Yongtan Liu

Super-resolution processing for HF surface wave radar based on pre-whitened MUSIC

Owing to the decametric wavelength, the angular resolution of high-frequency surface wave radar (HFSWR) is usually coarse, especially when dimensions of antenna arrays are restricted such as in shipborne HFSWR applications. In this paper, the relative strength of atmospheric noise and sea clutter that will heavily degrade the capabilities of HFSWR in target detection and resolution are calculated, then a method for estimating the spatial covariance matrix of background noise is presented. By introducing a pre-whitening procedure in multiple signal classification (MUSIC), the resolution performance of MUSIC is enhanced in spatially colored noise environment. Results with data from the aircraft detecting experiment conducted by the Research Institute of Electronic Engineering of Harbin Institute of Technology in 1994 and simulated data of two targets show that pre-whitened MUSIC can provide a better resolution and accurate determination of target number. Furthermore, a post-processing method is proposed to eliminate the sidelobe of spatial spectrum arising from the estimation errors of a noise covariance matrix.

A new method of motion compensation for ISAR

A detailed consideration of the accuracy requirement for the compensation for the inverse synthetic aperture radar is presented. A new scheme of the motion compensation is submitted, in which the overall target, not any particular target scatter, is tracked to do the motion compensation. Some experimental results are given. The results indicate that this scheme of motion compensation is effective and feasible.<<ETX>>

Progress in HFSWR research at Harbin Institute of Technology

The Experimental HF Surface Over-The-Horizon Radar set up in the late of 80s has been updated by installation of multi-channel receivers with digitization at the second IF stage, new designed transmitting and receiving antennas, wideband solid-state power amplifiers, frequency synthesizer with low phase noise and flexible signal generator, a new radar controller added to allow the automatic and manual control of the radar, a new designed signal and data processor built mainly with 16bit A/D converters and several multiprocessor boards with Share chip processors added in, and a electromagnetic spectrum monitor equipped in the updated radar system to be able to automatically survey HF spectrum occupancy for providing so called clear frequency channel suitable for radar operation. The purpose of the setting up of the updated radar system is to demonstrate some of new functions of the radar system, and to prove some new techniques to be possibly used in a near future. In the paper, some latest developments of countering measures with radio frequency interferences are presented, too.

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.

Method Of Precise Motion Compensation For ISAR

In this paper, a method of precise motion compensation for InverseSynthetic Aperture Radar (ISAR) has been submitted. It is the overall target and not any particular target scatter which is tracked to do the motion compensation. Thus track loss arising from target scintillation or shadowing effect can be greatly reduced. So, it is possible to process a large segment data in azimuth as to improve resolution. The results shown in this paper demonstrate that the method of motion compensation is effective and feasible against maneuvering target.

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.

Ship detection in heavy sea clutter echoes and man-made radio noise environment for an on-shore HF ground wave frequency agile radar

The spectral characteristics of sea clutter echoes and man-made noise disturbance in the HF band are analyzed. A new spectral method for ship detection in heavy sea clutter and man-made noise environment by an onshore ground wave radar is introduced. A T-R cell averaging detector with the assistance of a spectrum monitoring system operates on a frequency agile mode, and it can detect ships even if they are masked by the first-order Bragg spectra of sea clutter echoes.<<ETX>>

Optimum weights of DPCA processing for shipborne HFSWR

The displaced phase center antenna (DPCA) can be used to detect a ship in broadened first order Bragg lines due to platform motion in shipborne HFSWR (high frequency surface wave radar). In the paper it is shown that, by an analogy with the optimum weights of MTI (moving target indicator), the DPCA processing introduced by Xie et al. (1998) is optimized for the symmetrical 1st order Bragg lines with equal power and the asymmetrical 1st order Bragg lines with unequal power. The result is advantageous for 1st order Bragg line cancellation in shipborne HFSWR.

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.