A Lagrangian drifter with inexpensive wide area differential GPS positioning

Abstract

Autonomous drifting buoys that acquire position data using Global Positioning System (GPS) navigation have proven valuable in Lagrangian measurement applications requiring greater accuracy and data density than is available through other technologies such as ARGOS satellite tracking. The limits on measurement accuracy in a GPS drifter are set by the geographic accuracy of the GPS position. Standard civilian receivers are affected by the purposeful degradation of GPS accuracy (termed Selective Availability or SA), which result in position measurement inaccuracies of up to +/- 100 meters. A 100 meter error over a 30 minute timespan between drifter positions results in a 6 cm/sec error in computed current velocity, a potential error which grows larger as shorter position recording intervals are used. Improvement of the accuracy of GPS positioning would allow use of GPS drifters in an even wider range of current measurement applications that demand tine spatial and temporal resolution. Differential GPS (DGPS) refers to a technique that uses GPS data from a reference station at a well known location to calculate corrections that are then used to improve the accuracy of GPS positions at a less well known location. DGPS can increase the accuracy of standard civilian receivers to +/$20 meters or better. Conventional DGPS relies on real-time telemetry of correction data from one or more reference station to the GPS receiver that requires correction. This method can be difficult, expensive, and is not available at all locations. A drifter equipped for conventional DGPS must have a telemetry receiver which adds cost and requires power. Moreover, the telemetry link must be maintained reliably during data acquisition or DGPS position accuracy is lost. If real-time results are not required, however, a Wide Area Differential GPS (WADGPS) method can be used in pose-processing to remove the effects of SA from drifter data. The requirements in the drifter are some additional data memory, minor firmware changes, and a GPS engine configured to output raw GPS satellite data (called pseudoranges). During a deployment, the pseudorange data is stored in drifter memory while standard GPS positions are telemetered and stored to assist in deployment logistics. After retrieval, the pseudoranges are converted to DGPS-accurate positions using software and correction data provided by the Canadian Active Control System (CACS). CACS is a global network of automated GPS reference stations pioneered by the Canadian government for survey applications. Correction information is available from CACS at a modest fee with a few days delay from real time. Dockside testing of WADGPS correction showed a reduction in 2 dimensional rms position errors from +/- 33.5 to +/$9.3 meters. Several Brightwaters Modal 104AV autonomous GPS drifters have been fitted with a WADGPS upgrade. A full field test of WADGPS correction of drifter data will be conducted in August 1996.