I'm guessing you've never flown out of Newark then.I'm not sure how one would discover that "most major US airports" have any particular type of system "installed." The FCC does not list the type of system, only the frequencies, heights and locations of transmitter sites, basic emission mode and bandwidth, and authorized power levels, and the radio service under which the transmitter is licensed.
I am familiar with several "major" airports and not one of them has an iDEN system installed.
I was looking at various states airport operations, and i noticed that most major US Airports have private iDEN systems installed in the terminals.
What do they use these systems for?