R. König

The CHAMP-only earth gravity field model EIGEN-2

Abstract The German small geoscientific satellite CHAMP flies around the Earth since July 2000 in a highly inclined orbit with an altitude of initially 450 km and meanwhile at about 400 km. From the gravitational orbit perturbations, exploiting GPS-CHAMP satellite-to-satellite tracking and on-board accelerometer data over a time span of altogether six months, a new long-wavelength global gravity field model, called EIGEN-2, has been derived in a German/French effort. Thanks to CHAMPs dedicated orbit configuration, continuous GPS tracking and in-orbit measurement of non-gravitational satellite accelerations, the new CHAMP-only solution provides the geoid and gravity with an accuracy of 10 cm and 0.5 mGal, respectively, at a half wavelength resolution of 550 km. This is an improvement by almost one order of magnitude compared to any multi-satellite pre-CHAMP satellite-only gravity field model.

GRACE Time-Variable Gravity Accuracy Assessment

This study aims at the calibration of the formal variance covariance matrices of monthly GRACE-only gravity models that are known to give too optimistic accuracy measures for derived grav?ity functionals respectively surface mass anomalies. Based on 16 monthly solutions generated at GFZ Potsdam, a simple degree-dependent scaling of given variance-covariance matrices is considered, using difference degree amplitudes of monthly GRACE-only gravity field models that are sepa?rated by 12 months. It turns out that for the recent GFZ models the mean accuracy level of the monthly fields is about 20 – 40 times lower than the GRACE baseline accuracy. An observed scatter of the accuracy of the monthly models can be attributed to the amount of data used to generate the monthly model, but a more significant contribution arises from variations in the ground track coverage from month to month.

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

Abstract 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.

Surface forces parameterization of GFZ-1 orbits and gravity field recovery

The small laser satellite GFZ-1, in orbit since April 1995, experiences exceptionally large orbit perturbations by surface forces at its altitude below 400 km. The surface forces have to be modeled in an appropriate way in order to separate the gravity signal for gravity field recovery (the main objective of the mission). The study focusses on two points in the gravity field solution process: first to locate and to quantify the correlations between the cd-multipliers and the coefficients of the spherical harmonic expansion of the gravity field; and second to find an optimum weighting of GFZ-1 observations so that the normal equation system becomes solvable and the quality of the solution is not degraded.