The Two-Frequency Microwave Ocean Wave Scatterometer - A New Approach

Investigators in several countries have been studying a two-frequency microwave radar technique for the remote-sensing of direction ocean wave spectra and surface currents. This technique is conceptually attractive because its operational physical principle involves a spatial electro-magnetic scattering resonance with a single, but selectable, long gravity wave. Multiplexing of signals having different spacing of the two transmitted frequencies allows practical measurements of the entire long wave ocean spectrum to be carried out. The scatterometer signal/background ratio is, using conventional processing, only adequate for measurements made relatively near the dominant wave wavenumber and when sufficient temporal averaging of spectra is employed. A new scatterometer has been developed and experimentally tested which is capable of making measurements having much larger signal/background values than were previously possible. This new instrument couples in a hybrid fashion the resonance technique with coherent, frequency-agility capabilities. Frequency-agility provides a means of acquiring virtually instantaneously the signal degrees of freedom necessary to obtain accurate spectral estimates. The effect of increasing the number of coherently summed frequency-agile signals is to enhance the return from large ocean surface features such as long-wave trains which are correlated over the range of frequency changes used at the expense of signals from shorter centimetric wavelength waves which are decorrelated. It was, however, not known a priori whether or not the induced correlation of centimetric waves by the longer waves would be insignificant for the small scattering cells used in the experiments.