub.N produces a frequency spectrum of the radar return at a corresponding one of the N range channels 67.sub.1 -67.sub.N (i.e., ranges R.sub.1 -R.sub.N, respectively). The M frequency components, f.sub.1 -f.sub.N, are serially produced by each one of the N FFTs 80.sub.1 -80.sub.N at the bit rate f.sub.s, and are stored in a memory 82 in processor 24 as a Range-Doppler frequency map, or matrix, as shown. The processor 24 analyzes the Range-Doppler matrix to determine target range and Doppler frequency. For example, in the absence of clutter or received noise, the frequency component having the maximum energy level, here theoretically N, indicates that the target has such frequency component as its Doppler frequency and the range channel with such frequency component indicates the range to the target, as described above in connection with FIG. 2A. In order to enhance the signal from the target in the presence of noise or clutter (FIG. 2C), clutter suppression techniques, such as those described in the above and in the above referenced patent application may be used. In such case, an additional range channel, i.e., a so-called "image", or out-of-range, channel, here range channel 67.sub.N) is provided and the frequency components produced in the FFT 80.sub.N used in such range channel 67.sub.N are subtracted from corresponding frequency components produced by each of the FFTs 80.sub.1 -80.sub.N. That is, the clutter cancellation is implemented digitally at each of the FFT 80.sub.1 -80.sub.N output by coherently subtracting an "image" range channel FFT 80.sub.N from each range channel FFT 80.sub.1 -80.sub.N-1 output on a frequency component by frequency component, for those frequency components which cover the target Doppler frequency uncertainty. That is the Range-Doppler frequency map in memory 82 will be modified by the frequency spectrum in range R.sub.N =R.sub.MAX so that, for example, the energy at the frequency component f.sub.1 at range R.sub.1 will have subtracted from it the auto-correlation function residue energy (K+1) at the corresponding frequency component f.sub.1 at range R.sub.N, as described above in connection with FIG. 2D. In like manner, the energy at the frequency component f.sub.1 for each of the range R.sub.2-R.sub.N-1 will have subtracted from it, the energy at the corresponding frequency component f.sub.1 at range R.sub.N. Thus, in the general case, for the mth frequency component at range R.sub.n, i.e., f.sub.m,Rn, the FFT 80.sub.n will have subtracted from the frequency component f.sub.m produced by it the mth frequency component produced by FFT 80.sub.N, i.e., f.sub.m,RN. That is the processor 24 will have stored in Range-Doppler frequency cell f'.sub.m,RN, f.sub.m,Rn -f.sub.m,RN, where m=1 to M and n=1 to (N-1). The process is performed for each Doppler frequency component; however, it is unnecessary to perform this clutter cancellation on Doppler frequencies outside the expected target Doppler frequency band. The resultant range channel FFTs 80.sub.1 -80.sub.N-1 as well as the Nth range channel FFT 80.sub.N are passed to the processor 24 where constant false alarm rate (CFAR), target detection, electromagnetic counter-measure (ECM) and clutter avoidance algorithms may be performed.

Referring now to FIG. 3, a CW radar range measuring system 10' is shown, like elements to those in the system 10 shown in FIG. 1 having like numerical designation. Thus, the system 10' in FIG. 3 includes the transmitter 12 and a receiver 14'. The transmitter 12 is, as noted above in connection with FIG. 1, adapted to transmit a continuous wave radar signal phase encoded at a rate, f.sub.s, in accordance with a code. The code is here a sequentially repeating N-bit binary, pseudo-random noise code stored in a recirculating shift register 16, as shown.

The receiver 14', like receiver 14 in FIG. 1, is adapted to receive a target reflected radar return from the transmitted radar signal and includes: (a) a heterodyning section 18 for translating the radar returns to a baseband frequency; and, (b) an analog to digital converter section 20 for converting samples of the radar return into a series of corresponding digital words at the rate, f.sub.s ; (c) a digital correlator/integrator and frequency spectrum analyzer section 22' fed by the series of digital words and the differently time delayed replica of the successively repeating N-bit binary code for producing an output representative of energy in the radar return as function of range from the transmitter 12 for each of a plurality of frequencies; and (d) a processor 24 fed by the output of the digital correlator/integrator and frequency spectrum analyzer section 22' for indicating target range and Doppler frequency. Here, however, instead of the digital correlator/integrator section 63' being fed by the digital words produced by A/D converter section 20 and the frequency analyzer section 76' being coupled between the digital correlator/integrator section 63 and the processor 24, as in the system 10 of FIG. 1, here in system 10', the frequency analyzer section 76' is fed by the digital words produced by A/D converter section 20 and the digital correlator/integrator section 63' is coupled between the frequency analyzer section 76' and the processor 24.

Thus, referring in more detail to FIG. 3, the radio frequency (RF) carrier source 30 produces a signal having a frequency, f.sub.RF and the ultra-high frequency (UHF) reference source 32 producing a signal having a frequency, f.sub.UHF are fed via 2:1 power dividers 34, 36, respectively, to mixer 38 to thereby produce a CW signal on line 40 having a carrier frequency f.sub.RF +f.sub.UHF. The CW signal on line 40 is then bi-phase modulated in bi-phase modulator 42 with the pseudo-random noise code stored in the recirculating shift register 16. The bi-phase modulated signal is amplified in amplifier 44 and transmitted via transmit antenna 46. Radar returns are fed via receive antenna 48 and filter/limiter-amplifier 50 to the heterodyning section 18. The output of mixer 52 is fed to the anti-aliasing filter 54, here a single, Gaussian bandpass Surface Acoustic Wave (SAW) filter. The output of the anti-aliasing filter 54 is fed to quadrature mixer section 56 along with in-phase and quadrature components of the UHF carrier source 32 produced signal produced by quadrature modulator 57, thereby providing in-phase (I) and quadrature (Q) signals on lines 58, 59, respectively, as shown.

The in-phase and quadrature signals on line 58, 59 are fed to an analog to digital converter section 20, as shown. The A/D converters 60(I), 60(Q) convert samples of the baseband signals fed thereto on lines 58, 59, respectively, into corresponding digital words at the rate, f.sub.s.

A time multiplexor section 90 is fed by the series of digital words produced by the A/D converter section 20 for, during each of a series of periods, T=N/f.sub.s, coupling each nth one of N sequentially converted samples to a corresponding nth one of N channels 67'.sub.1 -67'.sub.N at the rate, 1/f.sub.s. More particularly, the time multiplexor section 90 includes a time multiplexor 92 in the in-phase channel 61(I) and a time multiplexor 94 in the quadrature channel 61(Q), as shown. Each one of the time multiplexor is fed the digital words produced by a corresponding one of the A/D converters 60(I), 60(Q), respectively, as shown. Each one of the time multiplexors 92, 94 has N output ports, 96.sub.1 -96.sub.N, 98.sub.1 -98.sub.N, respectively, as shown. The two time multiplexors 90, 92 operate synchronously with each other and are fed with the same bit rate signal, f.sub.s, as indicated. Thus, as N digital words, S.sub.1 -S.sub.N produced by A/D converter 60(I) are sequentially fed to the input of time multiplexor 92, they appear at output ports 96.sub.1 -96.sub.N sequentially (i.e., the digital words S.sub.1 -S.sub.N appear at output ports 96.sub.1 -96.sub.N, respectively, at the rate, f.sub.s. As the next N digital words produced by A/D converter 60(I) are sequentially fed to the input of time multiplexor 92, they again appear at output ports 96.sub.1 -96.sub.N sequentially (i.e., S.sub.N+1 -S.sub.2N). Thus, digital words S.sub.n, S.sub.n+N, S.sub.n+2N, . . . S.sub.n+(M)N, appear sequentially at output port 96.sub.n of multiplexor 92, where M is here 128, as noted above, and n is an integer from 1 to 256. More particularly, and considering an exemplary one of the output ports, here output port 96.sub.1, if the digital words sequentially produced by the A/D converter 92 are S.sub.1, S.sub.2, . . . , the digital words S.sub.1, S.sub.N+1, S.sub.2N+1, appear sequentially at output port 96.sub.1. Multiplexor 94 operates in a like manner with the digital words produced by the A/D converter 94 for the quadrature channel 61(Q). It follows then that pairs of output ports 96.sub.1, 98.sub.1 through 96.sub.N -98.sub.N, respectively, provide channels 67'.sub.1 through 67'.sub.N respectively.

The outputs of the time multiplexors 92, 94 are fed to the frequency spectrum analyzer section 76', as shown. The frequency spectrum analyzer section 76' includes a plurality of, N, frequency spectrum analyzers, here M point FFTs 80'.sub.1 -80'.sub.N, where M=128. Each one the analyzers 80.sub.1 -80.sub.M is fed by the digital words coupled to a corresponding one of the N channels 67'.sub.1 -67'.sub.N, for determining the frequency spectrum of the signal represented by the digital words fed thereto. A second time multiplexor 100 is provided for, during each of the series of periods, T=N/f.sub.s, coupling each nth one of N frequency spectrum analyzers FFTs 80'.sub.1 -80'.sub.N to an output at the rate, 1/f.sub.s.

More particularly, the pair of output ports 96.sub.1, 98.sub.1 through 96.sub.N -98.sub.N, respectively, (i.e., channels 67'.sub.1 through 67'.sub.N, respectively) are fed to M point FFTs 80'.sub.1 through 80'.sub.N, respectively, as shown. It is noted that the digital words fed to each one of the FFTs 80'.sub.1 -80'.sub.N appear at a rate f.sub.s /N. In response to the digital words fed thereto each FFT 80'.sub.1 -80'.sub.N the frequency spectrum is produced. Thus, the FFTs 80'.sub.1 through 80'.sub.N provide the frequency spectra for radar returns in channels 67'.sub.1 through 67'.sub.N respectively. More particularly, and considering an exemplary one of the ports 102.sub.1 -102.sub.N, here port 102.sub.1, the frequency components f.sub.1 -f.sub.M for the first channel 67'.sub.1 appear at port 102, (i.e., f.sub.1,R1, f.sub.2R1, . . . f.sub.mR1, . . . f.sub.MRN), each one of the frequency components appearing at such port 102.sub.1, at a rate f.sub.s /N. Thus, here the same frequency component f.sub.m, where m is an integer from 1 to M, for the N channels 67'.sub.1 through 67'.sub.N appears sequentially at ports 102, through 102.sub.N at the bit rate f.sub.s, as shown in the table in FIG. 4A, where f.sub.n,Rn is the frequency component f.sub.m for the range R.sub.n in the channel 67.sub.n.

The outputs of FFTs 80'.sub.1 through 80'.sub.N are fed to input ports 102.sub.1 through 102.sub.N, respectively, of the second time multiplexor 100, as shown. The time multiplexor 100 operates synchronously with time multiplexors 92, 94 at bit rate f.sub.s. The operation of multiplexor 100 is summarized in the FIGS. 4A and 4B, where the frequency component m at channel 67.sub.n, n, is designated by the f.sub.m,Rn ; the data at ports 102.sub.1 through 102.sub.N as a function of time being shown in FIG. 4A and the data at multiplexor 100 output port 103 as a function of time being shown in FIG. 4B.

A plurality of, here N, correlator/integrators 62'1-62'.sub.N, is fed, via lines .sup.71.sub.1 -71.sub.N by the second multiplexor 100 output 103 and a different time delayed replica of the N-bit binary code stored in register 16, each time delay corresponding to a different range from the transmitter. More particularly, the output port 103 is fed to one input of the multipliers 64'.sub.1 -64'.sub.N, as shown. The other input to the multipliers 64'.sub.1 -64'.sub.N is fed differently time delayed replica of the code stored in register 16 after passing through the time delay adjuster 69. Thus, the codes in stages C1-CN appear lines 71.sub.1 -71.sub.N, respectively. Lines 71.sub.1 -71.sub.N, are fed to multipliers 64'.sub.1 -64'.sub.N, respectively, as shown. The multiplications performed by multipliers 64'.sub.1 64.sub.N are summarized in FIG. 5. Thus, multiplier 64'.sub.1 multiplies the bits appearing in stage C1 at the bit rate f.sub.s with the frequency component f.sub.1 for the channels 67'.sub.1 -67'.sub.N, (i.e., ranges R.sub.1 -R.sub.N). respectively, as indicated. These multiplied products are accumulated in the accumulator ACCUM fed by the multiplier 64'.sub.1 which is initially reset (R). The accumulated products are loaded (LD) into the register REG fed by the ACCUM. Thus, If the target were at the range R.sub.1, and if the target had the Doppler frequency, f.sub.1, the contents in the REG would be +N, assuming an absence of clutter. The process repeats for each frequency component f.sub.2 -f.sub.M.

Thus, referring to FIG. 5, it is noted that at time 1/f.sub.s, a correlation function is performed between the code in stages C.sub.1 -C.sub.N (corresponding to range R.sub.1) and the returns having the frequency component, f.sub.1. Likewise, at time 2/f.sub.s a correlation function is performed between the code in stages C.sub.2 -C.sub.1 (corresponding to range R.sub.2) and the returns having the frequency component, f.sub.1. If the target has a Doppler frequency component f.sub.1 at range R.sub.n, a peak in the auto-correlation function, +N, will appear at the output of the correlator/integrator 62'.sub.n. The process repeats in like manner so that, over the period of time N/f.sub.s, an auto-correlation function has been performed between the frequency component f.sub.1 and each of the differently time delayed replica of the code stored in register 16. During the second period of time, N/f.sub.s, the process repeats for the frequency component f.sub.2. During the Mth period of time, N/f.sub.s, the process repeats for the frequency component f.sub.M. Thus, if the target has a Doppler frequency, f.sub.m, and is at range, R.sub.n, a peak in the correlation function +N, appears at the output of correlator/integrator 62'.sub.n for the frequency component, f.sub.m.

Thus, the output of correlator/integrator 62'.sub.1 provides for processor 24 an indication the top row, (i.e., the range R.sub.1 range channel 67.sub.1) of the Range-Doppler matrix stored in memory 82 of processor 24. That is, if a +N appears at frequency component f.sub.m, the target is at range R.sub.1, and has a Doppler frequency f.sub.m. Concurrently, multipliers 64'.sub.2 -64'.sub.N and the accumulators and registers fed thereby, operate in like manner, as summarized in FIG. 5 to provide range channels for ranges R.sub.2 -R.sub.N, respectively. Thus, each one of the plurality of, N, correlator/integrators 62'.sub.1 -62'.sub.N produces the frequency spectrum of a radar return at a corresponding range from the transmitter 12.

The processor 24 is fed by the outputs of the plurality of, N, digital correlator/integrators 62'.sub.1 -62'.sub.N for determining the target range and Doppler frequency as described above in connection with FIG. 1.

Before referring to FIG. 6, it is noted that with the system 10 and 10' described above in connection with FIGS. 1 through 5, the range resolution corresponding to a time delay 1/f.sub.s ; i.e., the reciprocal of the bit rate, f.sub.s. That is with the system 10 and 10', the bits of the code is transmitted at a bit rate, f.sub.s, and therefore the target range is resolved to a range resolution cell of 1/f.sub.s. Referring now to FIG. 6, the systems 10 and 10' are modified to provide range positioning to resolutions less than 1/f.sub.s. Thus, the system 10" in FIG. 6 includes, with like elements being again designated with the same numerical designations as those used in FIG. 1 and 3, a transmitter 12 and a receiver 14. The transmitter 12 is adapted to transmit a continuous wave radar signal phase encoded at a rate, f.sub.s, in accordance with a code. The code is here a sequentially repeating N-bit binary, pseudo-random noise code stored in a recirculating shift register 16, as shown.

The receiver 14 is adapted to receive a target reflected radar return from the transmitted radar signal and includes: (a) a heterodyning section 18 for translating the radar returns to a baseband frequency; (b) an analog to digital converter section 20 for converting samples of the radar return into a series of corresponding digital words at the rate, f.sub.s ; (c) a digital correlator/integrator and frequency spectrum analyzer section 22 fed by the series of digital words and the differently time delayed replica of the successively repeating N-bit binary code for producing an output representative of energy in the radar return as function of range from the transmitter 12 for each of a plurality of frequencies; and (d) a processor 24 fed by the output of the digital correlator/integrator and frequency spectrum analyzer section 22 for indicating target range and Doppler frequency.

More particularly, the receiver 14 includes a receive code generator 17 for storing a replica of the transmit code stored in register 16. The receiver code generator 17 produces differently time delayed replica of such transmit code on lines 71.sub.1 -71'.sub.N, respectively. The differently time delayed replica are produced a predetermined time, .DELTA., after the phased encoded radar signal is transmitted. Further, the transmit code stored in register 16 phase encodes the transmitted continuous wave signal at a predetermined rate, f.sub.s. The rate f.sub.s is produced by a source 32, here the UHF carrier source 32. Here the clock pulses are provided by replacing the 2:1 power divider 36 in FIGS. 1 and 3 with a 3:1 power divider 36', as shown. The third output of divider 36' provides, on line 90, clock pulses at a rate Pf.sub.s, where P is an integer greater than 1. The clock pulses on line 90 are fed to a divide by P circuit 91. The divide by P circuit 91 therefore produces clock pulse at the bit rate, f.sub.s, for the recirculating shift register 16, as indicated. The clock pulses produced by the source 32 are divided in rate by P in a second divide by P circuit 94, as shown, after passing through a time delay circuit 96, as shown. The time delay circuit 96 is fed by a digital word in register 98 representative of a desired time delay, .DELTA., and by the clock pulses produced by the clock pulse source 32 on line 90, as indicated. The time delay circuit 96 includes a count-down counter 101 which is loaded with a count corresponding to the desired time delay, .DELTA.. The count loaded into the counter 101 may be an integer, S, from zero to (P-1). Thus, the counter 101 changes the logic state at the output thereof after zero through P-1 clock pulses have been fed to it on line 90. The change in logic state at the output of counter 101 enables the second divide by P circuit 94. Thus, the divide by P circuit 94 produces, f'.sub.s, at the output thereof the clock pulses on line 90 delayed in time by the desired time delay, .DELTA., where .DELTA. is 1/Sf.sub.s. Thus, the desired time delay, .DELTA., is equal to S/(Pf.sub.s), where S is an integer less than (P-1). The receive code generator 17, in response to the output of the time delay circuit 96, more particularly, the output of the second divide by P circuit 94, f'.sub.s produces the differently time delayed replica of such transmit code on lines 71'.sub.1 -71,.sub.N. Further, the analog to digital converter section 20, in response to the clock pulses produced at the output of the time delay circuit 94, more particularly, the output of the second divide by P circuit 94, f'.sub.s, converts a sequence of samples of the radar return into a corresponding sequence of digital words at the rate f.sub.s. The digital correlator/integrator and spectrum analyzer section is fed by the digital words and the receive codes produced by the receive code generator to provide an indication of radar return energy as a function of range from the transmitter, as described above in connection with FIGS. 1 through 5. It is noted that where P is greater than 1, the receive codes are delayed with respect to the transmit code by less than 1/f.sub.s thereby enabling detection of a target with accuracies less than that corresponding to a time delay of 1/f.sub.s.

Other embodiments are within the spirit and scope of the appended claims.