Antonio M. Moreiras <[EMAIL PROTECTED]> wrote: > François Meyer escreveu: > > > For real time purposes (as NTP), formally this is > > UTC(ONRJ), TA(ONSP) and TA(ONBR) (just formal, no > > practical consequences if only NTP is concerned). > > Could you indicate some sources (books, websites) where I could learn > more about that? We are writing some documentation in portuguese and I > would like to get all the formalities correctly handled.
You might want to browse the time section ftp server of the BIPM at : http://www.bipm.org/jsp/en/TimeFtp.jsp The publication section will give you access to a wide panel of data (Circular T's, UTC-UTC(k), UTC- GPSTime and so on ; note that there is no UTC(GPS), but a scale called "GPS time" that is steered to within +- 10 ns of UTC(USNO), though official specifications are something like 1 us). That might give you an ensemble view. From a formal point of view, UTC(X) is a physical realization of the paper clock UTC. In order to deserve its UTC labelling, the clock(s) involved in the realization of a UTC(x) is(are) supposed to contribute to TAI via the BIPM and UTC(x) is supposed to be maintained within +-100 ns of UTC (or at least that there is an involvement of the institute x to fulfill these specifications). Circular T's show that UTC(ONRJ) hax no particular concern regarding this. Though it might seem easy to achieve, maintaining a UTC realization within these bounds is not an easy matter since UTC(x) is supposed to be a real time realization of a time scale that will only be available 1 month later. This generally involves a reasonable number of individual clocks and careful steering. Just to emphasize that the UTC(x) naming is not to be used lightly in a formal context. TA(k) (atomic time) is a more generic name that does not bear the involvement of a UTC(x) (though TA(x) may have a specific meaning in some countries as you can see in the section 2 of the circular T : TA(F) for example is a paper clock with an optimized frequency stability at a few days, while UTC(OP) is the real-time french realization of UTC. Circular T also shows there is a TA(ONRJ) that significantly differs from UTC(ONRJ) (by about 1.5 us) ; a foot note in circular T 222 reads : "(1) ONRJ: TA(ONRJ) is an independent local atomic time scale computed by ONRJ." Anyway, the point here is that TA(ONSP) and TA(ONBR) may well designate the time scale realized by any of the atomic clocks present on these sites. >> >> I cant see why you need a Rb clock here if UTC(ONRJ) >> (or a slightly degraded version in the case of the >> secondary observatories) is available locally. >> > > There are issues regarding the ONRJ Internet connections. Because of > this, the primary and secondary ntp servers are in other site (althought > it is also in Rio de Janeiro). Ok. That does not simplify things. Formally you cannot rely on GPS Time as a source of traceability to UTC(ONRJ). In the international GPS comparisons used by the BIPM, GPS is just an intermediate ; the obvious solution is a 'CGGTTS aware' GPS receiver that can output files following the BIPM schedule ; this will give you the possibility to continuously monitor your Rb clocks vs UTC(ONRJ) and to maintain the difference in a given interval of say +- 100 us). The need for yearly calibration with a clock travel or something similar disappears and after two years the investment in the GPS receivers is wiped out at the cost of some additionnal procedures to ensure both proper monitoring of the systems and data processing. -- François Meyer _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
