If you live in a city with tall buildings, take a look at the night skyline. The buildings probably have flashing lights on them to help aircraft pilots to identify the shapes of buildings and not fly into them.
You might notice that the buildings flash in synchrony, even though they are hundreds of metres apart. What's going on?
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The idea is that all the lights on a given building should flash together. That way, an aviator can visualize the solid space between lights that blink the same way. Blink same? Don't fly. Blink different? That's a gap.
And that's how it worked for a while. A building's owner would wire all the lights together and feed them with a centralized oscillator signal.
Later on, when wire got expensive and buildings got taller, they switched to short-range radio, with one transmitter sending the blink signal, and each light having a receiver in it.
This worked fine, because all oscillators will drift eventually, so every building gets a different blinking pattern.
Then GPS happened.
Now you can buy a beacon like this one, which is standalone and has a GPS receiver in it. Anyone who's cobbled together an NTP server will know that the GPS system can provide microsecond-accurate time as a side-effect.
So these beacons don't care where they are, but they all know exactly when "now" is. And they can synchronize to each other by synchronizing to the GPS signal. It makes maintenance _so_ easy.
Trouble is, these beacons come preset to a particular blink pattern, and when you buy one (ten, fifty) you're probably not going to bother to change the blink pattern in them all. What if you forgot to do one?
So what happens is that your building is synchronized to GPS, and so is your neighbour's building, and so is _their_ neighbour's building. The whole city flashes once every two seconds, like clockwork.
Completely negating the whole point of having one blink-pattern per building, and rendering aviation around tall buildings unsafe again.
It's an accidental own-goal, brought about by the efficiencies of global, perfect, timekeeping.
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(Postscript: a few of us here on the Fedi worked this out from first principles after someone noticed that the lights on several buildings in their city were all blinking together. It was a great piece of detective work.)
@futzle Very interesting thread! I can't help but wonder: if they have GPS, they not only sync but can also get coordinates of their location, ignoring height. So assuming resolution of <10meters and assuming buildings are usually >=5meters wide in any direction and have some space between them, wouldn't it be trivial to use the beacon's location as a seed to sync up all the lights of a building but ensure they are different from the beacons of the buildings next to them?
@jorgeyanesdiez I can't think of a way to do this without the beacon needing to have a copy of a map. Consider in a dense downtown: the corners of different skyscrapers may be across a street only 20 m wide but the buildings themselves fill a 200 m block. Closeness is actually the _opposite_ of what you want.
@futzle @jorgeyanesdiez hash the coordinates?