GNSS Observations of Earth Orientation

1
GNSS Observations of Earth Orientation

• 1. Polar motion observability using GNSS
• concepts, complications, error sources
• subdaily considerations
• 2. Performance of IGS polar motion series
• compare Ultra-rapid, Rapid, Final products
• assess random systematic errors
• 3. Utility of IGS length-of-day (LOD)
• assess value for combinations with VLBI UT1
• 4. Impact of errors in subdaily EOP tide model
• effects on orbits, EOPs, other IGS products

Jim Ray, NOAA/NGS
Wuhan University, May 2013
2
IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012) IGS Core Product Lines (2012)
Series Series Series ID code ID code Latency Latency Issue times (UTC) Issue times (UTC) Issue times (UTC) Data spans (UTC) Data spans (UTC) Data spans (UTC) Remarks Remarks Remarks Remarks
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) IGU IGU real-time real-time _at_ 03, 09, 15, 21 _at_ 03, 09, 15, 21 _at_ 03, 09, 15, 21 24 hr _at_ 00, 06, 12, 18 24 hr _at_ 00, 06, 12, 18 24 hr _at_ 00, 06, 12, 18 ? for real-time apps ? GPS GLONASS ? issued with prior IGA ? for real-time apps ? GPS GLONASS ? issued with prior IGA ? for real-time apps ? GPS GLONASS ? issued with prior IGA ? for real-time apps ? GPS GLONASS ? issued with prior IGA

Ultra-Rapid (observed half) Ultra-Rapid (observed half) Ultra-Rapid (observed half) IGA IGA 3 - 9 hr 3 - 9 hr _at_ 03, 09, 15, 21 _at_ 03, 09, 15, 21 _at_ 03, 09, 15, 21 -24 hr _at_ 00, 06, 12, 18 -24 hr _at_ 00, 06, 12, 18 -24 hr _at_ 00, 06, 12, 18 ? for near real-time apps ? GPS GLONASS ? issued with following IGU ? for near real-time apps ? GPS GLONASS ? issued with following IGU ? for near real-time apps ? GPS GLONASS ? issued with following IGU ? for near real-time apps ? GPS GLONASS ? issued with following IGU

Rapid Rapid Rapid IGR IGR 17 - 41 hr 17 - 41 hr _at_ 17 daily _at_ 17 daily _at_ 17 daily 12 hr _at_ 12 12 hr _at_ 12 12 hr _at_ 12 ? for near-definitive, rapid apps ? GPS only ? for near-definitive, rapid apps ? GPS only ? for near-definitive, rapid apps ? GPS only ? for near-definitive, rapid apps ? GPS only

Final Final Final IGS IGS 12 - 18 d 12 - 18 d weekly each Thursday or Friday weekly each Thursday or Friday weekly each Thursday or Friday 12 hr _at_ 12 for 7 d 12 hr _at_ 12 for 7 d 12 hr _at_ 12 for 7 d ? for definitive apps ? GPS GLONASS ? for definitive apps ? GPS GLONASS ? for definitive apps ? GPS GLONASS ? for definitive apps ? GPS GLONASS

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IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011) IGS Core Product Accuracies (2011)
Series Series Series ID Product Types Product Types Product Types Product Types Product Types Accuracies Accuracies Accuracies Accuracies Output Intervals Output Intervals Output Intervals
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) IGU ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits 5 cm (1D) 5 cm (1D) 5 cm (1D) 5 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) IGU ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits 10 cm (1D) 10 cm (1D) 10 cm (1D) 10 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) IGU ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks 3 ns RMS / 1.5 ns Sdev 3 ns RMS / 1.5 ns Sdev 3 ns RMS / 1.5 ns Sdev 3 ns RMS / 1.5 ns Sdev 15 min 15 min 15 min
Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) Ultra-Rapid (predicted half) IGU ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD 250 µas / 50 µs 250 µas / 50 µs 250 µas / 50 µs 250 µas / 50 µs 6 hr 6 hr 6 hr

Ultra-Rapid (observed half) Ultra-Rapid (observed half) Ultra-Rapid (observed half) IGA ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits 3 cm (1D) 3 cm (1D) 3 cm (1D) 3 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (observed half) Ultra-Rapid (observed half) Ultra-Rapid (observed half) IGA ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits 5 cm (1D) 5 cm (1D) 5 cm (1D) 5 cm (1D) 15 min 15 min 15 min
Ultra-Rapid (observed half) Ultra-Rapid (observed half) Ultra-Rapid (observed half) IGA ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks ? GPS SV clocks 150 ps RMS / 50 ps Sdev 150 ps RMS / 50 ps Sdev 150 ps RMS / 50 ps Sdev 150 ps RMS / 50 ps Sdev 15 min 15 min 15 min
Ultra-Rapid (observed half) Ultra-Rapid (observed half) Ultra-Rapid (observed half) IGA ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD lt50 µas / 10 µs lt50 µas / 10 µs lt50 µas / 10 µs lt50 µas / 10 µs 6 hr 6 hr 6 hr

Rapid Rapid Rapid IGR ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits 2.5 cm (1D) 2.5 cm (1D) 2.5 cm (1D) 2.5 cm (1D) 15 min 15 min 15 min
Rapid Rapid Rapid IGR ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks 75 ps RMS / 25 ps Sdev 75 ps RMS / 25 ps Sdev 75 ps RMS / 25 ps Sdev 75 ps RMS / 25 ps Sdev 5 min 5 min 5 min
Rapid Rapid Rapid IGR ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD lt40 µas / 10 µs lt40 µas / 10 µs lt40 µas / 10 µs lt40 µas / 10 µs daily daily daily

Final Final Final IGS ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits ? GPS orbits lt2.5 cm (1D) lt2.5 cm (1D) lt2.5 cm (1D) lt2.5 cm (1D) 15 min 15 min 15 min
Final Final Final IGS ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits ? GLONASS orbits lt5 cm (1D) lt5 cm (1D) lt5 cm (1D) lt5 cm (1D) 15 min 15 min 15 min
Final Final Final IGS ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks ? GPS SV station clocks 75 ps RMS / 20 ps SDev 75 ps RMS / 20 ps SDev 75 ps RMS / 20 ps SDev 75 ps RMS / 20 ps SDev 30 s (SVs) 5 min 30 s (SVs) 5 min 30 s (SVs) 5 min
Final Final Final IGS ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD ? EOPs PM dLOD lt30 µas / 10 µs lt30 µas / 10 µs lt30 µas / 10 µs lt30 µas / 10 µs daily daily daily
Final Final Final IGS ? Terrestrial frames ? Terrestrial frames ? Terrestrial frames ? Terrestrial frames ? Terrestrial frames 2 mm NE / 5 mm U 2 mm NE / 5 mm U 2 mm NE / 5 mm U 2 mm NE / 5 mm U weekly weekly weekly

• IGS aims for 1 cm orbit 1 mm terrestrial
  accuracies
• to satisfy most demanding mm-level user
  application requirements

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Ultra-Rapid Products
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IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle IGS Ultra-Rapid Update Cycle
day 1 day 1 day 1 day 1 day 2 day 2 day 2 day 2 day 3 day 3
? ? ? ? ? ? ? ?

? ? ? ? ? ? ? ?

? ? ? ? ? ? ? ?

? ? ? ? ? ?

? ? ? ? ? ?

? ? ? ?

? ? ? ?

? ?

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00h 06h 12h 18h 00h 06h 12h 18h 00h 06h

24 hr of Observations 24 hr of Observations 24 hr of Observations ? ? Observed ERPs Observed ERPs
24 hr of Predictions 24 hr of Predictions 24 hr of Predictions ? ? Predicted ERPs Predicted ERPs

IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval

05
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Ultra-Rapid AC Orbit Comparisons (over 48 hr)

• Performance among ACs is bimodal widely
  dispersed
• SIO USN have been rejected for 5 years NGS
  WHU added recently
• AC quality is more uniform over first 6 hr of
  predictions
• biggest differences come from 6 24 hr
  predictions

06
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Some IGU AC Orbits Have Large Rotations
0.5 mas 64 mm error _at_ GPS hgt

• SIO, USN, NGS have had large Z rotations
• NGS recently improved
• CODE sometimes has moderately large Z rotations

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Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009)
DX DY DZ RX RY RZ SCL RMS WRMS MEDI TOTAL ERR
IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions
mean 3.5 -0.6 0.3 0.3 0.8 3.1 -0.7 28.9 21.3 15.6 41.7
std dev 4.7 4.9 3.4 13.8 16.3 27.2 2.6 19.7 8.0 2.6

IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions
mean 1.1 0.3 -0.1 -0.5 -0.6 -0.9 -1.3 64.7 47.3 30.2 80.2
std dev 1.8 2.0 3.8 21.9 31.2 52.0 1.9 33.3 16.3 6.0

IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations
mean 1.2 0.3 0.1 -0.2 0.9 2.6 -1.2 9.0 8.0 7.2 16.3
std dev 0.8 0.9 1.3 3.4 3.4 12.7 1.5 1.6 1.3 1.2

• Orbit errors double when prediction interval
  increases by x4
• IGA total err only 40 worse than IGRs (but 175
  worse for RZ)

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Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009)
DX DY DZ RX RY RZ SCL RMS WRMS MEDI TOTAL ERR
IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions
mean 3.5 -0.6 0.3 0.3 0.8 3.1 -0.7 28.9 21.3 15.6 41.7
std dev 4.7 4.9 3.4 13.8 16.3 27.2 2.6 19.7 8.0 2.6

IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions
mean 1.1 0.3 -0.1 -0.5 -0.6 -0.9 -1.3 64.7 47.3 30.2 80.2
std dev 1.8 2.0 3.8 21.9 31.2 52.0 1.9 33.3 16.3 6.0

IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations
mean 1.2 0.3 0.1 -0.2 0.9 2.6 -1.2 9.0 8.0 7.2 16.3
std dev 0.8 0.9 1.3 3.4 3.4 12.7 1.5 1.6 1.3 1.2

Z rotation errors are largest RT error from
UT1 prediction errors

• Largest RT orbit prediction error comes from UT1
  predictions
• IGA accuracy also limited by RZ rotations

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Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009)
DX DY DZ RX RY RZ SCL RMS WRMS MEDI TOTAL ERR
IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions IGU 6-hr predictions
mean 3.5 -0.6 0.3 0.3 0.8 3.1 -0.7 28.9 21.3 15.6 41.7
std dev 4.7 4.9 3.4 13.8 16.3 27.2 2.6 19.7 8.0 2.6

IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions IGU 24-hr predictions
mean 1.1 0.3 -0.1 -0.5 -0.6 -0.9 -1.3 64.7 47.3 30.2 80.2
std dev 1.8 2.0 3.8 21.9 31.2 52.0 1.9 33.3 16.3 6.0

IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations IGA observations
mean 1.2 0.3 0.1 -0.2 0.9 2.6 -1.2 9.0 8.0 7.2 16.3
std dev 0.8 0.9 1.3 3.4 3.4 12.7 1.5 1.6 1.3 1.2

due to modelling of orbit dynamics
large X, Y rotation errors from PM prediction
errors

• Next largest RT limits from orbit modelling PM
  prediction errors

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Ultra-rapid Observed Polar Motion Accuracy
(Ultra Observed Final) PM Differences

• IGU accuracy improved
• greatly after 2003
• IGU dPM RMS errors lt40 µas in recent years
• errors in IGS Final PM reference no longer
  negligible
• IGS PM errors
• have low-frequency systematic components

dPM-x dPM-y
Polar Motion Differences (µas)
improved . . . ?
even more . . . ?
(25 Mar 2000 31 Dec 2011 daily noon epochs
only)
Annual mean std dev of (IGU-IGS) dPM
dPM-x dPM-y
Mean Std Dev (µas)
Final PM-x sigma Final PM-y -sigma
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Spectra of (Ultra Observed-Final) PM Differences
dPM-x dPM-y
inter-annual power is largest
(1461 d from 1 Jan 2008 31 Dec 2011)

• Subdaily tide model alias errors seen at 7.0
  14.19 d periods
• 7th GPS draconitic peak also strong in polar
  motion rates
• 3rd GPS draconitic peak also prominent in IGS
  orbit discontinuities
• note that differencing should remove common-mode
  errors!

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Ultra-rapid Predicted Polar Motion Accuracy
(Ultra Predicted Final) PM Differences
dPM-x dPM-y

• IGU PM prediction accuracy unchanged since 2006
• recent IGU
• 1-d RMS prediction
• errors
• 270 µas for PM-x
• 210 µas for PM-y
• PM prediction errors appear more random than
• systematic

Polar Motion Differences (µas)
(8 Nov 2006 31 Dec 2011 daily noon epochs
only)
Annual mean std dev of (IGU-IGS) dPM
dPM-x dPM-y
Mean Std Dev (µas)
Final PM-x sigma Final PM-y -sigma
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Spectra of (Ultra Predicted-Final) PM Differences
dPM-x dPM-y
(1880 d from 8 Nov 2006 31 Dec 2011)

• Subdaily tide model alias errors probably
  dominate sub-monthly band perhaps annual, but
  no distinct lines
• draconitic errors probably important in between

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Ultra-rapid Observed dLOD Accuracy
(Ultra Observed Final) dLOD Differences

• IGU accuracy improved
• steadily after 2002
• IGU dLOD RMS errors lt12 µs in most recent years
• errors in IGS Final dLOD reference not
  negligible
• IGS dLOD errors
• have high-frequency systematic components

Length-of-Day Differences (µs)
improved . . . ?
(25 Mar 2000 31 Dec 2011 daily noon epochs
only)
Annual mean std dev of (IGU-IGS) dLOD
Mean Std Dev (µs)
Final dLOD sigma
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Spectra of (Ultra Observed-Final) dLOD Differences
(1461 d from 1 Jan 2008 31 Dec 2011)

• Subdaily tide model alias errors seen at 7, 9,
  14 d bands
• long-period errors muted by calibration of AC
  LOD biases via comparison with IERS Bulletin A
  over sliding window of recent past results

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Ultra-rapid Predicted dLOD Accuracy
(Ultra Predicted Final) dLOD Differences

• IGU dLOD prediction accuracy slightly improved
• since 2006
• recent IGU
• 1-d dLOD prediction
• error 50 µs
• dLOD prediction errors have evident
• systematic signatures

Length-of-Day Differences (µs)
(8 Nov 2006 31 Dec 2011 daily noon epochs
only)
Annual mean std dev of (IGU-IGS) dPM
Mean Std Dev (µas)
Final dLODsigma
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Spectra of (Ultra Predicted-Final) dLOD
Differences
(1880 d from 8 Nov 2006 31 Dec 2011)

• Subdaily tide model alias errors seen at 9 14
  d bands
• plus strong 8th GPS draconitic semi-annual
  peaks

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IGS ERP Predictions Compared to Other Services

• IGS Ultra ERP predictions for 1 d after last
  observations compared to operational EOP services
• IGU ERPs issued 9 hr before prediction epoch
• predictions benefit from access to latest
  high-accuracy observations
• Compare to IERS (USNO Paris Obs) JPL EOP
  services
• results from IERS EOP Combination of Predictions
  Pilot Project (18.03.2012)
• IGU PM predictions better than any others
• IGU comparable to UT1/dLOD services due to their
  use of AAM predictions
• EOP prediction services should consider
  assimilating IGU predictions, as well as most
  recent IGU observations

Prediction Service RMS(PM-x) (µas) RMS(PM-y) (µas) RMS(dLOD) (µs)
Ultra (Pred) 270 211 48 (dLOD)
USNO/Bull A 442 290 55 (UT1)
Paris Obs 522 - 609 337 - 438 334 414 (UT1)
JPL 562 423 55 (UT1)
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Recent Ultra-Rapid ERP Accuracy

• IGA observed EOPs updated every 6 hr
• latency is 15 hr for each update
• each EOP value is integrated over 24 hr
• recent polar motion accuracy lt50 µas (1.5
  mm)
• recent dLOD accuracy lt12 µs (5.6 mm/day)
• reported formal errors are generally reliable
• IGU predicted EOPs updated every 6 hr
• for real-time applications
• issued 9 hr before EOP epoch
• recent polar motion prediction accuracy 250
  µas (7.7 mm)
• recent dLOD prediction accuracy 50 µs (23
  mm/day)
• reported formal errors are too optimistic by a
  factor of 3 to 4
• IGU PM predictions better than use IERS service
• IGU dLOD predictions similar to operational
  services
• IGU ERP observations predictions should be
  assimilated by operational EOP prediction services

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Rapid Products
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Rapid AC Orbit Comparisons

• Orbit performance dispersion among ACs is
  reasonable
• but ESA clearly dominates combination

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IGS (Rapid Final) Polar Motion Differences
gt98 RF sites ?
? lt55 RF sites

• Clear improvement in PM accuracy when IGb00
  reference frame adopted in 2004
• but systematic differences remain dominate
• probably mostly due to Analysis Center rotational
  deficiencies

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Spectra of (Rapid-Final) PM Differences
PM-x PM-y
(1024 d from Sep. 2006 Jul. 2009)

• High-frequency noise consistent with 30 µas
  accuracy recently
• but longer period errors are most significant
• fortnightly feature near 14.2 d signifies
  subdaily tide model errors

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Rapid AC LOD Comparisons with Final LOD

• EMR, JPL, SIO show strong annual LOD variations
• most other ACs show long-period variations
• similar features in Final LODs

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Final Products
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Final AC Orbit Comparisons

• Final AC orbit performance similar to Rapids
• Rapid orbits are consistently significantly
  better than any single AC Final

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A,C,R Spectra of IGS Orbit Day-Jumps
odd GPS draconitic harmonics likely
fortnightly signals
(1024 d from Mar. 2005 Dec. 2007)

• Jumps computed from Berne-model fit to adjacent
  orbit days
• stacked over all SVs lightly smoothed
• calibrated for errors due to (fit
  extrapolation) method
• Background errors follow flicker noise on
  seasonal time scales
• transition to whiter noise for lt14 d

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Along-track Spectra of AC Orbit Day-Jumps
smoothing effect of CODE 3-d arcs
(1024 d from Mar. 2005 Dec. 2007)

• AC along-track spectra show mostly flicker
  white noise
• Some AC peaks but good agreement only for
  fortnightly

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Cross-track Spectra of AC Orbit Day-Jumps
fortnightly band
3rd draconitic harmonic for most ACs
(1024 d from Mar. 2005 Dec. 2007)

• AC cross-track spectra show 3rd draconitic
  fortnightly bands
• Some spurious AC peaks lower white noise floor

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Orbit/PM Rotational Inconsistencies

• AC orbit PM rotational offsets should be
  self-consistent
• but orbit rotations show larger dispersion for
  all ACs
• most ACs show internal rotational inconsistencies
• part of problem was caused by IGS combination bug
  (fixed wk 1702)
• IGS orbit accuracy probably limited by such
  rotational effects

31
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Compute Polar Motion Discontinuities
midnight PM discontinuities
daily noon PM offset rate estimates

• Examine PM day-boundary discontinuities for IGS
  time series
• should be non-zero due to PM excitation
  measurement errors

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Power Spectra of IGS PM Discontinuities
PM-x PM-y
IGS Repro1 Combination (10 Mar 2005 29 Dec 2007)

• Common peaks seen in most AC spectra are
• annual 5th 7th harmonics of GPS year (351
  d or 1.040 cpy)
• probably aliased errors of subdaily EOP tide
  model (IERS2003)

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Spectra of Subdaily EOP Tide Model Differences
PM-x PM-y

• Compare TPXO7.1 IERS2003 (used by IGS) EOP
  models
• TPXO7.1 GOT4.7 test models kindly provided by
  Richard Ray
• assume subdaily EOP model differences expressed
  fully in IGS PM results

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Spectra of PM Discontinuities Subdaily EOPs
effects of orbit model coupling
HFEOP (J. Gipson) - IERS2003

• Aliasing of subdaily EOP tide model errors
  probably explains
• annual (K1, P1, T2), 14.2 d (O1), 9.4 d (Q1,
  N2), 7.2 d (s1, 2Q1, 2N2, µ2)
• Orbit errors presumably responsible for odd 1.04
  cpy harmonics

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3 Cornered Hat Decomposition of ERP Errors

• 3 cornered hat method is sensitive to
  uncorrelated, random errors
• for time series i, j, k form time series of
  differences (i-j), (j-k), (i-k)
• then Var(i-j) Var (i) Var(j) (assuming
  Rij 0 for i ? j)
• and Var(i) Var(i-j) Var(i-k) Var(j-k) /
  2
• but true errors also include common-mode effects
  removed in differencing
• Apply to IGS Ultra (observed), Rapid, Final PM
  dLOD
• consider recent 1461 d from 1 Jan 2008 to 31 Dec
  2011
• Surprising results
• apparently, Rapids give best polar motion
  Ultras give best dLOD
• Ultras give similar quality polar motion as
  Finals
• perhaps Finals affected by simultaneously solving
  for weekly TRFs

IGS Product Series s(PM-x) (µas) s(PM-y) (µas) s(dLOD) (µs)
Ultra (Obs) 25.8 27.6 4.99
Rapid 16.0 15.4 5.69
Final 25.3 31.3 9.19
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3 Cornered Hat PM Results with High-Pass Filtering

• Apply Vondrak high-pass filter before 3 cornered
  hat for PM
• test 4 cutoff frequencies pass all, gt0.5
  cpy, gt1 cpy, gt2 cpy
• results below from Paul Rebischung (IGN)
• IGU IGR PM errors nearly insensitive to
  frequency filtering
• IGS Final PM appears to improve when high-pass
  filtered
• implies low-frequency errors are in IGS Finals or
  common to IGU IGR
• source of low-frequency error (orbits?, frame?)
  not yet identified
• but internal Analysis Center inconsistencies
  strongly suspected

Freq Cutoff none none 0.5 cpy 0.5 cpy 1 cpy 1 cpy 2 cpy 2 cpy
(µas) sx sy sx sy sx sy sx sy
Ultra (Obs) 25.8 27.6 24.2 25.5 24.1 23.7 23.7 22.5
Rapid 16.0 15.4 16.2 14.6 15.6 16.1 15.2 16.8
Final 25.3 31.3 20.2 23.1 19.4 19.7 18.5 17.3
(filtered results from Paul Rebischung, IGN)
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Conclusions

• Since 2004.0 IGS Final polar motion accuracy lt30
  µas
• robust global network is prime factor
• Rapid PM is only slightly poorer, lt40 µas
• GPS PM nearing asymptotic limit for random errors
  (20 µas)
• smaller systematic errors possible with new
  GNSSs, better orbit modeling, better handling
  of solution constraints
• new subdaily EOP tide model required prospects
  currently unclear
• IGS Ultra-rapid observed PM accuracy currently
  lt50 µas
• updated 4 times daily with 15 hr latency
• could be used to provide some subdaily EOP
  resolution
• should be more heavily used by EOP prediction
  services !
• Leading error sources are systematic
• internal rotational inconsistencies by Analysis
  Centers
• errors in IERS subdaily EOP tide model alias into
  all IGS products

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