WHY DOES THE IGS CARE ABOUT EOPs?

1. 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-Rapidproductsverywidelyused for many • demanding real-time applications • e.g., very rapid tropo water vapor soundings for meteo models • & natural hazards monitoring • Ultra-Rapidproductqualitydepends on EOP • predictionaccuracy • latest observed orbits projected into future with EOP predictions • EOP prediction errors limit accuracy of IGS real-time orbits WHY DOES THE IGS CARE ABOUT EOPs? Jim Ray IGS Analysis Center Coordinator NOAA/National Geodetic Survey NGA Future EOP Prediction Workshop, Springfield, VA, 17 November 2011

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

5. Rotational Transform: Observed EOPs(t) Observed orbit: Crust-fixed frame Observed orbit: Inertial frame 1) • 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 + Projected orbit: Inertial frame Observed orbit: Inertial frame 2) Rotational Transform: Observed + Predicted EOPs(t) Observed + Projected orbit: Crust-fixed frame 3)

6. 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

7. Some IGU AC Orbits Have Large Rotations • 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 0.5 mas = 64 mm error @ GPS hgt

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

9. 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

10. 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

11. 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 ! 

12. 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