Acoustic and microwave backscattering of the sea surface

Results of the experiment SAXON Phase 2 which was conducted in November 1991 at the German Research Platform NORDSEE are presented. The experiment was designed to measure the acoustic and electromagnetic backscattering strength of the sea surface simultaneously and at the same sonar and radar wavelength. The grazing angle dependence was investigated at angles from 20° to 70°. Backscattering from the same spot of the sea surface was measured at a grazing angle of 18° for both systems. The sonar frequencies were 50, 26.5, and 5 kHz corresponding to radar frequencies of 10, 5.3, and 1 GHz (X-, C-, and L-band), respectively. Data were collected at wind speeds ranging from 0 to 21 m/s. The results exhibit excellent correlation between sonar and radar backscattering in the low wind speed regime at all frequencies. At higher wind speeds, bubbles caused by breaking waves are the dominant acoustic backscattering mechanism at high sonar frequencies. In this case both systems no longer see the same scatterers and the correlation decreases. The parameter combinations of wind speed and sonar and radar frequency, for which the backscatter occurs from the sea surface only, are given for a grazing angle of 45