ECA GROUP, expert in the development of satellite tracking systems, has furnished the French National Centre for Space Studies (CNES) with DORIS beacons since the 1980s. Since then, ECA GROUP has been developing three generations of ground beacons for the CNES’s DORIS system. In 2020, ECA GROUP delivered 20 beacons, and in the coming two years, the company will deliver 50 more for the renewal of the CNES’s satellite stations.
DORIS is a name given to the French space system for precise orbit determination and positioning that includes ECA GROUP’s ground beacons. The system has been developed by the CNES and the French National Institute of Geographic and Forest Information (IGN). DORIS stands for Doppler Orbitography and Radiopositioning Integrated by Satellites. The beacons work like an ‘inverted GPS’ as they send a signal from the ground to the satellite to know its correct position in orbit. The DORIS beacons send out a signal every 10 seconds in two different frequencies, allowing measurements with ultimate precision, thanks to a bench built by ECA GROUP and an antenna from COBHAM.
DORIS beacon in Saint Helena (South Atlantic): Beacons consist of an antenna and three boxes.
In the 1980s, the beacon’s measurements served only to localize the exact positon of a satellite. Today, the beacons contribute to analyzing climate change as they participate in measuring rising sea levels as well as the change in height of the Antarctic ice sheet.
Known to scientists, but still a surprise to the public: DORIS beacons have also proven the earth’s slight deformation. To the naked eye a perfect “blue marble”, our planet is in reality the shape of a football that lost air over time, slightly squashed at its poles and swollen at the equator.
DORIS is a microwave tracking system. It uses a network of 60 ground stations that send a radio-electric signal out to orbit – to the on-board receivers of satellites like SPOT, JASON, ENVISAT, CRYOSAT and SENTINEL 3 and 6.
The system works on the principle of the Doppler effect: The frequency of a wave changes when a transmitter and a receiver are in relative motion to each other. The frequency increases as the two objects get closer and decreases as they move apart. A simple example is the drop in pitch of an ambulance siren as it passes by.
The DORIS system uses this effect as it transmits and receives radiofrequency waves.
And these radiofrequency waves can help us analyse ocean levels and glacial melting or they can support us in cartography – science in the service of nature’s wonders, but also in the service of bigger concerns – climatic change.
Photorights: © CNES/ill./DUCROS David, 2013
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