[LEAPSECS] leap smear

[Poul-Henning Kamp]

In message <p06240806ca9c0133039d@[192.168.1.101]>, Joe Gwinn writes:

>At 7:56 PM +0000 9/18/11, Poul-Henning Kamp wrote:

>>In message <p06240805ca9bf8bc07d3@[192.168.1.101]>, Joe Gwinn writes:



>Not really.  The problem with leap seconds is that they are too rare 

>to allow for comprehensive testing of systems, and so such systems 

>tend to fail when a leap second comes along.


The fundamental problem is that a vast majority of the worlds
software is written as if leap seconds simply do not exist.

The fact that they do, are horribly expensive to test, and tend not
to get tested because *recently* they have not happened a lot, is
merely icing on the cake.


>What saved the software is that it is fed 

>only rotating-antenna data, with about 12 seconds between updates, so 

>plus/minus one second wasn't that big a fractional error.


In the ATC system I was involved in, radar data is collected from
an number of radars in a number of countries and glued togther
to form a coherent image for presentation.  As far as I recall,
the timestamp resolution from the radars are 1/128th second.

(On one of the runways, certain large planes have not completed
wheels-up by the time they are in a different countrys air-space.
This complicates matters so much, that it was one of the main
reasons to establish joint airspace between the two countries.)

Their solution for leapseconds, for which the system as a whole has
not been tested, is that they announce to all planes in their
air-space that they are "on their own, to remain on course until
further notice" and then they wait for the light-show to calm down.

Modern planes move 300m/s and with P-RNAV, anticollision detection
is set to pretty hysterical tolerances, so planes jumping 300m
forward or backward is reason for blinkenlights.

Fortunately, leap-seconds happen in the middle of the night here,
so there is no commercial passenger traffic in the airport, only
a couple of DHLs, FEDEX etc.

I'm sure LAX or HND will have much more fun, once they get a
modern ATC system.


>I bet it is known, even if we two don't know it.  And the stated 

>reason would be interesting.


I'm sure it's known, and I think the reason is either somebody
uninformed panicing, on somebody well-informed panicing.

I'd like to know which it is.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
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Never attribute to malice what can adequately be explained by incompetence.