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River discharge is an essential hydrological index of the global water cycle and is important for flood and drought forecast. Ultrahigh frequency (UHF) radar has gained much attention since it can remotely retrieve river discharge from the surface flow velocity measurement in real-time and all-weather condition. However, it becomes challenging for the mountain river case where the water level changes more significant than that of the plain river due to the deep-narrow channel, which breaks the previous empirical relationship between surface flow velocity and discharge. In this letter, an improved index-velocity method is proposed to address this issue. Through modeling the nonuniform variation of the cross-sectional area with water level, a more accurate surface flow velocity–discharge relationship is established. Experimental results show that the proposed method produces higher calculation accuracy than that of the previous methods, which enlarges the applicable scenario of UHF radar in discharge measurement. [ABSTRACT FROM AUTHOR]

High frequency (HF) radar often indirectly inverts wind field from wave height field, so the accuracy, range, and spatial resolution of the estimated wind field are usually limited by the wave field estimation performance. However, the sea surface current including the wind-driven current can often be accurately measured by radar. Thus, based on the wind-driven current estimated by HF radar, a new wind speed inversion algorithm is proposed in this letter. First, the relationship between wind speed and wind-driven current speed is established according to the ocean dynamics theory, but it contains several uncertain parameters. To avoid solving these parameters, an artificial neural network is used to train an accurate model between wind-driven current speed and wind speed. Final, the wind-driven current estimated by radar is substituted into the model to extract wind speed. A field experiment shows that the average correlation coefficient between the radar-estimated wind speed and the reference wind speed is 0.86, and the average root mean square error is 2.38 m/s. In addition, the proposed algorithm has larger measurement range and better spatial resolution than traditional algorithms. [ABSTRACT FROM AUTHOR]

The direction-finding high-frequency (HF) radar that employs a crossed-loop/monopole antenna as receiving has been widely applied in sea surface dynamics remote sensing due to its compact footprint and remarkable performance. Nevertheless, the direction-of-arrival (DOA) estimation of this system remains challenging in the case of coherent signals, which imposes a great threat to the radar measurement accuracy but has rarely been concerned in the past. In this article, a novel method combining the covariance matrix reconstruction process and subspace estimation technology is proposed to deal with this problem. By exploiting the purely real property of the array manifold, the rank loss of the covariance matrix due to the coherence of signals is mitigated. Both the theoretical derivation and numerical analysis prove that the proposed method is valid in estimating two coherent signals, and the DOA estimation accuracy is mainly related to the signal-to-noise ratio (SNR) and angle separation of the signals. Finally, field experiment data are also used to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Eddies are of great concern to ship operations and have an important influence on heat transfer in the sea. In this article, a highly concurrent and accurate method incorporating the winding angles and density-based clustering (WA-DBC) algorithms is proposed for automatic eddy detection from surface current maps generated by high-frequency radars (HFRs). As a case study, eddies and their characteristics, such as centers, radii, and lifespans, were extracted using both the WA-DBC and vector geometry algorithms from the HFR surface current maps of the north of Qiongzhou Strait, South China Sea. The eddy trajectories obtained show that most eddies in this region during the experiment period had a radius of 3–30 km and negative correlation existed between the radius and the life span (i.e., eddies with a smaller radius survived a longer lifespan). Furthermore, most of the eddies appeared in the central part of the study area and lasted for 40–100 min. This study shows that HFR is useful for detecting submesocale eddies for research works and applications in marine observation. [ABSTRACT FROM AUTHOR]

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]

River discharge measurement is of great significance, and as such, many methods and measuring instruments have been developed by many hydrologic researchers. However, most commonly used measuring instruments must be in contact with the water and are difficult to place and maintain. In this letter, a noncontact measurement method is proposed using ultrahigh- frequency (UHF) radar. The index-velocity method is employed to calculate the mean cross-sectional velocity, and the water-surface velocities on the cross section detected by the UHF radar are used to select the optimum index velocity. A field experiment was conducted in the Hanjiang River at Xiantao, Hubei, China, from March to July 2018. Data from March to June were processed for selecting the optimum index velocity, and the maximum water-surface velocity of the cross section was proven to be optimal. The fit relationship between the mean cross-sectional velocity and the index velocity was used to estimate the mean cross-sectional velocities and river discharges for June to July. The results were compared with those provided by the Hubei Xiantao hydrologic station, and the errors were mostly within ±5%. This confirms that the UHF radar can be used to accurately, automatically, and continuously measure river discharge. [ABSTRACT FROM AUTHOR]

Attributed to the maneuverability, the shipborne high-frequency surface wave radar (HFSWR) enables a larger coverage in ocean wind measurement than a shored-based system does. However, the antenna aperture of shipborne radar is usually limited by the ship size, which troubles the direction of arrival (DOA) estimation accuracy. In addition, the wind direction ambiguity caused by monostatic radar illumination remains challenging. In this article, an unambiguous wind direction estimation method based on a compact shipborne HFSWR is proposed. An equivalent dual-station model combining the successive radar data collected at two close locations is developed to solve the DOA ambiguity in wind direction estimation. A direction-finding algorithm incorporating the Doppler shifts due to ship motion and current shear is adopted to estimate wind direction from the ratio of the positive and negative Bragg peak powers. Moreover, three different wave directional spreading models are applied for performance comparison. Numerical simulation is conducted to evaluate the effect of dual-station configuration on the estimation accuracy. Finally, field experimental results are given to verify the correctness of the method. Radar-derived wind direction field agrees well with that provided by a numerical weather model. [ABSTRACT FROM AUTHOR]

OSMAR-S, a portable high-frequency surface wave radar (HFSWR), has been in operation for ten years in China for current mapping and shares more than 80% of the domestic HFSWR radar market. In this article, comprehensive quantitative validation and intercomparison of sea state parameter estimation from the OSMAR-S radar with multiple other sensors over a period of two months are presented. These sensors include five buoys temporarily deployed at range from 10 to 85 km away from the radar and another two types of commercially available phased-array (PA) radar made in China. A performance validation method of HFSWR compared with buoys deployed in both the high-precision area (HPA) and the edge area of the radar coverage is proposed for a full evaluation in a wide range area. A quantitative evaluation method using a variety of statistical metrics and their weights is proposed to obtain a comprehensive score for comparing the performance of different types of HFSWR. The good agreements with buoy data in a wide coverage area demonstrate the good capability of sea state parameters measurement using the OSMAR-S radar. Compared with the other two PA radars, OSMAR-S also shows a better performance in terms of accuracy and stability. [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 direction-finding high-frequency radars (HFRs) are widely used for remotely sensing the ocean surface currents. However, the performance of the direction-finding HFR is limited by the inevitable distortion of the actual antenna pattern of the compact monopole-cross-loop antenna, because the distortion of the antenna pattern leads to errors on the direction-of-arrival (DOA) estimation, which results in correctly determined radial velocities being placed into the incorrect bearing sectors. In this letter, a model determining the DOA estimation error (DOAEE) stemming from the antenna pattern distortion (APD) is presented with a detailed analytical derivation. This model suggests that the DOAEE only depends on the relative distortion of the two orthogonal loops. To validate the correctness of the proposed model, simulations are carried out. In addition, the simulation results show that the model well captures the actual relationship between the APD and the DOAEE. Moreover, a data set collected by a direction-finding HFR system and a synchronized automatic identification system receiver is used to further validate the proposed model. The precessing result of this data set demonstrates that the model-calculated DOAEE and the actual DOAEE are in good agreement with a correlation coefficient of 0.92 and a root-mean-square error of 9°, which provides solid evidence to verify the validity of the proposed model in determining the relationship between the DOAEE and the APD. [ABSTRACT FROM AUTHOR]

Wave–current interaction is a common and important phenomenon in the ocean. As an ocean remote sensing tool, high-frequency (HF) radar can be used to measure currents and wave parameters. In this paper, the possibility of studying wave–current interaction using HF radar is investigated. The first-order spectral power (FSP) of HF radar is used to explore the effect of current on the Bragg wave. By analyzing the FSP change with current (FSP-current distribution), we find that, in deep water, the wave–current interactions mainly belong to 2-D refraction case, while, over a relatively shallow shelf, the interactions are stronger and more complicated. Based on local topography and current field data at Taiwan Strait, the simulation results obtained using the SWAN model confirm the 2-D refraction of the Bragg wave. When the wave–current interaction is stable, we compensate the FSP with radar-measured currents according to the radar extracted FSP-current distribution and achieve a more accurate wind estimation. Comparisons between the original and refined wind fields show the effectiveness and necessity of the current-based compensation. [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]

Antenna pattern distortion incorporating the systematic differences among array elements is one of the most important factors that decrease the accuracy of radar direction of arrival estimation. In this letter, a new antenna pattern measurement (APM) method based on a fully-digital radar receiver and a portable signal source is presented. Compared with previous methods, the proposed method not only improves the measurement accuracy, but also reduces the complexity and time cost of measurement. APM results of a compact high-frequency (HF) antenna measured in two modes are depicted, both showing that two independent measurements at the same site are highly consistent. Moreover, the method is further verified by a comparison of radar measured radial current velocity with acoustic Doppler current profiler data. [ABSTRACT FROM AUTHOR]

Detecting and tracking small drones is significant for security requirements, but is challenging since small drones typically have a low radar cross section (RCS), small size, and hovering ability. Polarimetric analysis of returned radio wave is of great significance in detection and track of small drones. In this letter, the polarimetric analysis based on RCS and micro-Doppler signature of small drone rotor blade is first studied in VHF/UHF band. Both the simulated and measured RCS results confirm that the horizontal–horizontal polarization is preferred for drone detection in low-altitude angles at this frequency band. Moreover, the method of moments, a rigorous numerical method based on integral equation, is implemented to simulate the micro-Doppler signature generated by the rotating blade in VHF/UHF band. The results show that it is feasible to obtain the rotating speed from the micro-Doppler measurement, which is also verified by the experiment. [ABSTRACT FROM AUTHOR]

In this paper, Bragg and non-Bragg scattering mechanisms and frequency shifts from time-varying periodic water wave are studied from three aspects: wave tank measurements with an ultrahigh frequency radar, numerical simulation using the method of moments, and theoretical derivation applying the small perturbation method. The scattering field, radar cross section (RCS), and frequency shifts are discussed in both horizontal and vertical polarizations. The wave tank observations show that backscattering enhancement occurs when water wavelength is an integer multiple of Bragg wavelength, and there are several Doppler harmonics with frequency shifts of water-wave frequency and its integer multiples. Numerical simulations indicate that these Doppler harmonics except the one associated with the water-wave phase velocity are caused by the water surface edge effect. Moreover, theoretical analyses, numerical simulations, and wave tank experiments all show a clear exponential relationship between backscattering RCS and wave height. In addition, we further analyze the bistatic scattering and find that the scattering field is composed of plane waves propagating in the directions determined by water wavelength and radio wavelength, the bistatic frequency shift is the harmonic frequency of water wave, and the bistatic RCS also has an exponential relation with water-wave height. [ABSTRACT FROM AUTHOR]

Multiple-frequency observation and mobile platform deployment are the promising directions of high frequency surface wave radar (HFSWR). To adapt to this trend, previous transmitting antenna is challenged by the compatibility of multifrequency operation and small size. In addition, the radiation efficiency of previous compact HFSWR antennas has always been troubled by the rigorous grounding requirement. We developed a novel dual-frequency center-fed transmitting antenna to address these concerns, which can be easily extended to multifrequency case. The high-power omnidirectional radiation pattern is achieved at two frequencies within a single pole, and no grounding system is required. This antenna also provides the wider bandwidths at both frequencies with respect to that of a single frequency antenna, which enables a more flexible radar band selection. The antenna design scheme and the associated mathematical model are illustrated in detail. In addition to the mathematical analysis, both simulated results obtained using Fourier series treatment and field experimental measurements are given to verify the rightness and validity of the antenna design. [ABSTRACT FROM AUTHOR]

High-frequency (HF) radar receives the sea echo backscattered from the fluctuating ocean surface to remotely sense ocean surface currents over a large area with a high space-time resolution. To advance our understanding on this technology, we carried out an experiment for verifying the performance of ocean state monitoring and analyzing radar, model S (OSMAR-S) in ocean surface current measurement. The hourly radial currents measured by this radar system show a good agreement with acoustic Doppler current profilers (ADCPs) with correlation coefficients ranging in 0.40–0.93 and root-mean-square-error (rmse) values in 0.11–0.18 m/s. The u/v components of the radar-derived vector currents show a similar deviation from the ADCPs with correlation coefficients varying from 0.56 to 0.93 and rmse values varying in 0.13–0.18 m/s. Also, the bearing offset of this radar system in current extraction is in the range from $-$ 11 $^{\circ }$ to 18 $^{\circ }$. In consideration of the fact that the ideal antenna pattern was used in determining the bearing of the radials, we assume the main source resulting in the bearing offset is the antenna pattern distortion. Thus, a quantitative relation between the antenna pattern distortion factor and the bearing offset has been deduced. With this relation, we estimated the bearing error in the moorings’ bearing with the aid of the current information recorded by ADCPs. As expected, the result indicates that the antenna pattern distortion dominates the error of the bearing determination. [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]

High-frequency (HF) radars have great potential for maritime surveillance, and the multiple signal classification (MUSIC) algorithm is usually used to estimate the direction of arrival (DOA) of targets for a wide-beam radar. However, the performance of the MUSIC algorithm relies on the precision of the antenna pattern, which could be contaminated by nearby electromagnetic interference. Therefore, the actual antenna pattern must be measured and used. In order to remove the requirement of antenna pattern measurement, a new method for target DOA estimation from wide-beam HF radar data using support vector regression (SVR) is proposed in this letter. A system model that relates target bearing and radar data feature is obtained through the SVR-based machine learning using the automatic identification system data and data associated with the vessels successfully detected by the HF radar. Then, such a model is used to determine the DOAs of targets from new data. The field experimental results at two sites demonstrate that the performance of the SVR method is better than that of the MUSIC algorithm. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

High-frequency surface wave radar equipped with a crossed-loop/monopole antenna has been applied to estimate vessel direction of arrival (DOA). However, unavoidable distortion of the antenna pattern and complicated channel calibration are challenges for achieving high angular accuracy when the multiple signal classification (MUSIC) algorithm that relies on the antenna pattern is employed. Although various methods of pattern measurement and calibration have been proposed, high system complexity and considerable time cost are still troublesome problems, especially for the temporary use of radar. In this letter, we first analyze the characteristics of measured pattern and then rapidly calibrate the crossed-loop antenna using sea echoes and the automatic identification system data. Finally, vessels’ DOAs are estimated using MUSIC algorithm and the calibrated loop-only patterns. The high accuracy of DOA estimation is validated at an angle range of less than $\text{180}^{\circ }$. This method effectively reduces the system complexity and time cost when compared with the conventional method. A simulation and two field experiments are conducted. Both the rapid pattern calibration for 115 min and the comparison of processing results demonstrate the validity of this proposed method. [ABSTRACT FROM AUTHOR]

In addition to being able to extract current velocities and wind directions, the first-order Bragg peaks in high-frequency (HF) radar sea echoes also have the potential to independently map wind speeds. During a two-month experiment with the HF-radar ocean state measuring and analyzing radar, model S in the Taiwan Strait in 2013, the dependence of the first-order spectral power (FSP) on wind was found to be deterministic. An empirical model was thereafter proposed to estimate the wind speed, which gave a performance that was comparable with the popular second-order inversion method. Strong oscillations (up to ±3 dB) in the FSP caused by underlying currents were observed and cannot be readily explained by the classic wave–current interaction theory with the assumption of wave-action conservation. When we matched the local maximum and minimum values of the FSP with the local extreme and zero values of the radial-current velocities, we found that most of the extremes of the FSP occurred when the radial-current velocity was 0. To account for this phenomenon, additional smoothing with a 12.4-h moving window was applied, which led to a significant improvement in the wind-speed estimates. This knowledge of the FSP’s wind and current dependence helps us make another step toward the operational estimation of wind speeds using the first-order Bragg peaks. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

A compact-array high-frequency surface wave radar equipped with two crossed-loop/monopole receiving antennas has been established for vessel detection. Using two compact antennas of the same design, this system can obtain two extremely similar sets of radar range-Doppler spectra over the same period. To detect vessel targets efficiently, the spectra of two antennas are enhanced by performing a principle component analysis. A wavelet-based approach is then applied to suppress clutter and reduce noise. The signal-to-noise ratios and signal-to-clutter ratios of the echoes are thus improved. Finally, an adaptive threshold is used to extract targets. The real radar data detection results are compared with Automatic Identification System data as well as those from the conventional ordered-statistic constant false alarm rate method. The feasibility and the validity of method proposed here are thus demonstrated. [ABSTRACT FROM PUBLISHER]

In this paper, we focus on the characteristics of the surface currents in the Taiwan Strait observed by two high-frequency surface wave radars (HFSWRs). The current velocities correlate well with the in situ buoy records, which show the reliability of the radar data. Tidal constituents are then calculated from the 80-day-long sea current data. The current pattern in this area consists of a dominant semidiurnal tidal component and a mixed semidiurnal one. The influence of the shallow-water constituents is significant, and the bathymetry also produces an obvious effect on the spatial distribution characteristics of the tidal currents. The directions of the major axes of the M2 tidal current ellipses coincide well with that of the strait axis. Besides, the east–west components of the residual current velocity and the wind speed are found to be highly correlated with a correlation coefficient up to 0.808, while the north–south components are nearly uncorrelated with a correlation coefficient down to 0.238. This study shows the great capability of the HFSWR in oceanographic researches and applications, especially for the tidal analysis and a further study of the wind–current relation. [ABSTRACT FROM AUTHOR]