[time-nuts] TymServe 2100 1995 Issue - A Kludgy Fix

[Hal Murray]

Brian.Inglis at SystematicSw.ab.ca said:
> GPS provides only the current UTC offset from GPS time, which could be made
> available via a custom vendor message, or derived from the difference
> between messages which provide UTC and messages (e.g. $GPZDG) which provide
> GPS time.

I think it's more complicated than that.

The GPS satellites provide GPS time, the offset to UTC, and the the time of 
the next leap second.  (Obviously, the latter only works if one is scheduled 
and the folks who run the satellites have had time to update things.)

Some GPS receivers make that info available to the user.  Most of the low 
cost receivers default to NMEA which provides UTC and doesn't provide any 
leap-warning.

I think the GPS time via GPZDG is a proprietary hack, but maybe I didn't look 
hard enough.  NMEA wants $$$ for the official documents so google probably 
won't find any.  I didn't notice anything other than the NMEA driver in ntpd 
being able to process them.

Most of the proprietary/non-NMEA protocols provide the next-leap info.  My 
sample may be biased.  Mostly, I've been looking for the leap-scheduled bit 
which is what ntpd wants.  I think they also provide the when if you dig 
deeper, but I haven't been looking for that.


> Stratum 1 NTP servers need to be provided with a copy of the NIST leap
> second file and will propagate the warning to higher (numeric) stratum
> clients. 

I think ntpd will work without the file.  It may be simpler or less buggy 
with the file, but things should work without it.

Some GPS units provide a leap-scheduled flag.  I think ntpd will believe a 
refclock if it says leap-pending.

If you are stratum 1 and your refclock isn't smart enough to tell you about 
leap-pending, you can get the info from other servers.  I think it's 
something like a majority vote, but I'd have to look at the code and/or 
documentation to be sure.  It's the same logic as used by non-stratum-1 
servers.  (A long time ago, it used to believe just one server, but that got 
changed after a few broken servers caused a lot of trouble.)

You want a stratum-1 server to be watching other servers anyway as a sanity 
check.

It will be interesting to see how well things work.


-- 
These are my opinions.  I hate spam.