A Lagrangian Drifter with Inexpensive Wide Area Differential Gps Positioning

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 rerult in position measurement inaccuracies of up to +/100 meters. A 100 meter error over a 30 minute timespan between drifter poritions results in a 6 cmlsec error in computed current velocity, a potential error which grows larger as shorter position recording intervals are used. Improvement of the accuracy of GPS po5itioning would allow ure of GPS drifters in an even widcr range of current mearurement application\ that demand tine q)ati:il and temporal resolution. Differentiiil GPS (DGPS) refers to a technique that uses GPS dtita from a reference station at a well known location to calculiite correction, thiit a re then used to improbe the accuracy of GPS positionr a t a lcrs well hnonn location. DGPS ciin increaw the :iccuracy of standard civilian rccci\ers to +/20 meterr or better. Conventional DGPS relies on real-time telemetry of correction datu from one o r more reference stzition to the GPS rccci\ e r that requires correction. This method can be ditt’kult, cxpenri\e, and ir not s*:iilable a t a11 location\. A drifter equipped for con\cntioniil DCPS mu,t h:i\e ii telemetry receiver which addr co\t and require, power. Moreover, the telemetry link mu,t be miiintained reliably during data acquisition or DGPS pwition accuracy is lo+t. If red-time rerult, arc not required, howevcr, ii Wide Area Differciitial GPS (WADGPS) method 0-7803-3519-8196 $5.00