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Analysis of sea surface scatter in the time-varying impulse response


In ocean environments, acoustics is the primary means of signal transmission, and sea surface waves can cause significant propagation variability. Reflections from the moving sea surface waves cause transient, and often simultaneous, acoustic arrivals. The motivation for this study is to better understand scattered arrivals, which complicate processing of acoustic signals in communication systems. Numerical methods are presented that improve modeling of acoustic scatter in estimates of the time-varying channel impulse response. These methods are both intended to improve prediction of surface scatter, and also to relate scatter arrivals to features on the sea surface. Complementary analysis of experimental measurements also relates scatter observations to reflecting features on the sea surface. Taken together, these numerical and experimental analyses both show how surface waves lead to surface scatter, and also how scatter arrival observations can reveal properties of these surface waves.

Several numerical model methods, both approximate and exact, are used to calculate sea surface scatter. The approximate models are eigen-rays and the Kirchhoff approximation, and the exact models are the Rayleigh-Fourier method (RFM) and the limited duration integral equation method (DIEM). While the eigen-ray solution is often less accurate than the Kirchhoff approximation, it can be used to show the positions on the sea surface that serve as acoustic reflectors. The Kirchhoff approximation, in turn, often gives accurate results at close ranges but diverges significantly from the exact solution at moderate to long ranges. While the RFM was used to initially demonstrate these results, the DIEM method was developed as a more general method for exact calculations of surface scatter.

Experimental measurements of surface scatter are significantly more cluttered than predictions from the numerical models. This issue is addressed with Doppler sensitive probe signals, which enable resolving scatter arrivals with different Doppler shifts. The flat surface travel time demonstrates how the Doppler shift relates to both the position and velocity of each surface reflector. Doppler selective processing is then shown to select for surface reflectors from limited portions of the sea surface. These results improve the interpretation of scatter observations, and are intended to inform future studies that compare numerical results with experimental measurements.

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