Main types of satellite navigation systems - Doppler systems

From INVESaTWIKI

Doppler systems

The first operational systems were Doppler systems. The Doppler technique uses the Doppler shift of signals received from a low-orbit satellite. The frequency of the signal received from a moving source is shifted by an amount proportional to its velocity toward or away from the receiver. As the satellite passes the receiver, the Doppler shift will decrease from a positive to a negative value. The distance from the satellite track determines the magnitude of the shift. The crossover from a positive to negative shift will occur at the point of closest approach. The Earth's rotation during the satellite pass changes the shape of the curve in a way that indicates which side of the satellite track the receiver is on. These satellites orbit at relatively low altitude, about 900 km (550 mi), in order to produce a large Doppler shift.
Doppler systems have inherent limitations. They provide only two-dimensional position fixes. Altitude must be determined by some other means. They provide limited accuracy and no velocity information. Because of their low altitude and limited coverage area, they provide at most a few position fixes a day. As a result, these systems have been phased out.