Aircraft are identified by Secondary Surveillance Radar (SSR) using transponder codes comprised of four digits (0-7). These codes are assigned following the International Civil Aviation Organisation's (ICAO) Originating Region Code Assignment Method (ORCAM). ORCAM is based on the strategic allocation of sets of codes to individual States and on the alternate use of transit and local definitions of codes per participating area (PA). This allows the same codes to be used for transit in one PA and for local purposes in adjacent PAs. Together they cover the entire ICAO EUR Region.
However, this structure fails to provide sufficient SSR codes for today’s demand (due to increased levels of traffic and also changes in traffic flows). Besides, the current structure has difficulties coping with Functional Airspace Blocks (FABs) and direct routes.