[ntp:questions] NTP server with Garmin GPS 18 LVC

unruh unruh at invalid.ca
Thu Dec 19 18:55:40 UTC 2013

On 2013-12-19, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> On 18/12/2013 19:59, Adrian P wrote:

> What accuracy do you need?  I'm after millisecond level on Windows PCs, 
> which can be achieved with a PPS source, and should be even easier to 
> achieve in Windows 8.1 (compared to 7 or earlier).  To sync a PC to the 
> full accuracy which a time-keeping GPS can deliver would take 
> considerable effort.  It would be interesting to know whether anyone has 
> actually achieved that, and if so, how!

Certainly Linux and FreeBSD are capable of microsecond, not millisecond,
accuracy on modern equipment. For better than that, I think you would
need special interrupt or timestamping hardware to get around the
interrupt latency and variability of linux.
While averaging the fluctuations, as chrony does, could deliver sub
microsecond uncertainty, it is not clear you can get that accuracy due
to average latency (Ie, if the timestamp on the interrupts is on average
1us late, all the averaging in the world will not get rid of that.--
That was the kind of figures I got when I timestamped a parallel port
output pin transition and timestamped the parallel port interrupt that
that pin was tied to-- 1-2us latency. Now that was many years ago, but I
doubt that things have gotten better-- no reason for them to get better.
Noone else really wants that kind of low latency interrupts, and
interrupt conflicts cause much more variation than that. 

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