Application of preprocessing methods to improve time delay estimation for synthetic aperture sonar motion estimation

Abstract

Synthetic aperture sonar (SAS) provides the best opportunity for side-looking sonar mounted on unmanned underwater vehicles to achieve high-resolution images at longer ranges. However, SAS processing requires maintaining a coherent phase history over the entire synthetic aperture, driving strict constraints on resolvable platform motion. This has driven the development of motion estimation and compensation techniques that use the received ping data, in addition to the onboard navigation solution, to resolve ping-to-ping platform motion. The most common approach is to use the redundant phase center technique. Here the ping interval is set, such that a portion of the array is overlapped. The accuracy of the motion estimation depends on the accuracy of the time delay estimation between the data received on the overlapping channels. Given the stochastic nature of the operational environment some level of decorrelation between these two signals is likely, even without residual platform motion. This decorrelation results in inaccurate time delay estimation and image quality degradation. In this research various preprocessing techniques have been applied to the sonar data to reduce the influence of stochastic noise with the goal of improving the accuracy of the time delay estimates.