An analysis of energy spreading loss associated with tactical active sonar performance in a shallow water environment.
Bourke, Robert H.
James H. Wilson
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Energy spreading loss (ESL) is qualitatively defined as the reduction in peak echo level due to energy spreading of the transmitted acoustic pulse in time. An analysis of the impact of shallow water propagation on ESL was performed with the aid of a high performance computer using the FEPE_SYN and EXT_TD programs to compute the spreading of the received pulse due to multipath propagation in shallow water. A Blackman windowed pulse was used to model the transmitted pulse, which was centered at 3.5 kHz, with 200 Hz bandwidth. For input parameters, typical seasonal sound speed profiles and a Hamilton geoacoustic model of Area Foxtrot off the U.S. eastern seaboard was used. ESL's impact on sonar performance was determined as a function of range, source and target depth, sound speed profiles and geoacoustic properties. The impact of shallow water propagation on the correlation of the transmitted and propagated pulses through the quantitative definition of mismatch loss (MML) was also discussed. The results showed that strong ESL (5 to 10 dB) existed over a sand (reflective) bottom and was generally invariant with range. ESL was correlated with TL, i.e., areas of high spreading loss were found in regions of high TL. ESL was not as large (3 to 5 dB) over silt/clay (absorptive) bottoms due to the increased absorption of the bottom refracted path thus reducing the number of multipath modes. Broadband pulses were found to exhibit fewer fluctuation than single frequency signals, and generally the total TL loss was a few dB larger than a single cw case. To overcome the ESL, integration techniques based on an accurate prediction model in the post analyzing system are required with a high temporal resolution of the echo energy shape
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