IFF Mark X was the NATO standard military identification friend or foe transponder system from the early 1950s until it was slowly replaced by the IFF Mark XII in the 1970s. It was also adopted by ICAO, with some modifications, as the civilian air traffic control (ATC) secondary radar (SSR) transponder. The X in the name does not mean "tenth", but "eXperimental".[1] Later IFF models acted as if it was the tenth in the series and used subsequent numbers.
For most of World War II the standard IFF system used by the allied air forces was the IFF Mark III. Mark III responded on the same frequency as the trigger signal, returning a selected pulse pattern. Originally, the Mark X was simply a version of Mark III operating at a higher frequency, which has several practical advantages. Three return patterns, or Modes, were available. As it was being introduced, the new Selective Identification Feature, or SIF, allowed the response signal to be modified with bit encoding, providing the ability for each aircraft to produce a unique response using octal digits. This was initially handled through a separate box that connected to the original Mark X. For a brief time in 1957 this was known as IFF Mark XI before becoming IFF Mark X (SIF).
As the civil aviation market grew in the 1950s, Mark X was selected as the standard transponder system as the Air Traffic Control Radar Beacon System, or ATCRBS. For this role, a new series of four Modes was introduced, A through D. A is essentially identical to Mode 3, and these are now referred to as Mode 3/A. Mode C responds with a four-digit code encoding the pressure altitude in 100 foot (30 m) increments. Combining information from a radar with Mode A and C responses, the ATC system can build a complete picture of the airspace without the need for height finders or 3D radars. Using Mark X for the civilian role also allowed existing military users to be routed within the civilian network, as well as allowing civilian aircraft to use an existing and well-tested transponder design.
Mark X retained a key problem that was present in all IFF systems to date; the aircraft transponder would respond to any interrogation signal on the proper frequency with no way to tell if it was a friendly transmitter. This allows an enemy force to query the transponders and use triangulation to determine their location, or simply count the responses to look for increased activity. Military users had long desired a system that encoded both the interrogation and response, allowing the transponders to ignore signals from interrogators that did not present the right code. This led to the development of IFF Mark XII and its associated Mode 4 which began to be deployed in 1970.