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72 T he advent of unmanned systems in national airspaces has prompted conversations about safety, whether it be that of people on the ground or other vehicles in the air. For the latter, lessons can be taken from the manned aviation sector, which sees thousands of civilian and military aircraft traversing the skies every day, thankfully with few accidents – mid-air collisions rarely number more than one or two a year. Air traffic control (ATC) operators currently use primary radar systems combined with secondary surveillance radar (SSR) to gauge the identity and bearing of aircraft in order to coordinate them and keep them separate. SSR relies on the use of civilian transponders, which are based on military Identification Friend or Foe (IFF) devices and which transmit critical information about an aircraft’s identity and barometric altitude when they receive an interrogation signal from ATC. Transponder modes Interrogation modes – the formats of pulsed sequences of information given as replies by transponders – have been standardised for military and civilian transponders. Civilian transponders offer three principal modes of operation. The oldest, Mode A, transmits (or ‘squawks’) a four- digit octal code upon interrogation that is unique to each aircraft. This is called a Flight Identification code, which is issued by the ATC. Mode C transponders, which include Mode A functionality, reply with information about barometric altitude (for determining the performance and flight level of an aircraft) using a sensor in the transponder or integrated with it from elsewhere on the aircraft. These two sets of data enable ATC to identify individual aircraft and track their positions on primary radar displays, and communicate navigation instructions to Transponders will be key to UAVs operating in busy airspaces. Rory Jackson tracks the trends in the technology and highlights the need for regulations Crowd control December/January 2018 | Unmanned Systems Technology Air traffic control operators rely on primary radar systems and secondary surveillance radar to coordinate aircraft flight paths and keep them safely separated (Courtesy of NATS Holdings)