WHY DOES THE IGS CARE ABOUT EOPs? Summary of core products of the International GNSS Service (IGS) Ultra-Rapid (real-time), Rapid, & Final series outputs: orbits, polar motion/LOD, clocks, & station positions Ultra-Rapid products very widely used for many demanding real-time applications e.g., very rapid tropo water vapor soundings for meteo models & natural hazards monitoring Ultra-Rapid product quality depends on EOP prediction accuracy latest observed orbits projected into future with EOP predictions EOP prediction errors limit accuracy of IGS real-time orbits Jim Ray IGS Analysis Center Coordinator NOAA/National Geodetic Survey NGA Future EOP Prediction Workshop, Springfield, VA, 17 November 2011

IGS Core Product Series (2011) ID Latency Issue times (UTC) Data spans Remarks Ultra-Rapid (predicted half) IGU real-time @ 03:00, 09:00, 15:00, 21:00 +24 hr @ 00:00, 06:00, 12:00, 18:00 ● for real-time apps ● GPS & GLONASS ● issued with prior IGA (observed half) IGA 3 - 9 hr 03:00, 09:00, 15:00, 21:00 -24 hr @ ● for near real-time apps ● issued with following Rapid IGR 17 - 41 hr 17:00 daily ±12 hr @ 12:00 ● for near-definitive, rapid apps ● GPS only Final IGS 11 - 17 d weekly each Thursday 12:00 for 7 d ● for definitive apps

IGS Ultra-Rapid Update Cycle day 1 day 2 day 3 ● 00h 06h 12h 18h = 24 hr of observations = observed EOPs = 24 hr of predictions = predicted EOPs IGU updates every 6 hr are always 3 hr after the beginning of each prediction interval

IGS Core Product Accuracies (2011) Series ID Product Types Accuracies Output Intervals Ultra-Rapid (predicted half) IGU ● GPS orbits ~ 5 cm (1D) 15 min ● GLONASS orbits ~10 cm (1D) ● GPS SV clocks ~3 ns RMS / ~1.5 ns Sdev ● EOPs: PM + LOD ~250 µas / ~50 µs 6 hr (observed half) IGA ~ 3 cm (1D) ~5 cm (1D) ~150 ps RMS / ~50 ps Sdev <50 µas / ~10 µs Rapid IGR ~2.5 cm (1D) ● GPS SV & station clocks ~75 ps RMS / ~25 ps Sdev 5 min <40 µas / ~10 µs daily Final IGS <2.5 cm (1D) <5 cm (1D) ~75 ps RMS / ~20 ps SDev 30 s (SVs) + 5 min <30 µas / ~10 µs ● Terrestrial frames ~2 mm N&E / ~5 mm U weekly IGS aims for ~1 cm orbit & ~1 mm terrestrial accuracies to satisfy most demanding mm-level user application requirements

Errors in obs EOPs ~ cancel out in forward/reverse transforms Rotational Transform: Observed EOPs(t) Observed orbit: Crust-fixed frame Observed orbit: Inertial frame 1) + Projected orbit: Inertial frame Observed orbit: Inertial frame 2) Rotational Transform: Observed + Predicted EOPs(t) Observed + Projected orbit: Crust-fixed frame 3) Errors in obs EOPs ~ cancel out in forward/reverse transforms but EOP prediction errors fully embedded in crust-fixed orbit predictions typical prediction errors: ~0.4 mas/d for PM; 0.1 ms/d = 1.5 mas/d for UT1 0.1 ms = 1.5 mas = 4.6 cm @ Earth = 19.4 cm @ GPS

Ultra-Rapid AC Orbit Comparisons (over 48 hr) Performance among Analysis Centers has become bimodal SIO & USNO have been excluded for >2 year AC quality is more uniform over first 6 hr of orbit predictions biggest differences occur for 6 – 24 hr orbit predictions

Some IGU AC Orbits Have Large Rotations 0.5 mas = 64 mm error @ GPS hgt SIO & USNO have large Z rotational errors; also Y CODE sometimes also has moderately large Z rotations these AC rotations probably from poor orbit modeling, not EOP predictions

Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) DX DY DZ RX RY RZ SCL RMS WRMS MEDI TOTAL ERR IGU 6-hr predictions: mean 3.5 -0.6 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 IGU 24-hr predictions: 1.1 -0.1 -0.5 -0.9 -1.3 64.7 47.3 30.2 80.2 1.8 2.0 3.8 21.9 31.2 52.0 1.9 33.3 6.0 IGA observations: 1.2 0.1 -0.2 0.9 -1.2 9.0 7.2 1.3 12.7 1.5 1.6 Orbit errors double when prediction interval increases by x4 IGA total err only ~40% worse than IGRs (but 175% worse for RZ)

Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) DX DY DZ RX RY RZ SCL RMS WRMS MEDI TOTAL ERR IGU 6-hr predictions: mean 3.5 -0.6 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 IGU 24-hr predictions: 1.1 -0.1 -0.5 -0.9 -1.3 64.7 47.3 30.2 80.2 1.8 2.0 3.8 21.9 31.2 52.0 1.9 33.3 6.0 IGA observations: 1.2 0.1 -0.2 0.9 -1.2 9.0 7.2 1.3 12.7 1.5 1.6 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

Ultra-Rapid Orbit Diffs (mm) wrt IGR (2009) DX DY DZ RX RY RZ SCL RMS WRMS MEDI TOTAL ERR IGU 6-hr predictions: mean 3.5 -0.6 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 IGU 24-hr predictions: 1.1 -0.1 -0.5 -0.9 -1.3 64.7 47.3 30.2 80.2 1.8 2.0 3.8 21.9 31.2 52.0 1.9 33.3 6.0 IGA observations: 1.2 0.1 -0.2 0.9 -1.2 9.0 7.2 1.3 12.7 1.5 1.6 due to modelling of orbit dynamics large X, Y rotation errors – from PM prediction errors Next largest RT limits from orbit modelling (solar radiation pressure effects) & PM prediction errors

 Multi-technique EOP combinations mostly sub-optimal !  EOP Error Sources Station-related measurements: thermal noise instrumentation propagation delays multipath, etc σStation ≈ 1/√NStation Geophysical & parameter models: esp near S1, K1, K2 tidal periods AAM/OAM errors Source-related errors: orbit dynamics (GPS, SLR, DORIS) quasar structures (VLBI) σSource ≈ 1/√NSource σEOP = + + Possible improvements: new subdaily EOP tide model ? better handling of parameter constraints ? modern theory of Earth rotation ? more robust SLR, VLBI networks ? more stable site installations ? near asymptotic limit for GPS already new GNSS constellations better GNSS orbit models ? quasar structure models (VLBI) ?  Multi-technique EOP combinations mostly sub-optimal ! 

Conclusions Generally, IGA/IGU near- & real-time orbits & EOPs are of very high quality could use more & better input Analysis Center solutions Rotations are leading real-time orbit error due to UT1 & PM prediction errors used for IGU orbits models for orbit dynamics also add some rotational errors for some ACs EOP services could better use IGU products provide updates at least 4 times daily seek better input AAM + OAM predictions improve combination algorithms present IERS predictions generally not adequate for IGS requirements IGS ACs generate better 1-day PM predictions internally from their own latest measurements; we cannot do that for UT1 though Better model for subdaily tidal EOP variations also needed errors of IERS model alias into GPS orbit parameters