Junhao Xie

Second-Order Ocean Surface Cross Section for Shipborne HFSWR

To investigate the characteristics of sea clutter in monostatic shipborne high-frequency surface-wave radar (HFSWR) with uniform linear motion, the second-order patch scatter cross section is derived. With the previously proposed first-order ocean surface cross section, both cross sections could be reduced to the existing results in land-based HFSWR. Illustrations and discussions are provided on the basis of the simulation results for different situations. Although the spread Bragg lines vary with the ocean surface wind direction, the second-order contributions contaminate the energies of the Bragg lines in the high sea state, which may affect the application of shipborne HFSWR to wind direction extraction from the ratio of the positive and negative Bragg line energies. The derivations and analyses would be of importance to future investigations and applications, not only for target detection but also for ocean remote sensing.

Ocean Surface Current Measurement Using Shipborne HF Radar: Model and Analysis

The extension of onshore high-frequency surface wave radar (HFSWR) to shipborne HFSWR has the advantage of extended monitoring area and high application flexibility. However, additional modulation on the echo signal introduced by the forward movement and six-degree-of-freedom (6-DOF) ship motion may increase the surface current measurement error. Considering the radar array position on the ship, six-DOF ship motion, forward movement velocity, first-order Bragg scatter, and background noise, this paper models the backscattered Doppler spectrum of shipborne HFSWR. Based on this model, the performance of surface radial current measurement is analyzed, with the adoption of the MUltiple SIgnal Classification (MUSIC) to estimate the azimuth of the sea patches. Simulation results show that, rotation, sway and forward movement of the ship all have significant impacts on the surface radial current measure error. Nevertheless, if the ship sails at relative low speed and keeps its sailing direction (with small yaw amplitude), shipborne HFSWR may provide a better result than onshore HFSWR.

Remote sensing of surface currents with single shipborne high-frequency surface wave radar

High-frequency surface wave radar (HFSWR) is a useful technology for remote sensing of surface currents. It usually requires two (or more) stations spaced apart to create a two-dimensional (2D) current vector field. However, this method can only obtain the measurements within the overlapping coverage, which wastes most of the data from only one radar observation. Furthermore, it increases observation’s costs significantly. To reduce the number of required radars and increase the ocean area that can be measured, this paper proposes an economical methodology for remote sensing of the 2D surface current vector field using single shipborne HFSWR. The methodology contains two parts: (1) a real space-time multiple signal classification (MUSIC) based on sparse representation and unitary transformation techniques is developed for measuring the radial currents from the spreading first-order spectra, and (2) the stream function method is introduced to obtain the 2D surface current vector field. Some important conclusions are drawn, and simulations are included to validate the correctness of them.

Remote sensing of ocean surface wind direction with shipborne high frequency surface wave radar

Land-based high frequency surface wave radar (HFSWR) has been successfully employed for early warning and ocean remote sensing. However, there remain potential gaps for shipborne HFSWR in the application of ocean remote sensing. In this paper, the feasibility of extracting wind direction is studied based on the first-order ocean surface cross section in shipborne HFSWR. According to the spreading mechanism of the Bragg lines, a method of extracting wind direction without ambiguity is proposed. Furthermore, by use of a single receiving sensor rather than the receiving array, wind direction of the large sea area covered by radar can be obtained. Together with the simulation results, analyses of the concerning issues in an actual application verify the potential of the method proposed. Compared with the methods based on a huge receiving array in land-based HFSWR, it can provide higher transverse resolution and be more easily realized with less system cost.

High-Resolution Ocean Clutter Spectrum Estimation for Shipborne HFSWR Using Sparse-Representation-Based MUSIC

The spreading of the dominant first-order Bragg lines in shipborne high-frequency surface wave radar (HFSWR) severely obscures the detection of the slow-moving targets and the measurement of ocean clutter. Space-time adaptive processing (STAP) is an effective tool for solving the problem. It normally requires a large number of independent and identically distributed (i.i.d.) training samples to estimate the ocean clutter spectrum and design the filter to eliminate the ocean clutter from the test cell. However, the training samples are insufficient due to the system limitation of shipborne HFSWR, and the stationarity of training data is destroyed in the nonstationary and nonhomogeneous ocean environment, which result in decreased performance. Thus, the estimation of the ocean clutter spectrum with small training samples or even only the test cell is an important work for shipborne HFSWR. In this paper, by exploiting the intrinsic sparsity of the ocean clutter in shipborne HFSWR, the multiple signal classification (MUSIC) algorithm based on the sparse representation technique, called SR-MUSIC, is introduced to estimate the ocean clutter spectrum. The correctness of the ocean clutter sparsity and the validity of the SR-MUSIC algorithm for the high-resolution ocean clutter spectrum estimation are verified by the simulation results.

Ocean Surface Wind Direction Inversion Using Shipborne High-Frequency Surface Wave Radar

Shipborne high-frequency surface wave radar (SHFSWR) has exhibited great advantages over onshore HFSWR (OHFSWR) in ocean remote sensing. Unlike OHFSWR, SHFSWR suffers the problem of Doppler spectrum spread owing to platform movement, which is a great challenge preventing the extraction of ocean surface parameters for SHFSWR. To address this challenge, in this letter, the mathematical model of ocean surface wind direction is first investigated based on the first-order SHFSWR cross section. Furthermore, a method for the wind direction inversion without ambiguity from the spread Doppler spectrum is proposed using a single receiving antenna. Meanwhile, the wind directions of the sea area covered by radar can be obtained by sequentially utilizing the proposed method, which is more appropriate for the application of SHFSWR with limited deck space and less cost. Experimental results of the real data collected in Taiwan Strait preliminarily verify the detection accuracy and the distance limit of the wind direction inversion, as the root-mean-square error and the detection range are 9.85° and 120 km, respectively

An Improved Oblique Projection Method for Sea Clutter Suppression in Shipborne HFSWR

Sea clutter has a major impact on the detection performance of a shipborne high-frequency surface wave radar (HFSWR) system. Due to the platform motion of shipborne HFSWR, the Doppler spectrum of the first-order sea clutter suffers from some broadening so that the targets submerged in this broadening Doppler spectrum can be hardly detected. In this letter, an improved oblique projection (IOP) method, combining the oblique projection (OP) algorithm and the method of sea clutter suppression in the Doppler domain, is proposed to suppress sea clutter in both Doppler domain and spatial domain for shipborne HFSWR. Compared with the OP and the orthogonal weighting algorithms, the proposed IOP algorithm is shown to give far superior suppression results in the Doppler domain and can achieve better azimuth estimation results based on real data.

The first-order ocean surface cross section for shipborne HFSWR with rotation motion

To analyze the characteristics of ocean surface backscatter signals, the first-order high frequency (HF) radar ocean surface cross section is mathematically developed for the case of an omnidirectional transmitting/receiving antenna being installed on the shipborne platform with rotation motion. The developed cross section could be degenerated to a stationary monostatic land-based case. The simulations are carried out under different radar parameters and sea states. The results show that the rotation motion has an important influence on radar Doppler spectrum, which may bring significant implications in future investigations for detection of sea targets and ocean remote sensing.

Space-Time Distribution of the First-Order Sea Clutter in High Frequency Surface Wave Radar on a Moving Shipborne Platform

The first-order Bragg lines in ship borne high frequency surface wave radar (HFSWR) are spread due to the platform motion, and the space-time distribution of the first-order sea clutter manifests itself as two ideal diagonal lines. However, because of the interaction between platform and sea environment (such as ocean wave, current and wind), the first-order sea clutter spectra are influenced by the three oscillating motions of the ship borne platform, sway, surge and heave. In this paper, the effects of such three motions on the first-order sea clutter and the space-time distribution are investigated based on a uniform linear array consisting of seven omni directional vertically polarized pulsed dipole antennas.

Influence of bistatic shipborne HFSWOTHR platform oscillation on the sea clutter

Under certain sea state, the sea wave will lead to the ship oscillation when the ship is sailing on the sea. The influence that the ship oscillation has on the target echo is usually discarded when simulating the target in shipborne HFSWOTHR. In this paper, the characteristic of ship oscillation is analyzed, including rolling, pitching and yawing. The model of the ship oscillation is set up. The concept of the instantaneous synthetic rotational velocity and angle is introduced and their expression are deduced. Then, the dynamic geometry relation model of bistatic shipborne HFSWOTHR in the condition of the ship oscillation is set up. Computer simulations are carried out for the sea clutter under the condition of the swing and no swing of the ship. At last, the sea clutter spectrums that come from the two conditions of the oscillation and no swing are compared in order to analyse the influence of the oscillation. The study of this paper will improve the simulation of sea clutter in shipborne HFSWR, which makes the simulation more close to reality.