Using Wi-Fi for Navigating the Great Indoors

Lost and found: WiFiSLAM’s technology pinpoints the location of a phone on a map to within a couple of steps. This app uses that technology to enable a game of “real life Pac-Man.”
Credit: WiFiSLAM       

A phone can locate you indoors to within a few paces by combining Wi-Fi signals and the jolt of your footsteps.

The arrival of GPS receivers in cell phones led to a boom in location-based apps and services—everything from maps that show you where you are, to new kinds of social networking. But step inside a building and GPS often fails. Now a startup has technology that enables devices to know their position inside a building to within a few steps, and it hopes this could lead to a second wave of indoor location-aware services.
WiFiSLAM, which publically demonstrated its technology for the first time last week, enables a phone to work out its position by combining the "fingerprint" of nearby Wi-Fi networks with information taken from a device's accelerometers and compass. The company was founded by students from Stanford University, with the aid of the university's StartX accelerator program for startups.
Mobile devices already use Wi-Fi networks to refine outdoor GPS fixes by accessing databases maintained by companies including Skyhook and Google, created by driving around "sniffing" for wireless networks. However this technology can today only allow accuracy of 10 meters at best and is primarily aimed at outdoor use.

The technology is typically accurate to within a "couple of steps" of your current location, says Anand Atreya, cofounder of WiFiSLAM: "This accuracy will change how you interact with indoor environments." The technology could aid with navigation inside large and complex buildings such as hospitals or airports, he says, adding that app developers will likely find more imaginative uses, too.

"Think about going to the supermarket," says Atreya. "We can provide information relevant to the product right in front of you." Another possibility is allowing users to find the nearest store clerk, as long as that person is also being tracked.

When a gadget using WiFiSLAM wants to know its location, it analyzes the signal strengths and unique IDs of all the Wi-Fi networks around it. That is matched against a reference data set for the area either accessed over the Internet, or stored on the device. The estimate of location can be sharpened if a gadget moves slightly, because WiFiSLAM's algorithms can gather multiple fingerprints. Compass data and accelerometer signals capturing a person's footsteps are also used to refine the accuracy of subsequent location fixes as a person moves around.