[ntp:questions] ESR looking for good GPS clocks

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 6 20:59:59 UTC 2012


On 3/6/2012 3:28 PM, Rob wrote:
> Ron Frazier (NTP)<timekeepingntplist at c3energy.com>  wrote:
>    
>> 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.
>>      
> Look again.  You can find an example m and n in the word Garmin in the
> top caption.
>
>    
Gulp.  OK, I guess I have to eat some crow for that.  I apologize for 
the error.  Weirdest looking "m" I've ever seen.  But, now that you 
mention it, it does appear that the vertical scale is in milliseconds.  
So, what you and David are saying is that there is a 170 ms variance in 
the start time of the NMEA sentences for the Garmin.  Does it use a SIRF 
chipset?  Is this variance intrinsic to all GPS's?

I looked in the manual for the Garmin and for the Trimble Resolution T.  
All they say is that the NMEA is referenced to the PPS, but they don't 
give a spec for the delta between them.  Of course, my GPS has no PPS, 
but the NMEA output has to be referenced to something!

Sincerely,

Ron


-- 

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Ron Frazier
timekeepingdude AT c3energy.com



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