Seite - 23 - in Contributions to GRACE Gravity Field Recovery - Improvements in Dynamic Orbit Integration, Stochastic Modelling of the Antenna Offset Correction, and Co-Estimation of Satellite Orientations

The GRACE Satellites 4 TheGRACEtwinsatellitemissionwasa jointventurebetweenNASAandDLRwiththe primary purpose of mapping Earth’s mean and time-variable gravitational field. The satellites were launched into near-circular polar orbits at an altitude of approximately 500km on March 17, 2002 (Tapley et al., 2004). With one satellite trailing the other on its orbit at a distance of roughly 200km, the satellites primarily observed their position, their separation and its change, and the accelerations due to non-gravitational forces acting on the satellites. From this data, the gravitational acceleration that acts on the satellites can be inferred, allowing scientists to determine Earth’s gravitational field. After more than 15 years in orbit, the GRACE science mission ended with the final acquisition of data for gravity field processing in June 2017. This chapter gives a summary of the satellites mission, construction, and instrumentation. For authoritative and detailed descriptions of the satellites, the reader is referred to Bettadpur (2012) and Stanton et al. (1998). 4.1 Construction and Instrumentation The two GRACE satellites have an elongated prismatic body with dimensions of 3.1m×1.9m×0.7mandamassof487.2kgat launch.Thesatellitesarealmost identical inconstruction,differingonlyintheradiofrequenciesusedforgroundcommunications and the inter-satellite link (Stanton et al., 1998). The GRACE mission realizes both the high-low and low-low variants of the satellite-to-satellite tracking (SST) principle. In addition to flight control and instrument processing hardware, the satellites are equipped with the following science instruments which are of particular interest within the scope of this thesis (compare fig. 4.1): Accelerometer A three axis accelerometer (ACC) is mounted close to the center of mass (COM) of the satellite. The satellite COM is then calibrated through in-orbit satellite trim manoeuvres to coincide with the accelerometer COM. In addition to linear accelerations, the accelerometer also records angular accelerations in three axes. The accelerometer is used to observe satellite accelerations due to non-conservative forces. Star Camera Assembly The star camera assembly (SCA) consists of two star camera heads on each satellite, observing different views of the sky. The observations from both heads are combined to determine the satellite attitude. 23