New experimental results on GPS/INS navigation for Ocean Voyager II AUV

This paper presents preliminary experimental results on small-sized autonomous underwater vehicle navigation in shallow water environments. The vehicle was chosen to be our second-generation Ocean Voyager II which has been integrated with on-board GPS/INS sensors. These first-cut results reveal practical problems when using raw GPS fixes to perform high-precision real-time navigation. Among these, the most damaging factor is the observed correlated noise (which has a long time constant and large magnitude) embedded in the GPS data. With such disturbance, one cannot distinguish between instantaneous signal and noise over a short time scale (especially with sporadic fixes), and consequently the position estimator will be greatly biased. However, with regular incoming fixes, one can process the error between dead-reckoned and GPS signal in order to minimize the influence of the correlated noise under the assumption that any drift due to sensor bias or currents is relatively time-invariant. This processing thus enables one to estimate the drift and drift rate over time reliably for post-processed vehicle localization. In addition to the results in this paper, a differential GPS and Doppler-velocity-log sensor are currently implemented on our Ocean Explorer which is expected to provide much improved navigational performance. The chosen differential GPS sensor has so far demonstrated successful tracking at 2 feet below water surface, thus reinforcing a potential solution for clandestine operations.