Drifter

A drifter is a free-floating instrument that follows the current to measure flow in the Lagrangian frame, reporting its track and onboard properties by satellite. The standard Surface Velocity Program drifter pairs a surface float with a holey-sock drogue centered at 15 m depth, so it tracks the near-surface current rather than wind slip and Stokes drift. Position fixes from GPS or Argos give the velocity; most also carry a sea-surface-temperature thermistor, and many add a barometer for marine surface pressure. The Global Drifter Program maintains an array of roughly 1,250 active units that maps surface circulation and validates satellite winds and SST.