Doppler Velocity from Sea Surface on the Spaceborne and Airborne Weather Radars

The Doppler operation applied to the sea surface is studied for the spaceborne and airborne weather radars. In the space mission, a small amount of beam misalignment can cause large contamination of a platform velocity into a measured Doppler velocity. In this paper, theoretical bases for retrieving water droplet velocity via the Doppler signal from a sea surface are considered for pulse-pair operation in the nadir direction. On a fast-moving platform, Doppler fading is so crucial that the finite beamwidth of an antenna is taken into account in the formalism based on a scalar Kirchhoff theory. Correlation signals are derived for both the coherent and incoherent scatterings that have substantial differences in Doppler velocity. Furthermore, the incoherent Doppler velocity is shown to include a path-difference term, which is characteristic of the incoherent surface scattering, set apart from the incoherent body scattering. Applying the retrieval methodology to the Doppler signal from a ground surface is also discussed.