Doppler dependence of radar sea return

Abstract

A series of experimental tests have been performed in which the Doppler characteristics of radar sea return at C band were measured. Both monostatic and bistatic sea return at small depression angles with both horizontal and vertical polarizations have been measured. The mean Doppler shift of the radar sea return for horizontal polarization is 2 to 4 times as great as the Doppler shift for vertical polarization for the same or similar wind and wave conditions. The vertical-polarization Doppler shift is dependent on the wave height and is directly related to the orbital velocity of the gravity waves. The horizontal-polarization Doppler shift is dependent on the wave height and the wind velocity jointly and is directly related to the motion of a surface layer of the sea. This surface velocity is the sum of two velocities: the orbital velocity of the sea moving beneath this layer and a function of the wind velocity. For both polarizations, the mean Doppler shift is cosine-law dependent on the angle between the wind and wave direction and the radar propagation direction. The spectrum bandwidth is, however, relatively independent of this viewing angle. For small depression angles, the Doppler shift and the spectrum bandwidth is independent of the radar depression angle. For both horizontal and vertical polarizations, the spectrum bandwidth is directly dependent on the mean Doppler shift obtained looking into the wind and waves. Within the conditions measured, there is no dependence of the Doppler shift or the spectrum bandwidth on bistatic conditions. Thus, the results given above apply both to monostatic and to bistatic radar configurations within the conditions measured.