In Europe, the aircraft identification for ATC purposes is based on two data items transmitted by the transponders: the aircraft identification (also called the call-sign) and the so-called Secondary Surveillance Radar (SSR) mode-A transponder code. The transponder code is a 4 octal-digit code allowing a total of 4096 possibilities. According to Eurocontrol, out of the 4096 discrete codes, only about 2,000 can be currently used for IFR/GAT traffic. At the peak of summer 2019 in Europe, 37,000 flights needed to be managed… This often leads to change of codes at the boundaries between zones, which represents additional workload for pilots and controllers (e.g., 1,653 such code changes were needed when crossing the French boundaries on October 4th, 2018,). Several technology enablers try to tackle the possible shortage.
In a mode-S radar/ADS-B environment, a solution is to use a non-discrete mode A code (A1000) where the downlink aircraft identification (Aircraft ID or ACID) will be used to correlate the flight plan instead of the mode-A code. Therefore, all flights crossing such airspaces are using simultaneously only one code. This solution is required by the ACID European Implementing Rule.
France has now reached a 100% compliance level for all its airspace and aerodromes, allowing for a parsimonious usage of this scarce resource and for a reduction of workload (no code change when crossing boundaries with compliant adjacent ANSPs).
DSNA is very proud of this achievement in participating to this collective effort towards a seamless airspace.