Think back to the state of the art in the 70s when these were thought up. You can cite SIGSALY[1] but it took a really long time for these concepts to be flyab

Think back to the state of the art in the 70s when these were thought up. You can cite SIGSALY[1] but it took a really long time for these concepts to be flyable. AT&T was doing digital telephony terrestrially in the 60s [2] but radio links make this more difficult. Of course things are different today. [1] https://en.wikipedia.org/wiki/SIGSALY https://en.wikipedia.org/wiki/SIGSALY [2] https://en.wikipedia.org/wiki/T-carrier https://en.wikipedia.org/wiki/T-carrier