Doppler Spectrum of the Microwave Signal at Small Incidence Angles: Discussion of the Theoretical Model

The spectral and energy characteristics of the backscattered radar signal contain information about the scattering surface parameters. The backscattered normalized radar cross section (NRCS) is determined by the geometry of the underlying surface, while information on the sea surface movement is contained in the Doppler spectrum (DS) of the backscattered microwave signal. The paper deals with the analysis of the properties of the DS model at small incidence angles, when the quasi-specular backscatter mechanism is dominant. The dependences of the width and shift of the DS on the speed and direction of wind, nondimensional wind fetch, incidence angle, width of antenna beam and velocity of the radar movement are investigated. It is shown that even for a case of wind waves there is an ambiguous relationship between the wind speed and the DS parameters, which leads to ambiguity in solving the inverse problem. Numerical estimates have shown that the width of antenna beam strongly affects the width and shift of the DS and this can be used to determine the optimal measurement scheme. The case of swell and mixed sea is also considered and it is shown that a large difference in DS is observed even under the condition of equality of the SWH and the dominant wavelength of wind waves and swell. When measuring from a moving platform, the DS parameters remain dependent on the statistical characteristics of the sea waves for a radar with a wide antenna beam and can be retrieved during subsequent processing. When measuring the DS, information about the RCS is saved and can also be used during processing.