Using VLBI EOP Time Series to Test Precession Models

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Using VLBI EOP Time Series to Test Precession
Models
Patrick Wallace (RAL, HMNAO) Nicole Capitaine
(Observatoire de Paris/Syrte)
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IAU 2000A precession-nutation

• IAU 2000A Nutation
• luni-solar terms (678) planetary terms (687)
  (Mathews et al. 2002)
• based on
• rigid Earth nutation (Souchay et al. 1999)
• MHB transfer function/VLBI data ? 7 basic
  Earth parameters (BEP)
• Precession component of the IAU 2000 model
• Lieske et al. 1977 improved precession rates
  
• (VLBI estimate of H and d?A (e0 84 381".
  448)
• d?A (IAU 2000) ( -0.299 650 0.000
  400 ) "/c
• d?A (IAU 2000) ( -0.025 240 0.000 100
  ) "/c
• Celestial pole offsets at J2000 (VLBI estimates)
• ?0 (IAU 2000) (-16.6170 0.0100) mas
  
• ?0 (IAU 2000) (-6.8192 0.0100) mas

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VLBI measurements of celestial pole offsets
VLBI - IAU 1976/1980 precession-nutation
VLBI - IAU 2000A precession-nutation
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Improved precession
New models compatible with IAU 2000 with improved
dynamical consistency - Bretagnon P., Fienga
A., Simon J.L. (2003, AA 400) BO3 -
Fukushima (2003, A.J. 126) F03 - Harada
Fukushima (2004, A.J. 127) HF04 ecliptic -
Capitaine N., Wallace P., Chapront J. (2003, AA
412) P03 IAU Division I WG on
Precession and the Ecliptic established at the
2003 General Assembly (Chair J. Hilton) to
recommend a new model
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The P03 model improvements w.r.t. IAU 2000

• Improvement of the ecliptic precession
• Ecliptic based on both VSOP87 and fit to DE406
  (2000-yr interval)
• Choice of the integration constants
• MHB precession rates in longitude and obliquity
  corrected for perturbing effects on the observed
  quantities
• Dynamical solution for the precession of the
  equator
• Integration of the dynamical precession
  equations
• Non-rigid Earth model (MHB2000, Williams 1994)

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P03 precession expressions
Ecliptic
basic quantities (mas)
Equator
derived quantities
classical paradigm
new paradigm
rotation vector
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Comparing VLBI series

• For the purpose of comparing the different time
  series, the choice of precession model is
  relatively unimportant.
• Here we use the P03 model as it is the one being
  recommended as the successor to the IAU 2000A
  precession.
• Different precession models return distinctly
  different linear fit coefficients, but in most
  cases the plots are at first sight
  indistinguishable.

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VLBI EOP time series

• The time series were all from
• http//hpiers.obspm.fr/iers/series/operatio
  nal
• The files with names ending in _r were used
• (e.g. gsfc_r.eop).

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Comparison procedure

• For each record in the selected VLBI time series
• Read JD(UTC), ??, ?e, s?? and s?e
• UTC ? TT
• Evaluate IAU 1976/80 precessionnutation
• Apply the ??, ?e nutation corrections
• Form the VLBI-determined NPB matrix
• Transform s??,s?einto sX,sY
• Evaluate the precession-nutation model under test
• Add modeled FCN
• Form the trial NPB matrix
• Subtract elements (3,1) of the two matrices to
  obtain ?X
• Subtract elements (3,2) of the two matrices to
  obtain ?Y
• Log t, ?X, ?Y, sX, sY for subsequent plotting
  and fitting

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The fits

• Fitting was weighted according to the published
  standard deviations.
• Linear fits gave sensible and useful results, but
  a longer time span will be needed before the t2
  term can be reliably determined.
• The results from different series disagree by
  rather more than the predicted uncertainties.

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Linear fits to different time series
VLBI - (P03 MHB 2000 FCN), µas, Jcy
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Interpretation of C04 results
C04 differs from the other series in that before
1984 a model is used rather than real
observations. These data were excluded from the
fits.
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The plots

• Disregarding the early era, the different time
  series agree well.
• However there are small offsets and tilts that do
  seem to be significant.
• There seem also to be unexplained low-frequency
  components in the residuals, common to all series.

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Residuals in X and Y using different series
?X ?Y