GPS DRIFTING BUOYS

Model 104A GPS drifter with mini case and Argos satellite telemetry. Drifters can be changed from deep drogue (as shown) to Davis/CODE surface-following configuration in the field.

Model 104V large case drifter with custom sensors

Model 104V GPS drifter with large case and VHF packet telemetry. Sails and floats have been removed to show external sensors. The large case Model 104 has great flexibility and expandability for specialized applications. This drifter has been customized with 24 external analog channels plus power and control lines to accommodate three Satlantic multichannel irradiance sensors and a Falmouth Scientific salinity/temperature sensor.

Model 115 low cost autonomous GPS/Argos drifter.

Model 115 low-cost GPS drifter with Argos telemetry, rigged in Davis/CODE surface-following configuration.

Overview

Autonomous GPS Drifters are current following (Lagrangian) drifting buoys. They are released in a body of water and move with the currents over a period of days to months. Onboard electronics acquire a time series of positions using GPS as the drifter moves. This series of positions is telemetered via short-range packet radio and/or the worldwide Argos satellite network. Precision Lagrangian current data provided by GPS drifters are useful in current measurement, larval fish studies, oil spill or floating debris tracking, discharge dispersement calculations, and similar studies.

Models

Brightwater offers two models of GPS drifter. The Model 104 is our most capable drifting buoy. It offers internal storage of data, a variety of telemetry alternatives, optional differential GPS capability, and the ability to be easily interfaced to many additional sensors. The Model 115 is a low cost adaptation of the Model 104 that provides a basic level of functionality adequate for many applications while remaining compatible with software, deployment logistics, data format, and drifter characteristics of its more advanced sibling. Additionally, the Model 115 is optimized for simple deployment, allowing drifter launches from ships of opportunity by minimally-trained personnel.

Physical characteristics

Physical drifter design is similar to the Coastal Ocean Dynamics Experiment (CODE) drifter developed at the Scripps Institute of Oceanography. This design (also known as a Davis or SCULP drifter) provides excellent coupling to the surface layer and exhibits little wave rectification. An optional conversion kit allows the drifter to be changed in the field to a deep drogued configuration.
The Model 115 and the "mini" size 104 are enclosed in a three-inch diameter case. The three-inch case drifters are small and light (14 kg / 30 pounds) and are easily hand-deployed and retrieved by one person. The Model 104 is also available in a large size six-inch diameter case. The large case features longer battery life and can be fitted with more options.

Setup and operation.

Hardwired communication with the drifter is via a standard RS-232 serial interface. An external communications option brings the interface to an underwater connector on the drifter hull, allowing hardwired communications without opening the drifter case. Although the serial interface allows extensive tuning of drifter operational parameters, the drifter can also be operated with a single magnetic switch on the endcap. A "beep code" audible through the case allows personnel without a computer to monitor and confirm proper drifter operation before deployments. In addition to telemetering data, the Model 104 stores data taken during a deployment in CMOS RAM memory. Even if telemetry coverage is not complete, all deployment data can be retrieved from the Model 104 after recovery.

Telemetry.

        The Model 104 can be fitted with short range VHF/UHF packet radio telemetry (Model 104V), worldwide telemetry through the Argos satellite system (Model 104A) or both packet and Argos (Model 104AV). The Model 115 is available with Argos telemetry only.
         Argos telemetry provides worldwide data transmission through the US/French Argos satellite system. Data compression allows diagnostic data and up to 25 GPS drifter positions to be encoded in three alternating 256 bit error-checked messages using only one Argos ID number, a technique pioneered by Brightwaters. During deployments at mid latitudes, over 99% of 30 minute positions are typically received through Argos.
         Packet telemetry uses CD/CS-MA (carrier detect/collision sense-multiple access) communication that allows many drifters to share one radio frequency and provides error-free telemetry. Packet ranges of greater than 10 kilometers over water can easily be achieved, with the full range of Model 104 operational functions available over the remote link. In addition to the user-initiated bidirectional link, packet capable drifters can also send an unacknowledged "beacon" at regular intervals. This can be monitored and recorded or used to assist in locating strayed drifters.

Differential GPS A Wide Area Differential GPS (WADGPS) option adds the capability for the Model 104 drifter to store raw GPS data internally. After retrieval of the drifter, WADGPS data can be postprocessed to higher quality positions using commercially available GPS correction data and software. In field tests, WADGPS postprocessing reduced 2DRMS position errors from 30 meters to 2 meters. The Model 104 is the first (and to date the only) commercially-available drifter with DGPS capability.

The drifter tracks above are from Crater Lake, Oregon USA. The bars at upper right are 200 meters long. The left plot shows conventional GPS data. The right plot is the same data after WADGPS processing. Data courtesy of John Barth, Oregon State University.

Click for sample graphics screen from real time tracking software.

Software

Decoding and tracking software is included with all GPS drifters at no charge:

Postprocessing software decodes compressed Argos telemetry to flat ASCII files of position and diagnostic data.
Real time software tracks deployed drifters from a ship by monitoring packet beacon or Argos uplink transmissions. Text and graphical screens display drifter tracks, shorelines, ship position, range, bearing, and age of last valid telemetry.

Brightwaters president Thomas Wilson with four generations of GPS drifters. Right to left: Model 104V vintage 1992, Model 104AV vintage 1995, Model 104A vintage 1996, Model 115 vintage 1999. Note that all of these drifters are still in active use by their purchasers.

Serviceability

Unlike most competing products, our drifters are designed to be reused. Although Brightwaters offers complete refurbishing services, most routine maintenance can be accomplished by the end user. The drifter hull can be opened to change batteries. Replacement battery packs are simple to make locally or can be ordered from Brightwaters. External parts such as sails or floats are easily replaced if damaged. As can be seen from the picture at left, many of our customers have used the same drifters for years on multiple research projects.

Custom modification

The design of the Model 104 lends itself well to custom modification of hardware and software to meet individual user's requirements. Generous uncommitted analog and digital interface capability is available to integrate additional sensors. Alternative physical packaging such as WOCE SVP, Draper LCD, or extremely robust "crash cage" and "baseball bat" configurations are possible. There is no minimum quantity for custom modifications, and our wide previous experience often allows us to customize at surprisingly modest cost.

A history of innovation

Brightwaters Instrument Corporation has been building autonomous drifting buoys since 1991, with current products representing a sixth generation of design. Innovations developed at Brightwaters have been adopted across the drifter industry. Continuous design improvements reflect Brightwaters' commitment to offer the most technically advanced drifters in the world.

Downloadable data sheets

The following data sheets (containing high resolution graphics) can be viewed and printed using the free Adobe Acrobat Reader. Note: use of Version 4 or higher of Acrobat Reader is recommended for best results.