[IGSMAIL-7129] Re: Updates to the IERS Conventions (2010)

[Gerard Petit]

Author : Gérard Petit and Brian Luzum (IERS Conventions center)

Dear colleagues,

In reference to IGSmail-7125, a reply by Jim Ray to IGSmail-7112, we wish
to clarify a few points.

Indeed, as Jim noted, there is no difference in the goal of the new IERS
Conventional Mean Pole 2015 with respect to the  CMP (2010). Changes in
the wording of section 7.1.4 (first paragraph after equation 24) were
intended to more adequately describe the goal (maybe the new text was not
successful in this), but not to indicate a change of paradigm.

So the short answer is that the goal of the CMP(2015) is still to remove
the effects of annual + Chandler and therefore to leave longer-term
inter-annual deformations in the stations' coordinates. There is no change
with respect to 2010 or even 2003.

However in 2003 it was implicitly thought that a linear CMP was sufficient
for that purpose.

In 2010 it was clear that inter-annual effects were significant at the
chosen accuracy level (10 mas), but it was thought that a cubic+linear CMP
would be sufficient. Problems with the assumption became clear a few years
later.

Whether this goal (remove the effect of annual+Chandler only) is the right
one is another question that nobody clearly answered so far. The last
added paragraph was an attempt to address this question. At this moment,
it is beyond the scope of the present Conventions center to answer. But we
expect that directions will arise from scientific contributions.

Regarding the three items mentioned by Jim for the model change:

(1) the model change is clearly stated in the text of 7.1.4.

For (2) and (3) it all draws from the fact that, as mentioned in 2010 and
in the present text, the CMP aims at representing the “actual mean pole”
to within 10 mas (i.e. 0.3 mm equivalent radial displacement).

For (2) , the sentence “the motion of the mean pole is not likely to be
predictable (see e.g., Chen, et al. 2013) at the level required for the
pole tide correction” (i.e. 10 mas) could be complemented to state that
CMP(2010) failed in this respect for the X component of the pole sometime
around 2012.

For (3), the effect of the change on users' results is indeed not
explicitly quantified. However the magnitude of the effect is clearly
stated at 33 mm/as radial, 9 mm/as horizontal.



----------------------------------------
Gérard Petit
Time department, BIPM
Address: Pavillon de Breteuil
         92312 Sèvres  FRANCE
Phone: (33)1-45077067
FAX: (33)1-45077059
E-mail:  <mailto:gpetit at bipm.org> gpetit at bipm.org

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.igs.org/pipermail/igsmail/attachments/20150723/76aaab37/attachment.html>