Compensation of medium-induced fluctuations for synthetic aperture sonar

Abstract

There are two primary obstacles to achieving aperture coherence in synthetic aperture sonar (SAS); motion compensation (MOCOMP) accuracy, and medium fluctuations along the acoustic propagation paths. The feasibility of MOCOMP for high-resolution SAS using a combination of commercial MOCOMP sensor components with existing autofocus processing algorithms has been previously assessed. This paper discusses the experimental measurement of acoustic fluctuations induced by inhomogeneities in sea water, their impact on SAS imagery, and whether autofocusing algorithms can compensate for medium-induced fluctuations. Spatial as well as temporal fluctuations over propagation path lengths of 1000 ft and frequencies of 100 kHz are relevant to high resolution SAS. A historical survey discovered no data in this parameter regime. Therefore, a joint IR&D effort, the Acoustic Medium Stability Experiment (AMSE), was conducted to collect appropriate data. AMSE’s unique data set provides space-time statistical descriptions of the medium-induced phase fluctuations. Using the AMSE data and an end-to-end SAS simulation capability, it was found that medium-induced fluctuations do degrade SAS imagery significantly. However, available autofocusing algorithms are capable of estimating and compensating the medium-induced phase errors (in addition to MOCOMP errors), allowing fully focused imagery to be obtained.