Ground Wave Radar Research Articles

Probabilistic modelling of surface drift prediction in marine disasters based on the NN–GA and ARMA model

This paper studies the issue of target drift prediction based on ocean current from high-frequency (HF) ground-wave radar. A large number of unpowered drift observation experiments were carried out with a typical South China Sea offshore fishing vessel in the range of radar radiation. A target drift prediction model was proposed based on the neural network-genetic algorithm (NN–GA) and Auto-regressive Moving Average (ARMA) model. Replacing the traditional AP98 model, the NN–GA was used to establish the target wind-induced drift characteristic model, while the ARMA model replaced the traditional sub-grid model to refine the time series characteristics of sub-grid velocity. Firstly, trajectory simulation based on the Runge-Kutta method was used to preliminarily verify the improvement of target drift prediction accuracy by the ARMA model. In addition, the MonteCarlo method was used to calculate and compare the prediction range of different sub-grid velocity models. The study also investigated the NN–GA and ARMA model with different continuous prediction time steps. Compared with the traditional AP98 and sub-grid velocity model, the proposed method does not need to pay attention to the detailed force analysis of the drifting target and can significantly improve the performance of the target drift prediction model in terms of mean prediction error and separation. However, the ARMA model is also limited by its continuous prediction step size to a large extent. This study provides a new approach for trajectory analysis of drifting targets and verifies that the NN–GA and ARMA model with short-term continuous prediction can improve the performance of the target drift prediction model.

Review of the research on the application of ground wave radar current detection in China

As an effective new type of marine exploration equipment with high measurement accuracy, high spatial resolution and high repetitive sampling frequency, ground wave radar is widely used in the field of marine environment exploration around the world. This paper introduces the current status of international ground wave radar development and the operation of ground wave radar sea current detection in China. Then the current status of current observation date’s application of Chinese ground wave radar is described from four research directions, including tidal current prediction, calibration and validation, data quality control and numerical simulation. Finally, the development trend of ground wave radar is briefly analysed.

MIMO Ground Wave Radar Radio Frequency Monitoring

High Frequency (HF) ground wave radar is widely used in maritime target detection and marine environment observation. Its performance is often degraded by co-frequency interference of the HF band. It is necessary to detect the environmental radio frequency for a HF radar to select the proper operation frequency. In this paper, a radio frequency detecting module is designed and equipped on a double-frequency all-digital multi-input and multi-output (MIMO) HF radar. The operational frequency bands are scanned in the operation of the radar to help select an unoccupied frequency in the band. The application of this module reduced the influence of RF interference on the HF radar. The improvement in the signal quality is obvious.

Far-field calculation of an arbitrarily oriented electric dipole in horizontal layered confined space

The far-field calculation of arbitrarily oriented electric dipole in a stratified confined space is of great significance in analyzing electromagnetic properties in the lightning return stroke, submarine communication, over-the-horizon ground-wave radar, etc. Based on the mirror image method and the far-field approximation of an electric dipole in free space, a three-layer horizontal confined space model with an arbitrarily oriented dipole is established in this work. Through novel vector operations, the expression of the far field generated by an arbitrarily oriented electric dipole in the confined space model is derived, where the direct wave from the source point to the observation point and the waves reflected by the upper boundary and the lower boundary are all comprehensively considered. On this basis, the radiation characteristics of the electric dipole with frequencies of 100 kHz, 6 MHz and 10 MHz at different positions in the Earth-ionosphere waveguide are compared and analyzed, which are taken for example. Three different orientations of electric dipoles,i.e. vertical direction, horizontal direction, and 45º tilt are taken into account and the corresponding radiation patterns are presented. The results show that the radiation characteristics of electric dipoles in the Earth-ionosphere waveguide will change greatly with their frequencies, orientations and positions. For the electric dipole source at the same location, the higher the frequency, the more the number of radiation lobes is. In addition, when the source frequency keeps unchanged, the farther the dipole source is from the bottom boundary, the more the radiated lobes are. The proposed expression can conveniently and accurately consider the direct wave of a dipole source and its primary reflection from the upper interface and the lower interface in a confined space, and can also be further extended to solving the contribution of multiple reflections from the interfaces.

High-Frequency Radar Cross Section of Ocean Surface for an FMICW Source

The frequency-modulated interrupted continuous waveform (FMICW) has been widely used in remotely sensing sea surface states by high-frequency ground wave radar (HFGWR). However, the radar cross section model of the sea surface for this waveform has not yet been presented. Therefore, the first- and second-order cross section models of the sea surface about this waveform are derived in this study. The derivation begins with the general electric field equations. Subsequently, the FMICW source is introduced as the radar transmitted signal to obtain the FMICW-incorporated backscattered electric field equations. These equations are used to calculate range spectra by Fourier transforming. Therefore, Fourier transformation of the range spectra calculated from successive sweep intervals gives the Doppler spectra or the power spectral densities. The radar cross section model is obtained by directly comparing the Doppler spectra with the standard radar range equation. Moreover, the derived first- and second-order radar cross section models for an FMICW source are simulated and compared with those for a frequency-modulated continuous waveform (FMCW) source. Results show that the cross section models for the FMICW and FMCW cases have different analytical expressions but almost the same numerical results.

Multi-Channel Data Acquisition Card under New Acquisition and Transmission Architecture of High Frequency Ground Wave Radar

It is known that the data acquisition and processing system plays an important role in radar target detection system. In order to meet the requirements of real-time processing and accurate transmission of echo signals in high-frequency ground-wave radar (HFGWR) systems, a new acquisition and transmission framework utilizing the designed acquisition card based on the PCIe (peripheral component interconnect express) has been designed and is presented in this paper. The Xilinx FPGA (Field-Programmable Gate Array) chip Kintex7-XC7K325T is adopted as a hardware carrier in acquisition card. The hardware’s composition, analog front-end circuit, the DMA (Direct Memory Access) transmission, FPGA structure, ADC (Analog-to-Digital Converter) chip, and performance test of this card are showed and discussed. Currently, the acquisition card has been accomplished and applied in the practical system of HFGWR.

Advanced Sea Surface Target Detection System for Shipborne VHF Radar in Log Periodic Dipole Array Antenna

While navigating in sea target detection is an important problem in modern era communication. In the ocean to detect moving targets and to analyze active variables such as wind, waves and current we can make use of Very High Frequency Ground Wave Radar (VHFGWR). In shipborne VHFGWR the brag spectra widened by Doppler Effect of the ship motion, due to this vessel echo easily merged.An orthogonal projection method to suppress the extended Brag spectra using a log periodic dipole array is proposed in this paper. The advanced technique majorly covers two stages: 1) by utilizing sea echo parameters calculate the approximation of sea clutter in spatial domain and 2) design multi frequency classification Doppler bins method to suppress the extended Brag spectra. After echo suppression, for target detection we can use different algorithms. Simulation result gives a proof that signal- to- clutter noise ratio (SCR) is developed and sea clutter got removed. This method gives very good target detection for shipborne VHF radar.

A New Multigrid 3D-VAR Optimization Method for Bottom Friction Using HF Radar Current Observation

This paper proposes a new method for data assimilation of the surface radial current observed by High Frequency ground wave radar and optimization of the bottom friction coefficient. In this method, the shallow water wave equation is introduced into the cost function of the multigrid three-dimensional variation data assimilation method as the weak constraint term, the surface current and the bottom friction coefficient are defined as the analytical variables, and the high spatiotemporal resolution surface radial flow observed by the high-frequency ground wave radar is used to optimize the surface current and bottom friction coefficient. This method can effectively consider the spatiotemporal correlation of radar data and extract multiscale information from surface radial flow data from long waves to short waves. Introducing the shallow water wave equation into the cost function as a weak constraint condition can adjust both the momentum and mass fields simultaneously to obtain more reasonable analysis information. The optimized bottom friction coefficient is introduced into the regional ocean numerical model to carry out numerical experiments. The test results show that the bottom friction coefficient obtained by this method can effectively improve the accuracy of the numerical simulation of sea surface height in the offshore area and reduce the simulation error.

Calibration and Evaluation of a Circular Antenna Array for HF Radar Based on AIS Information

High-frequency (HF) ground-wave radars employing compact antenna arrays are advantageous in practical applications because of their convenient deployment. However, the direction-of-arrival (DOA) estimation accuracy may significantly degrade in the presence of array errors for compact phased-array HF radars. Consequently, wind, wave, and current measurements would become unavailable for further applications. In this letter, an array calibration method is proposed for an HF radar system based on a circular antenna array. First, an array manifold model containing array errors is established. Second, a calibrated array manifold is obtained by fitting and interpolating the responses of ship echoes using a least squares fitting method. Finally, the calibrated array manifold and the noise subspace matrix are substituted into the multiple signal classification (MUSIC) method to estimate the DOA of each ship echo. The results indicate that this calibration method significantly reduces the DOA estimation errors with the root-mean-square errors of less than 10°.

Sea Clutter Suppression for Shipborne HF Radar Using Cross-Loop/Monopole Array

The shipborne high-frequency ground wave radar (HFGWR) provides a better condition for sea surface target detection than the shore-based one because of its maneuverability and flexibility. However, the first-order Bragg spectra of the shipborne HFGWR would be broadened by the Doppler shift of the ship motion, which makes the vessel echo easily submerged. Even though some methods have been applied to suppress the broadened Bragg spectra for target detection, most of them are hard to achieve good result in a small-aperture radar. An orthogonal projection method to suppress the broadened Bragg spectra using a cross-loop/monopole array is proposed in this letter. The proposed method mainly includes two steps: 1) estimate the space of sea clutter in a spatial domain according to the ocean echo characteristics and 2) design an adaptive filter of single or multiple Doppler bins on the estimated clutter space to suppress the first-order spectra. After suppression, we use the multiple signal classification algorithm to estimate the incident direction of targets. The experimental data validate that the sea clutter is well suppressed and the target signal-to-clutter ratio is increased by the algorithm.

Evaluation on surface current observing network of high frequency ground wave radars in the Gulf of Thailand

Due to the high cost of ocean observation system, the scientific design of observation network becomes much important. The current network of the high frequency radar system in the Gulf of Thailand has been studied using a three-dimensional coastal ocean model. At first, the observations from current radars have been assimilated into this coastal model and the forecast results have improved due to the data assimilation. But the results also show that further optimization of the observing network is necessary. And then, a series of experiments were carried out to assess the performance of the existing high frequency ground wave radar surface current observation system. The simulated surface current data in three regions were assimilated sequentially using an efficient ensemble Kalman filter data assimilation scheme. The experimental results showed that the coastal surface current observation system plays a positive role in improving the numerical simulation of the currents. Compared with the control experiment without assimilation, the simulation precision of surface and subsurface current had been improved after assimilated the surface currents observed at current networks. However, the improvement for three observing regions was quite different and current observing network in the Gulf of Thailand is not effective and a further optimization is required. Based on these evaluations, a manual scheme has been designed by discarding the redundant and inefficient locations and adding new stations where the performance after data assimilation is still low. For comparison, an objective scheme based on the idea of data assimilation has been obtained. Results show that all the two schemes of observing network perform better than the original network and optimal scheme-based data assimilation is much superior to the manual scheme that based on the evaluation of original observing network in the Gulf of Thailand. The distributions of the optimal network of radars could be a useful guidance for future design of observing system in this region.

Radio Frequency Interference Suppression Algorithm in Spatial Domain for Compact High-Frequency Radar

High-frequency (HF) ground wave radar is an important tool for sea state measurement and low-altitude target detection. Dense radio frequency interference (RFI) in the HF band inhibits the extraction of sea state parameters and degrades the performance of radar. Though many interference suppression methods based on large arrays have been proposed, most of them are weak to deal with the interference in small-aperture radar. In this letter, a spatial subspace method is proposed to suppress RFI, which uses two crossed-loop/monopole antennas to construct interference subspace at the reserved range bins and projects the echoes onto its orthogonal subspace at the interested range bins. The processed results of measured data from OSMAR-SD verify the high performance of this algorithm for dense RFI suppression. Apart from that, the data utilization ratio and the accuracy of estimating wave height are both improved significantly after the RFI suppression.

Study on Pattern Distortion and DOA Estimation Performance of Crossed-Loop/Monopole Antenna in HF Radar

Crossed-loop (CL)/monopole antenna has been widely used in high-frequency ground-wave radar for ocean surface remote sensing owing to its compact size. The multiple signal classification (MUSIC) algorithm is commonly adapted to achieve a favorable angular resolution for this broad beam antenna. However, the direction of arrival (DOA) estimation is always troubled by the frequently occurring distortion of antenna pattern. Previous studies are mainly focused on the calibration by using the actual pattern or other assistant algorithms. In this paper, theoretical analysis is made to illustrate the reason for pattern distortion, which draws a conclusion that the distortion is essentially due to the different influence of environmental obstacles upon the electric field part and magnetic field part. This also implies that there is nearly no distortion between the two loops, which is validated by three different experiments. Based on this conclusion, the DOA estimation performance of the MUSIC algorithm is studied by creating a numerical relationship associating the estimation error with distortion level. Furthermore, a new method using ideal pattern is proposed to improve the estimation performance by removing the amplitude information of monopole and relying on the CL processing. Both simulation and experimental results are given to prove its validity.

The study of single station inverting the sea surface current by HF ground wave radar based on adjoint assimilation technology

This paper introduces the assimilation technology in an ocean dynamics model and discusses the feasibility of inverting the sea surface current in the detection zone by assimilating the sea current radial velocity detected by single station HF ground wave radar in ocean dynamics model. Based on the adjoint assimilation and POM model, the paper successfully inverts the sea surface current through single station HF ground wave radar in the Zhoushan sea area. The single station HF radar inversion results are also compared with the bistatic HF radar composite results and the fixed point measured results by Annderaa current meter. The error analysis shows that acquisition of flow velocity and flow direction data from the single station HF radar based on adjoint assimilation and POM model is viable and the data obtained have a high correlation and consistency with the flow field observed by HF radar.

Analysis of the Vector Current Velocity Detecting Precision of Bistatic High-frequency Ground Wave Radar System

Analysis of the Vector Current Velocity Detecting Precision of Bistatic High-frequency Ground Wave Radar System

Seasonal and interannual variations of surface current in the southern Taiwan Strait to the west of Taiwan Shoals

Seasonal and interannual variations of surface currents in the southern Taiwan Strait to the west of Taiwan Shoals were investigated by using measurements of high frequency (HF) ground wave radars from January 2006 to April 2009. The results demonstrate that surface currents in the water channel to the west of Taiwan Shoals are composed of a significant, seasonally fluctuating component and a relatively stable northward component. Forced by the East Asia monsoon, the annual variation of the surface longshore current is linearly correlated with the longshore wind. Behind the seasonal signal is a stable northward flow with speeds of O (10 cm/s). The observations also show that surface currents in the area are subject to distinct interannual variation, and the southward surface flow was more profound in winter 2007/2008 than in other winters. Observations from bottom-mounted ADCP also indicated that, in that winter, longshore currents in the west side of the Taiwan Strait are very different from the previous winter. The northward flow appeared much weaker, the currents were southward in most layers, and the interannual differences were observed throughout the water column. Hydrographic observations in the same period suggest that the large-scale southward intrusion of the Zhe-Min coastal water is the direct cause of the interannual difference. The enhancement of the East Asian monsoon during the 2007/2008 La Nina event is considered as the main reason for the winter current anomalies in the study area.

Bi-Station Multi-Target Tracking Using Range and Doppler Measurements

The azimuthal measurements of the high frequency ground wave radar are poor in an actual environment, which can cause the plots highly decentralized and damage the formation of the over-the-horizon tracks. To solve the problem, a new radar system is proposed to triangulate target tracks using range and Doppler measurements only. On the basis of the analysis of the characteristics of the range-finding location, a multi-target tracking algorithm under non-clutter condition is given in this paper, which further improves the tracking algorithm of this system. Simulation results show the effectiveness of this method.

The Application of a Novel Complementary Signal to High-frequency Ground Wave Radar

The Application of a Novel Complementary Signal to High-frequency Ground Wave Radar

Test for the Detection Depth with HF Portable Ground Wave Radar at Yangshan Area of Eastern China Sea

Test for the Detection Depth with HF Portable Ground Wave Radar at Yangshan Area of Eastern China Sea

An Inversion Method for Extraction of Wind Speed From High-Frequency Ground-Wave Radar Oceanic Backscatter

A new approach to extracting sea surface wind speed from high-frequency radar Doppler spectra is presented. Based on certain appropriate approximations associated with the Doppler region close to the first-order (Bragg) peaks, the second-order radar cross section equation is differentiated. Once Doppler shifts due to ocean currents are removed from the data, an expression that relates the wind speed to the frequency position of the second-order peak is derived. The method is applied to simulated noisy data as well as to field data obtained from a Seasonde (a product of CODAR Oceans Sensors) in Breezy Point, NY. In the latter case, the retrieved wind speeds are then compared to ground truth data measured by an anemometer from a National Oceanic and Atmospheric Administration weather station located in the vicinity of the illuminated patch of ocean. Subject to certain constraints as detailed in the manuscript, the algorithm shows significant promise.