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 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.
