High-frequency Hybrid Sky-surface Wave Radar Research Articles

Spreading Sea Clutter Suppression for High-Frequency Hybrid Sky-Surface Wave Radar Using Orthogonal Projection in Spatial–Temporal Domain

In recent years, the high-frequency hybrid sky-surface wave radar (HSSWR) has been increasingly used for target detection applications. Nevertheless, the specific bistatic system layout and the phase path disturbances induced by the ionospheric propagation channel may severely spread the sea clutter spectrum, thereby deteriorating the detection ability of the HSSWR for slow-moving targets. In this work, a novel subspace method based on the hybrid use of the amplitude and phase estimator (APES) and the orthogonal projection (OP) in the spatial–temporal domain, denoted as the APES-OP method, is proposed to suppress the spreading first-order sea clutter of the HSSWR. The distribution characteristics of targets and first-order sea clutter in the spatial–temporal domain were investigated, and a time-domain subspace signal model was adopted to describe targets perturbed by ionospheric phase path modulation. An APES filter was adopted to filter out the potential targets with a traversal approach to avoid attenuating desired signals while suppressing sea clutter. After that, sampling data from multi-channels and slow-time domains at the cell under test were employed to construct a spatial–temporal matrix, which was then utilized to obtain the sea clutter subspace by singular value decomposition. Simulation results indicate that the proposed algorithm can suppress sea clutter while retaining the target, even if the target is buried by sea clutter. The processing results of measured data further demonstrate the efficiency of the proposed algorithm. After sea clutter suppression, the target obscured by clutter can be revealed, and the signal-to-clutter ratio of the target is greatly improved.

Floating-Platform High-Frequency Hybrid Sky-Surface Wave Radar: Simulations and Experiments

A theoretical sea echo modeling of the floating-platform high-frequency hybrid sky-surface wave radar (HSSWR) is presented. The ionosphere is considered to be a no-tilt reflecting plane moving in the vertical direction. Subsequently, the first-order sea clutter cross section with consideration of the platform sway and yaw motion is derived. Simulations are conducted to explore the influences of the ionospheric movement and the platform oscillation motion on the sea clutter power spectrum. Simulation results indicate that the platform sway motion may induce additional peaks that distribute symmetrically concerning the Bragg peaks, while the platform yaw motion and the ionospheric movement may broaden even split the sea clutter. Experiments are conducted with the newly-developed HSSWR system, of which the receiving arrays are simultaneously deployed on a floating platform and the shoreside. Experimental measurements concerning the frequency shift, Doppler width, and the arrival angle of the E-layer/F-layer ionosphere reflected direct wave are compared and analyzed. The study demonstrates the feasibility of the proposed modeling and provides valuable guidelines for the sea clutter characteristics analysis and the data quality assessments of the floating-platform HSSWR.

Azimuth Resolution Improvement and Target Parameters Inversion for Distributed Shipborne High Frequency Hybrid Sky-Surface Wave Radar

In this paper, the aperture synthesis processing techniques for the distributed shipborne high frequency hybrid sky-surface wave radar (HFHSSWR) are proposed to improve the azimuth resolution and obtain the velocity vector and the azimuth estimation of the moving target. First, the system geometry and the signal model of the moving target for the distributed shipborne HFHSSWR are formulated, and then the azimuth resolution improvement principle is derived. Second, based on the developed signal model, we propose an azimuth resolution improvement algorithm, which can obtain the synthetic azimuth bandwidth and an improved resolution using sub-band combination. Finally, a target parameters inversion method is introduced to estimate the target velocity vector and the target azimuth, by solving the equations regarding the target geometry and echo signal parameters numerically. The simulations are performed to verify the proposed algorithms. The results indicate that the distributed synthetic aperture techniques effectively improve the azimuth resolution of this radar, and can obtain the target velocity vector and the high-precision estimation of the target azimuth.

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.

A Novel Main-Lobe Cancellation Method Based on a Single Notch Space Filter and Optimized Correlation Analysis Strategy

The high-frequency hybrid sky-surface wave radar (HFSSWR) is a new kind of over-the-horizon radar system. Combining the advantages of both the sky wave radar and the surface wave radar, HFSSWR has drawn much attention in recent years. But the new system also brings new challenges. One of the most severe problems is that the heterogeneous environment makes a big challenge for the sea clutter suppression processing. Due to the nonstationary properties of the ionosphere, the first-order sea clutter statistics change significantly among range bins. So the new efficient sea clutter suppression method is required, and the clutter characteristics analyses are also in urgent need of research in order to guide the design of the algorithm in the background of HFSSWR. In this paper, utilizing the measured data set, we first analyse the range and spatial characteristics of the nonhomogeneous first-order sea clutter in HFSSWR. Then, an improved main-lobe cancellation (IMLC) method based on single notch space filter and correlation analysis is proposed to get training data which contains precise clutter information. A main-lobe clutter canceller based on the single notch space filter has been developed to block the target component, and an optimized correlation analysis (OCA) strategy is presented to choose the efficient training data. Finally, the method is examined by measured data and the results indicate that the method has a good performance for broadening first-order sea clutter suppression than the traditional method.

Simulation Analysis and Experimental Study on the Echo Characteristics of High-Frequency Hybrid Sky–Surface Wave Propagation Mode

In high-frequency hybrid sky–surface wave radar, the echo characteristics of hybrid sky–surface wave propagation channel are extremely complex in spatial, time, and frequency domains, which seriously affect radar performance. The mechanisms and echo characteristics are theoretically and experimentally analyzed in this paper. Based on the hybrid sky–surface propagation model, the signal-to-noise ratio equation of the channel echo and ionospheric direct wave are initially derived. The simulation method is developed, which considers bistatic layout, ionospheric propagation, sea surface propagation, and radio noise. The echo characteristics by sweep frequency detection are simulated and analyzed. Using the newly developed hybrid sky–surface wave experimental system, the experimental results by sweep frequency detection are first achieved in China and abroad, which are consistent with the theoretical analysis, and the validity of the simulation is verified. Then, the characteristics of first-order sea clutter in different azimuths are theoretically and experimentally analyzed, and it is found that the bistatic layout and ionosphere play a significant role in the Doppler broadening and path distribution of the sea clutter. Finally, the experimental results by joint sweep and fixed frequency detections are analyzed. The echo characteristics are extremely complex with frequency. Specifically, owing to multimode echoes overlap, the spread of the sea clutter and ionospheric direct wave signals would seriously affect ship target detection in part of the operating frequency band. It is shown that the detection and diagnosis for hybrid sky–surface channel in spatial, time, and frequency domains are crucial for target detection performance. The research results on the propagation mechanisms and echo characteristics could provide important supporting function for layout design, frequency selection, target detection, and location of the radar.

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

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.

Experimental analysis of a HF hybrid sky-surface wave radar

High-frequency (HF) radar systems exploit refractive and diffractive properties of HF waves to sense targets beyond the line of sight. HF skywave radars use the ionosphere medium to reflect the signals and thus illuminate targets of thousands of kilometers away, but they suffer from the close-distance blind zone and severe ionosphere decorrelation effect. HF surface wave radars exploit the diffraction of vertically polarized waves to detect targets within hundreds of kilometers offshore. But the diffracted surface wave is highly attenuated, and the radar has the vertical incidence ionospheric clutter problems. High-frequency hybrid sky-surface wave radar (HFSSWR) is a novel radar configuration containing a skywave transmitting path and a surface wave receiving path, which is a natural combination of HF skywave and HF surface wave radar systems [1]. In addition, multiple separate receiving sites can compose a multistatic network and operate with only one transmitter. The system configuration is flexible.

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.

Ocean surface current measurement with high-frequency hybrid sky–surface wave radar

ABSTRACTHigh-frequency hybrid sky–surface wave radar (HFHSSWR) is a new remote sensing instrument with potential for applicability to large-area sea state monitoring. This study investigated the use of HFHSSWR for ocean surface current measurements. Based on the Doppler shift of the first-order Bragg peak of HFHSSWR, the component of the Doppler shift caused by the ocean surface current was calculated, and the elliptical velocity of the ocean surface current measured by HFHSSWR was derived. Land clutter was used as a reference source to correct the linear variation of the ionospheric effect, and the average of the current inversion results within many coherent integration times was used to eliminate the nonlinear variation of the ionospheric effect. Finally, the inversion results of the ocean surface current were validated using synchronised in situ measurements from a current meter. Their comparison (correlation coefficient: 0.81, root mean square error: 0.138 m s−1) demonstrated the feasibility of measuring ocean currents with HFHSSWR.