why does the igs care about eops
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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

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

slide4

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

    • to satisfy most demanding mm-level user application requirements
slide5

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

slide8

Orbit errors double when prediction interval increases by x4

  • IGA total err only ~40% worse than IGRs (but 175% worse for RZ)
slide9

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
slide10

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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