The MST-67A is a general aviation remote mounted Mode S Transponder. The MST-67A is packaged in a non-ARINC form factor that is optimized for installation in and operation on Business and Regional Aircraft. The MST-67A is a cooperative surveillance and communication system and designed to enhance the Air Traffic Control Radar Beacon System(ATCRBS). Mode S transponders reply to radio frequency interrogations. From these replies the distance to the target, its bearing, encoded altitude, and identity can be determined. A principal feature of Mode S is that each aircraft can be assigned a unique address code. As a diversity transponder, dual antennas mounted on the top and bottom of the aircraft can be used to more accurately determine the aircraft's position.

 The MST-67A has the capability of transmitting a specific 24 bit Mode S address. This transmission can be in response to an interrogation or carried out at a periodic rate known as a “squitter”. Using this unique address code, interrogations can be directed to a particular aircraft and replies unambiguously identified. This minimizes channel interference and improves the RF interference environment. The interrogation and reply format of the Mode S transponder contains enough coding space to allow the transmission of data to and/or from the transponder. This type of data link communications utilizes a combination of pulse amplitude modulation (PAM) and differential phase shift keying (DPSK) modulation techniques. Mode S data link transmissions are formatted in either a standard 56 bit message or an extended length message (ELM) of 112 bits. The minimum data link transponder supports all of the surveillance functions of the basic transponder as well as bi-directional air-to-air information exchange, ground-to-air data uplink, air-to-ground data downlink, and multi-site message protocol. The ground-air-ground link compromises a variety of services that can be implemented according to the mission requirements of the aircraft.