this post was submitted on 06 Jun 2024
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Four. GPS solves position in 4-dimensional space.
3 satellites for 2 dimensional space, 4 satellites gives you height as well (3 dimensional).
your wristwatch gives you your fourth dimension ;-)
GPS satelites are just fancy atomic wirstwatches hanging on Earth orbit
heh, this is true, but don't they still only give you a 3 dimensional location?
I'm trying to understand your question. They broadcast both their orbit parameters and their time.
No, more is preferred, but the way the signals are designed, some positioning slowly works with only two satellites.
Like old phones. Remember when GPS was slow and always a few meters off? Part of that was they were bad at or could not acquire more than two signals.
s/signals/receivers
I guess receivers that also measure angle to satelites do have 4 constants with only 2 satellites to get 4d solution(or rather 2 solutions, one of which is in future, while other is in the past, not sure which one is correct). Or maybe try to do some wierd math shit with Doppler shift. I was talking about original(cheap and easy) way receivers were solving coords with 4 latencies.
If we are really pushing the limits here, then receiver that knows it doesn't move relative to Earth can get coords from one satelites, but this is just speculation. And it may require atomic clock.
This doesn't sound like reason for it. Slow start? Receiver first needs to receive ephemerides and almanac to be able to solve position. Quoting wikipedia article on A-GPS:
Almanac can be stored on device for a long time to be used later in next start. It's called warm start. Ephemerides don't last as long. Start when they aren't stale called hot start.
Now goes quote from another article
So bright minds thought "what if almanac and ephemeris" will be downloaded from the internet? And this is how A-GPS was born.
Now about precision. Few meters is normal precision for normal GPS. Getting better precision requires very scary math hiding behind Differential GPS and additional correctional data. Not that normal GPS doesn't have scary math.
Lastly, about two satelites: old phones had one antenna per band(and usually only one band), so they did require at least 4 satelites.