Question; Twilight Imperium, PDS/space battles, first fire.
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PDS fire occurs on Step 3 while space combat comes a step after on step 4. Pre-combat abilities (assault cannons, action cards, anti-fighter barrage..etc..) occur on step 4 but before the regular combat occurs. The PDS fire is simultaneous and all PDS in range can fire(including deep space cannons),including 3rd party PDS .
The only requirements for PDS fire is that a. they are in range and b. the system has been activated. Note this is activation by anyone including 3rd parties. No ship movement is even required. Note that other means of activation are usually ignored because they don't trigger the normal activation of the system. For example using a strategy card or action card may cause a system to activate but you don't get to PDS fire because it is not a normal activation that make up a tactical action even if it allows say movement of ships into the system.