“" originally stood for "”
The postwar evolution of this concept transformed civil aviation. In 1952, the United States proposed IFF Mark X, operating at 1,030 MHz for interrogation and 1,090 MHz for replies. The "X" originally stood for "experimental," but it became the de facto worldwide standard. In 1953, the International Civil Aviation Organization (ICAO) adopted Mark X frequencies and the Selective Identification Feature (SIF), which allowed each aircraft to encode a unique four-digit octal "squawk" code. Mode A provided identification; Mode C, added later, encoded barometric altitude in 100-foot increments. The system became known as the Air Traffic Control Radar Beacon System (ATCRBS).
The engineering principles pioneered here—The postwar evolution of this concept transformed civil aviation—are still embedded in the aircraft you fly today.