FYI: CPDLC and ADS-C are both part of FANS 1/A. CPDLC is basically text messaging between the pilot and controller and ADS-C gives the controller your position information - ATC will set parameters as to when to report. For example ATC will tell the aircraft to give a position report every 5 minutes...

"what rf/radio systems are used as the communications link - 1) over land masses and 2) over oceans or non-radar areas. VHF? HF? Satellite? In what order of priority? If Satellite, which system? Does it vary from carrier to carrier? To the pilot, this is probably transparent, as the avionics takes ...