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Comment by roarcher | Hacker News Reader

> Including stationary stuff that ought not to be.

This has piqued my curiosity. What stuff is this? If something needs to know where it is and it's not going to move, can't someone just program its location once and be done with it?

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It's the cell network. The basic problem is that adjacent cells need to be in tight sync for time division agreement and handoff, but the obvious algorithm of "just sync to your neighbors" is highly problematic at scale because even if a big group of towers does a good job of drifting together there is always a boundary and the problems will just happen there. Since the cell network is highly profitable infrastructure, they just used the overkill strategy: "GPS sync everything and put atomic clocks everywhere for good measure." At least hobbyists get cheap and plentiful Rubidium clocks out of the deal! I don't know if newer standards are still as sensitive about timing agreement but it was definitely the case for a good long while.

Mostly for time synchronization. GPS/GNSS is cheap, widely available, and accurate down to nanoseconds, so it's displaced every other technology.

As an example, consider the blinking lights on top of wind turbines and other tall things. If you've got a lot of these in one area, the FAA recommends they blink in unison, to make them more visible and less distracting to pilots. Turns out the cheap and foolproof way to synchronize is to give each one a GPS receiver. This is fine until GPS goes down, and now something as innocuous as "blinking hazard light" is dependent on GPS.

This example fails safe if the lights fall out of sync ("oh no, anyway..."), but it's still emblematic of how widespread the problem can be.

Can I assume that they are meant to blink in one-second intervals and the GPS is just adjusting them to be synchronous, but they won’t stop blinking if GPD goes down?

Not that avoiding windmills is the priority on an airplane hit by a solar storm…

I would hope so, yes. But who really knows?

Someone on Hacker News, by judging from previous responses to the most obscure technical questions…

It's not even location data. A lot of stuff uses GPS to get time information and then propagate that using NTP or other timing infrastructure. Seeing a lot of stratum 0 or stratum 1 time servers go offline could potentially cause a lot of issues if systems start to drift away from each other.

Large CNC machines that are not exportable under ITAR use GPS and other sensors to detect movement.

I didn't know that, that's wild. Does the government actually track their location in real time?

I believe the machines generally disable themselves and need a factory technician to re-enable them should they fault out.

I've heard it justified as a safety measure - those machines are carefully balanced on installation, and if the sensors detect it being moved, the machine locks itself out to avoid damaging itself, workpieces and/or operators.

Are those not afraid of GPS spoofing either? Spoof GPS near a big CNC machine shop that a big competitor relies on, disable their machines...

I can't unlock a bikeshare in my city without GPS validating thru the app that I'm nearby. It's stupid. Assuming he means stuff like this.

As I understand, no one without a physical bikeshare rental card can check out a bike if GPS went down. Lots of apps that try to control user behavior and get access to permissions are probably similar

I'm guessing it's for time sync, but any clock worth it's crystals will keep good time for quite a while. Every substation generally has GPS time sync if it isn't fibre connected, for synchrophasor measurements or accurate timestamped waveform captures for example.

Most financial trading systems use GPS timestamps to coordinate all of their transactions.

But, but, Your Honor, at the time I was traveling at 0.5 c and in my hyperplane of simultaneity transaction A happened after transaction B!

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