[ntp:questions] does this make sense?

William Unruh unruh at invalid.ca
Thu Apr 5 17:50:17 UTC 2018

On 2018-04-05, Terje Mathisen <terje.mathisen at tmsw.no> wrote:
> Maria Iano wrote:
>> Thanks for your reply, David. Accuracy to within milliseconds is fine
>> for us. We currently have four old GPS appliances in four data
>> centers that we are replacing, and my thought is that some vendor
>> diversity would be good.
>> We are only staying in two of those data centers, so that was where I
>> was thinking of using GPS as we already have a cable to an antenna to
>> the roof. We have a third data center where we currently don't have
>> any ntp appliance, and that's where I was thinking it would be
>> convenient to put a WWVB device. But we do have access to the roof,
>> and from what you say it sounds like we would be better off putting a
>> GPS using all three services there.
>> We have a network of 12 stratum 2 servers that get time from our
>> appliances, and most of our infrastructure points to those, so maybe
>> a fourth stratum 1 server is overkill. Does this make more sense:
>> three GPS appliances using all three services at the three data
>> centers?
> I've designed similar setups a couple of times: Since we're located in 
> Europe my backup was the German radio transmitter since that one uses 
> spread spectrum modulation of the carrier, enabling 10-15 us precision.

That is about 2 miles light travel time. I suspect you are further than that
from the tranmitter. And propagation is all in the atmosphere, and the speed
of radio  varies in the atmosphere, depending on the pressure, the humidity,
clouds,.... Ie, I am doubting the 10-15us. Maybe 10-15 ms.?

> With a modern multi-system GNSS receiver and multiple geographical 
> locations I would probably be happy with a very good local clock (at 
> least TCXO, maybe a second-hand Rb?) to provide holdover and using 
> multiple internet sources as a sanity check.
> Terje

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