Official rule text
77.3 STEP 1—ROLL DICE: The player rolls dice for each of their units in the active system that has the “Space Cannon” ability; this is a space cannon roll. One hit is produced for each result that is equal to or greater than the unit’s “Space Cannon” value. a A unit’s “Space Cannon” ability is presented along with a unit’s attributes on faction sheets and unit upgrade technology cards. b “Space Cannon” is displayed as “Space Cannon X (xY).” The X is the minimum value needed for a die to produce a hit, and Y is the number of dice rolled. Not all “Space Cannon” abilities are accompanied by a (Y) value; a space cannon roll for such a unit consists of one die. c If a player has a PDS unit upgrade technology, they can use the “Space Cannon” ability of their PDS units that are in systems that are adjacent to the active system. The hits are still assigned to units in the active system. d Game effects that reroll, modify, or otherwise affect combat rolls do not affect space cannon rolls.