Rosum: Taking GPS Indoors

A Silicon Valley startup is blazing new trails for Global Positioning System technology

by Arik Hesseldahl

James Spilker and Bradford Parkinson were there in the early 1970s when a system of satellites designed to keep time and track movement on earth was literally just getting off the ground. As a young Air Force colonel, Parkinson was assigned in 1973 to take competing military proposals for satellite-based navigation systems and combine them into a single system. Spilker, an electrical engineer and professor at Stanford University, had expertise in wireless technologies and developed the signal systems for what came to be known as the Global Positioning System.

At the time, Spilker and Parkinson had little idea how essential to day-to-day life the network would become. Though GPS got its start in the military, today the 24 satellites that surround the globe from 12,000 miles up are integral to modern communications and commerce. GPS technology is used for navigation not only by motorists, pilots, and hikers, but also by vast networks—in industries from finance to medicine to telecommunications—that depend on precise timing to run properly.

But as useful as GPS is, the system doesn't work well once you bring it indoors. Sure, GPS-dependent wireless phones still keep time indoors, but it's hard to use GPS navigation features to keep tabs on a person or object inside buildings. The signals are simply not very strong.

Spilker and Parkinson are out to change that in a profound way.

In Sync With TV Signals

The pair aims to give GPS a jump-start by combining it with a different technology that operates in a similar way but is stronger on the ground: TV signals. Remember those rabbit ears and roof antennas you (or your parents) once used receive to TV signals? Although these days much TV is delivered via cable, satellite, and over the Internet, the airwaves can still carry a TV signal, and TV towers can still transmit them.

Enter Rosum, the Mountain View (Calif.) startup founded by Spilker and advised by Parkinson. As it happens, TV signals, like GPS, rely on precise timing to keep in sync, so they closely resemble the signals coming from the GPS satellites. "But the TV signals are a lot more powerful, and they come from a source that's much closer," Parkinson says. "They're something like 10 million times more powerful compared to GPS, so it's no problem for the TV signal to penetrate buildings."

Rosum has developed a technology that gets GPS and TV signals to work in sync. With a receiver designed to listen for both GPS and the combined TV-GPS signal, using a set of chips tuned for both, you have a powerful positioning system that can work indoors and show your 3D position, such as what floor of a building you're on. Already, Rosum's technology has been tested successfully in the San Francisco Bay Area and in Northeast locations from New Hampshire to Washington, D.C.

To what avail? Since objects and people aren't always outdoors and within sight of GPS satellites, you need more than GPS. "If you want position navigation everywhere, there is no single system that is going to accomplish that feat," Spilker says. Unlike Rosum's new technology, "GPS signals are transmitted with very limited power, so the system simply can't handle going indoors or underground."

Help for 911?

The technology can also enhance telecommunications, Spilker says. Cell phones already use GPS to help locate callers who dial 911. But Internet-calling services such as eBay's (EBAY) Skype that don't support 911 calls could be modified to help find users in an emergency. Add TV-GPS chips to a handset or computer and 911 operators will have the ability to know precisely where a call originates.