Coherence of Sea Surface Reflected Pulses

Abstract

Forward reflected undersea pulses are known to fluctuate and undergo severe distortion from arrival to arrival. The coherence of high-frequency surface reflected pulses has been measured by cross correlating the transmitted pulse with the reflected arrival. Both pulsed CW and linear FM pulses were utilized, with bandwidths ranging from 50 to 2000 Hz. Considerable care was taken to resolve the surface path from direct and bottom arrivals, thus enabling one to compute a normalized cross-correlation coefficient versus bandwidth. Data strongly shows that the normalized coefficient decreases with increasing bandwidth. Temporal elongation of the reflected pulse appears to be the major cause of decorrelation of the wider bandwidth signals. A statistical model of the sea surface is adopted based upon known properties such as the dependence of mean-square slopes and correlation lengths upon wind speed. Ray theory is used to compute the expected number of ray paths from facets and their attendant travel times. The problem is tractable, assuming Gaussian surface statistics. Woodward cross-ambiguity theory then allows one to predict signal coherence loss due to time spreading from the foregoing analysis, resulting in good agreement with experiment.