# Presentation: 370007

## IGS Contribution to ITRF 

_**Zuheir Altamimi **_**& Xavier Collilieux**
**IGN, France**

## Outline

**IGS/GPS contribution to ITRF**
**Some lessons from ITRF2005**
**IGS network and loading effects**
**Polar motion **
**Constraints**
**Network effect**
**Origin and Scale from GPS ?**
**Repeatability – Internal precision**
**Conclusion**

## GPS Contribution to the ITRF

**Strengths**
**Enforces the link between SLR and VLBI**
**Precise polar motion**
**Acces to ITRF and its densification via IGS products**
**Geographic density: Improve plate motion estimate & NNR condition implementation **
**Weaknesses**
**Discontinuities due to equipement changes**
**Antenna PCVs **
**Imprecise TRF origin and scale ? Something to improve here ?**

## Current ITRF Derivation

**TRF (X, V)+ EOP  (SINEX) **

**TRF (X, V)+ EOP  (SINEX) **

**TRF (X, V)+ EOP  (SINEX)**

**TRF (X, V)+ EOP  (SINEX)**

**Local Ties**

**Combination**
**ITRF**

**Stacking**

**S1 S2**

**S3**

**Sn**

**...**

**VLBI**

**SLR**

**GPS**

**DORIS**

**TRF (X, V)+ EOP  (SINEX)**

**Step 1**

**Step 2**

**W1**

**W2**

**Wn**

**...**

**W1**

**W2**

**Wn**

**...**

**W1**

**W2**

**Wn**

**...**

## ITRF2005: Distribution and number of the IGS stations

**ITRF2005: Distribution and number of the IGS stations**

**Percentage of stations**

- X

- Y

- Z

- Number of stations

## ITRF2005:IGS weekly WRMS

## Loading effects on IGS network

**Loading**

***Synthetic data***

**Loading effects on IGS network**

- WRMS - UP

- WRMS – 2D

## Loading effects on IGS network

**Loading**

**Loading+Flicker Noise**

***Synthetic data***

**According to Williams et al., 2004 variance values, rescaled by 60%**

**Loading effects on IGS network**

- WRMS - UP

- WRMS – 2D

## Loading effects on IGS network

**Loading**

**Loading+Flicker Noise**

**Loading+Flicker Noise****+Mes. Noise**

***Synthetic data***

**Loading effects on IGS network**

- WRMS - UP

- WRMS – 2D

## Loading effects on IGS network

**Loading**

**Loading+Flicker Noise**

**Loading+Flicker Noise****+Mes. Noise**

**IGS combined**

***Synthetic data***

***Real data***

**Loading effects on IGS network**

## VLBI

**Network effect evaluated using a loading model (Tonie van Dam)****Comparison with IVS and ILRS network**

**VLBI**

**SLR**

**GPS**

**East**

**North**

**Up**

## IGS weekly WRMS

**IGS weekly WRMS**

**Week 1400**

## Week 1400 effect

**Week 1400 effect**

**Week 1400**

## EOP constraints ? Ex from DORIS

**DORIS IGN**
**Free solution**

**DORIS LCA**
**EOP Continuity conditions added **

**Polar motion differences wrt IGS**

## Network effect on EOPsChange the RF Core sites by 10 sites

**Main **
**difference**
**Drift:**
**Xp: 2 μas/yr**
**Yp: 1 μas/yr**

## TRF Origin & Scale from GPS ?

## IGS Reprocessed solutions : Origin & Scale

**IGS Reprocessed solutions : ****Origin & Scale**

## Some Origin & Scale after W1400

**Some Origin & Scale after W1400**

## Impact of Orbit Parameters on TRF

**4 weeks from ESOC:**
**Default: ****	****18 orbit parameters fixed**
**Free: ****	****	****9 CODE RPRs are free **

## GPS PDR Origin & Scale wrt ITRF2005

## Repeatability – Internal Precision

## IGS Reprocessing: Repeatability – Internal Precision

**ESA**

**GFZ**

**MIT**

**PDR**

## IGS Reprocessing: Repeatability – Internal PrecisionCombined solutions

## PDR vs IGS weekly WRMS

**PDR**

**IGS**

## Conclusions

**Loading effects**
**GPS/IGS contains some noise, esp. at early dates**
**GPS/IGS network effect: ~1 mm /yr in the Up component**
**EOPs**
**Avoid continuity constraints**
**Network effect on the long-term:  ****2**** ****μas/yr**** **
**Repeatability (weekly): 2 & 4-5 mm Horizontal & Up **
**Origin and Scale from GPS ?**
**Origin: Agreement betwenn ACs 5mm in X & Y and 10 mm in Z**
**Origin: Orbit parameters impact the Z component**
**Scale: after W1400 & from reprocessing AC agreement at 0.5 ppb**
**Next ITRF: GPS Reprocessing is absolutely needed**