[time-nuts] Thunderbolt GPS TimeKeeper

David J Taylor david-taylor at blueyonder.co.uk
Tue Jan 24 03:23:42 EST 2012


> So back to time.   If the goal is keeping good time then it is best
> not to use Microsoft Windows.  There are good technical reasons having
> to do with the way MS Win. keeps and adjusts time.  The bottom line is
> that you will never be able to do better then about the millisecond
> level even with a directly attached GPS.

I agree that Windows is not the best OS for timekeeping, but with a 
directly attached GPS/PPS device I consistently see better than the 
millisecond level with Windows.  Please see:

  http://www.satsignal.eu/mrtg/performance_ntp.php

PCs Feenix (XP), Stamsund (Win-7/32), Alta (Win-7/64) and Bacchus (Win 
2000) demonstrate this.  All are connected to either GPS 18, GPS18x, or 
Sure GPS devices.  There is also interpolation code available allowing you 
to read time more precisely than 1 ms.

  http://www.lochan.org/2005/keith-cl/useful/win32time.html

> Using another OS, BSD or
> Linux you can do almost three orders of magnitude better.  (Three
> orders is huge.)  The OS and software is free and all you need is any
> computer that has a physical serial port, not USB but a real DB9
> connector.  This is a good use for a 10 year old notebook PC.  (The
> computational load is trivial so even a 486 class computer is OK)
>
> If you run BSD or a newer Linux then Pin-1 on the DB9 can be used for
> time interval measurement at almost 1 uSec precision.   You can use
> this for many things, NTP being just one of them.   If you have
> multiple DB9 type serial ports then you can time tag multiple channels
> to about a uSec or two. (certainly 3E-6 level)
>
> Then if the Windows PCs need correct time they could use the server
> running on the old BSD based notebook PC.  Zero added cost assuming
> you can find and old computer.  Then you also have a tool that can
> time tag any pulse at the uSec level.
>
>
> Chris Albertson
> Redondo Beach, California

My own experience suggests that you need rather more than a PC running a 
good timekeeping OS - such as FreeBSD.  Take a look at PC Pixie:

  http://www.satsignal.eu/mrtg/performance_ntp.php

This is an Intel Atom box running FreeBSD 8.0, and yet in an open domestic 
environment there are reported excursions of approaching 10 microseconds 
when the heating is switched on in the morning.  So put that PC in some 
sort of thermally-controlled enclosure if you want better than the 10 
microsecond level.  The GPS puck is outside, by the way, unaffected by the 
heating switch-on.

Comparing PCs Pixie and Feenix suggests that the FreeBSD system is only 
two orders of magnitude better than Windows - still a great improvement, 
of course!

Cheers,
David 





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