Circular 3The Web site of the JCR has been opened. You are invited to visit it at http://www.bipm.fr/WG/CCTF/JCR/welcome.html. It does not allow access to the content of the documents circulated in the working group, except for the circulars, but it gives information on the existing documents and on the topics covered. It also lists the members and allows direct e- mail connection to those who asked for this option. Some practical questions on the documents produced by the JCR: For the distribution of documents with scientific notations, many members asked for Tex/Latex only, while none requested Word only. Therefore the documents will be distributed as Latex files, and will be available as postscript files or in paper upon request. JCR03.2. Received contributions I maintain a list of the contributions which are distributed to the whole group. I start by using a simple identification of the type aa.x where aa is the number of the latest circular and x a consecutive letter. In the fortunate case when there are more than 26 contributions following a circular, we'll adapt it.The complete updated list of contributions is in the Web site.
JCR03.3. Realization of barycentric coordinate times The document "Realization of barycentric coordinate times" (contributors G. Petit and P. Wolf) is distributed with this circular and will be referenced as document JCR.03.a in our list. Its conclusions are presented below for quick reference and are open to discussion, with a view of providing new definitions and/or guidelines that might be submitted to the Unions at a later stage. Usage of L_C, L_G and L_B: These constants were introduced (see IAU Resolution A4, 1991) to express the mean rates between time scales but were not properly defined so that confusion appeared in their usage. We propose to use as definitions \Delta TCG / \Delta TCB = 1 - L_C \Delta TT / \Delta TCG = 1 - L_G \Delta TT / \Delta TCB = 1 - L_B We propose however to recommend not to use the quantities L_C, L_G and L_B beyond the first post-Newtonian order where they were defined. Realization of TCB: All ephemerides should be provided with the necessary information to allow the time transformation to TCB. The preferred form is the usage of formula (15) of document JCR03.a which needs the computation of the integral known as "time ephemeris" [Fukushima 1995, Astron. Astroph 294, 895]. An alternative is to provide explicitly both the formula that was used to compute the time argument T_{eph} and the constant K_{eph} to relate T_{eph} to TCB: dT_{eph} / dTCB = 1-K_{eph}. Gauge convention: The choice of a gauge convention in the metric is one of the tasks (of the Working Group RCMA and the Joint Committee). For the time transformations, it is possible to retain gauge freedom within some uncertainty limits which have to be specified with the formulas. For example the TCG/TCB transformation may be expressed by formula (15) of document JCR03.a down to the uncertainty level of 3.10^{-18} in a class of gauges including the standard PN and harmonic ones. For smaller uncertainty it is necessary to specify the gauge condition used. All the effects discussed here are important only for time transformations with a rate uncertainty of less than 3.10^{-16}, which might be considered superfluous. However we must consider two issues that are the bottom line of the work of the Joint Committee on Relativity: Time will come when it will be necessary to consider such uncertainties in the comparison of "clocks" in the solar system or elsewhere. In publications we should use properly defined quantities and common conventions. Publications exist that consider a rate uncertainty of less than 3.10^{-16}, which already leads to confusion. JCR03.4. Comments and call for contributions The item of the extension of the metric is to be discussed between the Working Group RCMA and the Joint Committee, with the RCMA conducting the study. From the contributions received since Circular 1, it appears that there are strong opinions for retaining as much generality as possible in the definitions (such as not choosing one particular gauge condition, see documents 02.a, b, d, f). Nevertheless, for the applications, the chosen hypothesis and the domain of validity of the formulae should be completely specified. Such a work has been started for barycentric coordinate times (see section above) and your input will be appreciated. The subject "definition and realization of geocentric coordinate times (including Terrestrial Time TT)", which was mentioned in circular 2, is the next item being studied, and will be presented in a coming circular. Looking forward to your contributions (to be sent to jcr@bipm.org or gpetit@bipm.org), (to be sent to jcr@bipm.org or gpetit@bipm.org), Gerard Petit |