[ntp:questions] ESR looking for good GPS clocks

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 6 20:08:29 UTC 2012

On 3/6/2012 11:00 AM, David J Taylor wrote:
> "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 looked at that chart 3 times and even zoomed in on it.  It looks like 
to me that the vertical scale is in ns.  So, the variation in start time 
for the NMEA sentence is about 170 ns if I'm reading it right, not ms.  
In that case, it's about 2/10 of a us.  This would have little impact 
compared to the 170 ms of possible variation we're discussing.

>> 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

I think that may be the wrong link.  I can't find any devices listed there.

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


(PS - If you email me and don't get a quick response, don't be concerned.
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Ron Frazier
timekeepingdude AT c3energy.com

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