I personally like to place my bases so that their radar cones will effectively cover Earth's major land masses. Do you think it's possible to show existing radar coverage whenever the user opts to construct a new base? Perhaps this showing of existing radar coverage should be in a different color so as not to confuse the player (e.g., cone when UFO is on radar == green; cone when merely displaying coverage == yellow).
If you want to be really nice, you can show what the radar coverage of the new base would be (if the player chooses to install radar) before the player commits to the new base's location.
I don't know what else players might want to know when they choose the location of a base, but IMHO, it's worth considering.