The impact of attitude control on GRACE accelerometry and orbits

Abstract

Since March 2002 the two GRACE satellites orbit the Earth at relatively low altitude (500 km in 2002, still close to 460 km mid of 2009). GPS-receivers for orbit determination, star cameras and thrusters for attitude control, accelerometers to observe the surface forces, and a very precise microwave link (K-band) to measure the inter-satellite distance with micrometer accuracy are the principal instruments onboard the satellites. Determination of the gravity field of the Earth including its temporal variations from the satellites' orbits and the inter-satellite measurements is the main goal of the mission. The accelerometers are needed to separate the gravitational acceleration from the surface forces acting on the satellites. They collect a wealth of information about the atmospheric density at satellite height as a by-product. These accelerations have not yet been analyzed thoroughly, because their interpretation is complicated due to numerous thruster spikes. We outline a method to model the thruster spikes and to clean the time series of the accelerations. The isolated effect of the modeled thruster pulses on the satellite orbits is studied and a first interpretation of the cleaned accelerations is given. A correlation between K-band residuals and regions of high atmospheric fluctuations was not observed, which is probably due to time variable signals of hydrological origin that dominate the residuals. © Springer-Verlag Berlin Heidelberg 2012.