Furious,
real radars can't "see" directly up. the beam usually is up to ~20 degrees. this is especialy true for WWII 2D radar, in which the EM pulses covers a banana shaped space. you can only tell if there is/isn't a target in the banana, you don't know where it is inside it. had the beam been too high and you spot a target at 4 miles, you wouldn't know if it's 4 miles away or strait up!
that is why radars are used in chains so each radar covers the space directly above it's neighbours.
we have to assume that there are radar stations outside of bases.
Bozon