Sea Surface Current Measurement And Characteristics Analysis Of The Currents Based On Compact High-Frequency Radar

High-frequency(HF)radars,which are based on the proporgation characteristics of the vertically polarized wave which can travel to an over-the-horizon distance along the sea surface,can real-time monitor the sea surface status over a large region with a fine temporal-spatial resolution.Moreover,they can operate under any weather conditions.At present,there are about 400 HF radar systems deployed around the word for observating sea surface currents.These radars can be devided into two types based on their receiving antenna,that is,phased array radar and direction-finding radar or compact radar.The compact radar is prefered because it is smaller,lower cost,and eaier installation than the phased array radar.Thus,the majority of the operating HF radar systems are the direction-finding radar.The ability of the direction-finding HF radar in observating sea surface current field has been widely acknowledged.And the focus,in the HF radar community,is on dealing with the antenna pattern distortion of the receving antenna for the past two decade.On the other hand,as the data collected by HF radar gets richer,using the HF radar for oceanographic study is also more and more popular.Therefore,this thesis studied the antenna pattern distortion and using the HF radar data to analyse the current characteristics.The detailed content includes the following four section.(1)studied the relationship between the antenna pattern distortion and the direction of arrival(DOA)estimation error.An analytic relationship quantifying the DOA estimation error steming from the antenna pattern distortion has been deduced.The analytic expression indicates that the DOA estimation only determined by the relative distortion of the two loops.Based on this,a calibration method,which aims at eliminating the effect of the antenna pattern distortions on DOA estimation error and producing high-quality current field,was proposed.And the calibration method was validated by field experiment.(2)studied the performance improvement in current observation of the dual monopole-crossed loop antenna array system relative to the conventional monopole-crossed loop antenna system.And a detailed theoretical analysis of the DOA estimation error resulting from antenna pattern distortion for this antenna system has been provided.The analysis result indicated that the DOA estimation error dominated by the relative distortion of the two groups of loops.And it also affected by the relative distortion of the two monopole-crossed loop element.Thus,a calibration method for the dual monopole-crossed loop antenna array was proposed in this thesis.By using this calibration method to process field experiment data for current observation,the validity of the calibration method has been validated.And it is able to improve the accuracy of the radar retrieved current velocity and the data rate.(3)using the high-quality current field data observed by HF radar to study the space-time characteristic of the tidal current in the southwest of the Taiwan Strait.This study reveals that the study area is dominated by semi-diurnal tides.And the tidal currents are severely affected by the coastline and the local topography.(4)studied the eddy detection algorithm for identifying eddies in current field mapped by HF radar,and analysed the characteristics of the submesoscale eddies in the southwest of the Taiwan Strait.The results suggested that the life spans of the submesoscale eddies within the study area are relative short.But the energy and size of the eddies clearly grow in initial stage and decaying in the last stage of their life span.Moreover,there is a positive relation between the spatial distribution of the occurrence of the submesoscale in the study area and the local topography.The studies mentioned above are siginificant for improving the performance of the compact HF radar in surface current obsevation and expanding the HF radar in practical applications.