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 repeat-pass SAS micronavigation updates from less than 1 to 10 m.
Highlights
A UTONOMOUS underwater vehicles (AUVs) with synthetic aperture sonar (SAS) are used for a wide range of applications, including search and survey [1], marine geology [2], habitat monitoring [3], environmental monitoring [4], naval mine countermeasures [5], change detection [6], [7], and underwater archeology [8], and there is a huge potential of Manuscript received December 13, 2019; revised August 28, 2020, January 5, 2021, and February 5, 2021; accepted February 15, 2021
The experiment was conducted on June 5, 2010 using the Royal Norwegian Navy’s HUGIN-MR AUV with its HISAS 1030 SAS [49]
We have investigated the spatial correlation of speckle for repeat-pass SAS micronavigation, and showed that spatial decorrelation can be effectively described through the three contributions: 1) speckle decorrelation; 2) footprint mismatch; and 3) stretching
Summary
A UTONOMOUS underwater vehicles (AUVs) with synthetic aperture sonar (SAS) are used for a wide range of applications, including search and survey [1], marine geology [2], habitat monitoring [3], environmental monitoring [4], naval mine countermeasures [5], change detection [6], [7], and underwater archeology [8], and there is a huge potential of Manuscript received December 13, 2019; revised August 28, 2020, January 5, 2021, and February 5, 2021; accepted February 15, 2021. Date of publication May 31, 2021; date of current version October 13, 2021.
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