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Comment by detritus | Hacker News Reader

This is probably a stupid thing to ask, but: Could this setup be hacked to provide a loose form of GPS?

Not stupid. Sort of. The term you're looking for multilateration (edit: i'm twisting the term a bit for my own use here, it's not exactly multilateration but it's not exactly triangulation either). Without looking at the RF specd in the FCC docs or the satellite hardware part list, I'm not sure if you could do it on the receiver, but it would be straightforward for the constellation visible to the receiver to triangulate a receiver's position and provide it over a maintenance channel to the terminal.

The answer is probably yes, but it would be a side-effect since data transmission will be the primary purpose. Actually it may turn out to be pre-condition for getting the best throughput - i.e. knowing when and where the sats are and when your transmission window is to within a tight tolerance will be necessary and that's what you need for GPS.

Thanks! I didn't know the term ( https://en.wikipedia.org/wiki/Multilateration ).

I was thinking of WW2 radio pinging when I asked. I'm sort of imagining a software simulation that has real-time updates of sat positions - I assume more than 3 in any useful domain - that checks delay/ping against where the sats actually are.

You need more than just any satellites to provide GPS. Each of them needs a very precise atomic clock (among other things I'm sure I'm not aware of).

You can build GPS (well, a GNSS would be more accurate) system without atomic clocks.

One way to do so requires user equipment that can transmit back to the satellites, but that will already be the case for SpaceX's customers.

You can also use Doppler measurements to get your location, although this tends to be less accurate, especially if you're moving quickly

You can also use Doppler measurements to get your location, although this tends to be less accurate, especially if you're moving quickly

There would still be a market for lower cost lower resolution location.

I wonder if the resolution would be better than cellular and/or wifi triangulation. Obviously those won't work in remote areas..

But GPS is cheap enough and widely enough deployed. Devices like laptops that don't usually have GPS are usually used indoors. I don't see the market.

An interesting use case for a "backup" GPS is to rely on GPS itself for the accuracy you need, but look at the other constellations to verify if GPS is being spoofed - in that way you can protect yourself against the GPS misdirection attacks (e.g. by relaying with a carefully calculated delay) that have been performed multiple times now; you'd know that you're getting a fake GPS signal and can't rely on it.

Yes. The Iridium (the closest analogue) already can do this. It's not very good, though. Worse than cell tower triangulation, I think.

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