BAE Systems have come up with a  positioning system that it claims works even when GPS is unavailable. According to New Scientist, GPS is a weak, highly jammable signal.  In fact anyone with an illegal handheld jammers can do it.

Spoofed GPS-format signal can be created to direct a vehicle (car, aircraft or ship say) in the wrong direction. BAE has come up with what it calls Navigation via Signals of Opportunity (NAVSOP) and 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.

It is based on the idea that in any area, the TV, radio, cellphone and radar signals will be at constant frequencies and power levels. Because of this positions can always be triangulated from them in the way that  Google Maps on a mobile uses local WiFi signals from homes and offices to improve positioning calculations based on cellphone mast and GPS signals.

BAE said that the infrastructure required to make it work is already in place. "Software defined radio" microchips that run NAVSOP routines can easily be integrated into existing satnavs. The firm believes the technology will work in urban canyons where GPS signals cannot currently reach. Ironically it will also use the signals from GPS jammers to build a radio picture of an area. It is accurate to a few metres.