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Tweefo
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« on: June 11, 2015, 06:54:57 AM »

http://www.slate.com/articles/health_and_science/science/2015/06/next_leap_second_june_30_dangers_to_software_military_gps_banking_air_traffic.single.html On the one hand, I like Christmas in summer, but on the other, I don't want a pilot to take my house roof (it is flat) for the runway. Should the system be changed?
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Faerie
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« Reply #1 on: June 11, 2015, 13:03:13 PM »

I didnt understand squat in the first part of the article, but the rest sounds like a really good plot for a blockbuster movie....

Is this not similar to the thoughts around the world blowing up when they started up the LHC?
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Tweefo
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« Reply #2 on: June 11, 2015, 14:08:29 PM »

I didn't understand squat in the first part of the article, but the rest sounds like a really good plot for a blockbuster movie....

Is this not similar to the thoughts around the world blowing up when they started up the LHC?
Not like the LHC at all. Time as we measure it keep us in line with the stars and seasons. That's why the Gregorian calendar is different from the Julian. The old people figured out that you need to add a leap year every 4 years to keep it all in sync. That turned out to be wrong and now the formula is a bit more complicated. But in modern times things need to be synced not just to the nearest day or hour. It is down to the nearest second or even part of a second. A GPS that does not take the time difference into consideration that it's signal needs to travel, whether you are on the highveld or at the coast, will be off by quite a few meters. The Earth's spin is not constant, what with earthquakes, so time needs to be adjusted, hence the leap second.
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BoogieMonster
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« Reply #3 on: June 11, 2015, 14:17:53 PM »

I would say yes and no. it's not similar because unlike the idea that the LHC would create a huge black-hole that swallows us all (as opposed to teeny tiny ones that fizzle instantly), the fact that atomic clocks are diverging from "normal" ones is actually true. I know, I'm a software developer and was talking about this very problem with some colleagues yesterday.

The idea that the sky will fall on our heads due to it though, is more debatable. Since we've been doing the adjustments for a long time and nothing crazy wrong has happened yet, we can take that as an INDICATOR that it doesn't cause too much fuss. Of course, that doesn't mean it won't all go pear-shaped someday. If it does, I expect the ramifications to be small and localised. My biggest concern would be the aging software of Nuclear Arsenals around the globe. Still, I think ti's much more rational to be concerned about the itchy trigger fingers of global "leaders".

The nice thing is that normal software guys like me are already painfully aware of issues like these (and DST, don't get me started....), so that helps us prevent real problems before they arise.
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Tweefo
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« Reply #4 on: June 11, 2015, 16:52:41 PM »

I would say yes and no. it's not similar because unlike the idea that the LHC would create a huge black-hole that swallows us all (as opposed to teeny tiny ones that fizzle instantly), the fact that atomic clocks are diverging from "normal" ones is actually true. I know, I'm a software developer and was talking about this very problem with some colleagues yesterday.

The idea that the sky will fall on our heads due to it though, is more debatable. Since we've been doing the adjustments for a long time and nothing crazy wrong has happened yet, we can take that as an INDICATOR that it doesn't cause too much fuss. Of course, that doesn't mean it won't all go pear-shaped someday. If it does, I expect the ramifications to be small and localised. My biggest concern would be the aging software of Nuclear Arsenals around the globe. Still, I think ti's much more rational to be concerned about the itchy trigger fingers of global "leaders".

The nice thing is that normal software guys like me are already painfully aware of issues like these (and DST, don't get me started....), so that helps us prevent real problems before they arise.
But is it not an accident waiting to happen? A big passenger aircraft, landing in thick fog relying on its navigational equipment, that was not updated?
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BoogieMonster
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« Reply #5 on: June 11, 2015, 17:35:46 PM »

I think there's a misunderstanding here. According to the article GPS uses atomic time, which is not subject to leap-second corrections. UTC is subject to leap-second corrections, because it drifts from atomic time.

Quote from: Article
Coders over the next few decades began pegging the clocks of various software programs to GPS system time, not Coordinated Universal Time—the one that gets tweaked by the occasional leap second.


Moreover it says...

Quote from: Article
Every time there is a leap second, that information has to be changed in the GPS message. In other words: It’s a window of vulnerability


Only IF the GPS system is not adjusted. It's a risk but it's a risk that has been, and is being managed.

Remember also that GPS does not allow pilots to LAND blind, there is still a requirement that a pilot has to physically SEE a runway before he can descend below a certain altitude (which is published seperately for each airport, though AFAIK it's generally around 400 feet above the ground, it may go as low as 200 feet, I'm not entirely sure). That is why airports can still get "fogged in" even though GPS exists... Pilots are not taught to trust GPS explicitly, since it can be slightly off even in great conditions, and if it fails they must still be able to fly and navigate. Moreover GPS is not the primary system that gets a big jet down to the runway in foggy weather. It can help you navigate but beyond finding the airport or getting more-or-less lined up it's of little use for precision work. That part is handled by ILS.

People over-estimate the importance of GPS in a cockpit, even big modern computerised planes can still be flown completely without it.
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Tweefo
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« Reply #6 on: June 12, 2015, 09:59:26 AM »

Going off topic here. It seems that the planes can land themselves.
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"It is fair to claim, therefore, that not only will the automatic system land the aircraft when the weather prevents the human pilot, it also performs the operation much more precisely".
http://en.wikipedia.org/wiki/Autoland But what about other things? I believe that some currency deals depend on second-time scales. But you are probably right, we should keep my birthday in autumn.
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BoogieMonster
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« Reply #7 on: June 12, 2015, 13:11:56 PM »

Here, this should make you feel better.
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Tweefo
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« Reply #8 on: June 12, 2015, 13:19:20 PM »

Looks very similar to a Hercules C130. The best skydive plane in my day.
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BoogieMonster
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« Reply #9 on: July 07, 2015, 15:11:02 PM »

So, today I learned that our "Previous leap second occurred in Johannesburg on Wednesday, 1 July 2015, 01:59:60."

Of course, that was only a fact that became clear in retrospect and was a tidbit of information that would've been extremely useful to have known up front.

Yes, I spent a good hour hunting a problem in a software system that just didn't make sense any way you cut it. Eventually I discovered that it was a bug in Java related to the system in question having experienced a leap second.

Within a minute or so the problem was all sorted out. Damage done: Lost a bit of productivity. So there ya go.
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