Wednesday, February 10, 2016

The perils of a connected world...

The perils of a connected world...  This story is about a GPS glitch that caused some GPS satellite clocks to broadcast a time that was 13 microseconds (that's millionths of a second) off.  That doesn't sound like much, but it's more than enough to cause major errors in GPS' major function (locating oneself on the planet).  That 13 microsecond error translates into a (roughly) three mile location error, if the broken satellite was one of three you're using.  The problem is less awful if your receiver is using more satellites, as is most often the case.

But ... that's only the beginning of the problem.  GPS satellites contain an atomic clock that keeps time accurate to billionths of a second (actually much better than that in the current generation of satellites).  Each satellite is broadcasting the time it has, continuously.  The position of each satellite is known to a fantastic degree of precision.  If you know the time that the satellite thinks it is (which it's broadcasting), and exactly where the satellite is (easy to get), then you can put a receiver anywhere on earth and use that to keep time that's almost as accurate as the clock on the satellite itself.  That receiver and the necessary electronics and software are (currently, at least) far less expensive than having your own atomic clock.  Because of that, systems on earth that require extremely precise timekeeping often rely on the GPS satellites as their time source.

I know of two such systems, but I'm sure there are more. 

First there's the global network of cell phones.  I won't go into the technical details here (that's a whole post on its own!), but here's a site that explains how cell phones work if you want to know.  The main point I want to make here is that the cell phone systems are utterly dependent on incredibly accurate time keeping – and they depend on the GPS satellites for that time.  That 13 microsecond error would translate directly into cell phones failing.

Then there's the global electronic trading systems, especially those for stocks, options, and currency exchange (probably others as well, but those are the ones I know well).  The exchanges for these electronic trading depend on knowing the exact order that trades take place.  That may seem like a trivial problem, but it's not – because (a) there are sometimes many thousands of trades occurring in a single second, and (b) the trades can originate anywhere in the world, (c) multiple exchanges that are simultaneously open must synchronize their trades perfectly.  The system can recover if an error occurs (and they sometimes do), but the process of “unwinding” erroneous transactions is expensive and painful, so the exchanges go to great lengths to avoid such problems in the first place.  Key to that is requiring a very high precision to the timekeeping used by all the components of that system.  That required precision has increased over the years, and it's now at the point where only atomic clocks can provide it.  Once again, that means a dependency on GPS clocks (except for a few big organizations that have sprung for their very own atomic clock). 

So when the GPS clocks suffer from a problem like the one described in the article, the consequences can be far more serious than you might expect!

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