Doppler Spectrum Analysis and Centroid Estimation of Ka-band Spaceborne Sea Surface Scatter Echoes

The ocean surface dynamic parameters reflect important air-sea interaction processes, such as the material and energy balance, and climate change. Under spaceborne measurement conditions, it is necessary to study the echo Doppler spectrum characteristics formed by the high operating speed of the satellite in conjunction with the sea surface dynamic parameters. In this paper, a time-varying dynamic sea surface model is established via the existing linear random superposition theory to simulate ocean surfaces. Based on the satellite parameters defined by the Ocean Surface Current multiscale Observation Mission (OSCOM), this work derives echo Doppler spectra involving different wind parameter effects under medium-incidence-angle Bragg scattering conditions. As wind speed increases, the sea surface roughness and root mean square height increase accordingly, resulting in the stronger backscatter modulation, and the echo Doppler shift increases significantly. When observed along the track, the echo Doppler centroid of the Doppler spectrum with wind direction is slightly asymmetric at the downwind and upwind, and reaches a minimum at a 90° wind direction. The analysis results of the wind fetch show that when the wind speed is 10 m·s^–1 and the length of wind fetch increases from a-10 km-developing wave to a fully-developed wave, the velocity of the sea surface increases, and the tilt modulation of the long wave increases, resulting in the Doppler shift increases, and the estimated Doppler centroid difference is 0.56 m·s^–1. Finally, this study considers the contribution of breaking waves to the co-polarized backscatter and analyzes the influence of both the Doppler centroid and velocity estimation. Analysis of echo Doppler spectrum under the condition of wave breaking shows that when the wind speed is 10 m·s^–1 and the observation azimuth is the same as the wind direction, the Doppler centroid offset is about 71.4 Hz, resulting in a deviation of about 0.3 m·s^–1 for the radial velocity estimation compared with the case without considering the breaking wave.