Re: [gpsd-dev] GPSD's assumptions about time

[Eric S. Raymond]

Hal Murray <address@hidden>:
> >> There was some discussion in the ntp lists a month or two ago about a NMEA 
> >> device that was broken and going to need this logic.  One of them might 
> make
> >> a handy test case.  I'll try to track down the details if you are 
> interested.
> 
> > I am.
> 
> It started on the time-nuts list:
>   [time-nuts] Warped back to 1993
>   http://www.febo.com/pipermail/time-nuts/2013-August/079197.html
> The time-nuts archive stuff often breaks threads into several chunks.  If you 
> want the whole thing, go back to the Thread index when you get to the end.  
> Look down a bit to pick up the next chunk.
> 
> Then it went over to ntp-questions:
>   [ntp:questions] Start of new GPS 1024 week epoch
>   http://lists.ntp.org/pipermail/questions/2013-August/035889.html
> Lots of noise here.  It's a long thread.
> 
> The problem has been observed in
>   FURUNO GT-74, GT-77, and GT-80
>   Truetime XL-DC (some, not all)
> 
> Furuno had a web page:
>   http://www.furuno.co.jp/en/news/notice/20130214_001.html
> 
> Somewhere in there I saw a comment about using the leap second offset to get 
> the right epoch.  Sounds like a neat hack.  That would give you 20 years from 
> the time you built the leap-second table.

OK, I've read the threads. Here are my thoughts.

First: Hal, thank you for the excellent explanation of why GPSes have trouble
finding sats on a cold boot. I did not know about the role of Doppler shift!
I have incorporated this into the FAQ.  You continue to be exceptionally good
at expository writing, an uncommon skill in our peer group.

(Harlan, scratch my gloomy prognostication of a few weeks ago.  Hal could
probably do about as good a job of rewriting the NTP docs as I could. Not
that I'm volunteering him...)

Second, I actually looked into trying to back the era out of the leap-second
offset a couple of years back.  It can't be made reliable enough.  Discussion:

http://esr.ibiblio.org/?p=2869
http://esr.ibiblio.org/?p=2882

Third, I now think a brutally simple solution to the GPS-era problem
will work acceptably, and is in any case better than what we have.
I will describe it in my next post.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>