Navsop

An intriguing developing technology:

Unlike GPS, Navsop doesn't need satellites once it's got going, so it doesn't rely on a signal coming from the sky. That means it can be used indoors and even underground, both places that GPS can't go because the signals are too weak, having journeyed all the way from space. By contrast, radio, TV and mobile signals are much stronger.

Most cell phones already use EGPS, which works in a similar way, but this sounds like a significant expansion of that concept.