EGM96: The NASA GSFC and NIMA Joint Geopotential Model
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### The entire satellite only solution was calibrated using the method of
Lerch [1991]. This method involves generating subset solutions that
specifically exclude that satellite from the general or master solution.
Then the calibration factor is computed for that satellite, where this
factor is the ratio of the sum of the coefficient differences between
the master solution and the subset, divided by the sum of the coefficient
sigma differences squared. The weight of the data in the normal equations
can be adjusted so that aggregate changes in the coefficients are commensurate
with the changes in the coefficient sigmas.
- All data sets were calibrated using subset solution testing.
- Select data sets, in particular those of important satellites (
TOPEX ,
Starlette
or those satellites that had continuous tracking (i.e. GPS/MET, EUVE) went through a second and a third iteration of calibrations. That is, after a new master solution was computed using the updated weights (normal equation scale factors), subset solutions were computed for these select satellites.
- We used comparisons with altimeter derived 5 deg by 5 deg gravity anomalies and GPS/Leveling tests to validate the weights selected.
- In general, to avoid deriving an error model that was too optimistic, we aimed for calibration factor, K, of between 0.8 to 0.9.
- Examples of the calibration factors for EUVE, and for Spot-2/
Stella
are shown in the attached figures.

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### Calibration factors, and
coefficient differences for two satellite only solutions, with
and without Spot-2 and Stella.

Average calibration factors:
- per degree (to 20) = 0.796;
- per degree (to 70) = 0.710;
- per order (to 70) = 0.661.

*Click on image for full size*

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Average calibration factors:
- per degree (to 20) = 0.713;
- per degree (to 70) = 0.881;
- per order (to 70) = 0.604.

*Click on image for full size*

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Send questions or comments to:

Frank.G.Lemoine@nasa.gov
or
Nikolaos.K.Pavlis@raytheon.com

Last revised November 22, 2004, 21:01 UTC.