The Doris system is used for precise (centimetre) orbit determination of satellites equipped with Doris receivers. It is based on measuring Doppler shifts of radio-frequency signals transmitted by a terrestrial network of Doris beacons, used as reference points on Earth's surface. There are 57 beacons around the world operating with more than 30 international host agencies. Conversely, the Doris system is able to locate ground positions with the same precision. This dual ability is highly valuable for altimetric oceanography or ice missions, for studying the Earth's shape and movements, and for a great number of location-based services.
Doris instruments are used for a range of applications:
- precise (centimetre) orbit determination for altimetry
- precise positioning of ground beacons (Doris is used by IERS, the International Earth Rotation and Reference Systems Service)
- enhancement of gravity field and geoid models
- on-board orbit determination
- on-board time tagging with respect to the TAI time scale
Missions flying a Doris instrument
Part of the scientific payload, Doris is a primary sensor for subdecimetric orbit determination which is required to achieve the large-scale ocean-monitoring goal of this mission. It was decided early on to achieve an in-flight validation of Doris before launching TOPEX/POSEIDON. This was done by flying a Doris instrument on the SPOT-2 Earth-observation satellite (launched in 1990) as a passenger experiment.
Recognising the ability of the system to provide precise positioning of ground beacons, Doris was approved to fly on SPOT-3 (launched in September 1993) and SPOT-4.
For SPOT-4 a real-time on-board orbit determination capability, called DIODE, was added to Doris to provide SPOT imagery users with satellite ephemeris data.
The Doris System was also approved by the European Space Agency (ESA) to determine the precise orbit of the Envisat-1 environmental monitoring satellite.
New Doris instruments were flown on Jason-1 (TOPEX/POSEIDON follow-on mission) and on SPOT-5. These new miniaturized second-generation instruments were similar to those on Doris/Envisat in terms of functions and performance, only with improved mass, volume, and power consumption characteristics, a more integrated design and a more extensive use of digital processing techniques.
The current DGxx generation of instruments is flying on Jason-2 (Jason-1 follow-on), Cryosat-2 (ESA ice-monitoring mission), and Saral/AltiKa (the French-Indian Ka-band ocean altimetry mission).
These instruments are equipped with 7 dual-frequency channels to allow simultaneous tracking of up to 7 beacons to fulfil the needs of the International Doris Service (IDS).
Such receivers are also flying on HY-2A ocean-observation mission (launched in 2011) and on Jason-3 (launched in 2016).
IDS (International DORIS Service)
Thanks to the achieved precision, added to the density and uniform coverage of its beacon network, Doris is now one of the precise location techniques used to define the International Terrestrial Reference system (ITRS) from IERS (the International Earth Rotation and Reference Systems Service). For that purpose, IDS (International Doris Service) was created 10 years ago to disseminate Doris data and products to the scientific community (geodesy, geophysics). The IDS service has been expanding rapidly thanks to good Doris performance and to an increased number of contributing satellites (more than 4). It achieved maturity in 2009 as 7 international analysis centres contributed to a high-quality combined Doris solution submitted to IERS for the ITRF2008. In 2010, the IDS extended the Doris product combination process to an operational service and established a Combination Centre, which is now fully operational.
The IDS is now a well-recognized technique-oriented service for the IERS, and is actively preparing the next ITRF2013 submission.
The IDS service has been expanding rapidly over the last few years thanks to the excellent performance of the DORIS system available on an increased number of contributing satellites. The IDS is also a participant in the Global Geodetic Observing System (GGOS).
IDS data & products
The IDS collects, archives and distributes DORIS observation data sets as well as the following derived products: coordinates and velocities of the IDS tracking stations, the geocentre and scale of the Terrestrial Reference Frame, ionospheric information, high-accuracy ephemeris data of Doris satellites and Earth rotation parameters.
DORIS has been part of the IERS (as an observer) since 1994. Other significant DORIS-related milestones include:
- July 1999, creation of the DORIS Pilot Project by the International Association of Geodesy (IAG) to foster international cooperation
- July 2003, formalization of the International DORIS Service as a service of the IAG during the IUGG in Sapporo (Japan)
- Since 2002, IDS Workshop every two years coupled with Ocean Surface Topography Science Team (OSTST) meeting: Biarritz (2002), Marne Valley (2003), Paris (2004), Venice (2006), Nice (2008), Lisbon (2010), Venice (2012)
- 2005, three analysis centres respond to the call for participation in ITRF2005
- 2009-2010, maturity: IDS official contributor to ITRF2008 (solutions with an accuracy of 1cm 3D), seven IDS Analysis Centre participated. Initialization of routine combination by the Combination Centre.
- 2013, active preparation for contribution to ITRF2013, expected significant improvements.