Drift meter

A drift meter consists of a small telescope extended vertically through the bottom of the aircraft with the eyepiece inside the fuselage at the navigator's station.

Drift meters were extensively used in World War II by bomber aircraft.

[2] USAAF Type B-5 drift meter was also equipped with a simple flight computer (a rotating wheel) to calculate the ground speed from the measured time that takes for a ground object to pass from the top to the bottom of the reticle.

An alternative method is to do it graphically by using a "double-drift diagram", which is a combination of two wind triangles (see the figure).

The sign of the difference indicates whether the wind direction is in the zero-drift heading or reciprocal to it.