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

Danny Mayer mayer at ntp.isc.org
Tue Oct 9 11:39:33 UTC 2007

Antonio M. Moreiras wrote:
> 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)?

Be aware that GPS is run by US satellites and their clocks are
synchronized to US UTC provided by the US Government agency NIST. I have
no idea how this may differ from that provided by the Braziian
Government ot BIPM for that matter.


> []s
> Antonio M. Moreiras.

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