Contacttower
I cringe slightly when I see in the response above that "all you need for A/G is an R/T license (sic)".
To phrase it slightly more realistically, you need to obtain a VHF Radio Operator's Certificate of Competence as a prerequisite to either providing an Air/Ground Communications Service or going on to study for an Aerodrome Flight Information Service Officer's licence. You will need a certain level of knowledge of aerodromes and aviation to be properly prepared for the AGCS examinations, both the written and practical - do not underestimate the latter. To then operate an A/G radio station, you will need the written authority of the radio station licence holder on your certificate. In practical terms, this means that you would undergo a period of training and that he/she must be satisfied that you are competent to provide the service correctly at the specific aerodrome.
2 s