Each planetary installation that is capable of putting up a defence should have a local map. This is a basic hex map that abstracts the environment around the installation. How big it needs to be depends on the defences. Anything outside of the reach of the installations defences doesn't need to be mapped. It is generally assumed that landing forces will be dropped just outside of the bases defence radius.
An installation with light defences is effective against anything trying to land within a range of 2 (the red area).
An installation with heavy defences is effective against anything trying to land within a range of 4 (the yellow area).
Ground artillery has a range of 5 (the green area).