1. Workshop, Miami, June 2008 IGS Contribution to ITRF Zuheir Altamimi & Xavier Collilieux IGN, France

2. Workshop, Miami, June 2008 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

3. Workshop, Miami, June 2008 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 ?

4. Workshop, Miami, June 2008 Current ITRF Derivation TRF (X, V)+ EOP (SINEX) Local Ties Combination ITRF Stacking S1 S2S3Sn… VLBI SLR GPS DORIS TRF (X, V)+ EOP (SINEX) Step 1 Step 2 W1W2Wn… W1W2Wn… W1W2Wn…

5. Workshop, Miami, June 2008 ITRF2005: Distribution and number of the IGS stations Percentage of stations X Y Z Number of stations

6. Workshop, Miami, June 2008 ITRF2005:IGS weekly WRMS

7. Workshop, Miami, June 2008 Loading Synthetic data Loading effects on IGS network WRMS - UP WRMS – 2D

8. Workshop, Miami, June 2008 Loading Loading+Flicker Noise Synthetic data According to Williams et al., 2004 variance values, rescaled by 60% Loading effects on IGS network WRMS - UPWRMS – 2D

9. Workshop, Miami, June 2008 Loading Loading+Flicker Noise Loading+Flicker Noise +Mes. Noise Synthetic data Loading effects on IGS network WRMS - UPWRMS – 2D

10. Workshop, Miami, June 2008 Loading Loading+Flicker Noise Loading+Flicker Noise +Mes. Noise IGS combined Synthetic data Real data Loading effects on IGS network

11. Workshop, Miami, June 2008 Network effect evaluated using a loading model (Tonie van Dam) Comparison with IVS and ILRS network VLBI SLR GPS East NorthUp

12. Workshop, Miami, June 2008 IGS weekly WRMS Week 1400

13. Workshop, Miami, June 2008 Week 1400 effect Week 1400

14. Workshop, Miami, June 2008 EOP constraints ? Ex from DORIS DORIS IGN Free solution DORIS LCA EOP Continuity conditions added Polar motion differences wrt IGS

15. Workshop, Miami, June 2008 Network effect on EOPs Change the RF Core sites by 10 sites Main difference Drift: Xp: 2 µas/yr Yp: 1 µas/yr

16. Workshop, Miami, June 2008 TRF Origin & Scale from GPS ?

17. Workshop, Miami, June 2008 IGS Reprocessed solutions : Origin & Scale

18. Workshop, Miami, June 2008 Some Origin & Scale after W1400

19. Workshop, Miami, June 2008 Impact of Orbit Parameters on TRF 4 weeks from ESOC: Default: 18 orbit parameters fixed Free: 9 CODE RPRs are free

20. Workshop, Miami, June 2008 GPS PDR Origin & Scale wrt ITRF2005

21. Workshop, Miami, June 2008 Repeatability – Internal Precision

22. Workshop, Miami, June 2008 IGS Reprocessing: Repeatability – Internal Precision ESA GFZ MIT PDR

23. Workshop, Miami, June 2008 IGS Reprocessing: Repeatability – Internal Precision Combined solutions

24. Workshop, Miami, June 2008 PDR vs IGS weekly WRMS PDRIGS

25. Workshop, Miami, June 2008 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