[mitchty]

Yes its essential. Because the satellites are moving at orbital velocity, their frame of time is ever so slightly different than ours. To keep accuracy timewise without special and general relativity would be difficult if not impossible without understanding relativity generally. Could we have done GPS without relativity? Maybe, but we'd have to discover it with the satellites to be able to use it.

A quick note from: (i'm too lazy to do the calculations right now) http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....

    If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time.

[hinkley]

I think I see what he's saying. Those satellites are moving at a fixed velocity. So the time dialation effect on them relative to the ground should be static.

It could reasonably be the case that we just adjusted the clocks on the satellites by .05% or whatever the drift rate is by measuring them against known points until we get it right. Good enough engineering.

Eventually someone would have asked some scientists to explain why it's happening.

[mitchty]

> I think I see what he's saying. Those satellites are moving at a fixed velocity. So the time dialation effect on them relative to the ground should be static.

The issue you have is that the dilation effects aren't static, they're all relative to each satellite and ground observer and are constantly shifting based on the orbits. Basically the premise you have to accept to allow for "just adjust the clocks" is too basic. This is why both general and special relativity come into play in GPS. You might get away with adjusting for a single observer, but not all observers.

Note that we already have the clocks purposefully skewed to account timewise for their orbital speed and still require constant updates. I just don't see that happening through "good enough engineering". If you're a nanosecond off you're off by over 1 kilometer and things get worse from there.

Plausible as a hypothetical gedanken experiment? I suppose, plausible in reality? I'm skeptical that you'd be able to do it. Would be akin to launching a rocket to the moon without understanding how to fly.

[soVeryTired]

Nice, thanks for the info! I'd never seen the inaccuracy quantified like that before.