spread-Doppler Clutter Research Articles

Low-cost solutions for mobile passive radar based on multichannel DPCA and NULA configurations

Abstract In this paper, we investigate low-cost solutions for enabling ground moving target indication applications with multichannel mobile passive radar systems. As known, in order to be competitive with their active counterparts, passive radars are typically characterized by severe constraints in terms of cost, complexity, and compactness, especially when installed on moving platforms. On the one hand, carrying out the computations onboard requires processing techniques as simple as possible. On the other hand, the need for lightweight and compact systems that can be installed on a moving platform requires using a limited number of receiving channels. To meet these requirements, we propose a series of nonadaptive detectors based on multichannel displaced phase center antennas, which allow suppressing the Doppler-spread clutter component without requiring computationally intensive space–time adaptive processing techniques. Moreover, we explore the use of nonuniformly spaced array configurations on receive, which represent a good alternative to conventional uniform linear arrays when a limited number of receiving channels can be implemented. The effectiveness of the proposed processing techniques and antenna design solutions is demonstrated via numerical analysis for the case of a DVB-T-based mobile passive radar system.

Model-based optimal action selection for Dyna-Q reverberation suppression cognitive sonar

The Doppler shift of low-speed targets is frequently disturbed by the reverberation Doppler spread clutter under the shallow sea. The clutter is generated by underwater scatterers, which increases the difficulty of Doppler estimation. To solve this problem, a reverberation target resolution function based on the Doppler spread clutter statistical model is proposed in this paper. Through the width of reverberation Doppler clutter, this function adjusts the waveform parameters by determining whether the target is discriminable. In addition, the reverberation Doppler spread clutter is time-spatial varying and affected by grazing angle, waves, wind speed, fish and other effects. Thus, the sonar waveform parameters need to be adjusted constantly. Therefore, this paper combines the cognitive sonar based on reinforcement learning with the reverberation target resolution function to evaluate different waveforms in different environments. Consequently, the sonar can adjust the waveform parameters in real-time and obtain the optimal waveform in different environments. Meanwhile, in this paper, the action selection strategy of Dyna-Q reinforcement learning is optimized, and the model-based maximum action selection Dyna-Q algorithm (Dyna-Q-Max-Action) is proposed. Compared with the traditional Dyna-Q and Q-learning algorithms, the proposed algorithm needs fewer episodes. Finally, numerical simulation verified the effectiveness of the proposed algorithm.

Doppler characteristics of HF spread Doppler clutter recorded at Alice Springs, Australia

Mean night-time peak power, Doppler shift and Doppler width of spread Doppler clutter (SDC) received by a high frequency backscatter radar located at Alice Springs, Australia from 2000 to 2018 is presented as a function of azimuth, sunspot number, time of year and frequency. The sampled region covers 90 degrees from West to North and includes the northern and southern equatorial anomalies.SDC peak power diminished across all azimuths during the winter solstice from around May to August (local winter) coinciding with the global decrease in F layer density due to the annual non-seasonal F2 anomaly but was generally constant during the equinoxes. In contrast, SDC Doppler width and inbound Doppler shift both increased during the equinoxes and exhibited azimuthal dependence related to the eastward equatorial plasma drift.SDC peak power increased with increasing sunspot number with frequency dependence during winter but not summer. Inbound Doppler shift and Doppler width increased with increasing sunspot number during equinox but not solstice with a strong dependence on azimuth and a weak dependence on frequency.

Adaptive suppression of main‐lobe spread Doppler clutter with high directivity for HFSSWR using oblique projection

The performance of high-frequency hybrid sky-surface wave radar (HFSSWR) is known to suffer from the spread Doppler clutter (SDC). As a major source of the SDC, ionospheric clutter generally possesses high directivity within the main detection range. Classical coherent sidelobe cancellation method provides an efficient way to suppress the highly directional clutter. However, it performs poorly when the directions of a target and the clutter are both in the main lobe. To address this problem, a novel clutter suppression method using oblique projection is proposed, which does not require priori knowledge of the targets, such as directions, Doppler frequencies, strength etc. By combining training data selection based on oblique projection with SDC suppression in the Doppler domain, this method is implemented in the interested range cell for all target directions. Experimental results indicate that the proposed method can achieve better clutter suppression performance based on real data.

Multi‐mode propagation mode localisation and spread‐Doppler clutter suppression method for multiple‐input multiple‐output over‐the‐horizon radar

Multi-mode spread-Doppler clutter (SDC), caused by a stratified ionosphere, affects the detection performance of over-the-horizon radar on slow ships. To improve a multiple-input multiple-output over-the-horizon radar (MIMO-OTHR) system's performance and to mitigate the lingering effect of SDC, a novel multi-mode SDC suppression method based on full-mode spatial separation is proposed here. Furthermore, authors' method can estimate the elevation angle of direction of departure (DOD) and direction of arrival (DOA), contributing to mode localisation. By applying the bi-orthogonality of matrices, the cost function of single-mode component analysis is constructed under a least square criterion. By iteratively solving for the cost function, and by applying a systematic multistage decomposition, the elevation DOD and DOA and the slow-time Doppler information of each mode are obtained sequentially. Compared with classical SOBI and SS-SOBI methods, the proposed method improves the output signal-to-clutter-plus-noise ratio for spatial filtering. The estimated DOD and DOA do not need to be paired, reducing the angle error inherent in the SR method while performing the transmit-receive cascade refinement searching step. Simulation results demonstrate the excellent performance of the proposed method, which is beneficial to improve the ability of MIMO-OTHR to detect slow ships and determine the propagation mode.

Reciprocal-Filter-Based STAP for Passive Radar on Moving Platforms

This paper addresses the problem of clutter cancellation and slowly moving target detection in digital video broadcast-terrestrial-based passive radar systems mounted on moving platforms. First, we show that conventional processing approaches based on the availability of multiple receiving channels might be ineffective in the considered scenarios due to the impossibility to control the employed waveform of opportunity. Therefore, an appropriate space-time adaptive processing scheme is proposed to cope with the Doppler spread clutter returns aiming at ground moving target indication applications. It exploits the benefits of the reciprocal filtering strategy applied at a range compression stage together with a flexible displaced phase center antenna approach. The effectiveness of the proposed scheme is demonstrated via application to a simulated dataset and then tested against experimental data collected by multichannel passive radar on a maritime moving platform.

Single dataset method for spread‐Doppler clutter suppression in HF hybrid sky‐surface wave radar

In high-frequency hybrid sky-surface wave radars, the spread-Doppler clutter (SDC) compromise low-velocity target detection performance. Generally, adaptive filter methods achieve clutter suppression by estimating a clutter-plus-noise covariance matrix from target-free training data, which are typically obtained from neighbouring range bins. Under certain circumstances, the clutter statistics change significantly across adjacent bins. To deal with this problem, a single dataset algorithm is proposed to suppress the SDC without secondary data. Firstly, a spatial orthogonal projection matrix is introduced to block the target component in the cell under test. Afterwards, a localised processing method is derived, using previously constructed clutter samples, to suppress the SDC. According to the experimental results, the proposed method can achieve better clutter suppression performance based on real data.

Multi‐mode clutter suppression of multiple‐input–multiple‐output over‐the‐horizon radar based on blind source separation

Spread-Doppler clutter caused by multi-mode propagation seriously affects the over-the-horizon radar detection performance for slow ships. To solve this problem, the authors propose to suppress multi-mode clutter using spatial information in the multiple-input–multiple-output over-the-horizon radar system. Traditional processing algorithms need prior information of the target direction. However, it is often not readily available. In this study, they propose a novel multi-mode clutter suppression algorithm based on the second-order blind identification (SOBI) technique. The proposed algorithm does not need prior information of the target direction and can effectively suppress the multi-mode clutter. Considering that the SOBI algorithm affected by the similarity in sources spectra, they improve the proposed algorithm using the spatial smoothing technique. Simulation results show that, in the single target scene, the improved algorithm can effectively eliminate the spectrum burrs caused by incomplete separation, making separated signals smoother. Signal-to-clutter-noise ratio is greatly improved. The good multi-mode clutter suppression performance is obtained. In the multi-mode scenario that double targets with the same radial velocity, that is, sources spectra have similarities, the algorithm based on SOBI cannot separate all targets. Although the improved algorithm can make all targets visible and effectively suppresses the multi-mode clutter.

MIMO adaptive beamforming for nonseparable multipath clutter mitigation

The performance of ground-moving target indicator radars is often degraded in complex terrain by multipath spread-Doppler clutter. Multiple-input, multiple-output (MIMO) beamforming with limited training data has been previously proposed with separable transmit and receive weights appropriate for discrete multipath scenarios. In this paper, partially adaptive MIMO beamforming is presented for suppression of a nonseparable continuum of multipath clutter with limited training data. Simulation results for a forward-looking vehicle-mounted radar in a simplified urban environment are described.

Adaptive Radar Detection in Doubly Nonstationary Autoregressive Doppler Spread Clutter

The problem of adaptive radar detection in clutter which is nonstationary both in slow and fast time is addressed. Nonstationarity within a coherent processing interval (CPI) often precludes target detection because of the masking induced by Doppler spreading of the clutter. Across range bins (i.e., fast time), nonstationarity severely limits the amount of training data available to estimate the noise covariance matrix required for adaptive detection. Such difficult clutter conditions are not uncommon in complex multipath propagation conditions where path lengths can change abruptly in dynamic scenarios. To mitigate nonstationary Doppler spread clutter, an approximation to the generalized likelihood ratio test (GLRT) detector is presented wherein the CPI from the hypothesized target range is used for both clutter estimation and target detection. To overcome the lack of training data, a modified time-varying autoregressive (TVAR) model is assumed for the clutter return. In particular, maximum likelihood (ML) estimates of the TVAR parameters, computed from a single snapshot of data, are used in a GLRT for detecting stationary targets in possibly abruptly nonstationary clutter. The GLRT is compared with three alternative methods including a conceptually simpler ad hoc approach based on extrapolation of quasi-stationary data segments. Detection performance is assessed using simulated targets in both synthetically-generated and real radar clutter. Results suggest the proposed GLRT with TVAR clutter modeling can provide between 5-8 dB improvement in signal-to-clutter plus noise ratio (SCNR) when compared with the conventional methods.

Multiple-input multiple-output over-the-horizon radar: experimental results

Results from an experiment that applied one class of multiple-input multiple-output (MIMO) waveform techniques to over-the-horizon radar (OTHR) are reported. The experiment objective was to demonstrate that adaptive transmitter beamforming could be used in an appropriately design radar to reject spatially discrete Doppler-spread clutter. In the particular MIMO radar architecture that the authors call non-causal transmit beamforming, conventional or adaptive transmitter beamforming occurs following waveform transmission, propagation, scatter from targets and clutter sources, return propagation and finally signal reception. In the case reported herein spatially discrete clutter was successfully rejected to the noise floor of the radar return with rejection in excess of 35 dB achieved using common adaptive algorithms and straightforward training data selection. As part of the rejection algorithm the transmitted waveform direction-of-departure (DOD) from the transmitter array to the target was estimated and used as the preserved steer direction in the adaptive beamformer. The DOD estimates agree well with the geometrically determined true values. The demonstration of non-causal transmit beamforming suggests that it will be possible to create multiple simultaneous adaptive range-dependent transmitter beams with an appropriately designed OTHR. This has several applications including for the mitigation of Doppler-spread clutter.

Utilization of a nonrecurrent waveform to mitigate range‐folded spread Doppler clutter: Application to over‐the‐horizon radar

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 brief 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.