User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-3217145-20130204112603

Here's a hypothetical. Say an invading fleet comes in to attack a planet. The defending fleet engages them in space over the planet. There are no PDCs on the planet.

Space is big, and with FTL travel, a ship can pretty much zip off anywhere else in space without you really being able to stop them. So the invading fleet sends several ships rushing headlong through the defending fleet, or around them (planets are big, no fleet can surround one completely), and into the atmosphere. The invaders may not care about firing on the planet, but the defenders wouldn't want to use high-power shots on their own planet and destroy their own resources and defenses. So the invaders surge onto the surface, obliterate the ground defenses with vastly superior ship-based weapons and maneuverability, and occupy the planet. They can take out ground troops, kill officers, free prisoners, do whatever they want, while the defenders are handicapped to weaker weapons to avoid collateral damage, and have to start turning around and heading planetside to deal with the invading force there, while the rest of the invading fleet is still firing on them from the other side.

Now assume that the planet has PDCs. The invaders can still reach the planet, but anything but super-tough ships will be taken out by the guns (even the Collector ship, which was a Cruiser and not capable of landing and taking off quickly, took a hammering under concentrated fire). As stated on the Starships page, only Frigates and Fighters are capable of landing on medium-gravity-or-higher worlds, and the PDCs would take out smaller ships being used to put troops on the ground. So the invaders can't send in light ships as troop transports, and heavy ships will be pummeled from below and possibly above (disruptor torpedoes and such). This means trying to get between the defending fleet and the planet is pretty much suicide, or not worth the resources you'll waste doing it.

In this scenario, this means that THE PDCS DON'T ACTUALLY FIRE ON ANYTHING. The invaders know that they WOULD be fired on if they ran right up to the surface, so they don't do it. The PDCs then act as a deterrent, and limit the battle to space, between ships.

Note that in battles where PDCs were really significant - like Horizon and Virmire - this is the role they filled. The ones on Virmire prevented the Normandy from landing. (Btw, "Frigates are light escort and scouting vessels. They often have extensive GARDIAN systems to provide anti-fighter screening for capital ships, and carry a squad of marines for security and groundside duty." This means the Normandy can't fire on the PDCs from thousands of kilometers away, it doesn't have the guns for it.) They never actually hit anything, they were just a deterrent. On Horizon, they were specifically built to ward off pirates and slavers, and there's no way a random bunch of criminals would have Cruisers; it would be like a street gang using a tank, they'd have no way of acquiring one. Someone mentioned that Hegemony-backed Batarian gangs might have larger ships, but I don't think the games ever confirm this, so if anything, it's probably extremely rare (IIRC the Hegemony tries not to give any actual proof that they're connected with criminals). So no, the Horizon guns probably wouldn't stand up to heavy forces (since they failed to destroy one Cruiser at point-blank range), but they'd be more than enough to stop the vast majority of criminals from being a serious threat.