Ka-band ocean wave/radar modulation transfer function: a comparative study

For radar systems looking at moderate angles of incidence most scattering theories assume that the prime scatterers are ripples. Various mechanisms relate the modulation of radar returned power to the long ocean waves. The most important contributors to the radar-signal modulation for real-aperture radars are the tilt and aero-hydrodynamic modulation. The tilt modulation is a purely geometric effect and can be evaluated from the ocean wave slope. The hydrodynamic modulation that results from the nonuniform distribution of the ripples on the long waves is less well understood. In this paper the authors compare the radar backscatter modulation measured by two different Ka-band radar systems to gain more insight into the modulation process. One system is a Ka-band unmodulated continuous-wave (CW) radar that measured radar cross section at HH and VV polarizations and Doppler frequency. The other is a Ka-band frequency-modulated CW Vector Slope Gauge (VSG) that combines a vertically polarized scatterometer with range measurements at three closely located spots on the surface, thus permitting measurement of surface elevation and two orthogonal components of the 2D wave slope. The modulation transfer function often used to describe radar/ocean interaction, depends on the assumption that the microwave signal power depends linearly on the long-wave slope. If linear wave theory applies, the slope can be extracted from measurements of either the surface elevation or the horizontal component of the orbital velocity