None that I'm aware of 1) A lot of expensive, specialized test equipment. Avionics Test Solutions - Aeroflex 2) The knowledge to use it. 3) Factory authorization as a repair station for their radios. 4) FAA approval as a repair station. 5) An FCC General Radiotelephone license to allow you to work on radio transmitters. FCC: Wireless Telecommunications Bureau: Types of Licenses: General Radiotelephone Without the last two you are unlikely to get the factory authorization and without the factory authorization you won't have access to the information needed to diagnose problems or the special replacement parts needed to fix them.