Clutter subtraction system

Abstract

A radar system having transmitted and received signals, the system including a feedback loop for modulating a sample of the transmitted signal to duplicate the phase and amplitude of the clutter echoes from nearby points of reflection. The modulated transmitted sample is subtracted from the received signal to remove the clutter. The feedback loop detects differences between the clutter-free signal and the transmitted sample to produce an error signal, the error signal being filtered and applied to the modulator for modulating the phase and amplitude of the transmitted sample. BACKGROUND OF THE INVENTION This invention relates to echo ranging systems such as radar and sonar systems and, more particularly, to an echo ranging system in which clutter returns from nearby objects are removed from echoes from distant objects. Echo ranging systems frequently utilize transmitted pulses of radiant energy in which the duration of the pulse is equal to a substantial portion of the overall round trip time of propagation of radiant energy signals from the echo ranging system to a distant object and back to the echo ranging system. The use of a long duration pulse of radio frequency energy permits the transmission of a large amount of radiant energy as compared to that which can be transmitted in a relatively short pulse from transmitting equipment which is limited in its maximum peak power capability. To produce enhanced discrimination in range between closely spaced reflecting objects, the spectrum of the transmitted pulse of radiant energy is broadened by a modulation of the carrier, such modulation being typically a phase modulation of the carrier, one such form of phase modulation employing a quadratic phase shift pattern which results in a frequency modulation pattern in the form of a linear sweep. Such a modulation pattern is conveniently employed with pulse compression filters in radar and sonar receiving equipment for the discernment of closely spaced points of reflection. A problem arises in that echo ranging systems are frequently employed in situations wherein nearby objects of reflection and distant objects of reflection, often referred to as targets, are simultaneously present. While the leading edges of echoes from nearby targets arrive at the radar or sonar system prior to the leading edges of echoes from distant targets, the relatively long duration of the transmitted pulses may result in extensive overlap of the major portion of the echoes of nearby targets with the echoes from distant targets. When the aforementioned linear sweep frequency modulation is employed, the instantaneous frequency of a portion of an echo from a nearby target differs from the instantaneous frequency of an overlaping portion of an echo from a distant target. Furthermore, the amplitude of an echo from a nearby target, such as the echo from a wave in a sea clutter situation, is much larger than the amplitude of the echo received from a distant target. In view of these differences in frequency, or phase, and amplitude of the echoes, the clutter of echoes from the nearby targets masks the echoes from the distant targets. Difficulties are encountered in echo receiving equipment in that automatic gain control used therein must have a large dynamic range to accommodate the large and the small amplitude echo signals. Such automatic gain control equipment, when tracking the large amplitudes of the clutter, tends to distort the waveforms of the echoes received from the distant targets with a resulting loss in data obtainable from such distant targets. The foregoing situation relates to both electromagnetic and sonic echo ranging systems and, since the invention to be described hereinafter applies to both electromagnetic and sonic echo ranging systems, the ensuing description will be given in terms of a radar system, it being understood that the description relates equally well to a sonar system. SUMMARY OF THE INVENTION The aforementioned problems of the prior art are overcome and other advantages are provided by a radar system having transmitted and received signals, and which, in accordance with the invention, modulates a sample of the transmitted signal to substantially duplicate received clutter signals, the modulated sample being subtracted from the received signal to provide a received signal free of clutter. In a preferred embodiment of the invention, the transmitted signal is phase modulated quadratically with time, this being a linear frequency sweep over the duration of the transmitted signal pulse. Such modulation provides a transmitted signal having an increased bandwidth suitable for the pulse compression of received signal echoes. In accordance with the invention, the aforementioned subtraction of the transmitted signal sample is accomplished by a feedback loop in which the clutter-free signal is compared to the transmitted sample by a detection of inphase and quadrature components of the clutter-free signal to develop an error signal of the feedback loop. The error signal is then filtered and applied to a modulator which modulates the phase and amplitude of the transmitted sample to match the phase and amplitude of the clutter. Thereby, the clutter component of the received signal may be subtracted from the received signal. The bandwidth of the feedback loop provides a sufficient speed of response for tracking clutter of nearby reflecting objects for which the frequency and phase of the echo approximates that of the transmitted signal phase. The loop bandwidth is sufficiently narrow to preclude the tracking of echoes from distant reflecting objects for which the frequency of the echoes differs substantially from that of the transmitted signal pulse. Thereby, distant objects, or targets can be observed since the echoes therefrom are essentially unaffected by the feedback loop.

Dependencies are inferred from the claim text.