This is good as far back as it goes. My employer’s Magnavox Research Lab participated in the first GPS simulation (1970s) test using small transmitters on the desert floor. I was visiting the lab when the results came in showing that the accuracy was twice as good as calculated. During the whoop la several of us private pilots realized that it could be used for a real 3D collision avoidance system, not a 2D bandaid like TCAS. Everything was classified so nothing happened until KAL007 was shot down, Reagan released the basic signal to the world, and the FAA finally accepted GPS (RTCA Task Force 1). A wise FAA management decided that the Korean War era technology 1090-ES version of ADS-B might not work and asked Mitre to take a clean sheet of paper and develop UAT. After four years, the 1090-ES standards group at RTCA was still screaming at each other so they authorized a UAT group. Within a year, UAT’s TSO was published (an FAA record) and 1090-ES took another year. At the Oct 2000 UPS/FedEx/Airborne-sponsored OpEval in Louisville, UAT was perfect; 1090-ES still had problems. Finally, who in their right mind would authorize two data links that don’t talk to each other and then require only the transmitter? (Unless they didn’t want the pilots to separate themselves?)