Physics, Instrumentation and Sensing

Ultra-Sensitive Accelerometry and Space Projects
Earth and Planetary Observation

CHAMP and GRACE Missions

The STAR accelerometer that Onera made for the CNES was launched from the Plessetsk base in Russia on July 15, 2000, on the German CHAMP satellite and has been supplying a continuous stream of scientific data since it was first put into service on July 19, 2000. In June 2002, a maneuver raised the satellite orbit to an altitude of 411 km and improved the orbit circularity. CHAMP completed its ⁶th year on orbit July 15, 2006, and will continue providing data for another 2-3 years.

Using the first data from the CHAMP satellite, a new model of the Earth’s gravitational field was presented in 2002. Dubbed Eigen-1S, this preliminary model of the global field, combining 88 days of CHAMP data with data already existing (Grim5-1S, Lageos1-2, Starlette and Stella), is already four times more precise than preceding models.

A second model was published in 2003 by GFZ, in association with OCA, using exclusively six months of data from the CHAMP missions. This model shows even better resolution: 10 cm in terms of geoid height or 0.5 mgal in terms of gravity anomaly for a geographic resolution /2 of 550 km.

The Eigen-CHAMP-03S model of 2004 integrates only the CHAMP data over 33 months. It is complete up to degree 120 of the decomposition in spherical harmonics. Its resolution is 5 cm for the geoid and 0.5 mgal in terms of gravity anomaly, with /2 reduced to 400 km.

The GRACE mission, with JPL as the prime contractor, tracks two low-altitude satellites with microwave measurement of their relative distance, to determine precisely the Earth’s gravitational field and its temporal variation. The two satellites, each carrying one of Onera’s SuperSTAR accelerometers, were launched from Plessetsk on March 17, 2002. The accelerometers, which have an enhanced resolution of 10^-10 ms^-2/Hz^½, were successfully started on March 21 and the results show that the measurements in orbit are in conformity with the expected performance.

In 2003, two new Earth gravity field models, Eigen-GRACE-01S and GGM01, were developed from the first GRACE data. One was published by GFZ from 39 days of measurements, and the other by the University of Texas’ CSR from 111 days of measurements. These models were already five times as precise as the latest CHAMP model, and 50 times better than previous models.

In early 2004, GFZ published the Eigen-GRACE-02S model, complete to order 150, with a geoid resolution of 1 mm for a /2 of 1000 km, which is two orders of magnitude better than the latest model from CHAMP data alone. In August, after the mission validation phase, the level 2 data (gravity field model) and level 1b data (calibrated instrument data) for the first two years of the mission were made available to the scientific community.

In October 2004, CSR published the GGM02 model based on 363 days of measurement, complete to order 160, with the GRACE measurements alone (index S) and extended to order 200 by constraining it with earthbound measurements (index C). Many new models are now being developed by the community from the data of these two missions, in the field of geodesics, oceanography, hydrography and climatology.

Representation of the earth geoid from the GGM01 model
(color scale : from -70 mgal [dark blue] to +70 mgal [dark red])

Improvement of the resolution of gravity anomalies [mgal]
between the Eigen-CHAMP 03S [left] and Eigen-GRACE 02S [right] models

Last Update: 9 June 2006 - © ONERA 2009 - Terms of use