A radio receiver called DORIS (Doppler Orbit and Radio positioning Integration by Satellite) detects and measures the Doppler shift on signals broadcast from a network of radio beacons spread around the world. Although the full accuracy of this system is only obtained after ground processing, DORIS is able to provide a real-time estimate onboard, good to about half a metre.
Real-time location: Diode
Precise orbit determination
Ionospheric electron content
By measuring and comparing the path delay of signals transmitted at two separate frequencies, DORIS is able to calculate the electron content in the atmosphere.
DORIS orbitography beacons transmit signals at two separate frequencies (2,036.25 MHz and 401.25 MHz) to the satellite. The receiver onboard 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 two centimetres on the radial component.