[LEAPSECS] Embedded software

[Joseph Gwinn]

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