GPS and AUVs: a solution to the problem of signal interruption

Autosub, an AUV, called surfaces periodically to communicate with its base and to determine its position. The satellite-based Global Positioning System (GPS) has been chosen as the prime candidate for this latter requirement because of its near-instantaneous global coverage, and because of the challenges inherent in adopting a leading-edge technology as a principal component in the overall design. However, GPS suffers from a major disadvantage, namely the difficulty of acquiring fresh satellite orbital parameters (the ephemeris) in the presence of periodic wave washover of the GPS antenna. This problem was tackled in two stages. Firstly, the dynamics of the AUV and the consequent antenna masking and interaction with the GPS carrier and code tracking loops were simulated, leading to the development and validation of a novel algorithm for reconstruction of the GPS satellite message from fragments. The algorithm was then embedded in the firmware of a production GPS receiver and tested under practical conditions of signal disruption, including sea trials in a full-size model AUV hull