GPS in autonomous underwater vehicles

The UK Natural Environment Research Council (NERC) is developing an autonomous underwater vehicle (AUV) capable of measuring, storing and transmitting full-depth oceanographic data over basin-wide tracks. An important feature of this AUV (known as Autosub) is that it will surface periodically for short periods in order 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. However, GPS suffers from a number of significant disadvantages in the wet environment of an AUV. The one discussed is the difficulty of acquiring fresh satellite orbital parameters (the ephemeris) in the presence of periodic wave wash-over of the GPS antenna. A collaborative programme involving members of the Autosub team based at the NERC Dunstaffnage Marine Laboratory (DML) and GPS specialists at Navstar Systems Ltd is addressing this problem in two stages. Firstly, the dynamics of the AUV and the consequent antenna masking and interaction with the GPS carrier and code tracking loops have been simulated, leading to the development and validation of a new algorithm for reconstruction of the satellite message. A later stage, not discussed in detail in the present paper, will embed the algorithm in the firmware of a production GPS receiver and test it under practical conditions of signal disruption. The results of the simulation stage are discussed. Some initial work on the development of a simple GPS antenna capable of withstanding immersion to full ocean depth is also presented.