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

Abstract

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.

At present a preliminary solution (PEGM78) has been accomplished, having the following characteristics:

• satellite-only model based on 695 arcs with 1.3·10⁶ optical, laser and doppler tracking data from 26 satellites (c.f. Tab.1);

• 3096 simultaneously adjusted parameters, comprising

  • 2323 spherical harmonic coefficients, including C₀₀ and Ċ₂₀, of the geopotential complete up to degree and order 40 plus zonal and resonant terms up to maximum degree 50

  • 96 ocean tide constituents for 12 waves:

    • long period (m=0): Sa, Ssa, Mm, Mf

    • diurnal (m=1): Ql, O1, P1, K1

    • semi-diurnal (m=2): N2, M2, S2, K2

  • 651 geocentric coordinates for all optical, doppler and laser stations and

  • 28 horizontal station velocity parameters for 14 laser stations.

Tab.2 gives a summary of the PEGM78 quality evaluation, performed by intercomparisons with external data sets and with the GSFC/NASA GEM-T2 model.

The application of PEGM78 on precise orbit determination for various satellites shows satisfactory results, e.g. orbital fits which are at the lOcm level for ST ARLETTE and AJISAI and some centimeters smaller for the higher satellites LAGEOS and ETALON-l.

The intermediate Earth gravity field model PEGM78, reveals competitive performance with other satellite-only models in satellite's orbit restitution and the long wavelength geoid approximation.

A major improvement is expected by the inclusion of GEOSAT doppler tracking data, presently being processed.