[time-nuts] Designing an embedded precision GPS time

Bob kb8tq kb8tq at n1k.org
Wed Nov 1 09:12:22 EDT 2017


> On Oct 31, 2017, at 11:33 PM, Leo Bodnar <leo at leobodnar.com> wrote:
>> From: Bob kb8tq <kb8tq at n1k.org>
>> Working all this back into a holdover spec in an unknown temperature environment is not at all easy.
>> Bob
> This is true, it is too easy to multiply figures from the datasheet and then start believing in them.
> We did extensive testing of real units in real life before committing to any specification figures.  They are based on statistical measurements followed by an expanded safety margin.
> Here is a typical holdover offset curve over 24 hours in non-DC environment (i.e. 5-10 degrees ambient temperature change during day/night period.)
> http://leobodnar.com/balloons/NTP/24hr-holdover.png <http://leobodnar.com/balloons/NTP/24hr-holdover.png>

Looking at the data, the DUT did some sort of discrete frequency shift around 4,000 seconds. The rest of the time it 
plodded along do nothing much ( = it was very stable). None of that is terribly unusual in terms of a holdover plot. 
The nasty question that always gets asked is “what if shift happened earlier?”. Depending on the test profile, that
may be unlikely or …. . Doing a lot of testing is about the only way to sort things out. 

> Time drift over 24h on this particular unit was below 0.7ms. This is pretty good for the device that consumes 1W of power (via PoE or USB) and fits in the pocket.
> I have used typical Raspberry Pi with a GPS add-on run-of-the-mill timeserver as suggested by Attila to monitor relative offsets - this is why reported timing is jittery and local (to RPi) 1PPS has an offset.
> It is really puzzling why holdover has suddenly come into focus.

This is TimeNuts ….

>  Due to NTP redundancy feature it is trivial to put several inexpensive time servers around the local or campus network and let clients do the standard NTP sanity checking and server selection.  And those building an NTP system able to cope with 24h+ global GPS outage know what they are doing anyway.

Based on some other posts, it appears that some of the applications are in *very* unusual environments. They are 
far more outage prone than one would normally expect. 


> Leo
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