Abstract
Subwavelength motion estimation is vital for the production of focused synthetic aperture sonar (SAS) imagery. The required precision is obtainable from the sonar data itself through a process termed micronavigation. Alongtrack micronavigation is achieved by a similar technique to that used in correlation velocity logs (CVLs), where sparse estimates of the spatial coherence function are interpolated to estimate the location of the peak coherence and hence estimate the interping vehicle motion. However, alongtrack micronavigation estimates made using this technique are biased, which limits the utility of these measurements for longterm navigation of autonomous underwater vehicles (AUVs). Three sources of alongtrack motion estimation bias are considered in this article. First, imperfect temporal registration between the signals results in coherence estimates that are negatively biased as a function of the temporal offset. Second, the sparse estimates of the spatial coherence function are obtained by crosscorrelation of complex baseband signals, a process which is known to result in positively biased coherence estimates, especially when the true coherence is low. Finally, mismatches between the underlying spatial coherence function and the interpolation kernel used to estimate the peak coherence location also result in alongtrack micronavigation bias. In this article, we describe and evaluate three methods for reducing alongtrack micronavigation bias. We introduce a temporal registration of the signals before coherence estimation, which reduces the impact of negative coherence bias due to temporal offsets. The remaining coherence estimation bias is reduced by combining multiple coherence estimates in a Bayesian coherence estimator. Additionally, an improved interpolation kernel is derived with a significantly improved fit compared to the current gold standard Gaussian interpolation kernel. The improvements in alongtrack micronavigation accuracy are demonstrated using two simulated data sets, which both allow comparison with ground truth. The first involves direct simulation of the spatial coherence from a given interping geometry using the pulseecho formulation of the van CittertZernike theorem, while the second involves simulation of raw sonar echo data using a pointscatterer model. Using these simulations, a reduction in alongtrack micronavigation bias of 48.5%99.5% is demonstrated, with reductions in alongtrack micronavigation error standard deviation of up to 34%. This improvement expands the potential for SASequipped AUVs to reduce their longterm navigation drift, facilitating longer underwater transits, improved target localization, and reduced track misalignment in repeatpass operations.
Original language  English 

Article number  9543700 
Pages (fromto)  162178 
Number of pages  17 
Journal  IEEE Journal of Oceanic Engineering 
Volume  47 
Issue number  1 
Early online date  22 Sept 2021 
DOIs  
Publication status  Published  1 Jan 2022 
Keywords
 Coherence
 correlation velocity log (CVL)
 micronavigation
 synthetic aperture sonar (SAS)
ASJC Scopus subject areas
 Ocean Engineering
 Mechanical Engineering
 Electrical and Electronic Engineering
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