How it works
The Doris system is based on the principle of the Doppler effect, which 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.
The DORIS system is composed of three main parts:
DORIS on board instrument
The Doris instrument is part of the satellite payload. It comprises a receiver that measures radial velocity (see Doppler effect) and an omnidirectional antenna. The receiver is driven by an ultra-stable oscillator (USO). Every 10 seconds, it measures the Doppler shift in the frequency of radio signals transmitted by beacons at 400 MHz and 2 GHz. Measuring the 400-MHz signal shift is vital for reducing ionospheric propagation errors.
Since the launch of the Spot 4 satellite in January 1998, the Diode (Détermination Immédiate d'Orbite par Doris Embarqué) navigator, which enables real-time calculation of a satellite's trajectory with a precision ranging from a few metres to a few centimetres depending on the mission, has been added to the satellite navigation. Successive Doris system upgrades have incorporated four generations of receivers.
|Antenna||DGxx dual-frequency instrument||USO|
Four generations of Doris instruments have been developed, each compatible with the station network. The two features that all instruments share are measurement acquisition every 10 seconds and the dual-frequency system. Thanks to the miniaturization of electronic circuitry, two receivers each with 7 dual-frequency channels and two USOs have been included in the current Doris unit.
A Doris station comprises a beacon (there are three generations of beacons), an omnidirectional antenna and a set of pressure, temperature and humidity sensors. Beacons transmit signals on two frequencies: 2,036.25 MHz and 401.25 MHz. Both signals are modulated to send messages containing an ID number, timing information, data from the meteorological sensors and engineering data (power, etc.).
The station network comprises:
- Around 6O active beacons of three successive generations
- Dual-frequency beacons at 401.25 and 2,036.25 MHz
- Three main beacons (able to upload commands to the instrument): in Toulouse, Kourou and Hartebeesthoek
- An additional one in 2007
- One time beacon (driven by a hydrogen maser) being installed at Yellowknife
Control and processing centre
The satellite relays acquire and store data at regular intervals at Ssalto, the Doris mission control centre in Toulouse, France. This centre monitors station operations, processes measurements, calculates the orbit of the satellites carrying the Doris instrument, and archives and distributes data. Since its maiden mission in 1990, Doris has acquired over 50 million measurements for the international scientific community.