Living Rules Reference · v2.0
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PDS
A PDS (planetary defense system) is a structure that allows a player to defend their territory against invading forces.
Each PDS has the “Space Cannon” ability.
The primary way by which players acquire PDS units is by resolving either the primary or secondary ability of the “Construction” strategy card.
A PDS unit is placed on a planet. Each planet can have a maximum of two PDS units.
If a player’s PDS is ever on a planet that does not contain any of their own ground forces and contains a unit that belongs to another player, that PDS is destroyed.
Source: Twilight Imperium: Fourth Edition — Living Rules Reference v2.0 (September 22, 2020), page 25. Rules text reproduced verbatim; the rulebook’s diagrams are omitted.