[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
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:
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.
The table here:
lists two SiRF devices with a claimed 1 us accuracy. PPS, not NMEA.
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