Application of Radar and Microwave Scattering to Ocean Wave Research

The high data rate and advanced technology of modern radar systems make them attractive instruments for measuring the properties of random wave fields. When the objective is wave research, it is particularly important that the relation between wave field properties and radar output be clear and unequivocal. This is generally the case in the measurement of radar range (altimetry), the measurement of line-of-sight speeds (Doppler shift) and under those conditions where the scattered electromagnetic field is proportional to the surface displacement. The latter situation is that of first order Bragg scattering, in which case the influence of all waves outside a narrow window at the Bragg resonance condition is strongly filtered out. In this case, it has proved possible to measure such wave properties as temporal growth and approach to equilibrium in wind-wave tanks and spectral energy transport in wave tanks or at sea. Furthermore, the Doppler shift is accurately the frequency of the Bragg wave in first order scattering. Thus phase speeds may be determined not only for their intrinsic interest but as a means of measuring surface currents and probing the profile of the mean flow on both sides of the air-water interface. Results of all these techniques are now available in the case of wind-wave tanks and numerous examples are presented. The techniques have so far been less fully exploited at sea, but some examples from HF groundwave scatter and dual frequency microwave radar returns from wind-generated seas are available. Finally, opportunities for exploiting second order Bragg and two-scale scattering results for wave research are also discussed