Yinsheng Wei

Analysis of first-order sea clutter spectrum characteristics for HF sky-surface wave radar

HF OTH radar based on the hybrid operating mode of sky-wave transmitting and ground-wave receiving because of its special system layout and propagating characteristics, its first-order sea clutter spectrum characteristics are not just about oceanic dynamics and frequency, but also influenced by grazing angle, bistatic angle and ionospheric conditions, so it needs to consider these factors comprehensively. In this paper, according to the geometry configuration and characteristics of sky-surface OTH radar detection, the positioning principle were firstly analyzed. The formula of the Bragg frequency of first-order sea clutter is presented derived from the system configuration, the frequency shift and the broadening mechanism of first-order sea clutter is analyzed, and the effects of various factors like ionosphere, grazing angle and bistatic angle on the first-order sea clutter spectrum and the characteristics of frequency shift and broadening of sea clutter in detection ranges at different distances are analyzed through the simulation based on the system layout. Finally, the results of theoretic analysis are examined with the experimental results, and the dominant factor that cause sea clutter broadening is discussed. Simulation results show that the results of experiment consist with that of theoretic analysis.

Over-sampled polyphase code design for physical implementation with spectral and correlation consideration

This paper presents an approach for designing continuous phase modulation (CPM) waveform with good spectral compatibility and low correlation properties. The suggested approach is to first design over-sampled polyphase code with stopband and correlation constraint, and then transform the polyphase code into CPM waveform. Applying over-sampling ensures that the resulting CPM waveform would well inherit the spectral and correlation properties from the designed polyphase code. To cope with such over-sampled code optimization, special considerations on both spectral and correlation penalty function are illustrated, and an efficient alternating projection method based on (I)FFT is proposed accordingly. Numeric results validate both the CPM waveform design approach and the over-sampled polyphase code optimization method.

Cascaded method for ionospheric decontamination and sea clutter suppression for high-frequency hybrid sky-surface wave radar

In high-frequency (HF) hybrid sky-surface wave radar, the first-order sea clutter broadening is very severely under the influence of ionosphere and bistatic angle, which affects the detection of ship target. In this study, a cascaded method for ionospheric decontamination and sea clutter suppression to enable detection of ship target is presented. Firstly, the radar configuration, sea clutter broadening characteristics, and model are introduced based on the newly-developed integrated HF sky-surface wave experimental system. In the cascaded processing method, the time-frequency analysis method based on S 2 -method is used to correct the ionospheric phase contamination with large amplitude. Then, a novel forward-backward linear prediction algorithm is proposed to suppress the broadening bistatic sea clutter caused by bistatic angle, which is based on the distribution characteristics and the multi-dimensional feature of first-order sea clutter. Finally, the proposed method is examined by measured data. Experimental results indicate that the proposed method can well suppress the broadening sea clutter and improve the ship target detection performance.

Spectrum optimization via FFT-based conjugate gradient method for unimodular sequence design

Abstract In this paper, we propose a unified framework for unimodular sequence design with different uses. We achieve the task by minimizing the residual error between the designed power spectrum density (PSD) and the target one. This unified metric includes the objective functions of PSD fitting, spectral mask reduction, improving signal-to-interference-plus-noise ratio (SINR), minimizing integrated sidelobe level (ISL), complementary set of sequences (CSS) design, and orthogonal set of sequences (OSS) design as special cases. We solve the PSD residual error minimization using conjugate gradient (CG) method, which enjoys reliable local convergence and good convergence rate. We derive a way to employ fast Fourier transform (FFT) in calculating the gradient with respect to the sequences phase, so that the CG method can be implemented efficiently. Due to the inherent nature of gradient method, the proposed method is very flexible and it can tackle the designs with composite objectives, which are more challenging and often intractable for the existing methods. Comparisons with the state-of-the-art methods indicate that the proposed method can achieve better or equally good results with much reduced execution time.

PCFM Radar Waveform Design With Spectral and Correlation Considerations

This paper considers the polyphase-coded FM (PCFM) waveform design with good spectral compatibility and correlation performance. In getting the desired PCFM waveform, we first design its sampled sequence with spectral and correlation considerations. To be specific, the power in prescribed stopbands is minimized to improve spectral compatibility, and correlation performance is enhanced by minimizing the integrated mainlobe energy (IME) and integrated sidelobe energy (ISE) of the autocorrelation function, which, respectively, control the range resolution and range sidelobe level. We deduce the optimal power spectrum density (PSD) condition for the considered problem and write the objective function as the residual error between the sequences PSD and the optimal one. We also devise a way to employ fast Fourier transform (FFT) in calculating the gradient of the PSD residual error such that it can be efficiently minimized using a general gradient descent method. With the designed sequence, we propose a new sequence-to-waveform implementation to reconstruct the PCFM waveform. Unlike sequences designed in many existing studies, the PCFM waveform is much closer to the real emission and thus expected to suffer less distortion in physical implementation.

A Beamformer Based on Sparse Array with Various Array Structures

The degrees of freedom (DOF) of the adaptive beamformer based on conventional linear array is limited by the number of physical array sensors. One way to enhance the DOF is to choose sparse array, whose DOF is greater than the number of sensors in the array. In this paper, we explore the adaptive beamformer based on sparse array with various sparse array structures. We evaluate the performance of different structures and get some useful conclusions on how to select the structure of sparse array. In addition, we also consider one robust beamforming technique to further improve the performance of sparse arrays. Simulations are provided to compare the results obtained by different strategies.

Ionospheric clutter suppression using Wavelet Oblique Projecting Filter

The detection performance of High Frequency Surface Wave Radar (HFSWR) is influenced by the nonstationary ionospheric clutter. For decades, ionospheric clutter suppression is always a challenging problem. Targets and ionospheric clutter have different 2D scale features in Range-Doppler map (RD map) of radar because of different stationarity and range correlation. 2D Discrete Wavelet Transform (2D-DWT) have advantages of separating such nonstationary signals with different 2D scales, so Wavelet Feature Vector (WFV) constructed by 2D-DWT is used to describe this feature in this paper. Based on the nonorthogonality and disjointness of target and clutter subspaces spanned by the WFVs, Wavelet Oblique Projecting Filter (WOPF) method is proposed to suppress the clutter. Experimental results show that this method can suppress clutter and extract targets effectively.

Sea clutter simulation for skywave radar considering ionospheric diffuse scattering

Ionospheric diffuse scattering is an important factor which leads to spread-Doppler clutter (SDC) in skywave radar. However, it is often ignored in a sea clutter model. In this paper, a sea clutter model for skywave radar considering ionospheric diffuse scattering was established. A general assumption about the independence of ionospheric time modulation functions in different spatial positions of the ionosphere is presented. Simulation results show the independence of time functions increases as the distance of their positions increases. It is verified by real data.

Steepest descent method for minimizing the peak sidelobe level of hopped-frequency waveform

Hopped-frequency waveform achieves high range resolution by transmitting a burst of narrowband pulses spanning the desired bandwidth. As its frequencies are irregularly or even sparsely distributed, hopped-frequency waveform could obtain a wider bandwidth with fixed pulses number and shows stronger anti-jamming ability than the linear stepped-frequency waveform; however, an undesired attribute associated is its high range sidelobe which would mask weak echo. In this paper, we optimally design the transmitted frequencies of the hopped-frequency waveform to suppress the peak sidelobe level (PSL). The considered design problem is formulated into a finite minimax optimization and a gradient-based descent method is proposed to solve this problem. This descent method uses the gradients of the largest several sub-functions of the considered minimax problem to fix a descent direction, and computes the step-length under the greedy criterion. Through iterative operation, this method reduces the function value monotonically and thus guarantees the convergence. Numeric results indicate that the proposed method can effectively suppress the PSL of the hopped-frequency waveform.

Ionospheric Decontamination for HF Hybrid Sky-Surface Wave Radar on a Shipborne Platform

This letter describes a method of correcting ionospheric frequency modulation for a high-frequency hybrid sky-surface wave radar mounted on a shipborne platform. In the proposed method, azimuth-dependent sea clutter signals are first decomposed into monocomponent signals based on distinguishable differences in their directions of incidence. Afterward, based on the decomposed monocomponent signals, the statistical mean of the time derivatives of the signal phases, weighted by the signal amplitudes, is used to estimate the ionospheric frequency modulation. Finally, the estimated result is applied to the received data to compensate for the ionospheric contamination. Numerical results on simulated data demonstrate the effectiveness of the proposed algorithm.