This paper presents a continuous active sonar (CAS) waveform which enables high revisit rates and wideband processing gains to be achieved with suppression of range-ambiguous returns. Non-recurrent wideband linear FM signals with circular Costas frequency-staggering across chirp repetition intervals is shown to provide ambiguity functions with good Doppler and range resolution, controllable Doppler and range ambiguities, and a high revisit rate. These slow-time Costas or “SLO-CO” CAS waveforms allow for the possibility of updating range-velocity estimates every pulse repetition interval while achieving the clutter suppression associated with the use of conventional wideband LFM signals.

Non-recurrent wideband continuous active sonar

An HF radar system comprises a transmitting system, a receiving system, a signal processing system and a frequency management/ionospheric sounding system. The transmitting system comprises a transmitting antenna array configured to transmit a beam in a near vertical direction and a transmitting device arranged to drive the transmitting antenna array at frequencies suitable for downward refraction by the ionosphere. The receiving system comprises a receiving antenna array configured to receive returning signals from a target area returning to the receiving antenna array via refraction at the ionosphere. The signal processing system comprises a digital data processing system. The frequency management/sounding system comprises cooperating transmitting and receiving systems sending HF signals to the ionosphere and analysing the returning signals. Alternatively, the system may have a duplexed antenna array. The receiving system includes means to discriminate the returning signal produced by a helicopter, other aircraft or surface vessels.

Near-vertical incidence hf radar

Radar Space-Time Processing for Range-Folded Spread-Doppler Clutter Mitigation by William W. Lee Department of Electrical and Computer Engineering Duke University

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Radar Space-Time Processing for Range-Folded Spread-Doppler Clutter Mitigation

—This paper seeks to expand the fundamental understanding of random pulse repetition interval (PRI) staggering radar by formulating a physically meaningful “Doppler manifold” signal model that incorporates slow-time coding and is examined in the context of monostatic, multiple-time-around (MTA) scattering, and multiple-input multiple-output (MIMO) configurations. In so doing, it is found that an intrinsic “range decoherence” effect arises for the MTA and MIMO cases, thereby expanding the means through which separability can be achieved, albeit with rather different degrees of “decoherence amplification”. Moreover, a closed-form solution for the average Doppler response due to random staggering is derived, yielding guidance on the degree necessary to suppress Doppler ambiguities.

Implications of Diversified Doppler for Random PRI Radar

This paper concerns the detection of radar targets masked by Doppler-spread surface backscatter from points beyond the radar's maximum unambiguous range. Current methods for mitigating this so-called “range-folded” clutter, such as reducing the pulse-repetition frequency or the use of nonrecurrent waveforms, typically involve some loss in the useable Doppler space fraction available for target detection. This paper uses blind source separation methods to exploit the known Doppler separation afforded by non-recurrent waveforms in order to estimate the spatial wavefront of the clutter across a linear receive array. Spatial adaptive processing using this estimated wavefront is then used to suppress range-folded clutter without sacrificing usable Doppler space. Simulation results are presented which quantify performance in terms of SCNR gain achieved.

Space-time adaptive processing for range-folded spread-Doppler radar clutter mitigation

This paper describes the application of a new nonrecurrent waveform (NRWF) to a relocatable over-the-horizon radar (ROTHR) located in southern Virginia. The waveform is designed to improve the detection of aircraft targets through the mitigation of long-range spread Doppler clutter (SDC) encountered during nighttime operations. A discussion of the NRWF and its principle of operation are included, along with a description of ROTHR modifications. The focus of the paper is the presentation of test results from engineering, clutter mitigation, and target detection tests conducted during January through April of 1997.

M.P. Hartnett

Papers

Mitigation of range folded clutter by a nonrecurrent waveform