Perpustakaan Digital ITB

Global monitoring of time-variable Earth gravity field from space – at first, observation of secular changes in the Earth flattening by SLR satellites and in recent years especially the regional hydrology variations revealed by the GRACE mission – has revolutionized the exploration of the Earth in many fields. To be able to obtain this rather weak part of the measured signal, it is necessary to measure/model and to subtract all other gravitational and nongravitational forces whose magnitude is not negligible compared to that of the time-variable gravity. In our paper, we analyze the influence of the perturbations arising from the general theory of relativity. Using the modelled data over one year of three example satellites at altitudes 250 km (GOCE), 450 km (Swarm A) and 5900 km (LAGEOS-1), we characterized the magnitude and frequency of occurrence of the three main general relativistic perturbing terms and compared their action to other relevant accelerations. Namely, for GOCE and Swarm A the accelerations due to time-varying gravity reach 9 10? 8 10?? 2 ms? , for LAGEOS-1 11 10? 10 10?? 2 ms? ; relative to the preceding, the dominant relativistic term is greater by one order of magnitude in the radial direction, the relativistic precession terms happen to be comparable in the cross-track direction. Therefore, for a proper extraction of the time-varying part of the gravity signal, all three relativistic terms have to be taken into account. This conclusion is validated by the real monthly gravity fields obtained through the inversion of GPS orbits of the Swarm A satellite.