For decades, high-frequency (HF) radar has played an important role in sensing the Earth’s environment. Advances in radar technology are providing opportunities to significantly improve the performance of HF radar, and to introduce more applications. This paper presents a low-power, small-size, and multifunctional HF radar developed by the Ionospheric Laboratory of Wuhan University, referred to as the Wuhan Ionospheric Oblique Backscattering Sounding System (WIOBSS). Progress in the development of this radar is described in detail, including the basic principles of operation, the system configuration, the sounding waveforms, and the signal and data processing methods. Furthermore, its various remote sensing applications are briefly reviewed to show the good performance of this radar. Finally, some suggested solutions are given for further improvement of its performance.

Wuhan Ionospheric Oblique Backscattering Sounding System and Its Applications-A Review.

In this chapter we discuss reduction of matrices to the canonical form by use of orthogonal transformations in the spaces of images and preimages. Such canonical form is called the singular value decomposition. In what follows we will use the well-known polar decomposition, which is recalled in Section 1 in course of discussion of singular value decomposition of square matrices.

Singular Value Decomposition

An analytic high-frequency (HF) radar cross section model for ionosphere–ocean propagation is presented. Based on earlier work, an expression for the first-order received electric field after a single scatter from each of the ionosphere and sea surface is derived and reduced to integral form. The field integrals are taken to the time domain, with the source field being that of a vertically polarized pulsed dipole antenna. Subsequently, the first-order radar cross section for the mixed path mode of ionospheric clutter is developed. The ionosphere reflection coefficient used in the analysis is assumed to be a stochastic process with an associated spectral density function to account for phase variations along the surface associated with nonuniformity of the signal path. Simulation results for the new cross section are also provided. It is shown numerically that the expected magnitude of the ionosphere clutter, under reasonable assumptions, exceeds the dominant first-order ocean clutter for the same apparent range by 50–60 dB. Further, it is spread in Doppler, depending on the ionosphere horizontal velocity and reflecting path nonuniformity. Also, this component can be presented from a certain minimum range, depending on ionosphere virtual height, to the maximum radar range.

On the Development of a High-Frequency Radar Cross Section Model for Mixed Path Ionosphere–Ocean Propagation

The high-frequency hybrid sky-surface wave radar (HF HSSWR) has recently been used to monitor large-area sea states. However, most of the HF HSSWR detection methods are based on the assumption of a no-tilt and constant height ionospheric model, and the influences caused by uneven electron density are ignored. This paper proposes a new surface current inversion scheme for the HF distributed HSSWR system, which considers the unknown ionospheric state as a black box and extracts the key parameters to compute the surface current based on a scattering model. The computational formula of the component of the current vector is explored using spatial scattering theory instead of an approximate bistatic model. In addition, the Fourier series expansion method is applied to the HF data to extract the real first-order Bragg frequency. Subsequently, the grazing angle and the bistatic angle can be found by inversion using the first-order Bragg frequency formula after searching out the common scattering patch of two receiving stations. Simultaneously, the coordinate registration of the currents can also be determined. The feasibility and effectiveness of this new algorithm are verified with field experimental results by comparing the current vectors derived from HSSWR and traditional HF SWR. The RMS differences of the magnitude and direction of the current vectors within the core common area of the two detection systems are about 10.2 cm/s and 9.5°, respectively.

Ocean Surface Current Extraction Scheme With High-Frequency Distributed Hybrid Sky-Surface Wave Radar System

The performance of the high-frequency hybrid sky-surface wave radar is often degraded due to the distortion of radar echoes induced by time-varying ionosphere. Here a modified S-transform method based on the direct wave for ionosphere decontamination of high-frequency hybrid sky-surface wave radar is presented. First, we extract the time-varying-ionosphere-distorted phase of the echoes from the direct wave of high-frequency hybrid sky-surface wave radar by using the modified S-transform. Then this phase function is used to decontaminate the ionosphere distortion of radar echo at each range bins. Simulations and experimental data are conducted to validate the performance of the proposed method on decontaminating the distortion. The experimental data are acquired by Wuhan Universitys ocean state measuring and analyzing radar. A mass of radar data is processed and analyzed, demonstrating that the decontamination effect is statistically significant. The method is not only effective to remove the Doppler shift of direct wave and Bragg peaks caused by ionosphere but also able to narrow the broadened direct wave and Bragg peaks. Meanwhile, the SNR is improved in most range bins; the background noise is reduced detectably, sometimes significantly; and some isolated radar echoes near direct wave or Bragg peaks are recovered or enhanced. These results indicate that the proposed decontamination algorithm works well.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018RS006645

Correction of Ionospheric Distortion on HF Hybrid Sky-Surface Wave Radar Calibrated by Direct Wave

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.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-spr.2014.0203

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

This paper based on a fast implemented multiphase screen method using DFT puts forward an ionospheric Es layer clutter model and uses the newly developed dimensionality reduction space-time adaptive processing- (STAP-) JDL algorithm to suppress Es layer clutter, which proves the validity of the proposed model. Firstly, the multiphase screen method was analyzed, and a fast algorithm using DFT was proposed. Then, based on the multiphase screen method and thorough simulation, we reached a conclusion of the high-frequency radio wave propagation’s fluctuation characteristics in the ionosphere. According to the results of the analysis, a new Es layer ionospheric clutter model was established and was compared with the measured data and verification was made. Finally, based on the built clutter model, JDL algorithm was applied to the high-frequency surface wave radar ionospheric clutter suppression, using the measured data to verify the validity of the model and algorithm. The simulation results showed that the built model can show the characteristics of the ionospheric Es layer clutter and that the JDL algorithm can suppress ionospheric Es layer clutter quite effectively.

/pdf/an-ionospheric-es-layer-clutter-model-and-suppression-in-hf-lvlr05vdu1.pdf

An Ionospheric Es Layer Clutter Model and Suppression in HF Surfacewave Radar

The invention provides an ionosphere propagation characteristic based phase diameter disturbance suppression method and belongs to the radar signal processing technical field. The problem that the suppression effect is limited and even fails when the disturbance suppression is performed on the ionosphere phase diameter by the existing ionosphere phase diameter disturbance suppression method from the perspective of signal processing is solved. According to the technical scheme, the Ionosphere propagation characteristic based phase diameter disturbance suppression method comprises performing the hybrid modeling on an international reference ionosphere model and an MQP (Multi-Quasi Parabolic) model to obtain an ionosphere space reflection time-varying parameter model; deriving a phase path calculation analytic expression of the MQP model by a ray tracing method on the basis of the electron concentration profile of ionosphere space reflection time-varying parameter model and obtaining change values of a phase path along with the time after the repeated operation at different moments; performing the suppression on the back wave spectrum with the ionosphere phase diameter disturbance through a phase disturbance compensation curve obtained through fitting by a least square method and finally obtaining the suppressed ionosphere back wave spectrum. The Ionosphere propagation characteristic based phase diameter disturbance suppression method is suitable for the processing on the broadening sea clutter spectrum with the ionosphere phase diameter disturbance under a high-frequency sky wave radar.

Ionosphere propagation characteristic based phase diameter disturbance suppression method

The HF hybrid sky-surface wave radar is a new system radar, and because of the complex characteristics of ionosphere as a propagation medium, its detection performance is widely determined by the state of ionosphere. Combined with the layout of the radar system and the character of receiving in wide beam, this paper analyzes the influence of ionosphere on the size of resolution cell and signal to clutter ratio (SCR). First of all, it analyzes and calculates the influence of ionospheric dispersion and irregularities on resolution. Then, it analyzes the calculation methods of the area of resolution cell under the condition of the bistatic radar and the wide beam, and analyzes ionospheric irregularities' influence on the size of resolution cell. At last, according to the basic equation of HF hybrid sky-surface wave radar, it deduces the influence of ionosphere on SCR, and do the simulation with parameters of typical system and ionosphere. The simulation results show that ionospheric dispersion and irregularities affect the size of the resolution cell, reduce the SCR, which go against the detection of ship targets.

Influence of ionosphere on resolution cell of HF hybrid sky-surface wave radar

Ionospheric phase perturbation with large amplitude causes broadening sea clutter's Bragg peaks to overlap each other; the performance of traditional decontamination methods about filtering Bragg peak is poor, which greatly limits the detection performance of HF skywave radars. In view of the ionospheric phase perturbation with large amplitude, this paper proposes a cascaded approach based on improved S-method to correct the ionospheric phase contamination. This approach consists of two correction steps. At the first step, a time-frequency distribution method based on improved S-method is adopted and an optimal detection method is designed to obtain a coarse ionospheric modulation estimation from the time-frequency distribution. At the second correction step, based on the phase gradient algorithm (PGA) is exploited to eliminate the residual contamination. Finally, use the measured data to verify the effectiveness of the method. Simulation results show the time-frequency resolution of this method is high and is not affected by the interference of the cross term; ionospheric phase perturbation with large amplitude can be corrected in low signal-to-noise (SNR); such a cascade correction method has a good effect.

/pdf/a-cascaded-approach-for-correcting-ionospheric-contamination-3ya0clzzn9.pdf

Rongqing Xu

Papers

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

An Ionospheric Es Layer Clutter Model and Suppression in HF Surfacewave Radar

Ionosphere propagation characteristic based phase diameter disturbance suppression method

Influence of ionosphere on resolution cell of HF hybrid sky-surface wave radar

A Cascaded Approach for Correcting Ionospheric Contamination with Large Amplitude in HF Skywave Radars