A Simulation Study Discussing the GRACE Baseline Accuracy
The twin GRACE-A and -B satellites are in orbit since more than 6 years and still perform well. They deliver gravity data of unprecedented accuracy, which enables hydrologists, oceanographers and geophysicists to study the temporal variations of the Earth
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A Simulation Study Discussing the GRACE Baseline Accuracy U. Meyer, F. Flechtner, R. Schmidt, and B. Frommknecht
Abstract The twin GRACE-A and -B satellites are in orbit since more than 6 years and still perform well. They deliver gravity data of unprecedented accuracy, which enables hydrologists, oceanographers and geophysicists to study the temporal variations of the Earth’s gravity field. But the baseline accuracy computed in a pre-launch simulation study (Kim, 2000) has not yet been reached by a factor of about 15. We therefore have to raise the question: are there improvements in the processing of GRACE data possible? To answer this question, a simulation study was performed, using the same software, processing strategy, background models and standards used at GFZ for the analysis of real GRACE data. We present the results and analysis of this simulation study. Initially a closed loop simulation shows, that GFZ’s EPOS software is numerically stable. The GRACE orbit geometry is sufficient and the sampling adequate to solve for monthly gravity fields at least up to degree and order 150. The estimation of instrument parameters as suggested by Kim (2000) does not absorb the gravity signal, but greatly reduces systematic effects in the observations. The accelerometer noise proved to be an important reason for not reaching the baseline accuracy with the processing strategy used so far. Additional accelerometer parameters do not really help, but the shortening of arcs gives promising results. Different background model errors were introduced and the ocean tide model turned out to be a probable error source, while atmospheric tides play a minor role. U. Meyer () Deutsches GeoForschungsZentrum (GFZ), Helmholtz Centre Potsdam, Wessling D-82234, Germany e-mail: [email protected]
23.1 Introduction The Gravity Recovery And Climate Experiment (GRACE, Tapley et al., 2004) satellites are approximately free falling test masses in the gravity field of the Earth. In addition to the determination of the satellite orbits by GPS-observations, which allows for a gravity field solution in spherical harmonics at least up to degree 70 (Prange et al., 2008), a K-Band intersatellite link provides a differential range measurement between the satellites with micrometer-accuracy, which for the first time allows to calculate satelliteonly monthly solutions up to degree and order 120 (Schmidt et al., 2007). Apart from the increased spatial resolution of the gravity field the low degree temporal variations can be observed as well, making the time series of monthly gravity fields suitable to study continental hydrological cycle (Schmidt et al., 2008), dynamic ocean topography (Flechtner et al., 2006b), polar ice mass changes and post glacial uplift (Barletta et al., 2008). At GFZ Potsdam GRACE daily orbits and monthly as well as long time mean gravity fields are produced on a routine basis. The satellite orbits are computed with the orbit determination software EPOS-OC (Earth Parameter and Orbit System – Orbit Computation) using a dynami
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