Ratboy and ATC Watcher,
Regarding the question of which data-link to use, it seems most likely at the mo that a multiple link system will be the eventual answer... obviously this leads to problems (either there has to be a translator on the ground between the different data-links - a potential application of TIS-B currently being trialed in Alaska - or aircraft will be required to equip with two or more equipment sets for the differing links). However, these are seen as lesser problems than the current wrangling over a single system. The Alaska CAPSTONE trials are using (as far as I know) only broadcast systems - but this does include TIS-B as well as ADS-B (the basic difference being that TIS-B uses a ground station to receive all the information from aircraft and ground users, then re-transmits it to the aircraft systems. ADS-B sends the info direct from aircraft to aircraft.)
FANS 1/A (so as not to exclude scarebus's offering!) is not just about ADS (Contract or Broadcast). The acronym stands for Future Air Navigation System but includes all aspects of comms, nav and surveillance... the system will potentially include a new aeronautical telecommunication network (ATN), ADS-C, and satellite navigation and communication (using an augmentation system for greater coverage and accuracy). It isn't just about oceanic airspace either - some airlines haven't equipped yet because they are waiting for the FAA to develop FANS for domestic use - it's just that the most discernible benefits are in the oceanic environment (e.g. user preferred routing, CPDLC).
A quick note on ADS - the contract version (ATC Watcher's 'pure' ADS) does not get interrogated as such - the ground sets up a contract with the aircraft's systems to provide information (for example, every 20 secs)... this info is then sent from the aircraft automatically, and can be used by all users to whom it is transmitted.
ADS-B is broadcast - however the information does go to the ground users as well as other airborne targets... the controller on the ground will have a consistent picture to that of the pilot watching his CDTI (cockpit display of traffic information).
Smurfjet,
The trials in Alaska are for pilots to use the system for 'enhanced see-and-avoid'... in other words, as an aid to visual acquisition... bush pilots would certainly be included, but the GA market would be the main beneficiary. It would probably be integrated with a GPS system, to provide terrain, navigation and traffic information in the same box!
ADS-B will be used for traffic separation sometime in the future (next 10 yrs?) - oceanic and domestic en-route.
RS.