IGS Contribution to ITRF

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IGS Contribution to ITRF

• Zuheir Altamimi Xavier Collilieux
• IGN, France

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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

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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 ?

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Current ITRF Derivation
Step 1
S1 S2
S3
Sn

VLBI
TRF (X, V) EOP (SINEX)
W1
W2
Wn

SLR
TRF (X, V) EOP (SINEX)
Stacking
W1
W2
Wn

GPS
TRF (X, V) EOP (SINEX)
W1
W2
Wn

DORIS
TRF (X, V) EOP (SINEX)
Combination ITRF
Local Ties
Step 2
TRF (X, V) EOP (SINEX)
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ITRF2005 Distribution and number of the IGS
stations
X
Percentage of stations
Y
Z
Number of stations
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ITRF2005IGS weekly WRMS
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Loading effects on IGS network
WRMS 2D
WRMS - UP
Synthetic data
Loading
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Loading effects on IGS network
WRMS - UP
WRMS 2D
Synthetic data
Loading
LoadingFlicker Noise
According to Williams et al., 2004 variance
values, rescaled by 60
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Loading effects on IGS network
WRMS - UP
WRMS 2D
Synthetic data
Loading
LoadingFlicker Noise
LoadingFlicker NoiseMes. Noise
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Loading effects on IGS network
Synthetic data
Real data
Loading
IGS combined
LoadingFlicker Noise
LoadingFlicker NoiseMes. Noise
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Network effect evaluated using a loading model
(Tonie van Dam)Comparison with IVS and ILRS
network
East
North
Up
VLBI
SLR
GPS
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IGS weekly WRMS
Week 1400
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Week 1400 effect
Week 1400
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EOP constraints ? Ex from DORIS
Polar motion differences wrt IGS
DORIS IGN Free solution
DORIS LCA EOP Continuity conditions added
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Network effect on EOPsChange the RF Core sites
by 10 sites
Main difference Drift Xp 2 µas/yr Yp 1 µas/yr
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TRF Origin Scale from GPS ?
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IGS Reprocessed solutions Origin Scale
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Some Origin Scale after W1400
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Impact of Orbit Parameters on TRF
4 weeks from ESOC Default 18 orbit parameters
fixed Free 9 CODE RPRs are free
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GPS PDR Origin Scale wrt ITRF2005
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Repeatability Internal Precision
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IGS Reprocessing Repeatability Internal
Precision
ESA
GFZ
MIT
PDR
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IGS Reprocessing Repeatability Internal
PrecisionCombined solutions
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PDR vs IGS weekly WRMS
PDR
IGS
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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