Microwave Bragg Scattering from the Sea Surface at Shallow Grazing Angles

Abstract

Wave tank experiments have shown that microwave backscatter from a water surface at moderate grazing angles is dominated by resonant or Bragg scattering. Although there are many examples of HF Doppler spectra from the open sea which confirm the Bragg scattering mechanism, little analogous data is available at microwave frequencies. Most microwave experiments are performed at moderate grazing angles (of order 4.5 degrees) in order to limit the area illuminated by the antenna. We have used FMCW techniques with an S band microwave radar to achieve range resolution down to 2.5 meters. A series of experiments has been performed at shallow grazing angles, typically 3 to 4 degrees, on an enclosed harbour during calm conditions. The resulting Doppler spectra show that resonant Bragg scattering by 5 cm (half the radar wavelength) short gravity waves dominates the scattering process under these conditions.