It is defined by the number of channels.
It is a VHF transmission, so there are no multi-path issues. That is how you can have APP and DEP on the same runway end.
Remember, GBAS system broadcasts a correction factor for the onboard system, that augments GPS. The signal also includes the information on the procedure for the aircraft to follow.
It is not a beam, just a signal.
If there is a terrain interference issue, another broadcast ant can be set up.
Yes there are already RNP to GBAS final approach procedures which are seemless to the ac. APCH to ILS is not seemless to the ac or the operator, and are very difficult to control.
All aircraft from Airbus and Boeing have had GBAS as a standard, at no cost option for the last 4 years. You just have to ask.
There are a few places that are currently using CAT III GBAS procedures with no issues.
Last edited by underfire; 27th Feb 2014 at 21:41.