Satellite dynamics of the CHAMP and GRACE leos as revealed from space- and ground-based tracking Original Research Article

The Low Earth Orbiters (LEOs) CHAMP and GRACE are precisely and continuously tracked by means of space-borne GPS. In case of the GRACE mission the distance between the twin satellites is also measured by a micrometer-precision ranging system. Thus the satellite dynamics are densely recorded and show up in the tracking residuals of the various data types in a dynamic Precise Orbit Determination (POD) solution. The signatures in the residuals time series are mainly caused by mis-modeling of the gravity field because the non-conservative forces such as atmospheric drag are taken care of by the on-board accelerometers. In addition all satellites carry laser retro-reflectors for centimeter accuracy range measurements from the ground. Examples of gravity signals in the residuals are displayed and discussed in view of the gravity field recovery and the POD tasks.