2Thesatellitemissions:observingtheEarthsystemfromspace
. Integrated observations to understand environmental and deep Earth´s processes
Besides the important application of gravity field observations for exploring the Earth interior, the knowledge of the Earth‘s gravity field and its variation with time is essential for the understand-ing of environmental processes.
The striking results in global gravity field recovery immediately obtained from the CHAMP and GRACE mission data have brought to evidence that data from a consistent long-term observation of the Earth‘s gravity field will open, when joined with satellite altimetry in multi-parameter data sets, new areas of multi-disciplinary research and application.
The multi-year data records, which will be collected with CHAMP and GRACE, and the high-resolution spatial gravity field recovery with GOCE, will demonstrate, that gravity is one of the key elements for an integrated geodetic-geophysical observing system, and that a permanent gravity mapping from space with advanced present-day satellite and sensor technology will be-come feasible. Such a permanent observation is urgently needed within the following fields of Earth system, environmental and global change diagnostics and prognostics:
a. ocean currents and heat flux
b. sea-level rise and Greenland/Antarctic ice sheets c. water cycling (ground water storage and snow/ice pack)
d. solid Earth processes (mantel flow & plate tectonics, post glacial adjustment)
Whereas the first and fourth point also require a high spatial resolution (down to some 10 km) of the gravity field, all points address temporal field variations with periods from weeks to centuries.
Although the three satellite gravity missions will not yet fulfil all stringent requirements concern-ing accuracy and resolution, these are to be considered as forerunners and concept missions for a long-term improved gravity field recovery from space.
Integrated Earth’s observation: benefits and objectives
The following paragraphs outline the importance of the combination of complementary data sets and its joint analysis to fully exploit the information content, to resolve ambiguities and to sepa-rate signal sources for a precise and reliable interpretation of the Earth‘s dynamics and the inter-actions between the various spheres.
• Global Gravity and Altimetry
Over the oceans and ice caps, satellite altimetry is employed to derive the surface geometry and its changes with time. Several European, French and American altimeter missions are presently operating: TOPEX/Poseidon, GFO, Jason-1 and ENVISAT. Two missions are de-signed for altimetry over the marine and continental polar ice sheets: the American IceSat (in orbit since 2003) and ESA‘s Cryosat, which are to be launched in 2004, respectively.
The altimetry records covering now continuously more than a decade can only be fully ex-ploited for climatologic processes if combined with highly precise and high resolution global gravity data. The gravitational potential in terms of the geoid is needed as a reference surface to derive the major ocean currents which control the climate of the Earth by transporting heat and CO2. The re-evaluation of the altimeter data being available back to the 1980ies would
also largely benefit from an improved global geoid to uncover precisely the evolution of ocean currents by assimilation in a global hydrostatic ocean circulation model.
Time varying gravity is needed to separate the mass effect due to changes in the ocean/
continent/atmosphere water mass balance and in the water/ice mass balance from pure ther-mal water volume changes and ice volume changes due to compaction. This applies to short period (seasonal, interannual) ocean surface currents and ice mass variations as well as to the global trend in sea level rise and ice coverage. Thus, ongoing climate processes can only be currently interpreted if altimetry is combined with gravity.
• Global Gravity and Land-based Hydrological Data and Models
Over land, it shall be for the first time demonstrated with GRACE, that satellites are able to globally probe the Earth for largely unknown soil moisture and aquifer changes on seasonal and interannual time scales. Being important for the understanding of the global water cycle, a satellite-based system shall continue to trace global hydrology after the five-years lifetime of GRACE.
An assimilation of global gravity field changes into models from land-based measurements of groundwater levels, snowloads and point measurements of soil moisture will be the tool to connect the small hydrological length scales with longer scales in order to estimate the dynamics of the water cycle and its evolution with time on continent-wide and global spatial scales.
• Global Gravity and Seismology
The worldwide seismic broadband station network enables the improvement of global els of the Earth‘s crust (density distribution and thickness) and of tomographic velocity mod-els of the Earth‘s mantle. The observed spatial structure of the gravity field gives boundary values for an isostatic model of the Earth‘s lithosphere to investigate its static equilibrium (medium to short-scale) and, by this, to infer the dynamic topography due to mantle dynam-ics for the mantle‘s temperature and density distribution by forward computations and veloc-ity-gravity inversions. For global solid Earth‘s physics studies, the knowledge of the Earth‘s crustal structure and the resolution and accuracy of tomographic models is compared to the knowledge of the gravity field, rather low. Therefore, for this application there is a need for improving the seismologic sounding and modelling rather than the gravity field recovery.
This situation changes, when turning to regional tectonic modelling, where accurate gravity down to wavelengths of some kilometers is required, and satellite gravity field missions will provide the longer wavelengths frame for a reliable detailed geoid and gravity field model-ling with a data coverage densified by terrestrial and ship- and airborne measurements.
Seismic tomography, dynamic topography, surface deformations, gravity and the vertical and lateral viscosity structure of the mantle are the key observables and parameters for a three and four dimensional modelling of mantle dynamics as the engine for plate tectonics.
• Global Gravity and Geodetic Networks
Continent-wide (ECGN) and global (GGOS) geodetic networks, including absolute and su-per-conducting gravimeters and GPS (Galileo) precise point positioning and height deter-mination, will deliver a picture of secular crustal deformations, height and gravity changes for Earth system science. Those networks, like the IGS and SLR networks, also provide the geometric reference frame and its evolution with time (Earth rotation, plate motions) to tie together all observations and for precise satellite orbit determination needed in global gravity field recovery and altimetry.
2Thesatellitemissions:observingtheEarthsystemfromspace
World wide web pages with further information on the satellite missions CHAMP http://op.gfz-potsdam.de/champ/
http://www.dlr.de/champ GRACE http://op.gfz-potsdam.de/grace/
http://www.csr.utexas.edu/grace/
GOCE http://www.esa.int/export/esaLP/goce.html http://www.goce-projektbuero.de
ICESat http://icesat.gsfc.nasa.gov http://www.csr.utexas.edu/glas/
CryoSat http://www.esa.int/export/esaLP/cryoSat.html http://www.cryosat.de
ENVISAT http://envisat.esa.int/
Jason-1 http://topex-www.jpl.nasa.gov/mission/jason-1.html ERS-2 http://earth.esa.int/ers/
TOPEX/Poseidon http://topex-www.jpl.nasa.gov/mission/topex.html Geosat FO http://gfo.bmpcoe.org/Gfo/
Vertical coastal crustal movements have to be analysed together with sea level observations by altimetry and tide gauges for a complete risk estimation. The observation of temporal grav-ity changes on a global scale reflects the gravitational effects of post-glacial crustal uplift and subsidence and therefore is the tool, combined with kinematic and stationary gravity measure-ments, to infer the elastic behaviour and properties of the solid Earth.
A precise high-resolution gravity field defines everywhere the ‚Mean Sea Level‘ which is the reference for the topographic heights. These heights are presently determined by time con-suming and man-power intensive geometric levelling. Once the ‚Mean Sea Level‘ is known with a precision compatible to levelling, the traditional surveying method can be replaced by modern satellite-based methods, called GPS-levelling and later on Galileo-levelling. The ground receivers using the American Global Positioning System (GPS) or the European Gali-leo navigation satellite system then could easily deliver heights above ‚Mean Sea Level‘, i.e.
topographic heights.
On the other hand, continent-wide distributed points, which are observed by both GPS and geometric levelling deliver the geoid heights defined in the national height reference system.
The comparison with the global geoid observed from space reveals the differences in the re-alization of the national height systems, thus leading to a unified global height reference sys-tem.