User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130128042915/@comment-4967682-20130129060915

Where are you getting this stuff about scanning range being limited? I'm not seeing anything about that in the codex...

The fact of the matter is that, compared to ships, planets move extremely slowly and predictably. As everything moves in a straight line in space, the computers should have absolutely no trouble compensating and destroying the guns.

What you said about frigates would make sense, I suppose, even if there's no hard evidence of it and it brings up a few questions. That explains how the AA guns in places like Virmire and Horizon can keep off pirates and normal frigates, but it still doesn't explain how big guns like the ones on Tuchanka are useful against cruisers.