[time-nuts] Adafruit Ultimate GPS timing message arrival times

Chris Albertson albertson.chris at gmail.com
Mon Aug 1 12:33:17 EDT 2016

You are right.  NTP, even over a poor internet connection can
typically do better then the tens milliseconds we see with some NEMA

But you eyes and human perfection is still even worse.  You can't
notice 40mS of error.

In fact that would be a good experiment:  Put two clocks up on a large
computer monitor and make one always tick some random number of
milliseconds away from system time and the other always thick on the
system time.  Then you click on the one you think is correct.  Can you
do better than a 50/50 guess. Keep incl=reasing the error until the
guesses are about 90% correct.   I bet you find you eyes are really
bad.   You ears are a little better and you might notice 40 mS by
listening to the "tick" sound

Another thought experiment is to show that the randomness of NMEA
jitter does not matter would be to try and build a GPSDO that uses
NMEA data.  If you averaged over a long enough time it would work.
Might be a way to set a Rb oscillator?

One reason NTP works so well even over poor Internet connections is
that it can use 5, 7 or even more other NTP servers to get the time
and all it needs is that a few of them are good.   I typically use
five pool servers when I set up NTP

On Mon, Aug 1, 2016 at 12:01 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> david-taylor at blueyonder.co.uk said:
>> From your data and my own measurements, I feel that using the serial NMEA
>> stream would, today, be a last resort, as an Internet sync would be
>> considerably better.  Would you agree with that?
> Depends on your internet connection and/or the specific GPS module you are
> using.
> I don't know of any good GPS modules that use NMEA.  I do know of really
> crappy internet connections.  Bufferbloat is the buzzword.
> --
> These are my opinions.  I hate spam.
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Chris Albertson
Redondo Beach, California

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