In this paper, the application of ultra-high frequency (UHF) radar for ship target detection over river is investigated for the first time. Due to the wide beam width of antenna, the ship detection of the UHF radar suffers from the broaden river clutters scattering from water waves. In addition, due to the high resolution, the ship echoes are seriously extended in both range and Doppler dimensions, which is different from the traditional point target signal. Extended target detection in strong river clutter is applied in this paper. Conventional constant false alarm rate (CFAR) detector is limited in this situation, which is applicable to the point target detection in clutter free background. An improved cell-averaging (CA)-CFAR method is proposed based on a joint estimation of threshold in range, Doppler and time serials of range-Doppler (R-D) spectra. Based on the time stationarity of clutter, this three-dimensional (3D) CA-CFAR combines several R-D spectra collected in continuous time to estimate the clutter threshold. The effectiveness of this improved method is validated using both simulated and field data.

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.

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.

This paper study both experimentally and numerically the backscattering radar cross-section (RCS) and Doppler spectrum characteristics from two-dimensional 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.

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.