Why does the igs care about eops
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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

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WHY DOES THE IGS CARE ABOUT EOPs?

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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-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


Why does the igs care about eops

  • IGS aims for ~1 cm orbit & ~1 mm terrestrial accuracies

    • to satisfy most demanding mm-level user application requirements


Why does the igs care about eops

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)


Ultra rapid ac orbit comparisons over 48 hr

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

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


Why does the igs care about eops

  • Orbit errors double when prediction interval increases by x4

  • IGA total err only ~40% worse than IGRs (but 175% worse for RZ)


Why does the igs care about eops

  • 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


Why does the igs care about eops

  • 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


Eop error sources

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

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


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