In 1973, two separate patents were issued in the United States for Radio Frequency Identification (RFID) concepts. The first, granted to Mario Cardullo, was for an active RFID tag with rewritable memory. An active tag has a power source (such as a battery) of its own, whereas a passive RFID tag does not. The second patent was granted to Charles Walton who proposed the use of a passive tag for keyless entry (unlocking a door without a key). Shortly thereafter a passive RFID tag was developed for tracking cattle (Fig. TF13-1), and then the technology rapidly expanded into many commercial enterprises, from tracking vehicles and consumer products to supply chain management and automobile anti-theft systems.