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

Horvath Bob-BHORVAT1 Bob.Horvath at motorolasolutions.com
Sun Sep 8 22:32:17 UTC 2013

>-----Original Message-----
>From: questions-bounces+bob.horvath=motorolasolutions.com at lists.ntp.org [mailto:questions->bounces+bob.horvath=motorolasolutions.com at lists.ntp.org] On Behalf Of David Taylor
>Sent: Saturday, September 07, 2013 11:27 PM
>To: questions at lists.ntp.org
>Subject: Re: [ntp:questions] GPS/PPS and "enable calibrate"
>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.

Thanks to you and all the others in this thread.  

I think what originally got me confused was I originally had my PPS wired wrong, so I WAS using the GPS time signal which was off compared to the servers from the NTP pool and various values got me close.   Then once I fixed the PPS, I didn't realize the fudge factor wasn't really contributing any more, and was focused on getting it "right".  

Assuming I finally get it, the only scenario in which it would be useful to get it "right" would be if I lost PPS, and had no other source of "time of day" with an accurate second other than the NMEA strings. 

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