A basic parameter of a ship's motion is its velocity, a knowledge of which is important to both seaman and ship designer. Often it is sufficient to know the ship's mean speed at given engine revolutions. Sometimes the knowledge of its instantaneous velocity may also be required.

Instantaneous velocity measurement is possible by means of a doppler radar. The radar is located on the ship and at a selected point of reference on shore, or on a buoy, a reflector is placed which strongly reflects the electromagnetic waves; the ship is then steered directly towards or away from the reflector. The radar emits a beam of electromagnetic waves directed toward the reflector. Due to the ship's motion in relation to the reflector, the received signal returned by the reflector is shifted in frequency from the transmitted frequency by a doppler frequency. This frequency is proportional to the instantaneous velocity. The mean velocity may be measured by counting the number of signal periods of doppler frequency. This number is the measure of the distance covered by the ship. Simultaneous measurement of the time enables the mean velocity over the given distance to be determined.

This method makes possible the measurement of the instantaneous velocity and acceleration of the ship. Simultaneous observation of instantaneous velocity when measuring the mean velocity allows a measuring length along which the velocity of the ship reaches a steady value to be chosen and the maximum velocity to be measured.