I know its by knowing where it Isn’t, but how does it actually work?
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Before GPS, **inertial navigation** (three spinning gyroscopes that tilt when “your” path varies from the path you’re supposed to travel) was used.
Thus, the missile (**or airplane**) didn’t know where it was; it just knew *how far away* it was from where it was *supposed to be.* Other hardware would detect such drift (from, say, crosswinds or slightly unequal thrust by the engines) and then automatically correct the course.
IOW, **it’s an autopilot**. (It’s how the first practical autopilots were designed.)
To use it, you must know:
1. where you are (starting point),
2. where you want to go,
3. what route to travel, and
4. at what speed to travel.
Put that into the (possibly mechanical) computer, and take off.
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