In Twilight Imperium 3rd ed you may get a technology advance that lets your PDS fire to a system adjacent to the one that contains them: The Deep Space Cannons.
The rules of firing in their same system stand but there is one extra scenario not described in the rules:
- If the enemy is at distance 2 and moves to an adjacent system, passing through an adjacent system will the PDS fire once or twice? (Meaning, will they fire once the enemy enters the first adjacent system and once more once the enemy reaches the destination adjacent system?)
Of course, what if the enemy is moving to the system that contains the PDS, would they shoot twice?