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.
To set up this system, customers have to put Rosum's radio chips and the modules that house them into their equipment. These modules, about as big as a matchbook, cost about $40 to make, but could become cheaper and smaller over time with high-volume production. One of the main computational tasks of these devices do is to filter out the wrong signals, such as ghost images that have been reflected off of an object.
"That's the part that took a lot of Stanford Ph.D.s," Speaks says.
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.
Flammel says TV signals are 10,000 times stronger than GPS signals. That means tracking through TV signals is much easier and quicker than via satellite.
Mark research firm Frost & Sullivan estimates the market for GPS equipment -- now at $445 million -- will double in the next six years.
There also are strategic uses for Rosum's technology. President Bush signed a directive in December requiring the government to find an alternative to GPS in case the system is compromised. Speaks says Rosum's positioning technology could serve as the GPS backup system.
Among the investors in Rosum, which has raised $16 million, is In-Q-Tel, Charles River Ventures, Allegis Capital, Motorola Ventures, Steamboat Ventures and KTB Ventures. Rosum says it has won nine patents and has applied for dozens more.
Rosum, which has only 27 employees, has a partnership with Sunnyvale-based Trimble Navigation to make a fleet-tracking device that would use both GPS and the Rosum TV-tracking system. Trimble expects to launch that product next year.
"The tests we've done show it works even in the basements of big buildings," said Dennis Workman, a vice president of component technologies at Trimble.
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.
"We aren't going for the commodity markets at first," says Speaks. "But you have to ask, what things do you really want to locate? What is it worth to have a 911 call that you can track regardless of what kind of phone initiates it?"