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Comment by sevensor | Hacker News Reader

What I don’t understand is why we would ever assume two clocks in two different places could be compared in a non approximate way. Your clock, your observations of the world state, are always situated in a local context. In the best of all possible cases, the reasons why your clock and time reports from other clocks differ are well understood.

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I believe it has some advantages that while you are waiting at the train station your clock shows exactly the same time as the train conductor’s several miles away from you.

Surely not! We could be a whole minute off and I’d still be standing on the platform when the train arrived.

in the US or parts of Europe you could wait there for 10m past the scheduled time and barely notice. In Japan if the train clock disagreed with the station clock by 30s, causing the train to arrive 30s late, they'd have to write all of the passengers excuse notes for why they were late to work.

GPS depends on widely separated (several times the diameter of Earth) clocks agreeing with each other down to the nanosecond.

and moving at such high speeds that relativity factors into the equations.

Speeds and altitude both! I believe time dilation from gravity is more significant but both are big enough to need compensation.

I think something like the small angle approximation applies. There are plenty of applications where you can assume clocks are basically in the same frame of reference because relativistic effects are orders of magnitude smaller than your uncertainty.

The approximation error is so small that you can often ignore it. Hence the concept of exact time.

Eg in most computing contexts, you can synchronize clocks close enough to ignore a few nanos difference.

How? Unless you have an atomic clock nearby, they will very quickly drift apart by many nanoseconds again. It's also impossible to synchronize to that level of precision across a network.

It's not only possible, you can demonstrate it on your phone. Check the GPS error on your device in a clear area. 1 ft of spatial error is roughly 1ns timing error on the signal (assuming other error sources are zero). Alternatively, you can just look at the published clock errors: http://navigationservices.agi.com/GNSSWeb/PAFPSFViewer.aspx

All the satellites in all of the GNSS constellations are synchronized to each other and every device tracking them to within a few tens of nanoseconds. Yes, atomic clocks are involved, but none of them are corrected locally and they're running at a significantly different rate than "true" time here on earth.

That's true, but it's not really the situation I'm thinking of. Your phone is comparing the differences between the timestamps of multiple incoming GNSS signals at a given instant, not using them to set its local clock for future reference.

A better analogy to practical networked computing scenarios would be this: receive a timestamp from a GNSS signal, set your local clock to that, wait a few minutes, then receive a GNSS timestamp again and compare it to your local clock. Use the difference to measure how far you've travelled in those few minutes. If you did that without a local atomic clock then I don't think it would be very accurate.

That's a common way of doing high precision time sync, yes. It's slightly out of phone budget/form factor, but that's what a GPSDO does.

The receiver in your phone also needs pretty good short term stability to track the signal for all of the higher processing. It'd be absolutely fine to depend on PPS output with seconds or minutes between measurements.

Basic hardware gets you a precise GNSS time once per second. Your local clock won’t drift that much in that time, and you can track and compensate for the drift. If you’re in a position to get the signal and have the hardware, then you can have very accurate clocks in your system.

Until somebody start spoofing GPS like they do in Ukraine, and you look embarrassing.

So use Galileo's OSNMA instead. That'll work until they spend $100 on a jammer.

I hate to break it to you, but all modern electronic warfare equipment has been targeting all GNSS for many years now. There's a reason why "GPS-denied", which is really referring to any form of satellite navigation, is a multi-billion dollar industry.

GNSS does in fact work fairly well even in conflict zones, but I'm not sure why you're trying to make this point. Everything is vulnerable to some level of warfare and civilian infrastructure is almost never designed with the expectation that it'll operate in a conflict zone. Are you suggesting we individually airgap all the networking equipment with local atomic clocks behind 10m of specially reinforced concrete instead?

Even the most secure civilian facilities (data centers) are fine sticking 2-3 receivers on the roof and calling it good.

WhiteRabbit achieves sub-nanosecond time synchronization over a network.

Oh wow, that's impressive. Is that over a standard internet connection? Do they need special hardware?

It does require a special switch yes.

The Precision Time Protocol is intended to solve this problem:

https://en.m.wikipedia.org/wiki/Precision_Time_Protocol

It does require hardware support, though.

camera cuts across to Newton, seething on his side of the desk, his knuckles white as the table visibly starts to crack under his grip

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