Range-coherent matched field processing for low signal-to-noise ratio localization

Range-coherent matched field processing (MFP) coherently combines snapshots to localize a moving, narrowband source. This approach differs from existing MFP approaches that treat each snapshot as having a random phase due to both unknown motion through the medium and imprecise knowledge of the source frequency. Range-coherent MFP requires determination of the source phase acquired between snapshots. With that information, MFP can be applied to the cross-spectrum of snapshots acquired at different times, since relative phase between snapshots is determined by the medium properties, source location, and source velocity. Viewed another way, range-coherent MFP is simply MFP applied to a passive synthetic aperture formed from a moving source. The synthetic aperture geometry depends on source velocity, which is included in the MFP search space. Range-coherent MFP produces robust velocity estimates at low signal-to-noise ratio (SNR), which permits the use of a longer fast Fourier transform in pre-processing. The synthetic aperture array gain plus the increased input SNR afforded by the enhanced pre-processing significantly lowers the required signal level for successful localization. In data from the SWellEx-96 experiment, range-coherent MFP successfully localizes a source that is too quiet for conventional methods to localize.