[time-nuts] Help in Locking a Windows Computer to GPS Time

David J Taylor david-taylor at blueyonder.co.uk
Thu Aug 22 02:45:24 EDT 2013


I am trying to lock a Windows XP computer to GPS time taking advantage of
both the NEMA sentence and the 1PPS with the hope of getting to within a few
ms.

I am using a Garmin GPS 18PC and the NMEATime program.  When I tick the box
to implement the 1PPS feature on NMEATime the program locks up each time it
attempts to correct the PC time.  Perhaps there is something I need to do to
configure the GPS 18PC to fix this.

I would be grateful for advice as to whether and how one can use NMEATime
for this purpose with a Garmin GPS 18 PC or advice on other programs to
achieve accurate locking of the PC.

Rex VK7MO
======================================

Rex,

Does the GPS 18PC even have a PPS output?  A module which does is this one:

  http://www.adafruit.com/products/746

and there are lots of others.

With Windows-8 and a bit of luck, you don't even need the PPS to get within 
a millisecond - see:

  http://www.satsignal.eu/mrtg/bergen_ntp_2.html

and that's over a Wi-Fi connection!  More typically, though, using PPS will 
get you within a millisecond = how much depends on the version of Windows 
you are using:

  http://www.satsignal.eu/mrtg/performance_ntp.php#windows-stratum-1

(PC Feenix is currently playing up, hence the glitches)  PC Alta is 
Windows-7, and PC Stamsund Windows-8.  I also have an XP PC whose graphs are 
not so regularly updated:

  http://www.satsignal.eu/mrtg/Old-Feenix/feenix_ntp_2.html

using a ublox NEO 6M module from China.

Rather than NMEATime, which uses just a small subset of the capabilities on 
NTP, use the standard NTP software from Meinberg, installed as per my 
instructions here:

  http://www.satsignal.eu/ntp/setup.html

Once that is done, and you have the serial GPS recognised (its performance 
won't be that good), add the PPS support with the special driver, as 
documented here:

  http://www.satsignal.eu/ntp/NTP-on-Windows-Vista.html

Performance of a Windows-7 system using that kernel-mode driver is here:

  http://www.satsignal.eu/mrtg/alta_ntp_2.html

Averaged jitter of under 35 microseconds.  The Windows-8 system (where the 
more precise timestamp instructions are available in the OS) has jitter 
under 20 microseconds.

73,
David GM8ARV
-- 
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk 



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