Unfortunately, the FAA has decided to keep the message strings the same. There are many, many comittees working on trying to work with the string, add to it or modify it, to no avail for at least eh last 10 years.
(Akro, the diagram doesnt show ADSB-in, is shows the ac being able to see each other)
As far as UAT, it doesnt matter how it is transmitted, the message bit is the message bit. The freq bandwidth of the 1090 and 987 Mhz is the problem, there are only so many messages that you can put into each frequency, how fast you do it doesnt matter.
From last year:
From the
Garmin site:
Even though it’s called a “Universal Access” transceiver, the 978 MHz UAT is actually less universal that the 1090 MHz “ES” transponder in terms of where it can fly and what airspace it can use.
For example, in its criteria for ADS-B “Out” compliance, the FAA will require 1090 ES transponders for aircraft operating higher than 18,000 ft MSL — while UAT is limited to aircraft that will operate no higher than 17,999 ft MSL.
So far, no country except the U.S. has accepted the 978 MHz UAT format for their ADS-B datalinks. (this may be why there is the issue)
Consequently, pilots who want to fly outside the U.S. — or operate at or above FL180 — will need the 1090 ES transponder for ADS-B “Out”. Since the FAA has decided to retain its Mode C and Mode S transponder requirements for flight in regulated airspace (to provide a secondary radar backup to ADS-B, as well as communicate with other aircraft that have traffic warning systems), the 1090 ES solution can satisfy both the ADS-B “Out” and functioning transponder requirement with the same piece of equipment.