"If you're inside a building, a GPS receiver cannot find you. But a $40 radio chip from Rosum Corporation will do it, with the help of TV signals," notes Roland Piquepaille. The CIA-backed start-up says TV signals are 10,000 times stronger than the ones from GPS, according to the Mercury News.
Rosum founder James Spilker, one of the original architects of the GPS satellite... realized a synchronization feature in digital and analog television signals could be used for other purposes than to lock the vertical hold for older TVs.The engineers created a radio receiver chip that could zero in on the TV signal and get the synchronization information. Using precision timing, they figure out how far a TV signal travels before it is picked up by a device equipped with Rosum chips. Next, they compare the measurements against other data that they collect with their own listening stations and then finally calculate the device's position. The Rosum engineers call this process "multilateration," which is akin to navigational triangulation...Rosum's vice president of engineering, Greg Flammel, says tests of the technology show it can track someone in the basement floor of the San Francisco Public Library. It also found a person in the heart of San Francisco's financial district...Rosum is best used with a GPS system, mainly because TV signals don't reach into places such as the Nevada desert or the middle of the ocean. The technology also isn't useful for tracking someone vertically. So it can locate a person in an office tower but can't determine what floor they're on unless the building is ringed with a set of Rosum antennas. (thanks to JF for the tip)