ADS-B was developed in the US to address areas of sparse RADAR coverage. The original project was called Capstone. It was used primarily in Alaska but also to serve some of the Gulf oil platforms where helicopters were below RADAR coverage much of the time.
The original Capstone protocol was very well designed and ran on 978MHz. It does an excellent job of not only allowing Aircraft-to-aircraft transmissions, but also aircraft-to-ground, and ground-to-aircraft. That is why it was chosen for the FIS-B (weather/RADAR) uplink.
The fly-in-the-ointment is TCAS. TCAS is mandated by law for transport aircraft so it MUST be present. TCAS runs on 1090MHz so some bright person (bright is actually spelled I-D-I-O-T) got the idea to put ADS-B on 1090 because that equipment was already installed in transport aircraft. Ah, but the devil is in the details and ended up requiring a change to the equipment in transport aircraft to allow for longer messages in the Mode-S transponders. This “extended squitter” (ES) is required to send the ADS-B position messages. Once they did that they ended up with TWO parallel, incompatible systems, one well-designed (Capstone UAT, 978MHz), and one badly hacked together (1090ES). Of course, as with most situation where you have competing systems, the worse system is chosen for political reasons. The ICAO, not known for its technical prowess, went with 1090ES as the “standard”. Now you know how we got here.
Now suddenly people in other parts of the world realized that to build a ground network like the US has would cost a LOT of money so maybe they could use some satellites instead of a ground network. It would be much cheaper for the various governments (zero in many cases). Is it too much to ask airliners to add extra hardware, cables, and antennas to allow their signals to be transmitted to satellites as well as ground stations and other aircraft? After all, the airlines are rich, right?
General Aviation? General Aviation? We don’t need no steenkin’ General Aviation!
By the way, uAvionix had a good and sneaky idea. Put the antenna on the tail and give it a radiation pattern that goes up as well as down. No diversity hardware needed. I keep waiting for one of the antenna manufacturers (RAMI, D&M, etc.) to make a tail-mounted transponder antenna to do what uAvionix did with their Tailbecacon. Heck guys, team up with Whelan and create an LED-tail-light-strobe-transponder-antenna device. That could be done for well under $1000. They’ll sell a lot faster than an $8000 diversity ADS-B/transponder box from Garmin.
Aw, what do I know.