Scintillation index of high frequency acoustic signals forward scattered by the ocean surface

Ocean measurements of the scintillation index (SI) of surface forward-scattered signals made in August 2002 are presented and compared with a model developed by Yang and McDaniel [Waves in Random Media 1, 419-439 (1991)]. The acoustic measurements employed continuous wave (CW) pulses and linear frequency modulated (LFM) sweeps with center frequencies of 20 and 40 kHz. Simultaneously, measurements of wind speed, directional surface wave height spectrum, and ocean sound speed profile were made. The sea state was between 0 and 1 during the four days of the experiment, in part because the location is very much in the lee of San Clemente Island. The measured values of SI are found to agree with Yang and McDaniel model predictions, except for measurements with the largest signal bandwidth and/or the narrowest beamwidths, which violate model assumptions of continuous signals and omnidirectional projectors and hydrophones. In addition, the data show that SI decreases with increasing signal bandwidth (or decreasing temporal extent). An extension to the Yang and McDaniel model is developed that accounts for a reduction in signal temporal extent or ocean surface ensonification. The extended model is in qualitative agreement with the measurements, in that SI is predicted to decrease with increasing signal bandwidth.