The Research Of Retrieving Sea Surface Wind And Ocean Wave Parameters From Synthetic Aperture Radar

Synthetic Aperture Radar?SAR?is a kind of active microwave imaging radar,the work is not affected by external conditions,with all-weather,all-day,high-resolution Marine remote sensing observation ability.Using SAR in order to develop the study of ocean has become an effective and convenient method,the high resolution SAR images in terms of sea surface wind field and the wave parameters inversion research is of great significance.In this paper,we do the further research on sea surface wind field and wave parameters retrieval from SAR image which is on the basis of predecessors' research.Geophysical model functions?GMFs?,e.g.,XMOD1,XMOD2,and SIRX-MOD were developed for X-band SAR data in vertical–vertical?VV?polarization to retrieve sea surface wind fields.To date,the polarization ratio model for X-band SAR?XPR?,which is only dependent on incidence angle,has only been available for sea surface wind retrieval from TS-X HH polarization images,in conjunction with an X-band GMF.In our previous study,it was found that the polarization ratio of TS-X dual-polarization data showed a particular relationship with sea surface wind speed.Therefore,in order to improve the accuracy of sea surface wind field retrieval from HH polarization TerraSAR-X image.In this study,we propose an improved X-band polarization ratio model,herein called XPR2,which is dependent on both incidence angle and sea surface wind speed.The XPR2 was tuned through 56 TS-X/TD-X dual-polarization?HH and VV polarization?images and the collocated European Centre for Medium-Range Weather Forecasts reanalysis wind data.The model was further validated by comparing the retrieved sea surface wind speed from another 38 TS-X HH polarization images against in situ buoy measurements.The comparison shows a root mean square error?RMSE?of 1.79m/s and a bias of 0.68m/s,which is better than the results achieved using the existing XPR with an RMSE of 2.31 m/s and a bias of 0.93m/s.In this paper,a semi-empirical algorithm for significant wave height?H_s?and mean wave period(T_mw)retrieval from C-band VV-polarization Sentinel-1 synthetic aperture radar?SAR?imagery is presented.We develop a semi-empirical function for H_s retrieval,which describes the relation between H_s and cutoff wavelength,radar incidence angle,and wave propagation direction relative to radar look direction.Additionally,T_mw can be also calculated through H_s and cutoff wavelength by using another empirical function.We collected 106 C-band stripmap mode Sentinel-1 SAR images in VV-polarization and wave measurements from in situ buoys.There are a total of 150 matchup points.We used 93 matchups to tune the coefficients of the semi-empirical algorithm and the rest 57 matchups for validation.The comparison shows the root mean square error?RMSE?of H_s was 0.69 m,scatter index?SI?of H_s was 18.3%,the bias of H_s was 0.11 m and 1.86 s RMSE of T_mw,24.8% of SI with the bias of 0.62 s.Results indicate that the algorithm is suitable for wave parameters retrieval from Sentinel-1 SAR data.