A story discussed recently over Slashdot is named NAVSOP Navigation System Rivals GPS.
Their summary: "BAE Systems has developed a positioning solution that it claims will work even when GPS is unavailable. Its strategy is to use the collection of radio frequency signals from TV, radio and cellphone masts, even WiFi routers, to deduce a position. BAE's answer is dubbed Navigation via Signals of Opportunity (NAVSOP). It interrogates the airwaves for the ID and signal strength of local digital TV and radio signals, plus air traffic control radars, with finer grained adjustments coming from cellphone masts and WiFi routers. In any given area, the TV, radio, cellphone and radar signals tend to be at constant frequencies and power levels as they are are heavily regulated — so positions could be calculated from them. "The real beauty of NAVSOP is that the infrastructure required to make it work is already in place," says a BAE spokesman — and "software defined radio" microchips that run NAVSOP routines can easily be integrated into existing satnavs. The firm believes the technology could also work in urban concrete canyons where GPS signals cannot currently reach."
I remembered 4 years ago we mentioned eLORAN as a possible backup system for the current satellite GNSS systems.