[LEAPSECS] alternative to smearing

[Warner Losh]

On Thu, Jan 5, 2017 at 3:42 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>
>> 1) The leap day in February can be handled by any isolated or autonomous
>> clock or timekeeping system. A leap second can only be handled with periodic
>> direct or indirect communication with IERS, or manual intervention with the
>> likes of keyboard input or toggle switches. For secure or embedded systems
>> this is a huge issue.
>
> If a system is isolated, does it matter if its clock knows about leap seconds?
>
> I could imagine that "isolated" meant nothing goes in for security reasons, but then time doesn't go in either.  So maybe you allow GPS to go in the back door, but that has leap info.

While still operational, LORAN-C timing stations were all isolated
systems. They needed to know the leap second data for a variety of
reasons. One of those reasons was that we had to be able to replace
the GPS receiver module at any time. It was replaced with a 'cold
spare' from the shelf, which might not have the most up-to-date leap
information on it. It was not possible to rotate all the spares in and
out every 6 months to get the new data for a variety of reasons I need
not go into here. These GPS receivers needed about 12.5 minutes to get
the proper UTC offset since cold ones had no valid almanac, and we
weren't allowed to have it get it from the network (not even one of
the 3 other redundant systems on said network that could have the
right data). This meshed poorly with a requirement that the GPS
receiver must report UTC time within 1 minute of the power being
applied to it, as you might imagine...

GPS also doesn't provide leap second tables. Just current and maybe
future offsets. It's often enough, but not universally enough.

Warner