A navigation system determining the distance of a mobile unit from two or more fixed stations, by measuring the time needed for a signal to travel between the mobile unit and each of the fixed stations. One measured distance locates the mobile unit on a circle with its centre at a known station and a known radius equal; a second measured distance locates the mobile unit on a second circle with known centre and radius. If the unit has not moved between the two measurements, it is located at one of the two intersections of the two circles. If it has moved, an estimated velocity and location are used to correct for the lapse in time.