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

David L. Mills mills at udel.edu
Tue Oct 9 18:29:13 UTC 2007


Guys,

To be accurate, there are two national timescales in the US, UTC(USNO) 
kept by the US Naval Observatory in Washington, DC, and UTC(NIST), kept 
by the National Institute of Standards and Technology in Boulder, CO. I 
am told the holy grail is to discipline these timescales with each other 
and other standards laboratories within one nanosecond. Twenty years ago 
the grail was one microsecond and may still be in some parts of the world.

With an ageing rate of 5e-11, the residual error after one day is 4.3 
microseconds, somewhat more than would ordinarily be expected of a 
national timescale. Our dedicated public NTP primary servers here 
typically keep within this nominal offset and jitter relative to a GPS 
with PPS.

Other timescales derived from UTC(USNO) include UTC(LORAN) and GPS. GPS 
does not run on UTC and has no leap seconds. It runs on International 
Atomic Time (TAI) with a 5-s constant offset. However, what you see in 
your GPS receiver is UTC as corrected by the GPS navigation message.

Dave

Danny Mayer wrote:

> 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 =5.765ms - 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.
> 
> Danny
> 
> 
>>[]s
>>Antonio M. Moreiras.
> 
> 




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