From gpetit at bipm.org Thu Jul 23 10:23:39 2015 From: gpetit at bipm.org (Gerard Petit) Date: Thu, 23 Jul 2015 19:23:39 +0200 (CEST) Subject: [IGSMAIL-7129] Re: Updates to the IERS Conventions (2010) Message-ID: <01a901d0c56c$509cd660$f1d68320$@bipm.org> Author : G?rard 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. ---------------------------------------- G?rard Petit Time department, BIPM Address: Pavillon de Breteuil 92312 S?vres FRANCE Phone: (33)1-45077067 FAX: (33)1-45077059 E-mail: gpetit at bipm.org -------------- next part -------------- An HTML attachment was scrubbed... URL: https://igscb.jpl.nasa.gov/pipermail/igsmail/attachments/20150723/76aaab37/attachment-0001.html