GPS-cellular drifter technology for coastal ocean observing systems

A drifter for observing small spatial and temporal scales of motion in the coastal zone is presented. The drifter uses GPS to determine its position, and the Mobitex terrestrial cellular communications system to transmit the position data in near-real time. This configuration allows position data with order meter accuracy to be sampled every few minutes and transmitted inexpensively. Near-real-time transmission of highly accurate position data enables the drifters to be retrieved and redeployed, further increasing economy. Drifter slip measurements indicate that the drifter follows water to within similar to 1-2 cm s(-1) during light wind periods. Slip values > 1 cm s(-1) are aligned with the direction of surface wave propagation and are 180 degrees out of phase, so that the drifter "walks" down waves. Nearly 200 drifter tracks collected off the Santa Barbara, California, coast show comparisons with high-frequency (HF) radar observations of near-surface currents that improve by roughly 50% when the average drifter values are computed from more than 25 observations within a 2-km square HF radar bin. The improvement is the result of drifter resolution of subgrid-scale eddies that are included in time-space-averaged HE radar fields. The average eddy kinetic energy on 2-km space and hour time scales is 25 cm(2) s(-2), when computed for bins with more than 25 drifter observations. Comparisons with trajectories that are computed from HE radar data show mean separation velocities of 5 and 9 cm s(-1) in the along- and across-shore directions, respectively. The drifters resolve scales of motion that are not present in HF radar fields, and are thus complementary to HT radar in coastal ocean observing systems.