Richard Laager via devel <devel@ntpsec.org>:
> I have (and was/am using) a clock using the Spectracom driver, with the
> two-digit year. It recently suffered from GPS rollover, so I disabled
> the GPS source in ntpd. I'm using the PPS source with network sources
> (which I was using anyway) to number the seconds. This works well. I
> didn't really care about the GPS source anyway. So if you were to remove
> the driver, I wouldn't care.
>
> That said, if we wanted to do a better job in that driver, the following
> pseudocode seems like it would produce the right results:
>
> We get a two-digit year and a day-of-year number. There is no
> month/date, which I think is used in the NMEA driver to detect and
> correct GPS rollover.
>
> ----
> // Calculate the year assuming the clock works (no GPS rollover).
> full_year = year + 2000;
> while (1) {
> date = get_some_date_thing(full_year, day_of_year);
> if (date > BUILD_EPOCH)
> break;
> full_year += 100;
> }
>
> // Calculate the year assuming GPS rollover
> if (year < 90) { // or some similar fixed value
> full_year = year + 2000;
> } else {
> full_year = year + 1900;
> }
> date2 = get_some_date_thing(full_year, day_of_year);
> while(1)
> {
> date2 = get_some_date_thing(full_year, day_of_year);
> if (date2 > BUILD_EPOCH)
> break;
> date2 = date2 + 1024 weeks;
> }
>
> if (date2 < date)
> date = date2;
>
> // Use date
> ----
>
> When my clock was working, it was outputting "18". Both approaches would
> output the same thing, which is correct. This would work until 2090.
>
> Once my clock's GPS rolled over, it started saying 99. Today, it is
> outputting "99 174". The first block would make that 2099 (as would the
> current code). The second block would get the correct value. Note that
> they will always be past the build epoch, so we just take the lower one
> to find the one closer to the build epoch, which here is the right date.
Please file this as an RFE, with some explanation of what get_some_date_thing()
needs to be doing - I can't quite get it from this.
--
<a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>
My work is funded by the Internet Civil Engineering Institute: https://icei.org
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