The Doppler Spectrum of The Microwave Radar Signal Backscattered by the Sea Surface at Low Incidence Angles

Results of the first experiment on measuring the Doppler spectrum of the microwave signal backscattered by the sea surface at low incidence angles (< 20°) made from a sea platform are presented. At small incidence angles (<10°), the reflection is quasi-specular and the Kirchhoff approximation is used to calculate the Doppler spectrum. Comparison showed a good agreement between the experiment and numerical estimates of the Doppler spectrum model. The measurements have shown that in the transition range of incidence angles (10°-20°), one should take into account the resonant scattering contribution to explain the features of the measured Doppler spectrum. To calculate the Doppler spectrum in the transition range, an advanced Bragg scattering model was used. It is shown that the employed quasi-specular and Bragg scattering mechanisms qualitatively explain the dependences of the width and shift of the Doppler spectrum on the incidence angle. In passing to quantitative estimates, tilt modulation and hydrodynamic modulation of the Bragg scattering mechanism should be taken into account, which will be done in a future study. The first measurements of the dependences of the width and shift of the Doppler spectrum on the incidence angle allow comparing well-known scattering models.