The climatic changes accelerates the polar icecaps melting. How can we value over the years the variations of the sea levels from space when the icecaps are lying on continents which are rising too, at a not enough known speed, sometimes lower than 1 cm per year?
The giant earthquakes, like the Sumatra-Andaman's in 2004 or the Tohoku's in Japan in 2011, are followed by regional movements which may extend over several years (or tens of years) with deformation speeds of the Earth's crust between a dozen centimetres to a few millimetres per year. Establishing a pattern of those post-seismic movements may help to better understand those catastrophic phenomena. How can we measure those movements over long periods with an accuracy of less than a centimetre, or even a millimetre?
Only space techniques can take up those challenges. In 1988, the International Astronomy Union (IAU) and the International Union of Geodesy and Geophysics (IUGG) created the International Earth Rotation and Reference Systems Service (IERS) and made it responsible for determining the International Terrestrial Reference Frame (ITRF).
The calculation of the stations' coordinates is based on multiple observation techniques, first developed for other scientific needs and the complementary sensitivities of which ensure the quality of the terrestrial frame:
- Very Long Baseline Interferometry (VLBI), radio-astronomy tool, developed to observe the quasars (radio sources of the remote universe), which make the distance measurements possible on thousands kilometres with an accuracy of a few millimetres;
- Satellite Laser Ranging (SLR), first developed to pinpoint the Earth's field of gravity, which make the accurate localisation of the Earth's mass center possible;
- Global Navigation Satellite Systems (GNSS) such as the GPS, which offer the best localisation performance for the geodesic stations;
- Since 1995, the Doppler Orbitrography and Radiopositioning Integrated by Satellite (DORIS) system.
DORIS is a French orbitography system designed by CNES, aimed to calculate the accurate path of satellite in medium or low orbits, such as the optical imaging (SPOT) or environmental monitoring (ENVISAT, JASON, CRYOSAT, etc.) satellites.
The applications to geodesy were developed in the 1990's, in particular by Pascal Willis. Since its conception, this system benefited from a network of ground beacons very well distributed throughout the planet. It brings elements which confirm the frame determined by its independent observations made by the three other techniques. In addition, its participation in the ITRF's determination allow the transfer of the frame qualities to the orbiting satellites fitted with such system.
Nowadays, 8 teams in the world are processing the DORIS' observations to determine geodesic results assimilated in the International DORIS Service (IDS) to contribute to the ITRF determination.
The DORIS system's geodesic performance are continuously growing. The analysis performed for the publication of the last determination of the international frame showed, for example, that the DORIS technique estimated the equatorial coordinates of the masses' center of Earth with an SRL-like accuracy.
The underlying objectives of the improvement of the terrestrial frame are still very important for the next decade, in particular for the characterization of the key climate variables. The accuracy of the frame of reference in which the space measurements are expressed has a direct influence of their interpretations.
On a global scale, the quality improvement of the ITRF in 2005, jointly with a better knowledge of the Earth's field of gravity, showed, for example, a measurement bias of 0.26 mm per year on the speed estimation of the rising average sea level on the basis of the TOPEX satellite's data. It also sharply modified our vision of the regional distribution of this phenomenon.
Beckley, B. D., F. G. Lemoine, S. B. Luthcke, R. D. Ray, and N. P. Zelensky (2007), A reassessment of global and regional mean sea level trends from TOPEX and Jason-1 altimetry based on revised reference frame and orbits, Geophys. Res. Lett., 34, L14608, doi:10.1029/2007GL0300022