The GRACE Formation: Science Mode Pointing Performance Analysis

The Gravity Recovery And Climate Experiment (GRACE) is a joint mission between the National Aeronautics and Space Administration (NASA) and the Deutsches Zentrum für Luft und Raumfahrt (DLR e. V.). Principle Investigator is the University of Texas Center for Space Research (UTCSR). Responsibility for operational activities, like satellite safety, telemetry and telecommanding, maneuver planning and execution and formation keeping, belongs to the German Space Operations Center (GSOC), where this work was conducted during the first phases of the mission (LEOP). The twin GRACE satellites were launched on-board the Eurockot Rockot launcher on March 17, 2002, form Plesetsk, Russia. The current mission phase is the transition from commissioning into the science validation phase. GRACE mission goals include the generation of models for the time-variable gravity field of the Earth, to produce a long-term mean estimate of the Earth´s gravity field and to provide radio occulation profiles over the predicted five-year lifetime of the mission. From a scientific point of view, GRACE improves, upon the conventional methods, the gravity field determination by using differential, range-change measurements between twin, co-orbiting satellites and improved accelerometers. This formation-flying scenario ensures global, homogenious and high accuracy gravity measurements. The fundamental requirement for normal science activities of each satellite is to point at the predicted position of the other satellite, keeping the bore sight of the microwave link within a dead band as small as +- 0,5 mrad (pitch and yaw) and +- 10 mrad in roll. The AOCS incorporates a trajectory generation algorithm, taking inputs from an on board SGP4 (Simplified General Perturbations) orbit propagator, such that the relative position between the two satellites is computed to be within 50m. This work was motivated by the necessity of verifying the correct behavior of the satellite during science operations and the attempt to improve AOCS pointing performance, especially after the failure of the inertial measurement unit on GRACE-1, resulting in a higher gas consumption and and more thruster activations. The pointing performance of investigated by the implementation of a reference/target attitude using as inputs GPS position and velocity telemetry data (WGS-84 frame), earth rotation service (IERS) parameters for a conversion to J2000 inertial frame, SGP4/Two Line Element (TLE) propagation (J2000 frame) and TLE records generated by the GSOC flight dynamics group. The current inertial attitude is delivered by on-board star tracker measurements. Furthermore the correlation between the attitude behavior and the mission environment (I.e. orbital position) is analyzed by the assesment of a GRACE science mode simulator including main disturbance torques (gravity gradient, aerodynamic drag, solar pressure and magnetic field) and validated using space telemetry data. The paper presents a description of the developed Matlab/Simulink model and the results of these investigations using latest GRACE space data