Real-time location systems (RTLS) are systems that allow you to identify an object and determine its approximate location in real time. RTLSs can use technologies other than radio frequency identification to do this. For instance, they can use ultrasound or infrared. But RTLSs employ active RFID tags that beacon; these tags transmit a signal every 2 seconds, 5 seconds, 5 minutes or whatever interval at which you set the beacon rate.
Reader antennas around an area pick up the signal, and software triangulates on the tag based on the time it took each signal to reach each reader antenna (other methods can also be used besides time of arrival). The software usually calculates the location to within 10 feet.
There are now also passive real-time location systems. Instead of focusing on the beacon, the readers use phased-array antennas that listen for the tag signal in a very narrow “tunnel” that enables software to calculate tag location.
Conventional RFID systems identify a tagged object and only tell you that the item is within the read zone—that is, the area covered by the reader antenna. So an active system would tell you, for instance, that a truck has pulled up at the gate of a distribution yard, while a passive system would inform you that a pallet has passed through a dock door. But these systems would not tell you an object’s location within a defined space.
—Mark Roberti, Editor, RFID Journal