(dop’ l∂r) 1. The apparent frequency of a constant frequency signal broadcast by a satellite varies because of the velocity of the satellite with respect to the observing station (Doppler effect, q.v.) (see Figure D-21). A count of the number of cycles in a given time interval (Doppler count) along with knowledge of the satellite’s position in its orbit (which the satellite broadcasts) can be used to determine an observer’s location and velocity. This forms the basis of the Global Positioning System (q.v.), as it did with the Transit system (q.v.). 2. Doppler counting is also used in connection with Doppler radar and Doppler sonar.