The holey-sock drogues contained by drifting buoys are centered at a depth of 15m and are tracked by the satellites. So, by taking the temporal difference of nearby drifter positions, the ocean currents are estimated.
Explanation:
A Buoy is an anchored floating mark used to read the properties of ocean currents for any future hazards.
An Ocean Current is the movement of sea water due to different forces acting upon it like temperature, Coriolis force, etc.
