USA As Represented by Secretary of the Navy

The United States of America

Other

Frank F Kretschmer, Jr

Bernard L Lewis

Karl Robert Gerlach

James P Hansen

Karl R Gerlach

An improved MTI radar system including a signal expander/compressor for providing palindromic P2 phase-coded upswept and downswept expanded signals that are alternately transmitted by a radar transmitter. The echos from the upswept and downswept signals are received by a receiver, compressed in the signal expander/compressor and inputted to an MTI subtractor. Since the autocorrelation sidelobes of the palindromic phase coded echos are real, the echos from stationary clutter are completely cancelled so that the system is capable of detecting weak echos from moving targets.

MTI using a polyphase code

A technique for improving sidelobe canceller performance by blocking the canceller output signal upon sensing reduced main channel interference accompanied by an increased sidelobe canceller output. Two signal processing networks form the difference between successive output signals from a radar antenna and sidelobe canceller respectively. The difference signals are multiplied together and the product used to control passage and preclude utilization of the canceller output signal when reduced jamming interference occurs.

Technique for processing interference-contaminated radar energy

A system for cancelling jammer signals entering the mainlobe of a monopulse adar and providing target locations with respect to the jammer position comprising for a sequential lobing embodiment: measuring the angular position of the jammer in the radar's mainlobe via the use of monopulse sum and difference signals, pointing the radar antenna so that the jammer is positioned within the 3 dB points of the mainlobe of the monopulse antenna, converting the sum and difference echo signals received with the antenna in this position from the next radar transmission into a second set sum and difference signals, applying this second set of sum and second set of difference signals to the inputs of an adaptive canceller to cancel the jamming signal therein and storing these signals in time sequential order, changing the orientation of the radar antenna by a small arbitrary elevation angle, converting the sum and difference echo signals received with the antenna in this new position from a third radar transmission into a third set of sum and difference signals, applying this third set of sum signals and third set of difference signals in reverse order relative to the second set of signals to the inputs of an adaptive canceller to cancel the jamming signal therein, reading out the adaptively cancelled stored echo signals from the second radar transmission into one input of a divider in time coincidence with the application of the adaptively cancelled echo signals from the same targets from the third radar transmission to the other input of the divider to obtain ratio signals proportional to the elevation angle separating the individual targets from the jammer.

Jammer reference target measurement system

A digital, open-loop, sidelobe canceller system includes an arrangement of uxiliary-channel preprocessing cancellers and a series of main-channel cancellers in which specific cancellers average a different number of samples for cancelling both barrage (continuous) jamming and false-target-repeater jamming, which enter the sidelobes of a radar antenna. The preprocessing cancellers and each of the main-channel cancellers, except the last succeeding main-channel canceller, use long-time averaging (averaging over a large number, n, of samples of the main and auxiliary signals, where n&gt;&gt;2) for removing barrage jamming from the main signal. The last succeeding main-channel canceller uses short-time averaging (averaging over approximately 2-4 samples) for removing false-target-repeater jamming from the main signal.

General purpose sidelobe canceller system

A method and apparatus exploiting the discovery that the crosscorrelation of constantly spaced rows of the matrices representing certain pulse codes sum to zero. In a ranging system, such as a radar, pulses are coded according to the rows of a such a matrix, transmitted sequentially and each return processed sequentially through a filter matched to one of the coded pulses. (A different preselected filter is used for each return.) The sequence of filters is chosen so that for returns for a given range interval, each filter is matched to the returning pulse, resulting in outputs from the filters representing auto-correlations of the returned pulses. These outputs are time delayed added coherently to form the compressed pulse, and annunciated as a target hit. Should the filters and returns be mismatched, as with ambiguous stationary clutter returns, the outputs of the filters are cross-correlations which, according to said discovery, sum to zero. Thus the invention operates to remove ambiguous range clutter from returns in such a ranging system.

Zero cross-correlation complementary radar waveform signal processor for

A method of preserving targets in the clear in an adaptive baseband MTI radar system with pulse compression is disclosed. The angle of the weighting signal is determined from the inverse tangent of the inphase and quadrature components of the weighting signal. This angle is then compared to a threshold angle in order to generate a modified weighting signal.

Adaptive MTI target preservation

A method and apparatus exploiting the discovery that the cross-correlation of rows of Frank or P4 matrices of a given spacing sum to zero. In a ranging system, such as a radar, pulses are coded according to the rows of a Frank or P4 matrix, transmitted sequentially and each return processed sequentially through a filter matched to one of the coded pulses. (A different preselected filter is used for each return.) The sequence of filters is chosen so that for returns for a given range interval, each filter is matched to the returning pulse, resulting in outputs from the filters representing auto-correlations of the returned pulses. These outputs are time delayed added coherently to form the compressed pulse, and annunciated as a target hit. Should the filters and returns be mismatched, as with ambiguous stationary clutter returns, the outputs of the filters are cross-correlations which, according to said discovery, sum to zero. Thus the invention operates to remove ambiguous range clutter from returns in such a ranging system.

Ranging method and system, especially for radar signal processing for

A method and apparatus for reducing cyclic losses due to doppler shifting frequency-derived phase coded expanded radar pulses using new expanded pulse codes which increase the number of phase elements without increasing compression ratios. These new expanded codes may be generated by sampling the phase characteristics of a chirp or step-chirp waveform above the Nyquist rate to derive the phases of the new coded waveforms and compressing the new expanded pulses with a compression ratio equal to the reciprocal of the signal bandwidth.

Doppler-improved polyphase pulse expander-compressor

A decoding device for use in pulse compression radars to decode a novel pe code that has the advantage of precompression bandwidth tolerance. The novel code type is described by the following formula (where .DELTA..phi..sub.k,p is the phase change at the kth subpulse of the pth subsequence within the width of a phase-modulated pulse):

Generalized polyphase code pulse compressor

A digital pulse expander-compressor for use in pulse compression radars and having the advantage of precompression bandwidth tolerance. The pulse expander-compressor employs a discrete Fourier transform circuit and multi-stage delay line feeding inputs x(n) to the discrete Fourier transform circuit to generate outputs in accordance with the formula

Method and apparatus for generating a modified P.sub.1 code

An adaptive N-pulse MTI processor for obtaining optimum clutter cancellation with a minimum of hardware, comprising a first input pulse line; first delay means for providing an interpulse delay T to pulses on this first input line; a first adaptive canceller with its auxiliary input connected directly to take pulses from the first input line, and with its main input connected to take delayed pulses from the first delay means; and N-2 series-connected canceller circuits, with each canceller circuit associated with a different level of pulses. The nth canceller circuit, where 1.ltoreq.n.ltoreq.N-2, includes an nth adaptive canceller; an nth input pulse line; n series connected delay means for providing a series of interpulse delays T to pulses propagating on the nth input pulse line, and with n+1 terminals for taking pulse outputs in front of each of the n delay means and after the last of the n delay means; and nth commutating switch means with a plurality of switch positions connected for applying pulses from the n+1 terminals to the main and auxiliary inputs of the nth adaptive canceller. The commutating switch means operates to provide appropriate sets of two pulses from the n+1 terminals to the main and auxiliary inputs to effect the n+2 canceller level of Gram-Schmidt pulse decorrelation on the output line of the nth canceller. The output from the n=1 adaptive canceller is connected to the first input pulse line. The output from the first adaptive canceller then provides the Gram-Schmidt decorrelation. With this configuration, only N-1 cancellers are required to implement the optimum Gram-Schmidt decorrelation for an N-pulse MTI.

Generalized adaptive MTI system

A digital pulse expander-compressor for generating polyphase coded pulses which are highly doppler tolerant and have autocorrelation functions which are identical under frequency sweep reversal. The expander-compressor comprises an input expansion circuit for generating N replicas of a pulse to be expanded, a matched filter FFT with phase weights disposed in each input thereto, and phase inverters disposed in every other output therefrom, and a time dispersion circuit for appropriately adding the pulses from the FFT output to yield an expanded or compressed pulse. The phase weights to be inserted at the inputs to the FFT are determined by the equation .phi..sub.n =.+-..pi.(n-1/2).sup.2 /.rho. for a PP-3 code or .phi..sub.n =.+-.[.pi.(n-1/2).sup.2 /.rho.-.pi.(n-1/2)] for a PP-4 code, where .rho. is the pulse compression ratio and n varies from 1, 2, 3 . . . .rho..

Palindromic polyphase code expander-compressor

A pulse expansion and compression system, especially useful for radar rang, comprising a pulse coder for expanding an input pulse and a pulse compressor of the matched-filter type. The coder consists of a plurality of delay stages into which the input pulse is fed, a discrete Fourier transform (DFT) circuit to which the output signals of the delay stages are fed by way of respective phase weights and for which every frequency port is phase-shifted prior to entry to a time-dispersion-means (TDM) comprising an arrangement of delay stages for differently delaying the output signals from the DFT. The TDM output is fed to a phase modulator and then to the transmitter. The echo signals are conjugated, time-inverted, and passed through the same DFT as the input pulse signal by way of the phase weights. The outputs of the DFT are then phase-shifted at every frequency port and passed through the TDM, but this time in time-inverted order. The outputs of the TDM are fed through an envelope detector to provide a cross-correlated facsimile of the original input pulse.

P2 Polyphase code expander-compressor

A pulse expansion and compression system, especially useful for radar rang, comprising a pulse coder for expanding an input pulse and a pulse compressor of the matched-filter type. The coder consists of a plurality of delay stages into which the input pulse is fed, a discrete Fourier transform (DFT) circuit to which the output signals of the delay stages are fed by way of respective phase weights and for which every other frequency port is inverted prior to entry to a time-dispersion-means (TDM) comprising an arrangement of adders interconnected by delay stages for differently delaying the output signals from the DFT. The TDM output is fed to a phase modulator and then to the transmitter. The echo signals are conjugated, time-inverted, and passed through the same DFT as the input pulse signal by way of the phase weights. The outputs of the DFT are then inverted at every other frequency port and passed through the TDM, but this time in time-inverted order. The outputs of the TDM are fed through an envelope detector to provide a cross-correlated facsimile of the original input pulse.

P1 Polyphase code expander-compressor

A method and apparatus for reducing the peak to peak-sidelobe ratio of the compressed pulse output signal of a symmetrical frequency derived phase coded pulse expander-compressor. The apparatus includes a bandwidth limiting device connected to the output of a symmetrical frequency derived phase coded pulse expander-compressor.

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Laurel, MD, United States of America

Frank F Kretschmer, Jr