Radar Measurements of the Modulation of Ocean Waves Using the Two Frequency Scatterometer

An experimental study has been conducted to measure the microwave modulation characteristics of a wide set of ocean wavelengths from an aircraft, and to determine how these functions depend on ocean wavenumber, the incidence angle of the radar wave and environmental conditions at the air-sea interface. The two frequency microwave resonance technique has been employed using the NASA LaRC two frequency Scatterometer (at -band) during the ARSLOE Experiment in November 1980. Accurate comparisons with surface conditions were possible with supporting results from the Surface Contour Radar, XERB buoy measurements and a single frequency AM-FM modulation measurement. These high altitude aircraft measurements of the modulation transfer function demonstrate a dependence on wavenumber, wind speed and sea state, and a gradual decrease with incidence angle. These results also demonstrate that the directional spectrum of ocean waves can be measured from high altitudes, and suggest favorable possibilities for higher altitude aircraft or spaceborne platforms. New insights have been obtained by comparing these measurements with tower based measurements of the modulation transfer function. While these results are a major advance for ocean-microwave radar remote sensing, they also point the way to new studies of the generation mechanisms of short gravity and capillary waves, their "wave-wave interactions" with the large ocean gravity waves, and their dependence on environmental conditions.