DORIS SYSTEM

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 ultrastable 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 has added real-time measurement processing capability for 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. Miniaturization of electronic circuitry has allowed to include in the currrent Doris box two receivers each having 7 bifrequency channels and two USOs.

Stations



A Doris station comprises a beacon (three generations of beacons exist), an omnidirectional antenna, and a set of pressure, temperature, and humidity sensors. Beacons transmit signals on two frequencies: 2036.25 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 beacons network

Around 6O active beacons of 3 successive generations
Bifrequency beacons at 401.25 and 2036.25 MHz
Three main beacons (able to upload commands to the instrument): Toulouse, Kourou, Hartebeestoek
An additional one in 2007
One time beacon (directed by an hydrogen maser) being installed at Yellowknife



 

Control and processing center



The satellites relay acquired and stored data at regular intervals to Ssalto, the Doris mission control centre in Toulouse, France. This centre keeps a check on 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.