On Thu, Jan 5, 2017 at 3:42 AM, Hal Murray <[email protected]> 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 _______________________________________________ LEAPSECS mailing list [email protected] https://pairlist6.pair.net/mailman/listinfo/leapsecs
