From jimr at ngs.noaa.gov Fri Sep 16 06:28:10 2005
From: jimr at ngs.noaa.gov (Jim Ray (NGS 301-713-2850 x112))
Date: Fri, 16 Sep 2005 09:28:10 -0400 (EDT)
Subject: [IGSMAIL-5211]: new routine for high-frequency nutation in
polar motion
Message-ID: <200509161328.JAA06277@ness.ngs.noaa.gov>
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IGS Electronic Mail 16 Sep 06:31:47 PDT 2005 Message Number 5211
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Author: Jim Ray
Dear Analysis Colleagues,
A new Fortran routine, written by Aleksander Brzezinski, has been posted
which implements the recommendations of section 5.4.2 of IERS Conventions
2003 concerning "high-frequency nutation in polar motion". The code can
be found at the Conventions Update site at:
ftp://tai.bipm.org/iers/convupdt/chapter5/PMsdnut.for
Background
----------
In Resolution B1.7 (2000) adopted at its 24th General Assembly, the IAU
redefined the celestial pole to eliminate ambiguities in the distinction
between "high-frequency nutation" and polar motion. Effects previously
regarded as nutations with periods less than two days (viewed from the
celestial frame) are to be considered using a model for the corresponding
polar motion. The forced nutations due to the lunisolar torque on the
triaxial Earth, having prograde diurnal and prograde semidiurnal terms, are
therefore now regarded as polar motion effects. The prograde diurnal
nutations correspond to long-period polar motions and are thus already
contained in the measured daily values; no changes are required for these.
The prograde semidiurnal nutations correspond to prograde diurnal polar
motions (the 10 terms in Table 5.1 of IERS Conventions 2003 with periods
near 1 day); these should be accounted for in data analysis in the same way
as the subdaily EOP tidal variations.
The original version of the Conventions did not contain any routine to
implement this recommendation. Aleksander Brzezinski has kindly written
such a routine, which is now being made available. This program was
numerically compared to an independent coding by Christian Bizouard and
found to agree to <0.0005 mas.
Magnitude of Effect
-------------------
The magnitude of the high-frequency nutation contributions (computed hourly
over the year 2005) are:
mean RMS min value max value
----- ------ --------- ---------
dX_nut 0.006 17.705 -43.993 44.061 microarcseconds
dY_nut -0.010 17.715 -44.168 43.676 microarcseconds
While small, this effect is significant compared to the size of the polar
motion formal errors reported by the IGS. In order to minimize any
impact on nutation offset estimates, this correction should also be applied
in VLBI data analyses.
Implementation
--------------
The diurnal components of these variations (namely, the 10 terms listed in
Table 5.1 with periods near 1 day, which are included in PMsdnut.for) should
be considered similarly to the diurnal and semidiurnal polar motion
variations due to ocean tides (namely, the effects computed by the subroutine
ortho_eop.f which accompanies Chapter 8 of the IERS Conventions 2003). They
should not be part of the polar motion values reported to the IERS or
distributed by the IERS, and should therefore be added after interpolation
in the data analysis.
The long-periodic terms, as well as the secular variation (the remaining
terms in Table 5.1), are already contained in the observed polar motion
values and need not be added to the reported values.
The technique coordinators may wish to synchronize any analysis changes
among the various groups, although it is possible that some have already
implemented their own codings of this recommendation.
For further information see the IERS Convention Center website at:
http://tai.bipm.org/iers/
--Jim Ray
IERS Conventions Advisory Board