| dc.description.abstract | Hundreds of high-frequency (HF) coastal radar systems are operated in the world for purposes related to marine salvage, oil spill response, coastal zone management, and understanding of upper ocean layer dynamics. At present, 25 HF coastal radar stations consisting of both cross/loop monopole and phased array systems were installed along Taiwan’s coastline and are primarily operated modes for observing ocean currents. In the next period, 21 phased array HF and very high-frequency (VHF) radar stations are on-going to install at the end of 2022 to monitor the evolution of ocean surface waves around the Taiwan island’s coastal area in the long term. In order to fulfill the demands for high-frequency radar application exploitation, this study focuses on two HF radar applications’ essential topics, which are assessing the uncertainty of ocean surface wave parameters under various sea-states and developing algorithms for coastal vessel monitoring.
First, the simulation of radar Doppler spectra was implemented to understand the theoretical relationship between radar cross-section (RCS) and sea-state parameters. Based on Barrick′s theory, the Doppler spectra of HF radar cross-section are simulated from the given directional wave spectrum, which can be generated by applying the wind-wave spectra model for steady wind conditions or using the 3rd generation wave spectra model for monsoon and typhoon conditions. As preliminary results, various sensitivity tests of the simulated Doppler spectra are implemented under different wind speeds, wind directions, operating radar frequencies, and spreading parameters. The result showed that the simulated Doppler spectra agree well with those in the literature. This indicated that our simulation toolbox works well.
To estimate the wave parameters. i.e., significant wave height, mean period, and wave spectrum from HF radar sea-echoes, existing methods are implemented to establish estimators for testing and evaluating the method’s performance. In the beginning, the numerical simulation is used to assess the bias estimation of wave parameters under various weather conditions, such as the steady homogenous wind, monsoons, and typhoons. The results showed that the uncertainty of wave parameters estimated from simulated Doppler spectra under typhoon conditions is lower than those in monsoon conditions. In addition, to assess wave parameters’ bias from the actual HF radar data, the backscattered signal of the 27.75 MHz HF radar (LERA MKIII) system is used. This system consists of 16 Rx antennas in a linear array installed at the northern of the Taichung harbor, Taichung City, Taiwan, in late November 2018 for wave monitoring in the long term. Estimation results are compared with those of in-situ data observed by an acoustic wave and current profiles (AWAC) deployed in the radar’s footprint. The comparison results indicated that the radar system and estimators perform very well. Furthermore, it is found that the connection coefficients of wave parameters (which might be the function of radar-to-wave angle, smallness parameters, and spectral width parameters) have existed. Correction algorithms are proposed to estimate the scaling factor for calibrating radar-deduced wave parameters. The results demonstrated the dependence of wave height scaling factor on the radar-to-wave angle, while wave period scaling factors are mainly influenced by smallness parameters.
On the other hand, this study implements two approach methods to identify coastal vessel locations using HF radar backscattered signals: the rang-Doppler (RD) (or Doppler-Range, D-R) spectra and range-Angle brightness distribution methods. The estimated position of coastal vessels is compared with ship’s information from Automatic Identification System (AIS) data for assessing the performance of the radar system and the efficiency of detection methods. The results showed that both approaches work well. Furthermore, while the RA method can monitor ships’ trajectories, the RD approach can provide the radial speed of those targets. Besides, we also found the influence of the ship’s characteristics, including the length, the direction, and the heading, on the detection number of targets. Overall, this study illustrated the advantages and the limits of the HF radar technique for wave monitoring and ship detection. | en_US |