# Presentation: 656702

## Use of G99SSS to evaluate the static gravity geopotential derived from the GRACE, CHAMP, and GOCE missions

- Daniel R. Roman and Dru A. Smith

- Session: GP52A-02

- Decade of Geopotential Research III

## 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

## How to analyze these models?

- By comparison with other currently available gravimetric data or models:

- NGS-held surface gravity measurements
- NGS-held height anomalies (GPSBM’s)
- G99SSS gravimetric geoid model
- EGM96 spherical harmonic model

## 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

## Slide 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

## 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

## Slide 8

## 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

## 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

## Slide 11

## 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

## 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

## Slide 14

## Slide 15

## 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

## 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

## 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

## 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

## A Revised Approach to Generation of the United States National Geoid

- A Paradigm Shift

## 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)

## 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

## 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