GPS-CHAMP high-low satellite-to-satellite and ground based laser tracking
Global models of the gravity field at the surface
of the Earth are derived from observed gravitational satellite orbit perturbations.
Due to insufficient data coverage and non-optimised orbit configurations,
tracking data from three dozens of satellites up to now have to be merged
together to get a state-of-the-art global gravity field model from satellite
tracking only. Satellite-only models represent, due to the attenuation
of the signal at orbit altitude, the broad structure of the global field.
Adding terrestrial gravity measurements and altimetry data over the oceans
allows to resolve within this given frame the smaller signatures of the
Earth's gravity field. Thanks to its dedicated orbit design, an unpreceded
low altitude in a (near-) polar orbit, and its continuous GPS satellite-to-satellite
tracking capability together with a direct on-board measurement
of the non-gravitational orbit perturbations, an improvement of up to two
orders of magnitude in accuracy shall be achieved with CHAMP in the recovery
of the broad to mesoscale structures of the Earth gravity field. This tremendous
breakthrough will open new insights and application areas in geodesy, solid
Earth physics and oceanography.
GPS-CHAMP high-low satellite-to-satellite and ground based laser tracking
In geodesy, the geoid, the special surface of equal gravitational potential
close to sea level, would become available with nearly cm-accuracy
up to a spatial resolution of about 650 km half-wavelength. The geoid reflects
the irregularities in the Earth's gravity field at the Earth's surface
due to the inhomogeneous mass and density distribution in the Earth's interior.
The long-wave geoid obtained with CHAMP then serves as a perfect reference
for higher resolution global or regional gravity field modelling.
The GRIM5 - geoid: surface of equal gravitational potential w.r.t. a best fitting ellipsoid of revolution
With CHAMP also temporal changes in the gravitational
field due to global mass redistributions, which are in general small, slow
and large-scaled, can be recovered with a higher signal/noise ratio and
an increased spectral resolution. By this, a discrimination between various
signal sources becomes possible, provided that the mission duration of
CHAMP will last several years. The most interesting temporal gravity variations
causing a global signature result from atmospheric mass redistributions,
ocean circulation, sea level changes due to polar ice melting or aggregation,
and the visco-elastic response of the Earth's lithosphere to past and present
loads. To give an example for the gain in information by
monitoring temporal gravity changes: gravity always senses the net mass
effect of water volume changes opposite to tide gauge or altimetry which
measure a combined mass and density (due to temperature changes) effect.
A combination of the various monitoring techniques will allow the separation
of the global temperature effect in water volume change and the mass exchange
between water and ice.
Dynamics of the Earth system
In geophysics, the observed irregularities in the gravity field constrain
Earth models obtained from observed seismic velocities. Seismic tomography
models of the Earth interior now approach resolutions corresponding to
wavelengths of about 2000 km. To verify and to discriminate among seismic
models, gravity at long-wavelength should be known to a homogeneous accuracy
of 1 % of its signal which is one order of magnitude more than what presently
can be achieved. Such an improvement will strongly enhance studies concerning
the structure and static/dynamic mass compensation at the core-mantle boundary
and the upper mantle discontinuities, and the conversion of seismic velocities
into density values to model lateral density variations in the Earth's
interior. This is to address the still unsolved question whether mantle
convection takes place in a single, double layered or a mixed system. This
will have a large impact on recent models of the driving forces of plate
tectonics and on the recycling of oceanic lithosphere to the interior. Besides seismic velocities and gravity, the magnetic field outside
the Earth is a direct observable originating from the Earth interior. The
evaluation of all three kinds of signals will allow a complete and integrated
modelling of the structure and dynamics of the Earth core and mantle generating
the gravitational and magnetic long wavelength signals to be observed by
CHAMP.
The most stringent requirement concerning accuracy
and resolution of global Earth gravity field models comes from oceanography.
Whereas altimetry monitors the ocean surface with cm-accuracy, the geoid
presently is known only to an accuracy of 1 m. The geometric difference
between ocean surface and geoid, the sea surface topography, is however
the essential information to derive directly the absolute mean ocean circulation
pattern and the hydrostatic pressure field, respectively.
The absolute ocean circulation reflects the currents down to large depths.
The knowledge of the absolute ocean circulation with its heat and CO2 transportation
is a prerequisite for global climate models and the interpretation of long-term
sea level changes. With CHAMP the resolution of the quasi-stationary ocean
circulation pattern will be increased from presently 4000 km to about 1000
km full wavelength.
Sea Surface Topography and Ocean Currents
In a very first experiment the feasibility of spaceborne GPS
altimetry shall be investigated using the nadir-looking GPS antenna aboard
CHAMP. Air-borne tests have shown that GPS signals can be received after
reflexion over a quiet water surface. With CHAMP and further aircraft tests
this principle shall be applied over water in an arbitrary state, ice and
snow. Monitoring the surface heights of ocean and polar ice by GPS altimetry
would give a valuable tool to collect supplementary data for combination
with CHAMP observed temporal gravitational variations. The figure below depicts
the coverage with GPS altimetry (reflection points) for a two weeks time
period with CHAMP. For the GPS satellites and CHAMP's orbit configuration
reflexions up to latitudes of 79° N and S could be evaluated, as simulation
studies have shown.
GPS altimetry - Data coverage with CHAMP for a two weeks time period
The geoid constitutes the surface which is most commonly known as 'sea level' in land surveying. Topographic heights are related to the geoid: heights above 'sea level'. Modern satellite positioning systems, like the American GPS, measure point heights which are related to a geometric surface the ellipsoid. In order to transform ellipsoidal heights to topographic heights, the geoid undulations have to be subtracted. To compete with the high GPS measurement accuracy, which is at cm-level, the geoid has to be known with the same accuracy. Then, effective GPS-levelling could replace the man-power intensive geometric levelling in surveying.
With CHAMP, the broad signatures of the geoid will be recoverable with an unpreceded accuracy, i.e. the CHAMP derived global geoid will serve as an ideal basis for a further refinement in regional and local areas evaluating existing or dedicated high resolution gravity information for a detailed geoid computation. The long-wavelength geoid will also give a reference for a unified global height system, being important when employing modern satellite surveying methods and constructing digital terrain models covering large land and ice areas for remote sensing applications and for cartography.
Any progress in the accuracy and completeness of long-wavelength gravity field models will also directly improve the reliability and accuracy of precise satellite orbit restitution as required in satellite altimetry and SAR-interferometry.
For the gravity field, CHAMP commences the international decade of
geopotentials. The follow-up missions GRACE (2001) and GOCE (2005) are
consequent extensions on the way towards a detailed gravity field
modelling for research of the Earth as a system.