[LEAPSECS] DCF 77

[Tom Van Baak]

> Do any sources of precise, accrued time have a leap second warning bit

> as DCF 77 does?    Is the philosophy of leap second warning in DCF 77 a

> good paradigm for helping implement the leap second broadly?


WWV (short-wave, USA), WWVB (LF, USA), and JJY
(LF, Japan) all include leap second information in the
form of a warning bit, along with another bit or method
to distinguish positive from negative leap seconds. See:

http://www.nist.gov/pml/div688/grp40/wwvbtimecode.cfm

The logic is when a receiver sees the bit on then there is
a good chance there will be a leap second at the end of
the current month. This information is present in the time
code for up to 30 days so there is usually sufficient time
to validate the information in spite of LF reception issues.
(This is much better than how DST is handled; more on
that if you wish).

Note there is no leap second history so a WWVB clock
cannot obtain TAI, or past UTC times; it can only keep
up with steps in UTC.

I doubt any consumer RC (radio controlled) clocks actually
look at the bit, however. They can drift on the order of a
second each day and are often sync'd once each night.
Thus the leap second is lost in the noise.

Note also only a few models of RC clock actually display
a leap second. First, no RC clock with analog hands is
capable of a leap second. Second, most of the digital
clocks implement 24x60x60 time internally and so don't
display a 61st second. There are exceptions, and this
gets some of us all excited. See:

http://www.leapsecond.com/notes/leap-watch.htm

GPS's model for handling of leap seconds is better: you
get both a UTC offset and a date when the leap second
is/was to be applied. Thus it is possible for you to obtain
TAI, GPS, or UTC out of a GPS receiver. One downside
is that you have to wait up to 12.5 seconds for the leap
second information to show up, which can cause timing
issues with cold-start receivers.

Are any of these a "good paradigm"? That all depends
on what your requirements are.

/tvb