The requirement to have radio licences originate from the International Telecommunication Union (ITU), an international organization responsible to manage the allocation of radio frequencies. They are also responsible to regulate the use of transmitting equipment with a view to ensure they operate within acceptable tolerances.
The Convention on International Civil Aviation requires (Article 30) that the transmitting equipment be installed and operated in an aircraft in accordance with a radio licence and that (Article 29) the radio licence be carried on board the aircraft.
The ICAO documents do not provide a detailed listing of equipment to be mentioned on a radio licence. However, all transmitting radio equipment (VHF, HF, ELT, radar, etc.) installed on an aircraft should be covered in the radio licence.
SSR equipment (transponder) should already be listed on the radio station licence. Therefore, as the ACAS systems and related equipment use the same frequency group, ACAS can be considered as covered by the listing of the SSR equipment of the radio station licence.