[ntp:questions] garmin 18x and linux

Mike S mikes at flatsurface.com
Sat Sep 10 01:53:58 UTC 2011

At 06:38 PM 9/9/2011, unruh wrote...

>The MT12+ also apparently has a delay of a few 
>10s of ns in sending out
>the pulse. Are these uncertainties? If yo udo 
>not know about them, they
>are. If you know about them you can subtract out 
>the systematic errors,
>but you cannot do that for the random errors.
>Note that this means that the Oncore is far far 
>from being within 2ns of
>the "truth" (ie UTC) More like 30ns.

What's the basis for that claim? Sounds like you 
didn't set the cable delay appropriately, if at 
all. What were you comparing against?

When properly corrected for, the PPS offset 
pretty much stays within +- 10 ns of UTC.

This paper pretty well summarizes the accuracy 
which an M12+ can provide, and they were 
comparing directly with UTC(USNO):

According to this presentation: 
ftp://ivscc.gsfc.nasa.gov/pub/TOW/tow2011/notebook/Hambly.Sem.pdf ,

"All the M12+ receivers, including the 4 
receivers in the 2002 test, appear to agree with 
UTC(USNO) to better than ±10 nsec." Of course, 
that would vary depending on UTC(USNO)-UTC(GPS) 
at any given time. But, that can only be corrected after the fact.

But, they also say that "The M12Ms show a bias 
errors up to ~30 nsec as compared with our 'Gold' 
reference Motorola receiver," so maybe you're 
confusing the M12+ with the M12M.

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