Whilst traditional Mode A/C secondary surveillance radar stations continuously interrogate all aircraft within their range, Mode S secondary surveillance radar stations establish addressed interrogations with aircraft within their coverage.
In order to avoid ambiguity in the operation of the system, it is essential that each Mode S radar is allocated an interrogator code (IC) which, in a coordinated operational environment, protects its operation from interfering with other Mode S radars that operates in overlapping or contiguous airspace.
Due to the limited number of interrogator codes, it is necessary to have a centralised IC allocation system to ensure an optimised and coordinated approach to the allocation and implementation of the ICs used by Mode S radars.
In the ICAO European region, the centralised IC allocation system is conducted by EUROCONTROL on behalf of the European regional office of ICAO. EUROCONTROL is also supporting the Middle East regional office of ICAO in Mode S IC allocation coordination of the ICAO Middle East Region. IC allocation requests for this region must be submitted through MID ICAO regional office.
The service is partially restricted to specific stakeholder groups.