A call sign is a unique identifier used in broadcasting, radio communications, and aviation to identify a transmitter station, aircraft, or ground facility. Call signs can be formally assigned by a government agency, informally adopted by individuals or organizations, or even cryptographically encoded to disguise a stations identity. In aviation, call signs are derived from several different policies, depending on the type of flight operation and whether the caller is in an aircraft or at a ground facility. In most countries, unscheduled general aviation flights identify themselves using the call sign corresponding to the aircrafts registration number. Amateur radio call signs are structured differently and are assigned in the international series. They normally consist of a one or two character prefix, a digit, and a 1-, 2-, or 3-letter suffix. Special call signs are also issued in the amateur radio service for special purposes, VIPs, or for temporary use to commemorate special events. In the United States, call signs are identifiers assigned to radio and television stations, which are issued by the Federal Communications Commission (FCC) . The vanity call sign system offers the opportunity to request a specific call sign for a primary station or club station.