Integrated Sensor Analysis for GRACE

GRACE measures the Earth's gravity field with great precision and detail. The gravity signal is deduced from the measured relative motion between the two satellites, corrected for all non-gravitational forces acting on the two satellites. Thus, a very sophisticated system of sensors with K-Band microwave inter-satellite ranging, GPS, micro-accelerometry, star-tracking and angular control together forms the gravimetric system. A simulator of this integrated sensor system has been constructed. Employing realistic models for gravitational as well as non-gravitational forces, satellite geometry and attitude, the expected GRACE signal can be simulated realistically. The noise modelling of the sensors is based on performance specifications. The simulator serves three purposes: (1) to deliver a thorough understanding of the sensor system, as well as its signal and noise behavior, (2) to compare anticipated and real system performance by analysis of real data and simulated data, (3) to identify processing errors and possible malfunctions. Based on a three day test data set K-Band and differential mode accelerometry have been analyzed in detail. Signal characteristics, the noise level as well as signal-to-noise behavior of the K-Band system are close to the anticipated performance. More complex is the situation for the differential accelerometry signal where actual, simulated and predicted performance show some disagreement that needs explanation.