Guidance,Nav,Control, and Comm as Disciplines

"Control" as an engineering discipline (or system in a spacecraft) is a broad term that means "make it do what you want it to do." That includes the actuators and electronics. When you get into the math, you'll find that the same equations apply whether you're calculating the internal pressures in a hydraulic actuator for an engine valve, or calculating the amount of thruster input required to return a spacecraft to its desired trajectory.

In the early Apollo stuff, they talked about a GNCC system, with communications included. But somewhere along the way Comm got separated out as a stand-alone system. Sort of. I think it's because the engineering disciplines for communication are wave theory, electromagnetic propogation, antenna design, and these days things like internal data busses and packet routing. So you hire Electrical Engineering graduates for Comm, while for GN&C you want Aeronautical and Mechanical Engineering guys. The GN&C guys treat the communication system like a black box: they send a message somewhere, and it magically arrives. They request data, and the data magically appear where they're supposed to be. All the GN&C guys worry about is the transport lag (the time it takes the data packets to get around), and even that number comes from the Comm guys.

You can see how that adds up. If we hire a GN&C guy, we expect him to be able to do guidance, navigation, and control. If we hire a Comm guy, we expect him to know antennas, networks, and electronics. If one engineer tried to do both, he spend all his time trying to keep up with the state of the art in both fields, so he'd have no time left over to do useful work. It'll really teach you to trust your co-workers.

Folks do switch among the disciplines, but they do it serially. Do some GN&C until that's solved, and then go over to Communication or Data Processing, and then get up to speed in the new discipline before getting down to serious work.

Still, you'll usually find Comm and GN&C in the same "Avionics" department, so they are not truly inseparable disciplines.