It's basically a high-tech digital version of mode C, which identifies you individually by an absolute code. Plan is (apparently) that everything leaving the ground (or heading for it) from an airliner to a glider to a parachutist is supposed to have it fitted over the next couple of years.
Causes all sorts of problems, including:-
- Concerns about irradiating crew and pax in non-metal aircraft
- Power supply for the likes of glidres
- Concerns that it'll be used for charging for airspace use
- Pointless because ATC won't be able to handle the information and will filter it out anyhow
- Costs too much money
- The kit doesn't exist yet for many aircraft anyhow.
- Some aircraft (microlights in particular) don't have the spare weight capacity.
In other words, most of GA regard it as something dreamed up by Eurocontrol to make life more difficult and allow them to make money out of us.
G