[ntp:questions] garmin 18x and linux

Chris Albertson albertson.chris at gmail.com
Fri Sep 9 18:30:42 UTC 2011


>> What we are talking about when we say "accuracy" is the amount of uncertainty.
>
> No we mean how closely the gps absolute time agrees with the utc
> absolute time (modulo leap seconds).

Accuracy is always expressed as an uncertainty.  For example I might
say  "the voltage is 6.32V plus or minus my meter's 2% error"  This
means I'm un-certain of the true voltage in the wire but think the
value I gave is within 2% of "truth"

When we buy a UT+ and read that it has 55nS accuracy what that is
saying is roughly that about 2/3 of the time the true UTC time will be
no more than 55nS from what the GPS says.    In other words Motorola
claims there will be an error that you can't know but does tell you
the statistical properties of the error.    I used the word
"uncertainty" to describe an known error.

Again this is all pretty much moot for NTP.  Once your GPS has an
error of less than a microsecond you are as good as "perfect" for NTP.

Chris Albertson
Redondo Beach, California



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