In the classic tale by the Brothers Grimm, Hansel and Gretel leave a trail of bread crumbs from their home so as not to get lost in the forest, but the plan fails when birds eat the crumbs. In the modern world, a GPS device could assist the fabled siblings. But what if they wandered into a place without GPS signals?

With that kind of problem in mind, a team of researchers at Microsoft set out to create a mobile device that could forge a trail of "digital bread crumbs." The device would collect the trail data while the user walked indoors, underground, or in other spaces where GPS signals are unavailable or weak--such as multilevel parking garages that can baffle people who forget where they parked.

The resulting Microsoft Research device, a prototype phone called Menlo, packs a suite of sensors: an accelerometer to detect movement, a side-mounted compass to determine direction, and a barometric pressure sensor to track changes in altitude.

While existing phones contain some of these sensors, what's new about Menlo is an app called Greenfield, which aims to solve the Hansel and Gretel problem by harnessing the data from the sensors. The goal is to count a user's sequence of steps, gauge direction changes, and even calculate how many floors the user has traversed by stairs or an elevator. The app stores the trail data so that a user can later retrace his path precisely.

The researchers call Greenfield an example of "activity-based navigation." In a paper to be presented at the MobileHCI conference in Lisbon, Portugal, next month, the Microsoft team positions Greenfield as an ideal method of navigation in places where maps haven't been constructed or aren't accessible. For the paper, computer scientist A.J. Brush and her team conducted a trial in which people had to retrieve an object from a colleague's parked car in a large garage, using the coworker's trail data to navigate the way.

"I knew this was possible, but I was wondering when someone would put all the pieces together," says Jeff Fischbach, a forensic technologist with SecondWave Information Systems, a consulting firm in Chatsworth, CA. Fischbach often serves as an expert witness in criminal trials in which GPS data is used as evidence. He says that trail data from an app like Greenfield could help determine whether a murder suspect is truthfully stating an alibi. "This kind of data is terrific for convicting people and terrific at exonerating people."