GPS–Cellular Drifter Technology for Coastal Ocean Observing Systems

Abstract

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 ∼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° 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 HF radar fields. The average eddy kinetic energy on 2-km space and hour time scales is 25 cm2 s−2, when computed for bins with more than 25 drifter observations. Comparisons with trajectories that are computed from HF 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 HF radar in coastal ocean observing systems.

Interpretation of Coastal HF Radar–Derived Surface Currents with High-Resolution Drifter Data

“…5). The standard deviation for the radial component of drifter observations is 3.4 cm s Ϫ1 , slightly larger than values for the COP radial cells and consistent with observed high frequency variations in the alongshore velocity component (Table 2; Ohlmann et al 2005). The RMS difference between hourly HF radar and drifter velocities is 3.3 cm s Ϫ1 , consistent with COP values, despite fewer hours where the radial sector is heavily seeded with drifters (Fig.…”

“…Accurate nearreal time positions allow recovery and redeployment of the drifters. The drifters follow the water to within ϳ1 to 2 cm s Ϫ1 and experience vertical shears of 1 to 2 cm s Ϫ1 from the top to bottom of the drogue (Ohlmann et al 2005). Observed error (standard deviation) in drifter position is responsible for a corresponding error in velocity less than 1 cm s Ϫ1 for the 10-min sampling interval (Ohlmann et al 2005).…”

“…Drifter data used in this study are collected with global positioning system (GPS) located, reusable, cellular instruments developed for high-resolution nearshore use (Ohlmann et al 2005). The drifters (manufactured by Pacific Gyre Corporation, Oceanside, California) are comprised of a corner-radar-reflector-type drogue attached to a surface float that houses the electronics.…”

“…The frequency resolution of computed radar cross-spectra, which depends on FFT length, limits radial velocity resolution to ϳ5 and 2.5 cm s Ϫ1 for 12-and 25-MHz systems, respectively. Drifters can slip at ϳ1 to 2 cm s Ϫ1 from the ocean water they follow (Ohlmann et al 2005). Second, vertical scales of measurement differ.…”

The Meandering Current Mobility Model and its Impact on Underwater Mobile Sensor Networks

“…Although the devices in use today are not able to communicate with each other, there is a growing trend of using lagrangian devices for monitoring regional and coastal areas [17]. In those settings the small distance between the devices makes it possible to acoustically interconnect them and deploy them as underwater mobile acoustic sensor networks.…”