You'd need to spoof at least three satellites to get a fix, right? And then you'd need to spoof different signals for different regions of the subway, because your signal sources aren't really in the orbit.

Sounds to me this could be very complicated and expensive. I wonder if it would even be possible because you'd need to have the same signal spoof the correct positions to everyone who hears that signal.

No, you need 3 satellites because they are far enough that their position and time gives you too much uncertainty.

If the entire sphere where I could be fits within 5m I guess I don't need other satellites and their time to start intersecting spheres.

I don't understand this. As far as I know, the satellite basically sends its identifier and precise time, and from this information (combined with the information in GPS calendar that tells the locations of the satellites) the recipient can determines its own location.

How could it be possible to determine the location from a single timestamp and information about which satellite it belongs to? I suppose if the recipient already has a fix, then it could perhaps survive with less than 3 satellites by making some assumptions, but I imagine this will result in lower quality location information.

Were you proposing to assume the location of a 5m sphere the recipient is in?

The satellite also sends information about all the other satellites that a receiver should be able to see. This is what makes adding other transmitters hard/impossible I guess - no way to tell the overhead satellites that your ground station also exists (there would presumably be thousands of them - most not in range of your receiver which would be even more problems.)

> I don't understand this. As far as I know, the satellite basically sends its identifier and precise time, and from this information (combined with the information in GPS calendar that tells the locations of the satellites) the recipient can determines its own location.

I'd assumed the GPS calendar was somehow broadcasted by the GPS network too.

> Were you proposing to assume the location of a 5m sphere the recipient is in?

I guess the proposal was to change the problem from pinpoint a single point in space, to figure out roughly where I am, in which case being anywhere in a tiny sphere is pretty much the same as being in a point after you account for errors.

Well, you are not a GPS satellite, so you are indeed spoofing military equipment.

On a serious note, I recall that smartphone location in metro in my city started to "just work" as soon as all stations and tunnels had indoor cell towers. Suddenly all apps worked fine and I forgot that problem ever existed.

Modern location detection is as scary as it is amazing.

Cell towers also transmit location data