Preliminary Analysis of Chinese GF-3 SAR Quad-Polarization Measurements to Extract Winds in Each Polarization

Abstract

This study analyzed the noise equivalent sigma zero (NESZ) and ocean wind sensitivity for Chinese C-band Gaofen-3 (GF-3) quad-polarization synthetic aperture radar (SAR) measurements to facilitate further operational wind extraction from GF-3 data. Data from the GF-3 quad-polarization SAR and collocated winds from both NOAA/NCEP Global Forecast System (GFS) atmospheric model and National Data Buoy Center (NDBC) buoys were used in the analysis. For NESZ, the co-polarization was slightly higher compared to the cross-polarization. Regarding co-polarization and cross-polarization, NESZ was close to RadarSAT-2 and Sentinel-1 A. Wind sensitivity was analyzed by evaluating the dependence on winds in terms of normalized radar cross-sections (NRCS) and polarization combinations. The closest geophysical model function (GMF) and the polarization ratio (PR) model to GF-3 data were determined by comparing data and the model results. The dependence of co-polarized NRCS on wind speed and azimuth angle was consistent with the proposed GMF models. The combination of CMOD5 and CMOD5.N was considered to be the closest GMF in co-polarization. The cross-polarized NRCS exhibited a strong linear relationship with moderate wind speeds higher than 4 m·s −1 , but a weak correlation with the azimuth angle. The proposed model was considered as the closest GMF in cross-polarization. For polarization combinations, PR and polarization difference (PD) were considered. PR increased only with the incidence angle, whereas PD increased with wind speed and varied with azimuth angle. There were three very close PR models and each can be considered as the closest. Preliminary results indicate that GF-3 quad-polarization data are valid and have the ability to extract winds in each polarization.

Retrieval of Sea Surface Wind Speeds from Gaofen-3 Full Polarimetric Data

“…Figure 5a shows variations in the PR with incidence angle, and error bars have been overlaid. In the diagram, the PR values present a firmly rising trend with increasing incidence angle, which indicates that the PR has a significant dependence on the incidence angle, which is consistent with the results found by Ren et al [45].…”

“…On the other hand, the QPS-CP model derived from a greater number of collocated agrees well with the one proposed by Li et al [42], indicating the stability of the statistical analysis in both analyses. The discrepancies between the QPS-CP model and the Ren et al [45] result are distinct. For SSWSs less than 13 m/s, the QPS-CP model yields higher estimates than the Ren model.…”

“…Previous studies [43,45] have shown that CMOD5.N is suitable for SSW retrieval from GF-3 SAR data, and this method is adopted in this study. The Introduction mentioned that the general accuracy achieved by a spaceborne SAR for SSWS is approximately less than 0.5 m/s in terms of absolute bias.…”

“…In addition to the three refitted PR models, the other two PR models dependent on incidence angles, i.e. the Zhang model [25], which is derived from RS-2 data, and the Ren model [45], which is based on the GF-3 QPS data, are added to the comparison. The gray solid lines are the error bars of the PR values in the incidence angle bins.…”

“…In addition, Ren et al [45] used QPS mode data from 1,787 scenes and the collocated Global Forecast System (GFS) SSW data to verify which GMF is suitable for SSW retrieval from the GF-3 data. Their conclusion is that CMOD5 is appropriate for SSWSs lower than 6 m/s, while CMOD5.N should be applied for SSWSs higher than 6 m/s.…”