# Presentation: 014211

## Evaluation of the Geosat Follow-On Precise Orbit Ephemeris

**Evaluation of the Geosat Follow-On Precise Orbit Ephemeris**

- _Frank G. Lemoine_, David D. Rowlands
- Space Geodesy Branch, NASA / GSFC,
- Greenbelt, MD 20771

-  Nikita P. Zelensky, Brian D. Beckley, Douglas S. Chinn
- SGT Inc, Greenbelt, MD 20770

-  John L. Lillibridge
- Laboratory for Satellite Altimetry, NOAA, Silver Spring, MD 20910

**GFO Sea Surface Height Variability**

**Adjusted GFO Sea Surface Height Variability**

-  The GEOSAT Follow-On spacecraft (GFO), launched in 1998, began continuous radar altimeter coverage of the oceans in 2000.   After an extensive series of calibration campaigns in 1999 and 2000, the spacecraft was finally accepted by the US Navy on November 29, 2000. By providing high quality altimeter data, GFO can supplement Jason, TOPEX/POSEIDON(T/P), and Envisat, providing a different synoptic sampling of the oceans with its 17-day ground track repeat cycle. Altimeter crossover analysis suggests GFO is capable of POSEIDON class altimetry, with orbit error the largest contributor to the GFO altimeter error budget. Satellite laser ranging (SLR) data, especially in combination with altimeter crossover data, offer the only means of determining high-quality precise orbits. SLR tracking is also augmented by Doppler (Tranet style) beacons. These data were used to tune the gravity field and macromodel (3D representation of the spacecraft geometry and surface properties). Beginning with January 2000 GSFC has produced a 6 year span of the Precision Orbit Ephemeris (POE) intended for use on the GFO GDR. The GFO orbits are currently produced with pgs7777b, a model based on CHAMP, GFO, and other satellite data. The pgs7777b had shown significant improvement in POD performance over all previous gravity fields.    Recent tests suggest even further orbit improvement using the Grace-derived GGM02C static gravity field. Other possible improvements are evaluated  such as using new ocean and earth tide models that conform to IERS2003 stands, a new Grace-derived time-varying gravity field, and forward modeling of  atmosphere gravity.  Orbit accuracy is evaluated using GFO  tracking data and altimeter crossover and collinear sea surface height residuals, and analysis of coefficients used to adjust GFO data into the TOPEX frame

| Orbit | pgs7727
 (2001) | pgs7777b 
(2004) | ggm02c
 (2005) | tvg 
(2006) |
| --- | --- | --- | --- | --- |
| Gravity (static) | PGS7727 
(70x70) | PGS7777b 
(110x110) | GGM02C 
(120x120) | GGM02C 
(120x120) |
| Gravity (time-variable) | C20dot, C21dot, S21dot | same | same | C20dot, C21dot, S21dot + 20x20 annual terms from GSFC GRACE solutions |
| Atmospheric gravity | Not applied | not applied | not applied | NCEP,  50x50 @6 hrs |
| Ocean Tides | Ray99 (T/P) + PGS7727   resonant  | Ray99 (T/P) + PGS7777b resonant | same |  (T/P-derived) GOT00.2 (20x20) |
| Solid Earth Tides | k2= 0.300; k3=0.093 + special handling for FCN. | same | same | IERS2003   |
| Albedo/IR | Knocke & Ries, 1988 | same | same | same |
| Atmospheric drag | MSIS86 | same | same | same |
| surface force 
macromodel | tuned 8-panel model | re- tuned 8-panel model | same | same |
| Data | SLR + Altimeter Crossovers + Doppler  | same | same | same |
| Parameterization and arc span | Cd/8 hrs + opr along+cross track/day;
6-day arc | same | same | same |
| SLR stations | ITRF2000 with ocean loading | same | same | same |
| Doppler stations | adjusted to the SLR CSR95L02 frame | same | same | same |
| LRA offset | tuned with 2-months SLR tracking data | same | same | same |
| precession | IAU1976 | same | same | IAU2000 |
| nutation | IAU1980+corrections | same | same | IAU2000 |

**GFO Orbit Modeling Development (red is new)**

| Gravity model | projected radial orbit error (mm) |
| --- | --- |
| JGM-3 (1995)
Update to JGM-2 with TOPEX/GPS, Stella and other satellite data | 49.8 |
| EGM96 (1996)
Model with new-satellite tracking data, altimetry, and surface gravity | 26.2 |
| PGS7727 (2001)
computed from post-EGM96 pgs7609g using GFO SLR, Doppler, GFO and TOPEX-GFO altimeter crossover data | 13.2 |
| PGS7777b (2003)
computed from pgs7727 using 87 days of Champ data and tracking data from GFO (SLR/Crossovers), TOPEX (SLR/DORIS), Jason (GPS), Envisat (SLR/DORIS), and other SLR data | 10.0 |
| GGM02C (2004)
GRACE-based combination model | 4.0 
(Ries 2006) |

**Gravity Model Summary**