Interferometric synthetic aperture sonar (InSAS) is an acoustic imaging technique that produces centimetre scale resolution 3D maps of the sea-floor. These sea-floor surveys are typically conducted from autonomous underwater vehicles (AUVs) which travel along a path, projecting acoustic pings and receiving their echoes from the sea-floor. Coherent processing of these echo data allows an image of the sea-floor to be formed. This processing requires sub-millimetre knowledge of the vehicle’s path, which must be derived using the sonar data because radio-navigation services such as GPS are unavailable underwater. Micro-navigation is the name given to this though-the-sensor navigation estimation. This project aims to develop new micro-navigation algorithms for interferometric synthetic aperture sonar.

Project outline

Micro-navigation algorithms typically exploit time delays measured between the echoes received by adjacent pings. These time delays are affected by the so-called triad of confounding factors; navigation, bathymetry and sound speed. Navigation encompasses the vehicle motion and sonar geometry. Bathymetry is the shape and depth of the sea-floor, the underwater analogue of topography.

The new algorithms developed in this project aim to account for the triad of confounding factors by simultaneously estimating the navigation, bathymetry and sound speed using the sonar data. This has potential to improve the accuracy of both the navigation and bathymetry estimates.

Science

The work performed during this project has been presented at numerous international conferences. A sample result presented at the recent Underwater Acoustics Conference and Exhibition 2019 in Crete is a synthetic aperture sonar image where colour encodes the de-trended sea-floor depth estimate. This estimate was made using a simultaneous navigation and bathymetry estimation algorithm developed for this project. A paper describing one component of this algorithm is currently in review with the Journal of Oceanic Engineering, and further paper is in preparation.

Impact

Interferometric synthetic aperture sonar already has multiple applications including military, such as mine hunting, and civilian, such as underwater archaeology, monitoring of underwater structures and environmental monitoring and surveying. However, the accuracy of the bathymetry estimates is currently limited by the size of the platform, but there is potential to improve bathymetry resolution by fusing data from repeated acquisitions of the scene. Advanced, phase preserving micro-navigation algorithms are required to realise this exciting potential.

This is the PhD project of Benjamin Thomas, supervised by Dr Alan Hunter and funded by The James Dyson Foundation.