Processing of raw analog echo data from a side-looking synthetic aperture radar receiver into images on board an airborne radar platform is made feasible by utilizing charge-coupled device (CCD) semiconductor technology. CCD circuits are utilized to perform input sampling, presumming, range correlation and azimuth correlation in the analog domain. These radar data processing functions are implemented for "single-look" or "multiple-look" imaging radar systems.

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Charge-coupled device data processor for an airborne imaging radar system

In a medium PRF pulse doppler type radar receiver, target detection is achieved in both search and track modes. In the search mode, the video signal from the receiver section is notch filtered to cancel the main beam clutter. The remainder of the signal is threshold detected, followed by ranging each of the thresholded signals to resolve the range ambiguity of the signals. The unambiguous signals are gated through a range sensitive threshold which operates along a power curve of R 4 , resulting in true target returns being thresholded by the range sensitive threshold circuit and blocking out discrete sidelobe returns. In the track mode, the video signal from the receiver is clutter cancelled to remove main beam clutter, as in the search mode, and the remaining signals pass through a velocity track filter with a narrow band window. The true target return signal ranged during the search mode, is passed by the velocity track filter which is adjusted to match the true target return doppler shift during the correct range gate. The velocity track filter is controlled to track the true target return signal and is narrow enough to reject discrete sidelobe signals which occur in the same range cell. The signals passed by the velocity track filter are subsequently monitored, and guard gates are established in both adjoining range cells to insure that lock on the tracked target will not be lost due to interfering signals such as returns from moving ground targets. When the interfering signals are detected in the guard gates, the system goes on memory until the interfering signal passes.

Target detection system in a medium PRF pulse doppler search/track radar receiver

A digital filter for generating a frequency domain output signal in response to an incoming signal arriving in successive receiving cycles whenever the incoming signal has any frequency component lying within the passband of the filter. A means for generating a timing signal is provided. An analog-to-digital converter responsive to the timing signal converts the incoming analog signal into a series of digital input pulses during each range gate interval. A reference generator responsive to the timing signal provides a train of digital reference pulses for each of the successive receiving cycles. A predetermined number of successive receiving cycles constitutes a single correlation frame. The amplitude of each train of digital reference pulses is determined by a composite correlation function. A multiplier multiplies each digital reference pulse with each digital input pulse to generate a train of discrete digital output pulses for each receiving cycle. An accumulator stores the discrete digital output pulses from the multiplier and generates a frequency domain output signal for each frame of reference pulses. The digital filter can also be constructed with either a two-channel or four-channel processor to provide a more sophisticated filter system.

Simplified digital moving target indicator filter

A digital technique is disclosed for filtering clutter in a pulse doppler I radar which allows a filter notch to be set for different doppler frequencies. In particular, the system includes two or more MTI filters cascaded to cancel different kinds of clutter signals that interfere with target signal reception. In each filter a digital detection circuit senses the mean-clutter-doppler frequency for a particular clutter type and automatically places a notch at that frequency. Each notch is fixed by treating the radar samples as complex numbers and replacing the conventional filter coefficients with complex coefficients indicating amplitude and phase as derived from the radar returns.

Digital mean clutter doppler compensation system

A clutter filter which reduces by (N-1) the number of input pulses requiredor operation of an N-pulse MTI canceler when L>N consecutive filtered outputs are desired. The clutter filter includes the two channels of an N-pulse MTI canceler which weight and sum the in-phase and quadrature-phase components of N radar returns, and a pair of delay circuits which introduce a delay of N interpulse time periods to the N and subsequent in-phase and quadrature-phase components of the radar returns. The stored outputs of the two channels of the canceler and the consecutive outputs of the delay circuits are combined in a complex multiplier to generate the consecutive in-phase and quadrature-phase outputs of the filter. The clutter filter has utility in a multiple-stage filter system employing a Doppler filter as the last stage.

Clutter filter using a minimum number of radar pulses

A radar system is disclosed utilizing digital techniques for implementing a moving target indicator (MTI) or pulse doppler type of radar system. More specifically, the radar system includes an MTI filter or canceler circuit for suppressing clutter signals corresponding to stationary and/or relatively slow-moving targets, while providing an output signal indicative of echo signals received from relatively fast-moving targets. The canceling circuit of this invention achieves moving target detection with digital apparatus having a capability of processing only a limited number of digital bits. The MTI filter of this invention incorporates modulo-N components having a modulus or linear range so that the minimum detectable signal of interest lies therein. The higher magnitudes of signals corresponding to echo signals received from stationary and/or slowly-moving objects will be canceled by the canceling circuit, normally. If echo signals are developed from moving targets that are of higher magnitudes than the modulus, these signals will be ''''aliased'''' by the modulo-N components about the levels N/2 and N/2 to provide a noise-like output. Regardless of the amplitude of the echo signals derived from a relatively fast-moving target, detecting means is provided to detect the absence or presence of such signals.

R Martin

Papers

Digital moving target indicator radar system incorporating modulo-n components