On the relationship between signal bandwidth and coherence for ocean surface forward scattered signals

The relationship between the signal bandwidth and the coherence of high frequency, surface forward scattered underwater acoustic signals has been investigated experimentally and compared with available theory. Acoustic measurements were made under ONR sponsorship (the ARL Program) during 2000 [J. G. Keranen et al., J. Acoust. Soc. Am. 109 (2001)] and again in 2002, both times in deep water off the coast of San Diego. Transmitted signals consisted of 0.1 to 1.0 ms CW pulses and 500 Hz to 22 kHz wide linear frequency modulated sweeps using center frequencies of 18 to 46 kHz. Concurrent environmental measurements included water temperature, current speed and direction, directional wave height and wind speed. An analysis of the data is focused on how the coherence of the surface forward scattered path depends upon the signal bandwidth. We evaluate the usefulness of the frequency coherence bandwidth parameter [P. H. Dahl, IEEE J. Oceanic Eng. 26(1) (2001)], and compare measurements with time spread-based theoretical predictions [J. C. Reeves, Ph.D. thesis, UCLA, 1973; M. H. Brill et al., J. Acoust. Soc. Am. 75 (1984)]. [Work supported by ONR under Award No. N00014-02-1-0156.]