Reply to post: Re: Big Outage

Experts: No need to worry about Europe's navigation sats going dark for days. Also: What the hell is going on with those satellites?!

Cuddles Silver badge

Re: Big Outage

"Can the clocks get so far off from where they're supposed to be that it can cause an unrecoverable degradation?"

No. While we don't usually think of it in that way, all clocks are simply counters - a pendulum clock counts the number of times the pendulum has swung, a quartz clock counts the number of electrical oscillations in a crystal, an atomic clock counts the number of oscillations in an electromagnetic wave. There's no measure of time actually inherent to any clock, all they do is count how many oscillations have happened since some arbitrary point.

In an everyday clock, that count is tied to some sort of human-readable display, and ideally that display can be adjusted if for some reason it ends up being inaccurate. But with an atomic clock, there isn't generally any display of "time", all it will do is tell you how many counts there have been since it was told to start counting. If you tell it to start counting from zero again, it will simply do so. So there's really no such thing as it's time being off from where it's supposed to be, because it doesn't know anything about time at all. All it's doing is counting oscillations, and no matter how far away from the expected count it might be it will never stop working or somehow become unrecoverable because as far as the clock is concerned such things don't have any meaning.

It's also important to consider that there are essentially two ways for a clock to be wrong - either the starting point is not what was expected, or the oscillations occur at a different rate than expected. The latter is the problem with almost all normal clocks - a pendulum doesn't always swing exactly once per second, so after an hour it may only have counted 3550 swings instead of the 3600 it should have. That can be fixed by just nudging it forward a bit occasionally, ie. resetting the start point. But with an atomic clock, it's essentially impossible for that to be a problem. The oscillations being counted are tied to atomic transitions that only radiate with very specific frequencies. Barring a fundamental change to the laws of physics, either the clock is counting at the correct rate (to within about 1 in 10^-15) or it's not counting at all; it can't slow down or drift around in the way a pendulum clock might. So essentially the only way for an atomic clock to be wrong is if the starting point of the counting is wrong.

Which brings us back to Galileo. Keeping a network of atomic clocks synchronised means making sure they're all told to start counting at the same time. And if that gets screwed up somehow, the clocks could all be functioning perfectly, but be completely useless since if one says "I've counted 10 billion oscillations" and another says "I've counted 100 billion", without knowing when either of them actually started counting it's all entirely meaningless. From the little information we have, it sounds like the problem is something along these lines. The clocks are probably all fine, but the ground station has somehow screwed up the synchronisation so the counts aren't all starting at the same time.

Of course, that's all talking about the actual clocks themselves, there could be all kinds of problems in the electronics and programming surrounding them. But given that there are multiple different systems built by different manufacturers involved, it seems very unlikely that they would all suddenly develop similar faults at the same time despite having worked fine until now. Given what we know of the nature of the issue, it's much more likely to be a problem in the wider network than anything specific to the actual clocks.

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