DORIS PRINCIPLE

The Doris system is based on the principle of the Doppler effect. This is the effect that causes the frequency of a wave to shift when a transmitter and receiver are in motion relative to one another. Consequently, the frequency of the received signal is not the same as that of the transmitted signal. The frequency increases as the two objects get closer and decreases as they move apart. The Doris system transmits and receives radiofrequency waves. The receiver is on the satellite and the transmitters are ground beacons.

DORIS orbitography beacons transmit signals at two separate frequencies (2,036.25 MHz and 401.25 MHz) to the satellite. The receiver on board the satellite analyses the received signal frequencies to calculate its velocity relative to Earth. This velocity is fed into orbit determination models to derive the satellite's position on orbit to within one centimetre on the radial component.