An electronic instrument for measuring the drift angle or the angle between the longitudinal axis of an aircraft and its path relative to the ground or the angle made by the path of a drifting vessel with its heading.

It may be an attachment to an airborne radar and it is an integral part of a Doppler navigator which refers to the change in electromagnetic frequency that occurs when the source of the radiation and its observer move toward or away from each other.

The faster they come together, the higher the frequency or the faster they move away from each other, the lower the frequency.