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Daniel R. Roman and Dru A. Smith

Use of G99SSS to evaluate the static gravity geopotential derived from the GRACE, CHAMP, and GOCE missions. Decade of Geopotential Research III. Session: GP52A-02. Daniel R. Roman and Dru A. Smith. Expected products of impending satellite gravimetric missions.

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Daniel R. Roman and Dru A. Smith

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  1. Use of G99SSS to evaluate the static gravity geopotential derived from the GRACE, CHAMP, and GOCE missions Decade of Geopotential Research III Session: GP52A-02 Daniel R. Roman and Dru A. Smith

  2. Expected products of impending satellite gravimetric missions • spherical harmonic models varying from degrees 120 to 250 • geoid accuracies to 2-5 cm for lower & higher degree models, respectively • gravity accuracies to 1-5 mgals for lower & higher degree models, respectively

  3. How to analyze these models? NGS-held surface gravity measurements NGS-held height anomalies (GPSBM’s) G99SSS gravimetric geoid model EGM96 spherical harmonic model By comparison with other currently available gravimetric data or models:

  4. NGS-held point surface gravity measurements • 2,155,662 surface gravity measurements scattered throughout North America • mixture of NGS & NIMA-proprietary data • terrain corrections are generally 30” with 3” terrain corrections in the northwestern United States

  5. Comparison of satellite models to NGS-held point gravity data • only a statistical comparison may be made because these data are point values • tidal systems differ between NGS (tide-free) and NIMA-proprietary (mean tide) data • data reference NAD27 & NGVD29 but can be converted to NAD83 & NAVD88

  6. NGS-held point height-anomalies (GPSBM’s) • 6169+ points scattered throughout CONUS • this data set is constantly being improved and expanded • it is used to generate the conversion surface between G99SSS and GEOID99

  7. Comparison of satellite models to NGS-held point height-anomalies • only a statistical comparison may be made because these data are point values • they highlight 400 km features not well represented in G99SSS as determined by an empirical covariance function • tidal system is tide-free • data reference NAD27 & NGVD29 but can be converted to NAD83 & NAVD88

  8. G99SSS gravimetric geoid model • incorporated 2,620,460 terrestrial, marine & altimetric gravity data points • remove-and-restore based on EGM96 • region covered: 24-58° N and 230-300° E • spatial coverage: 3500 km (degree 12) • resolution: 1 arc-minute

  9. Comparison of Satellite Models to G99SSS Height Anomalies • height anomalies are calculated at the surface from G99SSS geoid model • when combined with GPSBM’s, compares favorable at shorter (400+ km) wavelengths • model is mainly tide-free & ITRF96/GRS80 • upward continuation of combined G99SSS height anomalies & GPSBM’s necessitates trimming edges due to non-global coverage

  10. EGM96 geopotential model • degree 360 model with global coverage • generated from multiple satellite mission analyses and surface gravity observations • represents a least squares solution for the gravity field to a one degree resolution

  11. Comparison of Satellite Models to the EGM96 model • can generate either gravity or geoid values at any elevation including orbital • comparison may be made at all wavelengths • coefficients may be directly compared through the degree variances • possible discrepencies at degree 120 and lower based on comparison to G99SSS • model is tide-free & references WGS84

  12. An Ideal Comparison: A model at satellite elevation with all the wavelengths of EGM96 plus the adjustments implied by G99SSS & GPSBM’s for shorter wavelengths

  13. How to generate such a model? • upward continue G99SSS-EGM96 residuals and re-combine with EGM96 at elevation • create a spherical harmonic model from G99SSS values and re-calculate these at elevation

  14. Comparison of Satellite Models to hybrid G99SSS/EGM96 model • upward continuation is a stable process where downward continuation is unstable • analysis is at the level of the observations • long wavelengths based on EGM96 model ensure global coverage through degree 120 • shorter wavelengths are improved by the G99SSS & GPSBM’s

  15. A Revised Approach to Generation of the United States National Geoid A Paradigm Shift

  16. Use of Combined Dense Surface Gravity & Mission Models • new gravity mission models will provide 2-cm geoid signal through degree 120 • this could replace the use of GPSBM data in defining 400 km wavelengths • local gravity measurements and improved DEM’s will continue to provide the shorter wavelengths (below 400 km)

  17. Benefits to Revised Geoid Modeling • ease of comparison to other geoids for surrounding regions (Canada, etc.) • a new orthometric height datum based on only the geoid model and a GPS network such as the CORS may be implemented • the gravimetric geoid (with suitable transformations) for the United States would be the only geoid

  18. Conclusion • NGS-held point gravity & height anomalies may be compared independently and in conjunction with G99SSS & EGM96 to test spherical harmonic models from the GRACE, CHAMP, and GOCE missions • once confidence is established in these new models, they may be used as the basis for an improved national geoid

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