Drogue (Drifter)

A drogue is the subsurface drag element of a drifting buoy that locks the float to water motion at a chosen depth. On a Surface Velocity Program drifter the drogue is a holey-sock cylinder centered at 15 m, sized so its drag area is at least 40 times the surface float and tether area; that ratio keeps wind slip below about 1 cm/s. When the drogue is lost the buoy windages and the velocity record degrades, so drogue presence is tracked as a data-quality flag. The drogue is what makes a drifter a true Lagrangian follower of the current rather than a wind-driven surface object.