[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!
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
timekeepingdude AT c3energy.com
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