[ntp:questions] project ntp.br
nobody at nowhere.invalid
Tue Oct 9 12:37:11 UTC 2007
Antonio M. Moreiras <moreiras at nic.br> 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 :
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
More information about the questions