# Presentation: 000302

## Sylvain Loyer (CLS)
Félix Perosanz (CNES)
Hugues Capdeville (CLS)

*Sylvain Loyer (CLS)*
*Félix Perosanz (CNES)*
*Hugues Capdeville (CLS)*

**CNES-CLS Dynamical modelling of GPS orbits **
**and **
**SLR Residuals**

## GRGS

**CNES-CLS IGS Analysis Center:  **_**[igsac-cnes.cls.fr](http://igsac-cnes.cls.fr/)**_** **

**GRGS**

**IGS**

**IDS**

**ILS**

**IVS**

-  Submission & evaluation started July 2007
-  joined officially IGS ACs week 1478

**GPS**

**SLR**

**DORIS**

**VLBI**

***IERS/CRC***

## Several months of evaluation of CNES-CLS products indicate that the main discrepancies are on orbit products : 
around 3.5 – 4 cm WRMS3D vs IGS final orbits
a systematic scale of around 1 ppb (~2.64 cm at GPS     altitude) exhibits between CNES-CLS and IGS orbits

Pole and station coordinate solutions are very comparable to any other AC.

We wanted to investigate the source of the scale vs IGS : 
dynamic parameters modeling (i.e. SRP parameterization)
dynamic modeling (i.e. ALBEDO force model)
using SLR observations.

**Motivation of this study**

**Several months of evaluation of CNES-CLS products indicate that the main discrepancies are on orbit products : **
**around ****3.5 – 4 cm WRMS3D**** vs IGS final orbits**
**a systematic scale of around 1 ppb**** (~2.64 cm at GPS     altitude) exhibits between CNES-CLS and IGS orbits**
**Pole and station coordinate solutions are very comparable to any other AC.**
**We wanted to investigate the source of the scale vs IGS : **
- dynamic parameters modeling (i.e. SRP parameterization)
- dynamic modeling (i.e. ALBEDO force model)
**using SLR observations.**

## SLR residual analysis  is supposed to give an independent evaluation of the precision and accuracy of the computed orbit  
	(up to a level of ~ 5 mm)
In the framework of GRGS team activities SLR data are routinely processed using the GINS software (ILS, EIGEN gravity field modeling,...)

Questions :
Origin of a systematic bias between SLR data and IGS and ACs orbits solutions ?
What is the impact on GPS orbits of considering or not ALBEDO forces  in the orbit computation ?
Is the orbital precision derived from the statistical distribution of SLR residuals consistent with the expected value ?

**SLR tracking of SV35 and 36 **

**SLR residual analysis  is supposed to give an independent evaluation of the precision and accuracy of the computed orbit  **
**	****(up to a level of ~ 5 mm)**
**In the framework of GRGS team activities SLR data are routinely processed using the GINS software (ILS, EIGEN gravity field modeling,...)**
**Questions :**
- Origin of a systematic bias between SLR data and IGS and ACs orbits solutions ?
- What is the impact on GPS orbits of considering or not ALBEDO forces  in the orbit computation ?
- Is the orbital precision derived from the statistical distribution of SLR residuals consistent with the expected value ?

## Full description available at : http://igscb.jpl.nasa.gov/igscb/center/analysis/grg.acn
	(see also Jim Ray’s presentation in this session)
Main significant differences:
48h Arc length ; 15mn data rate ; real ambiguities
Radiation pressure :
Solar direct + Earth albedo and IR flux
GINS Box and wing model (19 planar surfaces)
Nominal satellite attitude (no yaw steering model)
Empirical parameters (6+6 per arc) :
1 scale/arc  in factor of solar pressure 
1 {Y}-bias/arc 
1/rev. sin & cos in {X’} {D}
Albedo and IR :
Systematic ~ 5. 10-10 m/s/s mainly in {Z}
Flux from ECMWF 6h grids 
Include overcast and umbra effects

**Dynamical model and processing strategy**

**Full description available at : **_**[http://igscb.jpl.nasa.gov/igscb/center/analysis/grg.acn](http://igscb.jpl.nasa.gov/igscb/center/analysis/grg.acn)**_
**	****(see also Jim Ray’s presentation in this session)**
**Main significant differences:**
- 48h Arc length ; 15mn data rate ; real ambiguities
- Radiation pressure :
- Solar direct + Earth albedo and IR flux
- GINS Box and wing model (19 planar surfaces)
- Nominal satellite attitude (no yaw steering model)
- Empirical parameters (6+6 per arc) :
*1 scale/arc ** in factor of solar pressure** *
*1 **{Y}**-bias/arc *
*1/rev. sin & cos in **{X’} {D}*
- Albedo and IR :
*Systematic ~ 5. 10**-10** m/s/s mainly in **{Z}*
*Flux from ECMWF 6h grids *
*Include overcast and umbra effects*

## Data from 10 October 2007 to 10 may 2008 ; (2099/1983) valid  normals points for GPS(35/36). 
Data weigthing :  station quality factor 
                             + from LAGEOS residuals analysis
                             + uncertainties from ITRF2005 coord.
Fixed IGS or GRG orbits + one SLR bias per station

**SLR data set and SLR processing strategy**

**Data from 10 October 2007 to 10 may 2008 ; (2099/1983) valid  normals points for GPS(35/36). **
**Data weigthing :  station quality factor **
**                             ****+ from LAGEOS residuals analysis**
**                             ****+ uncertainties from ITRF2005 coord.**
**Fixed IGS or GRG orbits + one SLR bias per station**

## CoM offsets : 
			XY values : 862.6 mm / -524.5 mm

**SLR CoM offset**

**CoM offsets : **
**	****	****	****XY values : 862.6 mm / -524.5 mm**

|      (values in millimeters) | GPS35 | GPS36 |
| --- | --- | --- |
| ILRS Z axis (radial)(recomanded)
(see also A. Davis & J. Trask, 08/2007). | 669.5 | 671.7 |
| Used Z axis  (« old » values) used in this study. | 658.4 | 658.4 |
| Differences | 11.1 | 13.3 |

## Statistics on SLR residuals

| (cm)
GRG
 IGS  | GPS35
2.88 (1983) 
2.44 (1983) | GPS36
2.45 (2099) 
1.98 (2099) | 
Global |
| --- | --- | --- | --- |
| GRG
 IGS  | 1.72 (1516) 
1.46 (1516) | 1.97 (1656) 
1.77 (1656) | Non-eclips periods |
| GRG
 IGS  | 4.65 (467) 
3.57 (467) | 3.65 (459) 
2.27 (459) | Eclips periods |

**Statistics on SLR residuals**

## 
SLR bias differences  : 

IGS=GRG -2.4 cm (0.91 ppb)

GRG bias nearest 
      from 0 cm 
    (i.e. compatible with Laser Station calibration)

**SLR adjusted biases**

**SLR bias differences  : **
**IGS=GRG ****-2.4 cm (0.91 ppb)**
**GRG bias nearest **
**      ****from 0 cm **
**    ****(i.e. compatible with Laser Station calibration)**

## Dynamic impact of Albedo force.

**Dynamic impact of Albedo force.**

- 1.6 cm (0.6 ppb) in syst. radial with nearly no change
-      for other statistics.

## }

- IGS Analysis Center Workshop, 2-6 June 2008, Miami, USA

- JPL  ESA and GRG use an Albedo model in their orbit processing.
- (Scales computed by G. Gendt & T. Nishan)

**Albedo**

**CoM**
**offsets**

**All values together...**

## SLR bias still not compatible with IGS scale
Discrepancies between ACs remain large (1 ppb)
CNES-CLS orbit closer to « old » CoM offset value
Albedo forces explain 0.6 ppb of the scale
     and maybe part of scale discrepancies between ACs.
CNES-CLS scale is less than 0.5 ppb relatively to Laser data
ACs with Albedo modeling (except EMR) are closer to current laser CoM offset value
Radial error of CNES-CLS orbits increase SLR RMS by 2.5 mm

**CONCLUSIONS**

**SLR bias still not compatible with IGS scale**
**Discrepancies between ACs remain large (1 ppb)**
**CNES-CLS orbit closer to « old » CoM offset value**
**Albedo forces explain 0.6 ppb of the scale**
**     ****and maybe part of scale discrepancies between ACs.**
**CNES-CLS scale is less than 0.5 ppb relatively to Laser data**
**ACs with Albedo modeling (except EMR) are closer to current laser CoM offset value**
**Radial error of CNES-CLS orbits increase SLR RMS by 2.5 mm**

## => Main discrepancy concern orbits scale and WRMS.

| Orbits vs IGS Combined orbit:

TX = 2 +/- 1.5 mm ;  TY = 0.3 +/- 1 mm ; TZ = -2 +/- 3 mm

RX = -17 +/- 35 uas ; RY = -75 +/- 65 uas  ; RZ = 38 +/- 60 uas 

Scale  = 1 + /- 0.05 ppb ;  WRMS3D : 3.2 +/- 0.35  cm |
| --- |
| Stations vs IGS05 (bias + rms)

Nord = 0 +/- 2.5 mm ;  Est = 0 +/- 1 mm ; Up = 0 +/- 6 mm |
| Pôle vs IGS solution  (bias + rms)

Xp = 5 +/- 25 uas ;  Yp = 43  +/- 30 uas ;  LOD = -1.5 +/- 32 usec  

Xprate = -56 +/- 90 uas/day ; Yprate = -6.5 +/- 90 uas /day |