How the Doris system works
On the satellite, an antenna pointed towards Earth receives radio waves emitted by ground stations in the Doris network. An electronic receiver measures the frequency shift caused by the Doppler effect.
Mathematical processing of the measurements, after modelling to take into account the forces acting on the satellite (mainly Earth's gravity), accurately reproduces the satellite's trajectory. These same measurements can also be used for location missions in order to determine the exact position of points on the ground and thus detect displacements.
|Did you know?|
|The Doppler effect
We have all observed this phenomenon at one time or another, without considering its scientific or technical applications. A common example is when a passing car hoots its horn; an immobile observer perceives a variation in the sound depending on the vehicles position. The tone is higher pitched as it arrives and becomes lower as it moves away. This change in frequency of the sound results from the movement of the source. By analogy, we can determine the position of a transmitter by analysing the frequency of its radio waves. We call it the Doppler effect in honour of the Austrian physicist who explained the phenomenon in the 19th century.
Since 1986, CNES and IGN have deployed an international network of autonomous stations used as reference points on the ground to continuously cover satellite trajectories. Today, the DORIS system has approximately 60 stations spread evenly around the globe.
The Doris system also includes:
- The Doris control and processing centre, in Toulouse, France, which is responsible for centralizing measurements, monitoring onboard instruments and beacons, and establishing work plans for the instruments
- Master beacons, a special type of orbitography beacon that enables the control centre to transmit useful operational data to the Doris transmitter, each time the satellite passes
- The orbit determination service, which calculates the orbit of the satellites carrying the Doris instrument off line, to verify its accuracy