parent_page_battlestationsxcom Placement of bases, the range of their radar installations and their proximity to alien incursions are all vital elements of an X-Com campaign. Once bases have been placed, the GM rolls for where aliens appear and what their terror target location is. Where possible, alien ships move in straight lines. An alien ship cannot be attacked by a bases weapons unless the aliens are specifically targetting a base, which they won't do at low terror levels.