[ntp:questions] Time server question

Mike Cook mike.cook at orange.fr
Wed Jul 24 10:44:26 UTC 2019


> Le 24 juil. 2019 à 11:19, William Unruh <unruh at invalid.ca> a écrit :
>> 
>> The hardware under consideration can time the pulse arrivals more
>> precisely than the interrupt delivery time, thanks to special hardware.

That tickled a grey cell. There was/is a timing product family bc635/637 time and frequency processors sold by Microsemi which can timestamp a PPS input event  to 100ns resolution.  Various OS drivers are available, but no ntp refclock driver AFAIK. 

> 
> Does that hardware read the local clock of the computer, or its own
> internal clock, which then means you have to also figure out what the
> relation is between that hardware clock and the system time. 
> It also means that you have to be careful of termination resistances in
> the lines from the gps to that hardware and drive power from the clock. 
> Remember the "faster than light" neutrinos, which cam down to a bad
> fibre optics connection from the gps to the underground detector, making
> the underground clock sightly late, making it look like neutrinos got
> there faster than than they did.
> 
> The application of the corrections  should all get handled in an 
> ntp driver for the gps unit, which can
> apply the corrections and deliver the corrected readings to ntpd. ntpd
> has about 50 different refclock drivers and one might well cover your
> case. Otherwise one might need to be written.
> 
> 
>> 
>> Once that has been set up (in the future), the next problem becomes
>> applying the higher precision offset to the time source data input to
>> the ntp algorithms.
>> 
>> At a higher abstraction level this means telling ntp that "at
>> hhmmss.xxxxxxxxx (local clock), a time stamp of hhmmss.yyyyyyyyy
>> arrived from this hardware time source".
> 
> OK, that should work. The main problem is that usually that correction
> comes long (seconds) after the actual pulse itself as I understand. 
> 
>> 
>> 
>> 
>> Enjoy
>> 
>> Jakob
> 
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«  What’s the point? »
J.C.










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