Doppler Spectrum Analysis of Ground Echoes from Spaceborne Doppler Scatterometer

Sea surface current fields are important oceanic and climatic variables. Due to its capability for global coverage and direct observation of sub-mesoscale sea surface current fields, the Doppler scatterometer has become a frontier in ocean remote sensing technology research. The calibration and quantitative measurement of the Doppler scatterometer are the foundations and prerequisites for current field observations, as well as a critical core issue that needs to be addressed. In this study, a ground-based scattering echo simulation model was developed to simulate and analyze the Doppler spectrum characteristics of potential natural ground extended targets that could be used for external calibration of spaceborne scatterometers. The changes in these characteristics were compared under different platform motion speeds, incident angles, and azimuth angles. The results indicate that platform motion speed is the primary factor affecting Doppler spectrum characteristics, while variations in incident and azimuth angles also have significant impacts. Simulation results for ground extended targets with different height variations show that the greater the height variation of the target, the larger the Doppler spectrum shift, while changes in the central height have minimal impact on the Doppler spectrum characteristics. Therefore, in the selection of calibration targets, relatively flat extended targets should be chosen. Finally, an analysis of the ground echo Doppler frequency shift was conducted and validated using the DEM model. The findings of this study will provide support for further research on Doppler scatterometer calibration.。