Let's Dissect a Drifter
There are two basic sizes of drifters: the original, relatively heavy Surface Velocity Program, or SVP, drifter and the newer “mini” version. The less expensive, easier-to-deploy mini design was proposed in 2002 and is currently produced alongside original SVP drifters.
The surface float ranges from 30.5 cm to 40 cm in diameter. It contains: batteries in 4-5 packs, each with 7-9 alkaline D-cell batteries; a transmitter; a thermistor to measure sea surface temperature; and possibly other instruments measuring barometric pressure, wind speed and direction, salinity, and/or ocean color. They also have a submergence sensor or a tether strain sensor to verify the presence of the drogue.
The drogue is centered at 15 meters beneath the surface to measure mixed layer currents in the upper ocean. The outer surface of the drogue is made of nylon cloth. In the original design it is 7 sections, each 92 cm long and 92 cm in diameter, for a total length of 6.44 m. Mini drogues are 4 or 5 redesigned sections of diameter 61 cm, length 1.22 m per section. Throughout the drogue, rigid rings with spokes support the drogue’s cylindrical shape. The drogue is a “holey-sock”: each drogue section contains two opposing holes, which are rotated 90 degrees from one section to the next. These holes act like the dimples of a golf ball by disrupting the formation of organized lee vortices.
Once deployed, a modern SVP drifter lives an average of around 400 days before ceasing transmission. Occasionally, drifters are picked up by fishermen or lose their drogue and run aground.