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

unruh unruh at invalid.ca
Sat Sep 7 23:45:46 UTC 2013

On 2013-09-07, Horvath Bob-BHORVAT1 <Bob.Horvath at motorolasolutions.com> wrote:
> -----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 unruh
> Sent: Saturday, September 07, 2013 11:20 AM
> To: questions at lists.ntp.org
> Subject: Re: GPS/PPS and "enable calibrate"
> On 2013-09-07, Charles Elliott <elliott.ch at verizon.net> wrote:
>> Or use PPS and be accurate to  a microsecond or so.
> 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. 

Teh PPS occurs at the second. The nmea labels those seconds so you know
which second the PPS occured on. But the nmea sentences occur late by
something like .1 to .6 (or occasionally with a defective GPS 18x, by
1.1 sec) Now you can tell your computer that the nmea occurs say .4 sec
late. so that its idea of when the nmea sentece comes through is not on
the second but on the second plus .4 sec. It does not really matter,
because as long as that labeling is withing 1 sec, ntpd will use PPS to
determine when the second occured, that the nmea to say which second
that was. 
Ie, if you use PPS that fudging does nothing about the time your
computer shows. So unless your GPS is way out, that parameter is
If your PPS is not working, then you make your clock accurate to about
.1 sec by using that parameter, rather than .4 sec late.

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