[ntp:questions] ESR looking for good GPS clocks

Rob nomail at example.com
Tue Mar 6 17:27:14 UTC 2012


Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> 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.  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.  

This is not correct.  The SiRF itself does not show this wobble.  If
it would, it would also show a position inaccuracy of 170ms * c meters,
and it would not be usable as a GPS.

What is showing this wobble is the output of NMEA sentences.
NMEA is unsuitable for timekeeping, because the time in the sentence
is not the current time, but the time at which the GPS last calculated
a position fix.  The time delay between position calculations and NMEA
output is varying, because these two functions are carried out by separate
tasks in the multitasking OS that is running on the processor of the
GPS device, and there is no tight synchronization between them.

All of this is not determined by the GPS device, but by the method it
is being (ab)used in the setup, i.e. by using NMEA to convey timing.

SiRF chips should be used in SiRF binary mode.  ESR should know that.



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