| Date: | 2005-09-16 23:40:00 | |
| Sender: | Jim Ray <Jim Ray (NGS 301-713-2850 x112) <jimr@ngs.noaa.gov>> | |
| Subject: | [SLR-Mail] No. 1378: new routine for high-frequency nutation in polar motion | |
| Author: | Jim Ray | |
| Content: | ******************************************************************************** SLR Electronic Mail 2005-09-16 23:40:00 UTC Message No. 1378 ******************************************************************************** Author: Jim Ray Subject: new routine for high-frequency nutation in polar motion 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 From: Jim Ray ”(NGS” 301-713-2850 ”x112)” ******************************************************************************** |