A Planetary Defense Array or PDA was a defensive weapon. It consisted of a constellation of nuclear mines, controlled through heavily encrypted links, usually from the surface of the defended planet.
Each mine when triggered used magnetic lenses to focus ions through the target; a single mine could destroy even a battleship.
The array could be swept by projectiles launched from beyond the range of the ion jets, but individual mines had a degree of mobility which made the process time consuming as well as dangerous. Further, warships from the defended world could attack the sweepers while remaining within the minefield themselves.6:2