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
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.
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