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Zezong Chen, Longgang Zhang, Chen Zhao, Xi Chen, and Jianbo Zhong

Abstract

Wind sea and swell representing different weather conditions generally coexist in both open waters and coastal areas, which results in bimodal or multipeaked features in directional wave spectrum. Because they make wave parameters such as significant wave height and mean wave period of the mixed sea state less meaningful, the processes of separation and identification of wind sea and swell are crucial. Consistent wind sea and swell results can be obtained by a commonly used method based on wave age (WA) with the directional wave spectrum and wind velocity. However, the subjective dependence of wave age threshold selection and the required wind information restrict the application of this method. In this study, a practical method based on the overshoot phenomenon (OP) in wind-generated waves is proposed to extract wind sea and swell from the directional wave spectrum without any other meteorology information. Directional wave spectra derived from an S-band Doppler radar deployed on the coast of the South China Sea have been utilized as the datasets to investigate the performance of both methods. The proposed OP method is then validated by comparing it with the WA method and the verifying results are presented.

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Yan Jin, Zezong Chen, Lingang Fan, and Chen Zhao

Abstract

A new method is proposed to detect small targets embedded in sea clutter for land-based microwave coherent radar using spectral kurtosis as a signature from radar data. It is executed according to the following procedures. First, the echoes of radar from each range gate are processed by the technique of short-time Fourier transform. Then, the kurtosis of each Doppler channel is estimated from the time–Doppler spectra. Last, the spectral kurtosis is compared to a threshold to determine whether a target exists. The proposed method is applied to measured datasets of different sea conditions from slight to moderate. The signal from a small boat is detected successfully. Furthermore, the detection performance of the proposed method is analyzed by the way of Monte Carlo simulation. It demonstrates that the spectral kurtosis–based detector works well for weak target detection when the target’s Doppler frequency is beyond the strong clutter region.

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Zhao Chen, Chen Zezong, Jiang Yanni, Fan Lingang, and Zeng Gengfei

Abstract

For operations across a wide range of oceanographic conditions, a radar system able to operate at more than one frequency is theoretically and experimentally recommended for robust wave measurement in recent years. To obtain more sea-state information by HF radar, a multifrequency HF (MHF) radar system, which can simultaneously operate at four frequencies at most in the band of 7.5–25 MHz, was developed by the Radio Wave Propagation Laboratory of Wuhan University in 2007. This paper mostly focuses on detailing the data process method of MHF radar wave-height estimation. According to different bands of operating frequencies, a least-mean-square (LMS) linear fitting method is adopted to calibrate wave-height estimation formulation, which is introduced by Barrick to extract significant wave height from backscatter Doppler spectra. Both the wave-height measurements of the initial and modified methods are compared with wave buoy measurements. Afterward, a data fusion algorithm of multifrequency estimates based on relevant factors quantification is discussed step by step. Three comparisons between radar-derived and buoy-measured estimates are presented to illustrate the performance of the MHF radar wave-height measurement. The statistics of the MHF radar wave-height measurements are listed and analyzed. The results show that the wave-height measurements of the MHF radar are in reasonable agreement with the measurements of the wave buoy.

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Chen Zhao, Zezong Chen, Gengfei Zeng, and Longgang Zhang

Abstract

One pivotal factor affecting the accuracy of HF radar current measurements is the direction of arrival (DOA) estimation performance of the current signal. The beamforming technology or superresolution algorithm cannot always perform best in practical applications because of the phase errors existing in array channels. These phase errors, which cause uncertain estimation of DOA, lead to confused values in radial current maps. To solve this problem, this paper is focused on discussing the performances of two autocalibration methods using sea echoes for multifrequency high-frequency (MHF) radar current measurements. These two array calibration methods, based on maximum likelihood (ML) and multiple signal classification (MU), first seek single-DOA sea echoes and then gather them for array calibration using different cost functions. The ML and MU methods provide approximate mean phases, while the standard phase errors of the MU method are smaller. After array calibration using these two methods, the results show significant improvements in current retrievals. Comparisons between the MHF radar and ADCPs reveal that array calibration using the ML and MU methods also improves the estimation of radial currents clearly, with correlation coefficients over 0.93 and rms differences of 0.09–0.18 m s−1 at different operating frequencies and sampling locations. The performance of the bearing offset is also improved. Only small bearing offsets less than 10° exist in radial current measurements. Therefore, this paper demonstrates that array calibration is a crucial part for current measurements, especially for direction-finding HF radar.

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Chen Zhao, Zezong Chen, Gengfei Zeng, Longgang Zhang, and Fei Xie

Abstract

A multifrequency high-frequency (MHF) radar system was designed and developed by Wuhan University in 2007. This system can simultaneously operate at four frequencies mainly in the 7.5–25-MHz band. This paper focuses on discussing the performances of an MHF radar system deployed along the coast of the East China Sea based on comparisons with multidepth ADCP datasets, which were obtained from ADCPs deployed at different locations in August 2010 during a small storm. The comparisons illustrate that radar-derived radial currents are correlated with ADCP data at mainly a 2–4-m depth with correlation coefficients over 0.95 and RMS differences less than 0.12 m s−1 for both operating frequencies. Bearing offsets at points A, C, and D are computed for different operating frequencies with magnitudes of 0°–11°.

The capability of MHF radar to measure currents at different depths is explored. The results indicate that the effective depth of current measurements by MHF radar increases with decreasing operating frequency. A linear regression (with a regression coefficient of 0.0576) of the responses in the mean effective depth on the predictors in radio wavelength is obtained. The dominant semidiurnal and diurnal constituents are also analyzed. The radial current amplitudes of the M2 and K1 constituents are strong in this area during this experiment. The residual currents vary with wind speed, with a correlation coefficient of 0.52. A correlation coefficient of 0.79 between nontidal currents and the radial wind speed after a clockwise rotation of the wind vector by about 50° was obtained.

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