On Sun, 19 Jan 2014 16:02:54 -0700, Warner Losh wrote: > > On Jan 19, 2014, at 2:12 PM, Hal Murray wrote: > >>> The olson database updates will take care of that. >> >> One of the problems with leap-seconds is that they aren't >> predictable and the >> software in embedded gear doesn't get updated. That also means the >> time zone >> data doesn't get updated. >> >> What sort of embedded gear uses time but not time zones? Or are people >> willing to gamble that Congress won't mess with DST anytime soon? > > All the embedded gear I've done runs on UTC. But it also has > requirements for 'cold start' that are incompatible with GPS and > couldn't be met if the system had been off across a leap second. It > didn't deal with localtime, so DST insanity didn't matter so much...
Many Air Traffic Control (ATC) radars run on UTC (not local time) in a loose sort of way. What saves them is that most ATC radars are mechanically rotated, with a typical rotation period of 12 to 15 seconds (precision approach radars can be 2 seconds, but that's a different discussion). The effect is one full circle of detections that appear to be out of position by one second's travel time, which causes a disturbance in the tracks, which disturbance soon dies out. Full-scale tests have been done on operational hardware and software in simulation, where a leap second was emulated by manually stepping the clock. For positive leaps (insert a second), there was no visible disturbance to the crawling-worm displays. For negative leaps (omit a second), there were quite visible disturbances, but these also settled out. The percentage error for omission of a second is larger than that for adding a second, and the difference in visible effect may be partly due to detections missing the coarse gate and being dropped in the rotation following the jump. In later rotations, the fractional error is smaller, and the tracker picks up where it left of. This is the bounding case. ATC systems typically do not jump at the leap, they slide over 10 seconds or so. Joe Gwinn _______________________________________________ LEAPSECS mailing list [email protected] http://six.pairlist.net/mailman/listinfo/leapsecs
