[ntp:questions] ESR looking for good GPS clocks

Richard B. Gilbert rgilbert88 at comcast.net
Tue Mar 6 21:51:50 UTC 2012

On 3/6/2012 3:59 PM, Ron Frazier (NTP) wrote:
> 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

It would appear that your GPS receiver is NOT a timing receiver.

Any GPS receiver knows what time it is.  Not all receivers are equipped
to output the time in any usable form.  A timing receiver generally has
a Pulse Per Second (PPS) output.  One edge marks the start of a second.
See your manual to determine WHICH edge, rising or falling, is used.

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