Workshop, Miami, June 2008

IGS Contribution to ITRF

Zuheir Altamimi & Xavier CollilieuxIGN, France

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

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 ?

Workshop, Miami, June 2008

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 CombinationITRF

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…

Workshop, Miami, June 2008

ITRF2005: Distribution and number of the IGS stations

Percentage of stations

X

Y

Z

Number of stations

Workshop, Miami, June 2008

ITRF2005:IGS weekly WRMS

Workshop, Miami, June 2008

Loading

Synthetic data

Loading effects on IGS network

WRMS - UP WRMS – 2D

Workshop, Miami, June 2008

LoadingLoading+Flicker Noise

Synthetic data

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

Loading effects on IGS network

WRMS - UP WRMS – 2D

Workshop, Miami, June 2008

Loading

Loading+Flicker Noise

Loading+Flicker Noise+Mes. Noise

Synthetic data

Loading effects on IGS network

WRMS - UP WRMS – 2D

Workshop, Miami, June 2008

Loading

Loading+Flicker Noise

Loading+Flicker Noise+Mes. Noise

IGS combined

Synthetic data Real data

Loading effects on IGS network

Workshop, Miami, June 2008

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

VLBI

SLR

GPS

East North Up

Workshop, Miami, June 2008

IGS weekly WRMS Week 1400

Workshop, Miami, June 2008

Week 1400 effect Week 1400

Workshop, Miami, June 2008

EOP constraints ? Ex from DORIS

DORIS IGNFree solution

DORIS LCAEOP Continuity conditions added

Polar motion differences wrt IGS

Workshop, Miami, June 2008

Network effect on EOPsChange the RF Core sites by 10 sites

Main differenceDrift:Xp: 2 µas/yrYp: 1 µas/yr

Workshop, Miami, June 2008

TRF Origin & Scale from GPS ?

Workshop, Miami, June 2008

IGS Reprocessed solutions : Origin & Scale

Workshop, Miami, June 2008

Some Origin & Scale after W1400

Workshop, Miami, June 2008

Impact of Orbit Parameters on TRF4 weeks from ESOC:Default: 18 orbit parameters fixedFree: 9 CODE RPRs are free

W1471 TX(mm) TY(mm) TZ(mm) Scale(ppb) Default -6.8 -0.3 -9.2 0.64 Free -7.0 0.2 4.1 0.54 W1472 Default -3.9 0.6 -5.2 0.41 Free -3.6 1.2 6.3 0.37 W1473 Default -6.9 0.6 -9.5 0.31 Free -7.7 1.8 6.0 0.27 W1474 Default -6.6 5.5 -9.3 0.43 Free -7.1 7.3 41.7 0.40

Workshop, Miami, June 2008

GPS PDR Origin & Scale wrt ITRF2005

Workshop, Miami, June 2008

Repeatability – Internal Precision

Workshop, Miami, June 2008

IGS Reprocessing: Repeatability – Internal Precision

ESA GFZ

MIT PDR

Workshop, Miami, June 2008

IGS Reprocessing: Repeatability – Internal PrecisionCombined solutions

Workshop, Miami, June 2008

PDR vs IGS weekly WRMS

PDRIGS

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