It wasn’t an ADS-B, Transport Canada (the FAA equivalent in Canada) doesn’t like ADS-B on drones yet, so the solution was to have an SDR (BladeRF, full-duplex for Tx/Rx), the on-board SBC had a server the received the voice sample (either direct through ground station MIC or automated reading directions, alt, etc every X period of time) and then broadcasting it to the airband, so it’s simply:
Ground station mic -> internet -> server on SBC -> SDR -> airband (AM, I think it was ~120Mhz that time) -> other pilots
If you have the manual communication (where the drone pilot comms and not an automated broadcast), you can pretty much talk with pilots as if you are on-board.
I had a better write up if interested on how it works in here, in the “SDR” section.
Radio would require the planes to be listening on a specific frequency and ADSB Out would require the planes to have ADSB In which is not guaranteed.