On 3/6/2012 3:59 PM, Ron Frazier (NTP) wrote:
On 3/6/2012 3:28 PM, Rob wrote:
Ron Frazier (NTP)<timekeepingntpl...@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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