Abstract
The Geophysical Model Function (GMF) XMOD1 provides a linear algorithm for sea surface wind field retrievals for the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR). However, the relationship between the normalized radar cross section (NRCS) and the sea surface wind speed, wind direction and incidence angles is non-linear. Therefore, in this paper, XMOD1 is revisited using the full dataset of X-SAR acquired over the ocean. We analyze the detailed relationship between the X-SAR NRCS, incidence angle and sea surface wind speed. Based on the C-band GMF CMOD_IFR2, an updated empirical retrieval model of the sea surface wind field called SIRX-MOD is derived. In situ buoy measurements and the scatterometer data of ERS-1/SCAT are used to validate the retrieved sea surface wind speeds from the X-SAR data with SIRX-MOD, which respectively yield biases of 0.13 m/s and 0.16 m/s and root mean square (RMS) errors of 1.83 m/s and 1.63 m/s.
Highlights
The Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was present on two flights of the Space Shuttle Endeavor in April and October, 1994
Prior to the launch of TS-X, we developed a linear geophysical model function (GMF) called XMOD1 [12] to retrieve the sea surface wind from
Simulated using the SIRX-MOD model agrees well with the X-SAR observations, the difference in the normalized radar cross section (NRCS) between the up-wind and cross-wind conditions of real X-SAR data is larger than the prediction of SIRX-MOD for incidence angles greater than 25°
Summary
The Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was present on two flights of the Space Shuttle Endeavor in April and October, 1994. The multi-frequency capability is the most attractive feature of SIR-C/X-SAR, and many studies analyzed the different radar signatures of oceanic and atmospheric processes in the X-, C-, and L-bands. Ufermann and Romeiser [5] compared the simulated radar signatures for different settings of oceanic and atmospheric parameters with the observed multi-frequency/multi-polarization SIR-C/X-SAR signatures of the Gulf Stream front. Prior to the launch of TS-X, we developed a linear geophysical model function (GMF) called XMOD1 [12] to retrieve the sea surface wind from. A dedicated wind retrieval algorithm for other applications using SIR-C/X SAR data, such as oil spill monitoring, sea surface wave retrieval and ship detection, may be useful. The development of an updated nonlinear GMF to derive sea surface winds from X-SAR data is presented. The developed model is further validated using in situ buoy measurements and the scatterometer onboard ERS-1 (ERS-1/SCAT)
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