Spatial Coherence of Speckle for Repeat-Pass Synthetic Aperture Sonar Micronavigation

Abstract

Accurate positioning of autonomous underwater vehicles is a major challenge. The long-term drift is problematic if global position updates are not available, and for applications such as repeat-pass interferometry and coherent change detection, millimeter accuracy is needed. Repeat-pass synthetic aperture sonar (SAS) micronavigation is one potential technique for countering both challenges. While single-pass SAS micronavigation enabled successful coherent processing within one track, the potential is that repeat-pass SAS micronavigation can support coherent processing between passes. Both techniques are based on recognizing the speckle pattern in the seafloor return, but repeat-pass SAS micronavigation has additional challenges with the larger temporal and spatial separations between the observations. In this study, we investigate the spatial correlation of speckle as observed from SAS systems. We divide the different contributions to spatial decorrelation into three groups: 1) speckle decorrelation; 2) footprint mismatch; and 3) stretching. We examine each contribution separately and develop simplified formulas for their decorrelation baselines. When correlating synthetic aperture images, decorrelation from stretching dominates. When correlating single-element data recorded at low grazing angles common to SAS, speckle decorrelation dominates. We validate our findings on experimental data, and by combining elements into larger effective elements, we demonstrate increasing the across-track baseline for repeatpass SAS micronavigation updates from less than 1 to 10 m.

Synthetic Scan Formation for Underwater Mapping With Low-Cost Mechanical Scanning Sonars (MSS)

“…SAS is a very active field of research, see, e.g., [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. But while the inspiration comes from SAS, there are also some substantial differences to it in SSF.…”

Near range breakdown of the phase centre approximation

“…Statistical estimation error introduces bias and variance to the magnitude of complex correlation coefficient, both of which increase with decreasing population coherence [29]. This effect of estimation bias has been observed in SAS systems for speckle coherence [31] and for DPC micronavigation [25]. The challenges with observing this ripple effect are apparent in the PoSSM simulated time‐series data.…”

An Overview of Advances in Signal Processing Techniques for Classical and Quantum Wideband Synthetic Apertures

“…Similarly, time delay estimation between signals recorded on overlapping along-track positions between pings provides an across-track micronavigation estimate. Across-track micronavigation is particularly challenging in repeat-pass SAS processing for coherent change detection, where aggregated navigation errors between passes can result in baseline decorrelation not attributed to scene changes [265][266][267].…”

“…The authors in Ref. [267] show that, by combining elements in the multi-channel system into larger effective elements, the along-track and across-track decorrelation baseline increases. A machine learning approach based on variational inference has been recently proposed for robust data-driven estimation of the three-dimensional platform translation between pings from spatiotemporal coherence measurements of diffuse backscatter [269].…”