G. Balmino

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.

Improvement of GRIM4 Earth Gravity Models Using Geosat Altimeter and SPOT-2 and ERS-1 Tracking Data

The fourth generation long-wave global Earth gravity field models, GRIM4, accomplished by a joint German/French effort, undergo continuous improvement with respect to accuracy and resolution by the incorporation of more satellite observations and the consideration of the most recent Earth surface data sets.

Grim4-C1, -C2p: combination solutions of the global earth gravity field

On the basis of the GRIM4-S1 satellite-only Earth gravity model, being accomplished in a common effort by DGFI and GRGS, a combination solution, called GRIM4-C1, has been derivcd using 1° × 1° mean gravity anomalies and 1° × 1° Seasat altimeter derived mean geoid undulations. In the meantime improvements could be achieved by incorporating more tracking data (GEOSAT, SPOT2-DORIS) into the solution, resulting in the two new parallel versions, the satellite-only gravity model GRIM4-S2 and the combined solution GRIM4-C2p (preliminary). All GRIM4 Earth gravity models cover the spectral gravitational constituents complete up to degree and order 50.In this report the emphasis is on the discussion of the combined gravity models: combination and estimation techniques, capabilities for application in precise satellite orbit computation and accuracies in long wavelength geoid representation. It is shown that with the new generation of global gravity models general purpose satellite-only models are no longer inferior to combination solutions if applied to satellite orbit restitution.

GRIM-4 — A New Global Earth Gravity Model (Status Report)

In view of the SPOT-2 and ERS-1 satellite missions DGFI and GRGS are jointly engaged in the generation of a new global gravity model within the GRIM series, called GRIM-4. It will consist of a satellite-only model derived purely from satellite tracking data and a combination solution additionaly incorporating Earth surface gravity data and altimeter data.

Detailed gravimetric geoid for the North Atlantic

A detailed gravimetric geoid in the North Atlantic Ocean, named DGGNA-77, has been computed, based on a satellite and gravimetry derived earth potential model (consisting in spherical harmonic coefficients up to degree and order 30) and mean free air surface gravity anomalies (35180 1°×1° mean values and 245000 4′×4′ mean values). The long wavelength undulations were computed from the spherical harmonics of the reference potential model and the details were obtained by integrating the residual gravity anomalies through the Stokes formula: from 0 to 5° with the 4′×4′ data, and from 5° to 20° with the 1°×1° data. For computer time reasons the final grid was computed with half a degree spacing only. This grid extends from the Gulf of Mexico to the European and African coasts.Comparisons have been made with Geos 3 altimetry derived geoid heights and with the 5′×5′ gravimetric geoid derived byMarsh andChang [8] in the northwestern part of the Atlantic Ocean, which show a good agreement in most places apart from some tilts which porbably come from the satellite orbit recovery.