[time-nuts] Adafruit Ultimate GPS timing message arrival times
kb8tq at n1k.org
Mon Aug 1 12:48:14 EDT 2016
The issue with any of these approaches is how long it will take to converge.
If I start with a pps that is good to 10 ns and my goal is 10 ns or 10 ppb, I’m there in a second.
If I start with a serial string that is good to 10ms and my goal is 10 ppb, I’m waiting for a
million seconds per sample.
If I want to get an Rb to 1x10^-12, I need to wait for 10,000 seconds per sample with the pps.
To do the same thing with the serial string is 10 billion seconds for each independent sample.
The serial string is fine for an "eyeball clock". It’s not so fine for a GPSDO.
> On Aug 1, 2016, at 12:33 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 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
>> 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.
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> Chris Albertson
> Redondo Beach, California
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts