But while Google Earth uses GPS to create its 3-D renderings, the team had to figure out another way to model interiors in 3-D (GPS isn't very reliable indoors). The solution was an inertial management unit (IMU) like those used in guided missiles. The IMU figures out where the backpack is relative to where it just was, the cameras gnerate a four-direction view, and the lasers record the geometry of the world around them.