[time-nuts] Motorola M12+ or M12M questions

M. Warner Losh imp at bsdimp.com
Tue Jul 7 14:56:25 UTC 2009


In message: <4A533C36.1030608 at rubidium.dyndns.org>
            Magnus Danielson <magnus at rubidium.dyndns.org> writes:
: There are (at least) three interpretations of when leap-seconds may be 
: inserted:
: 1) At the end of every month.

This is the ITU standard.  It says that leap seconds can be inserted
at the end of each month.  Almost no gear allows this, and the gear
that does doesn't always do it in a sane way.

: 2) At the end of every quarter.

This is also ITU standard.  These are the secondary times.  They have
never been used.

: 3) At the end of every half-year.

These are the primary times.

: GPS is operated under the assumption that case 3 holds.

Why is this the case?  The GPS data just tells you which week the next
leap second will happen, as well as the last time a leap second
happened.  How is it the case that you can say that 3 holds?

: There exist equipment assuming that case 2 holds.
: The actual definition allows for case 1, making case 3 the primary 
: preference and case 2 the secondary preference.

Yes.

: Lovely mess, isn't it?

Leap seconds are evil and must die.  For such a simple thing, there's
so much complication that getting leap seconds right can be rather
hard.

At least there's no leap second this December...

: Anyway, if I don't recall it incorrectly (OK lazy to check), the GPS 
: signal actually indicate WHEN then upcomming leap second will occur.
: This reduces to a up-comming leap second flag out of the GPS OEM board 
: which can trigger pre-maturely execution of leap-second algorithm on the 
: timing receiver, as we have seen in the Z3801A for instance.

You are correct.  There's an indication when the next leap second will
happen.  The Z3801A uses this to turn on a simple 'leap second
pending' which causes a leap second to happen at the next leap second
opportunity rather than at the week published.  The GPS operators turn
on next leap second about 4 or 5 months early, which triggers the bug.

Warner



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