[ntp:questions] project ntp.br

François Meyer 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 :

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




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