Precise accelerometry onboard the GRACE gravity field satellite mission

The twin satellites of the GRACE (Gravity field recovery and climate experiment) mission are equipped with three-axes capacitive accelerometers measuring the non-gravitational surface forces acting on the satellites with a sensitivity which is unprecedented for low earth orbiters. In this paper, we study 10 Hz raw accelerometer observations from both GRACE satellites. We identify and analyse numerous acceleration spikes related to switching activity in the circuits of onboard heaters. These spikes are very well observed, but their source is not yet understood. Based on various indications, we suggest the presence of mechanical forces due to electrical current changes. The acceleration spikes demonstrate the difficulty to create a quiet environment on a low earth orbiter platform. We use spike-free data from time spans without heater activity to show that the accelerometers indeed achieve the expected sensitivity, with a noise level slightly below 10-10m/s2/Hz for the along track and radial axes. We derive average models for the switching spikes and use them to construct a synthetic spike time series. Based on preliminary modeling results, we expect at most a slight influence of the acceleration spikes on the GRACE gravity field determination. However, the acceleration spikes are not negligible for the investigation of satellite aeronomy and of the sources of non-gravitational forces.