Which actually leads to something I've wondered about before: Why couldn't you (with appropriate FCC clearance) set up GPS transceivers around a shopping mall calibrated to rebroadcast the signal at the appropriate time offset? It would work with everyone's existing hardware and be completely transparent.
Differential GPS is not about rebroadcasting the actual GPS position, but the error from a known position. This allows the device to enumerate the error caused by atmospheric effects.
So that's why you couldn't do this with existing receivers, but I don't know of any reason why you couldn't make new receivers that cooperate with a new type of ground-based transmitter built into cell towers or similar. Multipath might be a problem but it seems surmountable. Do any radio experts here know why nobody has done anything like this for cell phones? (The Wi-Fi/Bluetooth positioning talked about in the article is based solely on signal strength which is far inferior to the GPS method).
That said, here's a paper describing various positioning methods - http://emits.esa.int/emits-doc/1-5200-RD25-Signal-processing...
EOTD looks rather interesting - https://en.wikipedia.org/wiki/E-OTD
The main problem is that wireless signals simply aren't reliable. You could "boost" GPS indoors, but that doesn't get around the fact that there are corners, concrete walls, mirrors, humans, and so on.
Anyway, we're splitting hairs. The point is that we don't currently have an IPS. It doesn't matter if we extend GPS to work indoors or use other signals to create an IPS -- the point is, we need to create an IPS :)