Remote sensing of the sea surface using one‐ and two‐frequency microwave techniques

Abstract

The capabilities of one‐ and two‐frequency, coherent, pulsed, microwave radars for the remote sensing of the sea surface have been investigated both theoretically and experimentally. A combined instrument recording data independently in both modes is a compact and versatile probe for the study of two‐scale wave systems. An expression for the output of a dual‐frequency microwave system operated at intermediate grazing angles has been developed and quantitatively confirmed. This expression predicts that, for large illuminated areas, the dual‐frequency signal/background ratio will be inversely proportional to the area of the scattering cell, a dependence which has been confirmed experimentally. Furthermore, it has been shown that if a scattering cell of radial dimension small in comparison with the dominant ocean wavelength and an azimuthal dimension large in comparison with the dominant ocean wavelength is used, frequency demodulation of coherent single‐frequency microwave backscatter yields a signal proportional to the orbital velocity of those long waves which are propagating radially. This directional discrimination has been verified experimentally, and directional waveheight spectra have been shown to compare favorably with those measured by an array of pressure sensors. Finally, a double‐modulation technique has been conceived and tested experimentally. This technique allows measurement of interactions between waves with lengths of a few meters and much longer waves. Representative modulation transfer functions for these wave‐wave interactions have been obtained by using the technique.