EGM96: The NASA GSFC and NIMA Joint Geopotential Model
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We compare the projected error from the two satellite only solutions, JGM-2S,
and the new satellite only solution, EGM96S (the foundation of EGM96).
Note, that the
error at the higher degrees is higher in EGM96S, since we are using a
different * a priori * constraint than in JGM-2S. All satellite
only solutions must be conditioned by the application of a power law constraint,
to prevent the high degree terms from developing excessive power (see the
discussions of previous satellite only solutions for details; cf. Nerem
et al., [1994], and Marsh et al., [1990]). In JGM-2S, the power law
constraint that was applied was Kaula's power law, divided by the
square root of two. However, this power law underestimates the power in
the Earth gravity field, as determined from surface gravity observations.
Therefore, we use a a power law constraint derived from the signal of the
surface gravity data to spectrally constrain the coefficients of EGM96S.
As illustrated in the figure below, the addition of the new tracking data
results in a factor of two to four improvement in the formal errors, as
measured by the ratio of the coefficient sigmas at the lower degrees.
Since the satellite only solution is the foundation of the high degree
quadrature solution, it is important that the spectrum of the errors in
the satellite only model be consistent with the signal observed by the
surface gravity data.

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Send questions or comments to:

flemoine@olympus.gsfc.nasa.gov or npavlis@geodesy2.gsfc.nasa.gov.

Last revised February 26, 1998, 23:16 UTC.