Use of GPS drifters for measurement of near surface currents in a wide strait

The authors have recently undertaken a program of measurements and numerical modeling of the strong currents in Juan de Fuca Strait. Previous current measurement programs have under-sampled the near surface region (0 to 15 m) because of the vulnerability of conventional moored instrument arrays in this commercially active water way and because instrumental limitations preclude reliable measurements near the surface. Satellite tracked surface drifters, like those developed for the World Ocean Circulation Experiment (WOCE), have been used in great numbers to observe the surface currents in the open ocean. However, the accuracy and frequency of the position fixes provided by satellite-based location system (Argos) are insufficient to resolve the short space and time scale of the flow in the coastal zone. The Global Positioning System (GPS) has improved both the frequency and accuracy of drifter positioning, and its wide spread use has made it feasible to incorporate this technology into the field proven WOCE surface drifter. In the last two years, the authors have modified, tested and successfully used WOCE-GPS drifters from three different manufactures. An indispensable component of their drifter program is a ship-based, real-time tracking system. The tracking system comprises a commercially available uplink receiver to acquire the direct data transmissions from the drifters, together with a software program which we developed in-house to process the data messages so that they may monitor the drifter positions and their onboard sensor data in real-time. With the use of the real-time tracking they are able to recover and re-seed the drifters when they leave the experimental area or run aground, and they have been able to adjust their sampling strategy based on the observed surface circulation patterns