Easy engineering/physics Q: what's a "Contained Beam"?

Sorry that Google was not my friend today, but I’m sure the SDMB is. Can someone tell me in plain English what “contained beam” means in this sentence? (It is in reference to how airplanes used to navigate for landing before the advent of satellite technology.)

Thanks in advance.

I think what they mean is that the ILS system sends it’s radio beams in a particular direction. (like the blue areas in this.) If your aircraft is outside those beams, you won’t be able to pick it up.

An older way of doing things was non-directional beacons (NDBs). Your navigation radio could pick up the signal from anywhere in range, you worked out which direction it was coming from and flew towards it (or just used it to work out where you were.)

Any pilots reading this are probably pulling out their hair at the inaccuracy of my description, but I think thats close-ish :stuck_out_tongue: