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Based on the sea surface scattering mechanism of radio wave, compact high-frequency surface wave radar that employs the direction finding technology also shows promising potential for remotely mapping of ocean wave parameters. However, due to the low signal-to-noise ratio (SNR) of scattered echoes, the diverse external interference and clutter signals, other unresolved measurement uncertainties, the quality (such as accuracy, temporal, and spatial coverage rates) of wave maps are often limited. In this article, a novel, real-time, data quality control method is proposed to alleviate these issues. A comprehensive three-stage processing scheme is established, including the range-Doppler spectral processing, the spatial-grid processing, and the temporal-scale Kalman filtering. The first two stages aim to improve the echo signal quality and reduce the spatial gaps, respectively. The third stage is designed to mitigate the estimation error using an autoregression prediction model and to relate the observation error variance to the SNR of second-order Doppler spectral peak. A detailed verification and performance analysis between the field radar data and in situ ground truth data over one-month period is carried out, indicating that the proposed method can improve the reliability of wave maps with respect to the conventional Doppler spectral smoothing or averaging method, particularly in low sea state (i.e., low SNR) scenarios. [ABSTRACT FROM AUTHOR]

This article studies both experimentally and numerically the backscattering radar cross section (RCS) and Doppler spectrum characteristics from 2-D linear and nonlinear time-varying sea-like surface at UHF band under low grazing incidence. The small slope approximation and choppy wave model are applied to solve rough surface scattering and generate nonlinear sea waves, respectively. A coherent Doppler radar operating at 340 MHz was deployed at the tip of Huangqi peninsula in the southeast coast of China to measure the echo spectrum from the actual sea surfaces. At the same time, an ocean buoy was placed in the radar coverage to monitor the sea states. The RCS and Doppler spectrum are analyzed and compared comprehensively between radar measurements and numerical predictions. After compensating the influence of wind direction, the responses of radar echo power and numerical predicted RCS to wind speed are basically consistent and are more sensitive in low sea states. The correlation of Doppler spectra between radar measurements and numerical simulations exceed 0.96 during the whole experiment. The intensity of the high-order peaks increases rapidly with the wind speed, while the intensity of the Bragg peak decreases slightly. Doppler spectrum will be shifted by an amount proportional to radial surface current. [ABSTRACT FROM AUTHOR]

Ocean wave height measurement using a wide-beam high-frequency surface wave radar (HFSWR) remains challenging due to its poor spatial resolution, which significantly limits the application of such compact systems. In this article, a novel method for wave height field extraction from the first-order Doppler spectra of a dual-frequency wide-beam radar is proposed. A model relating significant wave height to the ratio of the first-order spectral powers associated with two radar frequencies is put forward and studied numerically. Through theoretical analysis and experimental validation, it is confirmed that the first-order Doppler peaks of two radar frequencies have arisen from an approximately same direction of arrival (DOA), and their amplitudes are also affected by a similar wave directional spreading. Hereby, an algorithm combining beamforming and direction finding is developed to determine the spatial distribution of the first-order spectral power ratio and derive the significant wave height field. Finally, experimental results are given to verify the algorithm. The radar-derived wave height field agrees well with that obtained using a numerical wave model. Furthermore, the radar-measured wave heights are compared with the data collected by two buoys at the distances of 12.7 and 73 km, respectively. The comparison shows that the corresponding root-mean-square errors are 0.3 and 0.5 m and the correlation coefficients are 0.85 and 0.88, respectively. [ABSTRACT FROM AUTHOR]

Compact high-frequency surface wave radar has been widely applied to the measurement of sea surface current, but its accuracy and direction resolution of wave parameter estimation are always limited due to the wide beam of the antenna. In this article, a novel wave-height mapping method based on the second-order harmonic peak (SHP) of radar Doppler spectra is proposed to address this concern. The characteristic of the SHP at the Doppler frequency of $\sqrt {2}$ times the Bragg frequency is studied through the theoretical derivation and numerical simulation. A relationship between the ratio ($R$) of the SHP power to the Bragg peak power and significant wave height ($H_{s}$) is derived. Furthermore, the $R$ – $H_{s}$ model is improved by incorporating influences, such as background noise and antenna beamwidth. With this improved model, a wave-height mapping algorithm based on the direction finding technique is presented. This approach enables the significant wave-height map extraction using a broad-beam radar. Finally, wave-height maps obtained at different sea states are depicted and analyzed, and the wave heights appearing on the maps are compared with buoy data over a one-month experiment to verify the validity and robustness of the algorithm. During this period, the significant wave height varies from about 0.5 to 4.5 m, and the radar measured wave heights at different range/distance bins show an overall root-mean-square error (RMSE) of 0.33–0.77 m and a correlation coefficient (CC) of 0.78–0.94, with respect to the buoy measurements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Wind spreading functions (WSF) are crucial for high-frequency radar (HFR) wind-direction inversion. The popular half-angle cosine WSF always fails to describe observed HFR Doppler spectra and tends to provide almost fixed relative angle estimations. In this letter, analysis of the data from a broad-beam HFR radar, deployed on the Taiwan Strait’s west coast, shows that a modified WSF (based on the cosine WSF) has a better wind-direction estimation performance. The modified WSF fits average Bragg ratios of 15-day data well with the aid of data from buoys. The data of the next 13 days are used to test the modified WSF. The wind direction estimated by the modified WSF has an advantage when Bragg ratios have adequate average processing, and directions of arrival are around the upwind or downwind direction. The root mean square error of the modified WSF wind-direction estimate is 32.59° for the entire observation, decreasing to 14.18° when a significant wave height is between 1 and 2 m. [ABSTRACT FROM PUBLISHER]

Herein is proposed a low-power-density signal based on time-staggered frequency modulated continuous waveform (TS-FMCW) for portable high-frequency (HF) radar. According to the principle of pulse compression that the signal-to-noise ratio (SNR) is related to the total energy, the proposed signals can carry more energy to get higher SNR within same level power density in time–frequency plane. The range grating lobes and Doppler grating lobes shown in the ambiguity function of our proposed waveform can be avoided in the practical HF radar application. A signal processing architecture containing low-pass filtering and fast Fourier transform is proposed. Experimental results demonstrate that higher SNRs are achievable with the same range and Doppler resolution when the same power density is used as the conventional FMCW. [ABSTRACT FROM AUTHOR]

Wind speed is an important sea surface dynamic parameter which influences a wide variety of oceanic applications. Wave height and wind direction can be extracted from high frequency radar echo spectra with a relatively high accuracy, while the estimation of wind speed is still a challenge. This paper describes an artificial neural network based method to estimate the wind speed in HF radar which can be trained to store the specific but unknown wind-wave relationship by the historical buoy data sets. The method is validated by one-month-long data of SeaSonde radar, the correlation coefficient between the radar estimates and the buoy records is 0.68, and the root mean square error is 1.7 m/s. This method also performs well in a rather wide range of time and space (2 years around and 360 km away). This result shows that the ANN is an efficient tool to help make the wind speed an operational product of the HF radar. [ABSTRACT FROM AUTHOR]

Study of wave height inversion with High-Frequency Surface Wave Radars (HFSWRs) has been going on for more than 40 years. Various wave inversion methods have been proposed, and HFSWRs have achieved great success in local wave measurements. However, the method of wave mapping is still under development, especially for the broad-beam HF radars. Existing methods of wave mapping are based on narrow-beam radar with beamforming. This paper introduces a way of wave height inversion, using the ratio of the second-harmonic peak (SHP) to the Bragg peak (RSB). A new wave mapping method is proposed, which can be used in both narrow and broad-beam radars, according to the way of wave inversion based on the RSB. In addition, radar wave measurements at the buoy position are compared with the in situ buoy, which show a good agreement. At last, the results of wave mapping on the two-hour timescale are given. [ABSTRACT FROM AUTHOR]

High-frequency (HF) radars have achieved great success in the remote sensing of sea states. Most operational methods for wave extraction resort to the second-order Doppler spectral continuum based on Barrick's formulas. However, accurate division of the first-order Bragg peak and the second-order continuum is often unavailable due to spectral broadening and splitting when the radar beam is not sufficiently narrow. The second-order peaks, particularly the second-harmonic peaks (SHPs), can also introduce large errors to the wave height estimates. In this letter, we describe some properties of the SHP that are directly learned from the experimental observations of sea echoes by HF radars in both the 25- and 13-MHz bands. The SHPs exist throughout two radar experiments with comparable strengths to the maximum of the second-order continuum. They have similar spectral structures as the Bragg peaks in both the Doppler and spatial domains, showing that these peaks are representations of some particular sea waves. The power ratios of the SHPs to the Bragg peaks (RSBs) in decibels have strong correlations with the wave heights recorded by an in situ buoy or a known model, and the linear regression models between the RSB and the wave height can be determined by data fitting. These findings give new insights into wave height estimation from the spectral peaks only, which can provide supplemental information to the conventional results from the second-order continuum to improve both the accuracy and robustness of the wave extraction by HF radars. [ABSTRACT FROM PUBLISHER]

High-frequency (HF) radars have gained much attention in recent years due to their remarkable capabilities in remote sensing of sea surface states. Radio frequency interference (RFI) should be suppressed before extraction of useful information from the radar echoes. Element instead of array-based interference suppression method is preferred for a small-aperture radar, and it is still more difficult to deal with the nonstationary interference. In this letter, a short-time range domain cancellation method for RFI suppression is proposed, whose advantage over the global one is twofolded: better suppression performance and lower computation cost. Processing results of real data collected by OSMAR-S, a portable HF radar, show the validness of the short-time method for nonstationary RFI suppression. The method can greatly improve the anti-interference capability and detection performance of the HF radar. [ABSTRACT FROM PUBLISHER]