# Presentation: 014219

## Towards the GEOSAT Follow-On Precise Orbit Determination Goals of High Accuracy and Near-Real-Time Processing

- Frank G. Lemoine
*Planetary Geodynamics Laboratory*
*NASA GSFC, Greenbelt, Maryland USA*
- Nikita P. Zelensky, Douglas S. Chinn,
- Brian D. Beckley
*SGT Inc.,*
*Greenbelt, Maryland USA*
- John L. Lillibridge
*Laboratory for Satellite Altimetry, NOAA*
*Silver Spring, Maryland USA*

- AIAA Paper  2006-6402
- AIAA/AAS Astrodynamics Conference, Keystone, Colorado
- August 21-24, 2006

## Outline

_**Outline**_
**Introduction**
**Data  **
**Description of GFO POD System**
**Gravity Modelling Improvements**
**Macromodel**
**Medium precision orbit (MOE) results**
**Precise orbit (POE) results.**
**VIII.****	****Summary**

## NAVSOC: Operates s/c.

- _Manufactured by_: Ball Aerospace for the US Navy.
- _Launched:_ February 10, 1998.
- _Declared Operational_: Nov. 29, 2000.
- _Orbit_:
- 	Altitude: 		784 km
- 	Eccentricity: 	0.0008
- 	Inclination:	108.04°
- 	Arg. of perigee:	90.5° 			(frozen orbit)
- _Repeat Period_:	244 revs in 17 days.
- _Payload_:
- 	Radar Altimeter
- 	Water Vapour Radiometer
- 	SLR Retroreflector
- 	Doppler Beacon
- 	GPS antenna (not operational)

- _NAVSOC:_ Operates s/c.
- _NASA:_ Coordinates SLR tracking with ILRS. Computes daily medium precision and precise orbits.
- _NOAA:_  Distributes altimeter data (IGDR and GDR)

## Altimeter Measurement Schematic

## GFO Orbit Determination Challenge

- =>> Altimeter range measurement accuracy depends on orbit quality.
- =>> In  light of the failure of GPS on GFO, can the other GFO tracking systems (SLR, Doppler, Altimeter) deliver sufficient data to meet POD requirements, especially since GFO altitude (784 km) is more challenging than Topex/Poseidon altitude (1336 km)?
- =>> Can SLR+Doppler data be use to compute operational orbits (latency of < 24 hrs)?
- =>> How do we measure orbit accuracy?

## Satellite Laser Ranging

- Greenbelt, Maryland, USA

- Mt. Stromlo, Canberra, Australia

**Up to 40 stations worldwide operate under the aegis of the International Laser Ranging Service (ILRS)**
**URL: **_[http://ilrs.gsfc.nasa.gov/](http://ilrs.gsfc.nasa.gov/)_
**The best stations deliver ranging accuracy of a few mm. **

## ILRS NETWORK in 2005

## Number of SLR Passes for GFO, January 2005 to March 2006

_**Number of SLR Passes for GFO, January 2005 to March 2006**_

| Stations (36 total) | percent passes |
| --- | --- |
| Yarragadee (YARA) | 15 % |
| NASA (MNPE, GRF1, MCDO)
 (no HOLL7210) | 11 % |
| Other | 74 % |

## SLR Tracking History: January 2005 - March 2006

**SLR Tracking History: January 2005 - March 2006**

## Doppler Data: Three stations: Guam, Point Mugu, California; Maine.  Dual-frequency 150/400 Mhz. Noise 1.5 - 2.0 cm/s.

_**Doppler Data**__: _Three stations: Guam, Point Mugu, California; Maine.  Dual-frequency 150/400 Mhz. Noise 1.5 - 2.0 cm/s.
_**Altimeter Data**__: _Use data from NOAA IGDR (Intermediate Geophysical Data Record). Form altimeter crossovers.

**Doppler and Altimeter Data**

| Ocean Tide | GOT00.2 (Topex derived tide model) |
| --- | --- |
| Earth Tide | Cartwright & Eden (updated) |
| Dry Troposphere | NCEP |
| Wet Troposphere | GFO WVR or NCEP |
| Ionosphere | IRI95 |
| Inverse barometer |  ƒ (dry troposphere) |
| EM bias | 3.8% SWH |

_**Altimeter Range Modelling for the GFO IGDR**_

## GFO Altimeter Crossover Modelling  

**GFO Altimeter Crossover Modelling  **

_**Editing Criteria**_
- Bathymetry:
- (Reject depth < 500 m)
- Sea surface variability: (Reject   > 20 cm)
- Max Residual:
- (Reject > 20 cm)

- _Sea Surface Variability (TP+ERS)_

- _Example of Crossover _
- _Data Distribution _

## GFO Precision Orbit Determination System  

**GFO Precision Orbit Determination System  **

## Typical Processing Scenario  

**Typical Processing Scenario  **

- =>Import SLR Data and Doppler data by early afternoon
-  	(local time, or 17:00-18:00 UT).
- (SLR data delivered hourly to ILRS data centers)
- =>Import IGDR altimetry data from NOAA
- (Lag of 48 hrs in data delivery).
- Import updated Earth orientation parameter info (IERS) and solar flux/geomagnetic index info (NOAA/NGDC)
- Process data with GEODYN Orbit Processor and Geodetic Parameter Estimation Program.  Medium precision orbits (MOE’s) have five day sliding window.
- By COB, or 21:00 to 23:00 UT, deliver MOE orbit to users at NOAA and the US Navy.
- =>Send new ephemeris predict based on daily MOE orbit to SLR stations.
- Precise orbits have a latency of ~3 weeks.
- 	(6-day arcs with 1-day overlaps).
- => Maneuvers introduce complications!!

## GFO Processing Standards  

**GFO Processing Standards  **

| POE Generation | 1st: 
PGS7727
(2001) | 2nd:
 PGS7777b (2004) | New:
GGM02C
(2006) |
| --- | --- | --- | --- |
| Gravity | PGS7727
(70x70) | PGS7777b
(110x110) | GGM02C
(120x120) |
| Time-variable
Gravity | C20dot, C21dot S21dot | Same +zonal annuals | 20x20 annuals from GRACE |
| Ocean Tides | Ray99 + pgs7727 resonant | Ray99 + pgs7777b resonant | GOT00.2 (20x20) |
| Solid Earth tides | k2, k3, + FCN | Same | IERS2003 |
| Albedo/IR | Knocke/Ries, 1988 | Same | Same |
| Drag | MSIS86 | Same | Same |
| Parameterization | Cd/ 8hrs, opr along+cross/day | Same | Same |
| SLR coordinates | ITRF2000 | ITRF2000 | ITRF2000 |
| Doppler coordinates | Tuned with CSR95L02 | Same | Same |
| LRA offset | Estimated with CSR95L02 | Same | Same |

## GEOSAT Gravity Model Error     

**GEOSAT Gravity Model Error     **

## GEOSAT Gravity Model

**GEOSAT Gravity Model**
**Radial  Orbit Error    **

## GFO Macromodel 

**GFO Macromodel **
**(Nonconservative Force Modelling)    **

## GFO LRA Offset Modelling    

**GFO LRA Offset Modelling    **

## Estimate GFO LRA Offset using June ’98 SLR Data

**Estimate GFO LRA Offset using June ’98 SLR Data**

| Description | Spacecraft body-fixed coordinates (cm) |  |  | SLR residuals (cm) |  |
| --- | --- | --- | --- | --- | --- |
|  | X | Y | Z | Mean | RMS |
| A priori CoM | 89.7 | 0.8 | -6.6 |  |  |
| A priori LRA offset | 114.2 | 77.2 | 42.7 | 2.5 | 10.7 |
| Estimated LRA offset | 107.9 | 76.1 | 53.3 | -0.1 | 10.0 |

## GFO MOE RMS of Fit    

**GFO MOE RMS of Fit    **

- SLR avg. RMS =  6.1 cm
- Crossover avg. RMS = 7.3 cm

- Due to latency issues, MOE arcs, have altimeter crossovers for first three days of each arc only.

## GFO MOE Orbit Overlaps

_**GFO MOE Orbit Overlaps**_

**Radial**

| MOE Orbit Statistics (cm)
(Avg. RMS Overlaps) |  |
| --- | --- |
| Radial | 5.11 |
| Cross-track | 11.89 |
| Along-track | 23.82 |

- (Only show statistics since we started routinely including crossovers in MOE orbits in February 2004)

## GFO POE RMS of Fit    

**GFO POE RMS of Fit    **

- SLR avg. RMS =  4.37 cm
- Crossover avg. RMS = 7.51 cm

## GFO POE RMS of Fit  Summary  

**GFO POE RMS of Fit  Summary  **

| Year | No. 
of Arcs | SLR 
(cm) | Crossovers
(cm) | Doppler
(cm/s) |
| --- | --- | --- | --- | --- |
| 2000 | 67 | 4.68 | 8.41 | 1.74 |
| 2001 | 60 | 4.70 | 8.64 | 1.93 |
| 2002 | 66 | 5.39 | 8.12 | 2.10 |
| 2003 | 63 | 4.45 | 7.12 | 1.93 |
| 2004 | 62 | 4.49 | 6.80 | 1.75 |
| 2005 | 71 | 3.26 | 6.57 | 1.90 |
| 2006 | 34 | 3.18 | 6.58 | 1.54 |
| ALL | 423 | 4.37 | 7.51 | 1.89 |

## GFO POE Orbit Overlaps

_**GFO POE Orbit Overlaps**_

| POE Orbit Statistics (cm)
(Avg. RMS Overlaps) |  |
| --- | --- |
| Radial |  1.84 |
| Cross-track | 11.56 |
| Along-track | 13.50 |

**Radial**

## GFO Orbit Error Assessment from analysis of mean of the GFO sea surface variability 

_**GFO Orbit Error Assessment from analysis of mean of the GFO sea surface variability **_

**Before**
** ****Empirical Correction**

**After**
** ****Empirical Correction with Topex/POSEIDON**

**Orbit Error (relative to Topex) from RSS difference = 4.62 cm.**
**Including Topex error (2.5 cm) => GFO orbit error   = 5.25 cm.**
**This assessment done with PGS7727 orbits early in mission.**

## Summary 

_**Summary **_

- • The GFO mission was rescued by the laser retroreflector and the demonstration of near-real-time POD using SLR, Doppler, and altimeter crossover data.
- • MOE (medium precision) orbits are exported daily, with a probable radial accuracy of 15 to 20 cm.
- • POE (precise) orbits are exported with a ~3-week latency with a radial precision of about 5 cm.
- • GFO altimeter data have many scientific applications, especially in combination with data from other missions such as Jason-1, Envisat, ERS: mapping of eddies; near-real-time monitoring for hurricane forecasts; inland lake monitoring; detection (ex post facto) of Indian Ocean tsunami.
- • Further orbit modelling improvements are planned using GRACE gravity models, better CG modelling, improved drag and radiation pressure modelling.