RE: Using non-PPS receiver for ntpd?

[Stein, Josh]

Thanks Greg,

  So in this instance (not using PPS), do I just ignore any mention of 
configuration when moving forward with the guide? Also just leaving out the PPS 
section of the ntp.conf?

e.g. do not enter the lines :

   # GPS PPS reference (NTP1)
   server 127.127.28.1 prefer
   fudge 127.127.28.1 refid PPS


--Josh
-----Original Message-----
From: Greg Troxel <gdt@lexort.com> 
Sent: Wednesday, June 9, 2021 9:16 AM
To: Stein, Josh <josh@anl.gov>
Cc: gpsd users <gpsd-users@nongnu.org>
Subject: Re: Using non-PPS receiver for ntpd?


"Stein, Josh" <josh@anl.gov> writes:

>   I currently only have access to a USB GPS receiver connected to an 
> rPi which does not supply PPS data. Despite the lack of PPS accuracy, 
> I would like to use this receiver as a clock source to the ntp daemon 
> running on that Pi. This Pi in turn will serve time to the other 
> devices on my setup giving me a relative synchronization of their 
> clocks. In this particular instance, I can tolerate some pretty 
> egregious jitter - on the order of tens of seconds.
>
> My first question is fundamental - given the discussion in the guide 
> here (https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html), is it 
> even feasible to use a non PPS receiver in this role?

Yes, it is entirely feasible.    The time synchronization will have two
accuracy problems:

  a delay of very roughly 100 ms

  variance in that delay

You can calibrate the delay by observing other systems and using fudge in the 
NTP config.

The resulting accuracy problems can be viewed as within specification or 
terrible, depending on your requirements.  The HOWTO tends to take the view 
that every microsecond must be right, or the apocalypse is at hand, and doesn't 
really address the (common IMHO) situation of people with non-PPS setups that 
would like time that is good to 100ms or maybe even
10 ms and works reliably without an internet connection.   My own view
is that time sync within 1s is vastly better than no time sync, even though I 
tend to want better.

I don't have my notes handy, but I ran a Truetime XL-DC in non-pps mode for 
years, and (separately) did calibrations for a few USB GPS (u-blox).
I remember delays in the 90-150 ms range.


If you don't care that your whole setup is 150 ms slow, you can just ignore 
that.  But if you have internet sometime, I'd advise adjusting fudge to make 
the local GPS-derived time line up with the Internet
sources.   Or just guess 100 ms, which is a better guess than 0.