[ntp:questions] project ntp.br - discrepancy from UTC

Antonio M. Moreiras antonio at moreiras.eng.br
Fri Oct 5 00:28:32 UTC 2007

The Cesium clock at observatorio nacional (ON) is UTC. In fact, the ON
is one of the metrology laboratories that colaborates with the Bureau
International des Poids et Measures (BIPM) in generating the UTC (as
NIST does in USA, for example).

The Rubidium clocks are synchronized with the UTC at least one time per
year and the manufacturer says that the Rubidum reference has a monthly 
aging less than 5e-11 and a yearly aging less than 5e-10.

It gives us a 16ms maximum discrepancy from UTC (31,536,000,000 ms/year 
* 5e-10 * 1 year = 15.765ms - is this correct?)

After one year, with the discrepancy at 16ms it will be probabily of the 
same order than the half round trip time for the majority of the clients.

Given this, do you you think it will be necessary any modification?

If so, what would be the maximum discrepancy allowed to not need any 
modifications? As the primary servers are appliances from Symmetrycom or 
Spectracom probabily it will be very difficult to get customized 
firmware versions... Maybe we should study the possibility of 
synchronize the Rubidium reference clocks more frequently with UTC.

I don´t know if I correctly understood what this discrepancy can cause. 
If a client is using other sources attached directly to GPSs, for 
example, there is a risk that our servers being considered falsetickers. 
Is it? Or is there other problems?

A discrepancy of 16ms in a Internet NTP primary server is acceptable?

In Brazil time stamps should be less than 100ms accurate to be legaly 
valid (for financial or government institutions, for example). I think 
it is the same in other parts of the world. Then, 16ms appears to 
reasonable for an Internet service.

The majority of the Internet NTP primary servers are GPS based. For 
curiosity: what is the discrepancy of GPS time from UTC (without 
considering the leap seconds)?

Antonio M. Moreiras.

David Woolley escreveu:
> In article <1191513268_6661 at sp12lax.superfeed.net>,
> Antonio M. Moreiras <antonio at moreiras.eng.br> wrote:
>>                 |(periodically assures the
>>                 | accuracy with the official
>>                 | brazilian time - that is
>>                 | in last instance UTC)
>>                 #
>>        ** Rubidium clock **
> How far is this allowed to get from UTC?  If the maximum discrepancy isn't
> very small compared with the combination of "precision" and half minimum
> round trip time to a client, and given this is a high profile server, you
> should modify the source code to add the maximum error into the root 
> dispersion calculation.
> If you don't do this, someone using your source and something more 
> directly tied to UTC may find that the error bounds don't overlap, and
> one gets thrown out.

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