Surface Velocity Program (SVP) Drifter


SVP Drifter Schematic
Surface Velocity Program (SVP) Drifter

Technical Description

  • 35cm sphere surface float
  • GPS based tracking
  • Iridium Short Burst Data (SBD) telemetry
  • Sea Surface Temperature (±0.05K accuracy)
  • Holey Sock Drogue centered at 15m depth
  • Variable sampling rate down to 5 minutes
  • 2 year lifespan

Hardware

The SVP drifter’s hull has a diameter of 35 cm and is made of acrylonitrile butadiene styrene (ABS). The drogue is a cylindrical tube of 600 den, or higher, rip-stop fabric nylon connected to the surface buoy with an impregnated steel wire-rope tether. A thermistor potted with a thermally conductive resin encapsulated into a stainless-steel enclosure is mounted in the bottom part of the surface buoy, 18 cm below the floatation line. The base of the thermistor enclosure that communicates with the inside of the drifter is thermally insulated from the inside of the buoy with a special resin to avoid biased SST readings that may occur when the drifter’s hull overheats under the sun, especially at low latitudes. The bench-accuracy of the SST measurements is ±0.05C after 5-point bath calibration across the sensing range of the thermistor (-5C to 40C).

Electronics

A LDL designed digital controller handles the duty cycle of the data collection (typically once per hour, but it can be changed underway using the two-way Iridium communication) and formats the data (SST and position) for satellite transmission. A comprehensive set of diagnostic data, that are crucial to monitor the health of the drifter array, such as the battery voltage, the hull’s internal pressure, temperature and humidity, are also formatted and appended to the data message.

Telemetry

All drifters carry a satellite modem for data telemetry. The Argos satellite system has been used in the past to transmit the data to shore and to compute the drifter position using the Doppler shift of the transmitter carrier frequency, yielding a geo-location accuracy that ranges from ~200 m to ~2 km (Lopez et al. 2014).

In 2016, the GDP has begun a transition to the the Iridium satellite system. The main advantages of Iridium telemetry are that the data are received by the drifters’ operators in a shorter time (typically 1 minutes compared to 90—120 minutes with Argos) and the positions are measured with a Global Positioning System (GPS) engine that yields typical geo-location accuracy of 2—50 m (rms).

Batteries

The GDP drifters are fitted with diode-protected, alkaline D-Cell, 12 V, 56 Ah battery packs. Although alkaline D cells are not designed to withstand shocks and vibrations, they are preferred because less hazardous than primary lithium batteries. Since the target lifetime of the drifter is in excess of 400 days, it means that through its life the surface buoy undergoes several millions of impacts resulting from surface waves with mean periods ranging from ~ 3 s for wind waves to ~ 20 s for swell (Kinsman 1984).

Drogue

The presence of the drogue is detected with a strain gauge mounted inside the hull and near the tether attachment. The strain-gauge senses the deformation of the hull due to the pull of the tether, that is enhanced when a drogue is attached and surface waves interact with the drifter’s hull.