[ntp:questions] GPS/PPS and "enable calibrate"

David Taylor david-taylor at blueyonder.co.uk.invalid
Sun Sep 8 04:26:33 UTC 2013

On 07/09/2013 21:30, Horvath Bob-BHORVAT1 wrote:
> I guess this is the fundamental question I don't understand.
> I thought with PPS, which I do have using the Adafruit GPS and a Raspberry Pi, I would be fairly accurate off of the GPS time and PPS together.   Then I started seeing fudge parameters of 0.496 which seem to work well, but seem like a large amount of time to be "fudging" for something that is supposed to already be accurate.  That is in seconds, right? So it is about a half a second?  I am confused what that parameter does, what it needs to be set to, and how one figures out what to set it to, or whether I am just really confused.

Yes, the PPS should be "microsecond level" accurate, when configured and 
working (you see a "o" as the tally code from ntpq -pn).  However, the 
PPS signal itself is just a transition on voltage - there is no 
information to tell NTP /which/ second has just started, that is what is 
provided by the other servers or by the serial NMEA data.  Provided that 
serial data is less than one second delayed from the PPS second 
transition, there is no problem.  Setting an offset for the serial NMEA 
data is almost optional, especially if you also have other NTP servers 
listed which will get NTP right to better than a second.

Unusually, some GPS receivers (with poorly programmed firmware) have a 
delay just around or just exceeding one second, which makes 
identification of the exact second problematical without the correct 
delay factor being added to the configuration.  There is also the 
possibility that serial NMEA data is being used without PPS, as it is 
the best source available, and calibration would be important in that 
case.  Serial alone can be worse than just using Internet servers, so 
it's a situation to avoid if possible.
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