What Use Is An F-call?

What use is an F-call? As I've said in the past, a Foundation License is an introduction into Amateur Radio, the first step on a path to a hobby that can take you places you never knew about. Your newly minted license opens the door to exploration, curiosity and education. It's a license to learn. For some reason, I don't recall what prompted it, I became interested in finding out a little more about how Voyager communicates with us here on Earth. That lead me down a rabbit hole filled with amateurs aiming to replicate the feat and on the way I bumped into something called Unified S-Band, the communications system developed for the Apollo program by NASA and the JPL. It consists of carriers, sub-carriers and a whole lot of interesting stuff, including being able to determine the distance to the space craft within 15 meters. I'm not going into the detail of it here, look it up on the 'net at Wikipedia, it's a fascinating read and serves as a jump off point to even more reading, phase locked loops, Quindar t