RFID Background ...

United States Patent Application: 0030028787

... "One solution involves the use of Radio Frequency Identification (RFID) technology. RF ID technology is more commonly associated with aircraft identification systems, in which each aircraft has a transponder that emits a code in response to receiving radio waves of the correct frequency. The emitted code helps aircraft identify one another and thereby avoid collision or, in the case of military aircraft, avoid firing on each other. This technology has also been used in door entry systems, in which employees of a company are issued security cards, each card having a transponder that transmits an ID code in the presence of an electromagnetic field. To enter a secured door, an employee places his or her security card in close proximity to a card reader. The card reader emits radio waves that cause the security card to respond with an ID code. The security system then determines whether or not to open the door based on the ID code. " ...