The shape of our planet deviates substantially from an ideal sphere in many ways. First of all, the Earth is roughly an ellipsoid because of its rotation, bulged at the equator and flattened at the poles. Additionally, high mountains on the continents and deep valleys in the oceans crumple its surface. More interesting from a geoscientific point of view, however, are the spatial and temporal variations of the Earth's gravitational field that produce additional deviations from the ideal sphere. These deviations are caused by large scale mass variations due to mantle convection currents resulting in motions of the whole Earth body (e.g. variation of Earth’s rotation) or parts of it (e.g. deformation of the lithosphere visible in the Mid Atlantic ridge, at subduction zones, in plate kinematics, volcanism, or Earth quakes). Additionally, luni-solar and planetary gravitation, atmospheric pressure and winds, ocean circulation and tides, ocean loading, variations in the continental water cycle or melting of ice in Polar regions and glacier systems cause spatial and temporal variations in the gravity field.
To observe these mass distribution and mass variation in System Earth on a global scale, with homogeneous accuracy and over long time periods dedicated gravity satellite missions are required.
Section 1.2 develops and operates these missions in strong collaboration with German Industry, NASA, and the German Satellite Operations Center (GSOC) of the German Space Agency (DLR). Analysis of the mission science data is performed using our Earth Parameter and Orbit System (EPOS) software using up to date international standards.
GFZ-1, the first satellite of the GFZ, designed as a small, passive satellite and equipped with 60 retro-reflectors to be illuminated from the ground by the global network of satellite laser ranging (SLR) systems has been used end of last century together with various other geodetic satellites to improve our knowledge of the Earth’s gravity field. A completely new generation of Low Earth Orbiting (LEO) satellites, equipped with onboard GPS receivers and highly precise inter-satellite and accelerometry instrumentation, enabled scientists since beginning of this century not only to observe the Earth‘s gravitational field with much higher spatial resolution but also, for the very first time, its temporal variability.
The individual satellites CHAMP (CHAllenging Minisatellite Payload, 2000-2010) and GOCE (Gravity field and steady-state Ocean Circulation Explorer, 2009-2013), as well as the twin satellites of the GRACE (Gravity Recovery and Climate Mission, 2002-2017) mission can exactly measure even the smallest accelerations caused by gravity changes. GFZ played and still plays a leading role in the development, operation and analysis of these modern satellite missions. We are part of the joint US/German GRACE Science Data System and provided the Deputy Operations Mission Manager. We were member within ESA’s GOCE High Level Processing Facility and we develop within a Memorandum of Understanding with NASA a GRACE-FO (Follow-on) mission due for launch in early 2018.
The analysis of these data includes instrument data pre-processing, precision orbit determination and routine generation of various static and time-variable EIGEN (European Improved Gravity model of the Earth by New techniques) gravity field models which are used for many applications in Earth system science such as monitoring the continental global water cycle, melting of large glacier systems, or analysis of surface and deep ocean currents. These so-called 'static satellite only models (purely derived from satellite data)' are of special interest as they have been derived independent from terrestrial data. These models are further combined (within Topic 2) with terrestrial gravimetry data to ultra-high spatial resolution global models.
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Dahle, C., Flechtner, F., König, R., Michalak, G., Neumayer, K.-H., Gruber, C., König, D. (2014): GFZ RL05: An Improved Time-Series of Monthly GRACE Gravity Field Solutions. In: Flechtner, F., Sneeuw, N., Schuh, W.-D. (Eds.), Observation of the System Earth from Space - CHAMP, GRACE, GOCE and future missions, (GEOTECHNOLOGIEN Science Report; 20; Advanced Technologies in Earth Sciences), Berlin [u.a.] : Springer, p. 29-39, doi.org/10.1007/978-3-642-32135-1_4
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