Global ADSB is a promising service, but it is only part of the system. The primary intent (I believe) is to provide surveillance over remote areas. Remote areas, and in particular oceans, have very little comm capability. If separation is to be reduced in these areas then reliable communications is going to be needed. Theoretically it may be possible to drop to 5nm over the ocean, but not with HF Voice. CPDLC is a possibility, but I can't see that working with minimum separation. Is there anything in the works fo communications systems to work in conjunction with these satellites?