The Doris system is a precise orbit determination and positioning system based on the principle of the Doppler effect.
The Doris antenna onboard satellites receives signals emitted by the terrestrial stations network. When the receiver and the source are moving each other, the receiving wavelength differs from the emitting wavelength : it is the Doppler effect.
When the satellite moves closer to the emitting beacons, the frequency of the signal received by Doris instruments onboard the satellite is higher than one of the emitted signal, and lower when it moves away. If the emitting and receiving frequencies are equal, the emitting beacon is on the perpendicular to the satellite ground track. On a plot of the frequency received by the satellite as a function of time, the slope of the curve at the point of near maximum (TCA point: Time of Closest Approach) enables to calculate the distance between the beacon on the ground and the transmitting satellite.
Further information on Doris system.