Space-Time Matched-Field Depth Estimation for Active Sonar

Jeffrey Krolik and Granger Hickman
Duke University
Department of Electrical and Computer Engineering
Durham, NC  27708-0291


Abstract This paper concerns the use of complex multipath propagation for discriminating target depth with a low-frequency active sonar. Although matched-field processing techniques which exploit full-wave modeling of multipath propagation have been previously demonstrated in passive sonar applications, their use in active sonar has been precluded by uncertainty in the complex backscattering characteristics of the target. In this paper, a matched-field technique is proposed which exploits changes in the complex target return seen between consecutive sonar pings at a horizontal sensor array. Because the method exploits only changes in the coherent combination of multipath arrivals, it does not require a priori knowledge of the target backscattering characteristics assuming its aspect is only slowly changing from ping-to-ping. Space-time matched-field depth estimation (MFDE) has been developed as an extension of a successful technique for altitude estimation in over-the-horizon HF radar. Important differences between the techniques are the increased number of multipaths occurring in the sonar case as well as the availability of sensor outputs from a horizontal line array. In this paper, simulation results are presented which are modeled after a realistic Mediterranean scenario taken from the Shallow-Water Active Classification (SWAC) experiment performed in November, 1995. MFDE simulation results indicate that target depth accuracies of better than 20% of the channel depth can be achieved at ranges of 10 to 15 km. Preliminary results with real SWAC data achieved a depth accuracy of approximately 10 m for a 15 km target in a range-dependent channel whose bathymetry varied between 70 and 250 meters.

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