Analysis of Doppler spectrum of a spaceborne Doppler scatterometer using an echoed signal simulation model

ABSTRACTThe Doppler spectrum of a microwave scatterometer signal is determined by the relative motion between the scatterometer and the ocean surface. It is also determined by the modulation from the ocean surface backscattering coefficient. The relative motion includes contributions mainly from the motion of the platform, the phase velocity of the resonant Bragg waves, the orbital motions of ocean waves, and the ocean surface current. Estimating the Doppler centroid from the complex Doppler spectrum of the received signal is an important issue for the application of a spaceborne Doppler scatterometer in ocean surface current retrieval. In this study, we use an echoed signal simulation model based on a two-dimensional rough dynamic ocean surface to estimate the Doppler spectrum of a spaceborne Doppler scatterometer. We analyze the Doppler spectrum with different observation geometry and ocean state, and the effects of modulation on the Doppler spectrum with the pulse-pair method. Our results show that the high platform velocity and the modulation of the backscattering coefficient from the ocean surface are the main influencing factors that affect the shape of the Doppler spectrum of the received signal, which affects the accuracy of estimation of the Doppler centroid. For a Ka-band pencil-beam rotating Doppler scatterometer using the pulse-pair method, the modulation can lead to a measurement error of 1.2 m·s−1. Disclosure statementNo potential conflict of interest was reported by the author(s).Data Availability statementNSCAT-4 GMF data were provided by the Koninklijk Nederlands Meteorologisch Instituut (KNMI) and are publicly available at https://scatterometer.knmi.nl/nscat4_gmf/.Additional informationFundingThis work was supported by the "Strategic Priority Research Program" of the Chinese Academy of Sciences under [Grant No. E029122A1S].