It was not until the 1960s that FM stations in the United States started to challenge the popularity of the AM band, helped by the development of FM stereo by General Electric, followed by the FCC's FM Non-Duplication Rule, which limited large-city broadcasters with AM and FM licenses to simulcasting on those two frequencies for only half of their broadcast hours. Armstrong's FM system was also used for communications between NASA and the Apollo program astronauts. (He is of no known relation to Apollo astronaut Neil Armstrong.)