GRACE Gravity Field Results and Validation Schmidt R., Flechtner F., König R., Meyer U., Neumayer K.-H., Reigber Ch., Schwintzer P., Zhu S.Y. GeoForschungsZentrum Potsdam (GFZ), Department 1 Geodesy and Remote Sensing, Telegrafenberg A17, 14473 Potsdam, Germany, E-Mail: rschmidt@gfz-potsdam.de
Abstract The US-German twin-satellite mission GRACE (Gravity Recovery And Climate Experiment) was successfully launched on March 17th, 2002 into a low-altitude, almost polar orbit. Primary mission goal is the recovery of the long- to medium-wavelength features of the global gravity field and its temporal variations. The basic mission concept is to exploit inter-satellite measurements (satellite-to-satellite tracking SST) between the low-flying GRACE satellites and the sender satellites of the Global Positioning System (GPS) and the inter-satellite observations between the twin-satellites. In particular the measurements of the micrometer-precise low/low link, which correspond to the differential gravitational accelerations of the center-of-masses of the GRACE satellites along the inter-satellite range, are needed for
Figure 1: GRACE mission concept.
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an increased resolution of the gravity spectrum to medium wavelengths. Dominant non-gravitational forces such as air drag, Earth albedo and solar radiation pressure present at GRACE’s altitude – to be separated from the gravity signal part in the SST observables – are accurately measured by three-axis accelerometers, precisely located at the center-of-mass at each spacecraft. The orientation of the accelerometers‘ axes with respect to a celestial reference frame are measured by star cameras rigidly connected to the satellite body. Figure 1 depicts the GRACE mission concept. For the determination of the global gravity field parameters the classical method of combining precise dynamic orbit determination (POD) and least-squares adjustment is applied at GFZ. Linearized observation equations