Research On Waveform Retracking Of Coastal Satellite Radar Altimetry Data

In recent decades,coastal sea level changes have had a major impact on the production and life of coastal cities.In the past,the tide gauge measurements are the main source of monitoring the sea level changes near the shore,but it can only give a very limited observation of discrete points,and it is always unavailable to obtain the data.Instead,Satellite altimetry technology has the characteristics of all-weather,global coverage and high accuracy,it is an important means of monitoring sea level changes today.However,in near-shore areas,echoes returned from footprint sea surface will be contaminated by land echoes and deviate from the standard ocean waveform,which leads to a poor accuracy of Sea Surface Height(SSH).Waveform retracking is a key way to improve the height measurement's quality.Currently,many waveform retrackers have been proposed.Although they have improved the accuracy comparing with the Ocean retracker's results,their applicability and accuracy levels are different.In addition,since the implementation of the satellite altimetry mission,its observation mode has evolved from the traditional Low Resolution Mode(LRM)of the footprint pulse limited radar altimeter to the Synthetic Aperture Radar altimeter(SAR)mode.The accuracy and resolution have been greatly improved,and SAR mode will be the main working mode of the future altimetry mission,but there is still little research on the waveform retracking for SAR mode.Therefore,evaluating and perfecting various types of retrackers make a great importance to improving the accuracy and availability of height measurements in coastal areas.This paper systematically summarizes the waveform retrackers for LRM and SAR altimetry waveforms,analyzes the advantages and disadvantages of each retracker,and proposes two improved retrackers for LRM and SAR mode altimetry data in nearshore respectively.The data of the global tide gauge stations were used to compared with the retracking results of Jason-2 and Sentinel-3A missions.The main work of this paper is as follows:(1)Analyze the characteristics of theoretical waveform of LRM mode and SAR model,summarizing most popular waveform retrackers which has been used in LRM and SAR mode height measurement data,and giving the advantages and disadvantages of each retracker and their applicability.(2)In recent years,a retracker called Adaptive Leading Edge Subwavefom(ALES)is widely recognized,but some problems had been found when there are a single peak noise or wide peak noise in the trailing edge of coastal waveforms in this paper.As a result,the midpoint of the leading edge of the ALES retracker is higher,which makes a smaller SSH.To solve this problem,an Adaptive Weighted Subwaveform(AWS)retracker is proposed.By adaptively lengthening or shortening the stopgate of the subwaveform as well as the weight dropping of the noise gate,the peak noise and the rayleigh noise of trailing edge noise are suppressed.By using the Jason-2 mission data and observations from 69 tide gauge stations worldwide,we compared the results of the ICE1,OCEAN,BETA5 and ALES retracker in all,it is verified that AWS make a more comprehensive accuracy improvement than ALES within 25 km offshore,and it is obviously superior to other retrackers at the range of 10 km offshore.(3)As for SAR mode data,a SAR multi-subwaveform threshold retracker were developed based on the basic idea of a Multiple Waveform Persistent Peak(MWa PP)retracker and the improved threshold retracker.The retracker not only takes advantage of the peaks of the SAR theoretical waveform,but also uses the results of the singlesubwaveforms,which derive a SSH with a relative good accuracy.A median of the normal height of the series of along-track single-subwaveform were used as a reference normal height.The data of Sentinel-3A mission and 30 global tide gauge stations were obtained to compared with each other,it is verified that the retracker is superior to the ICE1,SAMOSA and Ice Sheet retracker within 25 km especially 5km offshore.(4)Based on the LRM and SAR model data integrated by Sentinel-3A altimeter,the ICE1,OCEAN and LRM multi-subwaveform threshold retracker were also used to retrack the LRM waveforms within 25 km offshore,and the 30 global tide gauge measurements above were compared with the retracking results.The accuracy of the SSH obtained by the two modes shown that the SAR mode has a significant improvement in performance compared to the LRM mode in all distance but especially within 3km offshore.