Coherent Recombination of Acoustic Multipath Signals Propagated in the Deep Ocean

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Abstract
The multipath acoustic impulse response of the deep ocean has been measured over a 250-NM propagation distance and has been used in an attempt to remove the multipath distortion of a broad-band signal propagated over the same range interval. The process can be considered a coherent recombination of multipath arrivals, or a deconvolving of the received signal from the ocean impulse response. The measured impulse response is cross correlated with the distorted received signal to produce an estimate of the transmitted signal. Cross correlating this estimate with a replica of the original signal, and comparing this result to a cross correlation of the distorted received signal with the replica, shows a significant improvement in the former. Sidelobes due to undesired arrivals have been reduced to the point that, with a suitably chosen threshold, the message resolution time can approach the system inverse bandwidth rather than the multipath spread time, an improvement in resolution time of more than an order of magnitude. The experiment also demonstrates short-term (e.g., 6-sec) stability of the ocean impulse response, without establishing an upper limit for this stability.

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