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A multipurpose system

Origins of the programme

In the 1960s, satellite trajectories could be determined to within several metres. This level of accuracy soon became insufficient, particularly in the 1980s with the advent of new ocean-observing missions, which required very precise knowledge of a satellite's orbit. In collaboration with IGN and GRGS, CNES thus began the Doris project.

This orbit determination system was tested for the first time in 1990 on the SPOT 2 satellite, and then in 1992, aboard the French-US Topex/Poseidon satellite. In 1993, the instrument was used on SPOT 3. Its objective was to locate fixed points on Earth.

Satellites equipped with Doris. Crédits : CNES/Ill.D.Ducros

Satellites equipped with Doris. Crédits : CNES/Ill.D.Ducros

Continuous enhancements

In March 1998, a Doris instrument took off aboard SPOT 4. It was enhanced with the Diode experimental navigator, which calculates the satellite's trajectory in real time based on measurements taken by the Doris receiver. This software has since been incorporated into all Doris instruments, enabling onboard, real-time calculations of the satellite's position with extreme accuracy.

Integration of Doris on SPOT 4. Crédits : CNES

Integration of Doris on SPOT 4. Crédits : CNES

Diode offers practical advantages for navigation-related applications. It not only increases the autonomy of the Doris instrument, but also that of the satellite, which will ultimately be able to control its own altitude and the thrust of its motors automatically.

Another upgrade is the miniaturization of the Doris receivers. Their mass has been cut in half for Envisat and reduced fourfold for Jason-1 and SPOT 5. The 2nd-generation Doris receivers have 2 signal reception channels, as opposed to only one originally. This technical improvement enables the instrument to simultaneously acquire measurements from 2 different beacons and minimize conflicts between orbit determination and location missions. Processing capacity has been increased in current instruments, called "DGXX", to better meet the needs of the International Doris Service and to improve the system’s autonomy, robustness and accuracy.

Evolution of the Doris instument. 18 kg in 1990 (left), 5 kg in 2001 (right)

Evolution of the Doris instument. 18 kg in 1990 (left), 5 kg in 2001 (right)

Doris instruments are now also more compact, providing internal redundancy inside a single unit, i.e. the equivalent of 14 first-generation Doris instruments. The signal received by the antenna is automatically switched to the active system. An effort has also been made to reduce the number of operations, and thus to reduce system operating costs at the satellite mission control centre.

Closer coupling of Doris with the altimeters on the Jason-2 and AltiKa missions will enable data to be acquired in coastal zones and over continental water bodies (lakes and rivers), making Doris a crucial system for this type of application.

A technical, operational and scientific success

Since entering service, Doris has performed far beyond the expectations of the scientific community. Over the years, system performance has constantly increased, and with the Jason mission has reached centimetre accuracy.

CLS, a CNES subsidiary, performs a big proportion of data exploitation tasks and provides support to CNES in promoting data use. Furthermore, the International Doris Service (IDS) was created on 1 July 2003, at CNES's behest, by the IAG Executive Committee and the IUGG General Assembly in Sapporo. The IDS is an IAG Service providing Doris products to users in the geodesy, geophysics and operations communities.

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