|Space geodesy, solid earth and fluid envelopes
In the distant past, geodesy was devoted to measuring the shape and dimensions of the Earth. For a few decades, the use of space techniques have opened new applications areas: the Earth's gravity field, Earth rotation fluctuations, tectonic plate-induced global, regional and local crustal deformations, seismic and volcanic activity, core dynamics, and more recently ocean dynamics, sea level change, land hydrology and the mass balance of the ice sheets. Measurements of distance and relative speed between the Earth station networks and a satellite are the conventional space geodesy tools. These measurements are used to calculate the satellite's trajectory in space. The knowledge of the orbit allows to determine the forces acting on the satellite, primarily the Earth's gravity field and its variations over time. Various geodesic parameters are calculated along with the orbit, such as the position and movements of Earth stations, the Earth's rotation speed and the movement of its rotation axis, the movement of the Earth's mass centre, etc. Space geodesy also uses different satellite-borne sensors to measure a number of parameters. Radar altimetry is particularly useful for mapping the sea's surface and its spatio-temporal variations. Space gravimetry measures spatio-temporal change of the Earth's gravity field.
The gravity field
The study of geodesy satellite orbits has enabled very accurate Earth gravity field maps to be plotted at long wavelengths. The geographical variations of the gravity field arise from the distribution of matter in the globe's different envelopes. Major results have been obtained from the gravitaty field on the large-scale convective structure of the Earth. Altimeter satellites have mapped the gravity field over the oceans in great detail at short wavelengths (10-500 km). They have highlighted how complex the seafloor tectonic structures are. Through the GRACE space gravimetry mission, "average" gravity field precision has now improved by a factor of 10 over the best models based on precise orbitography.
Crust and tectonic plate deformations
Crustal deformations at various scales have been measured using space techniques for over a decade. The optical imaging of SPOT satellites has been used to map the active continental faults to a resolution of a few metres. The deformations of regions at the plate boundaries have been broadly studied using the GPS positioning system to a precision level better than 1 cm/yr. Radar interferometry, developed in the early 1990s has proved to be extremely powerful for studying co- and post-seismic deformations, volcanic deformations, glacier deformations, landslides and soil subsidence, oil or gas pumping. The DORIS, Laser, GPS and VLBI space geodesy systems have used for very precise measurements of current tectonic plate motions and have demonstrated that these are identical to the average movements of the past three million years.
Temporal variations of the gravity field
Recent advances in space geodesy techniques have made it possible to measure temporal variations of the gravity field, or what is the equivalent of the geoid. The seasonal and interannual variations, whose amplitude is less than 1 cm in terms of geoid height, result from redistributions of air mas in the atmosphere and water in the oceans, atmosphere, continental reservoirs and ice sheets. Recent studies carried out by our team on the basis of GRACE space mission observations, demonstrate that it is possible to measure fluctuations of land storage and of the snow pack for all continents. These are key data for the hydrological cycle and its link with climate variability and human activities.
The Earth's rotation
It is known for a long time that most of the fluctuations in the Earth's rotation result from variations of atmospheric zonal winds. Recent studies have revealed that ocean currents also play a significant role.
The vertical movements of the Earth's crust
In contrast to the horizontal movements of the earth's crust that are mainly due to the drifting of major tectonic plates, the origins of its vertical movements are highly diverse: post-glacial rebound, seismic and volcanic deformations, man-induced subsidences, deformations due to surface loads, combined with air and water mass distributions in the fluid envelopes of the surface. Current space geodesy techniques (DORIS, Laser, GPS and VLBI) enable these very low amplitude movements (1-10 mm/yr) to be measured. We have better understanding of the phenomena that generate these movements from these results.
Motions of the Earth's mass centre (geocenter)
In a reference system linked to the solid Earth, the mass centre of the Earth system (including surface fluid envelopes) is not stationary. This motion, albeit of only a few mm in amplitude, whose main component is seasonal, is the result of mass redistributions in the atmosphere, oceans and continental water reservoirs. Space geodesy is now capable of measuring the motions of the geocentre. These observations provide constraints on models describing water exchanges among the surface fluid envelopes.
Variations in mean sea level
One important application of satellite altimetry is the measure of mean sea level variations to a high precision level, global coverage and high temporal resolution. These variations result from different climatic contributions in response to global warming. The Topex-Poseidon satellite has enabled us to detect global mean sea level rise of ~3 mm/yr since 1993. Now, Jason-1, the successor to Topex-Poseidon, launched late in 2001, also provides very accurate sea level measurements. At the seasonal time scale, water with change of continental water reservoirs, in particular seasonal change of the snow pack, is the main cause for the annual mean sea level variations measured by Topex-Poseidon. It is primarily thermal expansion of the oceans and land ice melt that are responsible for the mean sea level rise at the multiyear timescale.
Hydrology from Space
Space observations are increasingly important for the study of terrestrial waters and their evolution in response to global climate change. Lakes, rivers and flood plains water levels are now routinely measured using space altimetry. In certain conditions, the river water level variations can be converted into discharge rates. Spatio-temporal variations of the surface water volumes can also be measured by combining altimetry with radar or visible imagery. As previously mentioned above, space gravimetry is used to quantify land water storage fluctuations (soil water, surface water, underground water and snow) for the entire continental domain. This new technique also provides information on the mass balance of the ice sheets and variations in ocean mass.