Second-order scattering based wave height measurement with high-frequency (HF) radar has always been subjected to problems such as distance limitation and external interference especially under low or moderate sea state. The performance is further exacerbated for a compact system with small antennas. First-order Bragg scattering has been investigated to relate wave height to the stronger Bragg backscatter, but calibrating the echo power along distance and direction is challenging. In this paper, a new method is presented to deal with the calibration and improve the Bragg scattering based wave height estimation from dual-frequency radar data. The relative difference of propagation attenuation and directional spreading between two operating frequencies has been found to be identifiable along range and almost independent of direction, and it is employed to effectively reduce the fitting requirements of in situ wave buoys. A 20-day experiment was performed over the Taiwan Strait of China to validate this method. Comparison of wave height measured by radar and buoys at distance of 15 km and 70 km shows that the root-mean-square errors are 0.34 m and 0.56 m, respectively, with correlation coefficient of 0.82 and 0.84.

OSMAR-S, a portable high frequency surface wave radar (HFSWR), has been in operation for 10 years in China for current mapping and shares more than 80% of the domestic HFSWR radar market. In this paper, comprehensive quantitative validation and intercomparison of sea state parameter estimaton from the OSMAR-S radar with multiple other sensors over a period of two month are presented. These sensors include five buoys temporarily deployed at range from 10 km 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 fully 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 to the other two PA radars, OSMAR-S also shows a better performance in terms of accuracy and stability.

This letter proposes a uniform linear array (ULA) self-calibration method for ultrahigh-frequency (UHF) river flow detection radar (URFDR). URFDR usually uses ULA with less array elements, thus traditional self-calibration methods are all ineffective. This method using river echo signals with single direction of arrival as calibration sources mainly includes two steps: 1) A cost function is constructed based on eigenstructure decomposition, and the array error with Vandermonde fuzziness is estimated by minimizing the cost function using the gradient descent method. 2) The fuzzy array error estimation is corrected by nonlinear least square method based on the characteristics of river echo signals. A field experiment was conducted in the Yangtze River in October 2016, the processing results of which verify the effectiveness of the proposed method.

Practical and stable channel calibration method is one of the most interesting technology for array radar engineers. This paper proposes a single source active calibration method (SAM) for ultra high frequency (UHF) radar with uniform linear array (ULA) Yagi antennas. This SAM measures the amplitude and phase errors of receiving channel at a known direction of arrival (DOA) in the antenna pattern measurement (APM) mode of radar without a transponder, then the calibration values at other angles are determined by a formula derived from array signal model. The results of simulation and field experiment show that (1) the antenna pattern (AP) caculated from SAM calibration is consistent with the simulated and measured AP of the Yagi array. (2) after calibrated by SAM values, DOA estimantion of digital beamforming (DBF) and signal to noise ratio (SNR) gains of Doppler power for 5 ship targets are in good agreement with the theoretical performance of 8 arrays. and (3) the azimuth angle of the single active source is insignificant and can be user-defined according to the field environment. It is proved that the proposed SAM calibration provides a feasibility and idea for the practical and reliable calibration of UHF Yagi ULA.

Shore-based ultrahigh-frequency (UHF) radars have similar characteristics as the high-frequency (HF) ocean radars, for example, the sea waves generating Bragg resonance are gravity waves. Compared to HF radars, UHF radars are more sensitive to the small changes of sea waves, have finer spatial resolution and measure sea areas closer to the coast, hence are expected to complement the nearshore sensing capacity of HF radars. Two wind direction inversion methods for coastal UHF radars are adopted. One is based on the Bragg peak power ratio (BPPR) and the other is based on the modified Doppler spectrum centroid (MDSC). The latter is proposed to address the problem that it is often difficult to calculate the BPPR at high wind speeds. In both methods, several models describing the relationship between the Doppler spectrum characteristics and wind direction are used, respectively, in combination with the least square method (LSM) to estimate the wind direction with the ambiguity removed. Using the buoy wind sensor data as the ground truth, the results of a three-week experiment demonstrate the feasibility of the two methods. It is also shown that the former method has slightly smaller errors and the latter method applies to a wider range of wind speeds. For both methods, the most recommended model is the one based on the cosine-type directional spreading function with variable spreading parameter, because it has small errors and does not require data fitting.

A pair of ultrahigh-frequency (UHF) radars system for measuring the two-dimensional river flow patterns is presented. The system consists of two all-digital UHF radars with exactly the same hardware structure, operating separately at 329–339 MHz and 341–351 MHz. The adoption of direct radio frequency (RF) sampling technique and digital pulse compression simplifies the structure of radar system and eliminates the distortion introduced by the analog mixer, which improves the SNR and dynamic range of the radar. The field experiment was conducted at Hanjiang River, Hubei province, China. Over a period of several weeks, the radar-derived surface velocity has been very highly correlated with the measurements of EKZ-I, with a correlation coefficient of 0.958 and a mean square error of 0.084 m/s.