[ntp:questions] ESR looking for good GPS clocks

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Mar 6 16:00:17 UTC 2012


"Ron Frazier (NTP)" <timekeepingntplist at c3energy.com> wrote in message 
news:4F562BBF.7050401 at c3energy.com...
[]
> I haven't been following this thread extremely closely, but I did read 
> ESR's blog post and exchanged a few emails with him.  He says SIRF GPS's 
> exhibit a "wobble" of variance of outputting the NMEA time of 170 ms or 
> so.

Yes, there is a variation in the start of the NMEA data, which is why PPS 
is the recommended route to precision time keeping.  There are some 
measurements on the GPS 18x LVC here:

  http://www.satsignal.eu/ntp/Garmin-GSP18x-LVC-firmware-issue.htm
  http://www.satsignal.eu/ntp/Garmin-18x-3.7.png

which support the 170 ms value you mention, and this makes NMEA alone no 
better than Internet servers, and possibly worse.

> I've personally observed with my USB BU-353 this effect, where the 
> Meinberg Server Monitor shows my computer locked into the GPS time 
> within a few ms and a number of internet servers that I have programmed 
> will show offsets of say 50 ms.  However, when I originally set the 
> fudge factor, I had almost all the internet servers showing single digit 
> offsets when the PC is locked to the GPS time.  Sometimes, it shows a 
> positive offset for the internet servers and sometimes it shows 
> negative.  Anyway, ESR says SIRF GPS's aren't suitable for timekeeping. 
> I have not done anything to corroborate his statement, other than 
> observing this strange behavior.  I'm going to start a new thread 
> talking about what I've seen in more detail, since it's really a 
> different topic.  I just mentioned it since you brought up SIRF.
>
> Sincerely,
>
> Ron

The table here:

  http://www.satsignal.eu/ntp/Garmin-18x-3.7.png

lists two SiRF devices with a claimed 1 us accuracy.  PPS, not NMEA.

Cheers,
David 



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