5.6: Beacon-based Sensors

Localizing an object by triangulation goes back to ancient civilizations orienting themselves using the stars. As stars are only visible during unclouded nights, seafarers have eventually invented systems of artificial beacons emitting light, sound, and eventually radio waves. The most sophisticated of such systems is the Global Positioning System (GPS). GPS consists of a number of satellites in orbit, which are equipped with knowledge about their precise location and have synchronized clocks. These satellites broadcast a radio signal that travels at the speed of light and is coded with its time of emission. GPS receivers can therefore calculate the distance to each satellite by comparing time of emission and time of arrival. As not only the position (x,y,z), but also the time difference between the GPS receiver’s clock and the synchronized clocks of the satellites is unknown, four satellites are needed to obtain a “fix”. Due to the way information from the satellites is coded, getting an initial fix can take in the order of minutes, but eventually is available multiple times a second. GPS measurements are neither precise nor accurate enough (see below) for small mobile robots, and require to be combined with other sensors, such as IMUs and compasses. (The bearing shown on some GPS receivers is calculated from subsequent positions and is therefore meaningless if the robot is not moving.)

There exist also a variety of indoor-GPS solutions, which consists of either active or passive beacons that are mounted in the environment at known locations. Passive beacons, for example infrared reflecting stickers arranged in a certain pattern or 2D barcodes, can be detected using cameras and their pose can be calculated from their known dimensions. Active beacons instead usually emit radio, ultrasound or a combination thereof, which can then be used to estimate the robot’s range to this beacon.