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

I'm going to try my best to answer this question, but the in game material regarding PDCs is scarce at best. Probably the best answer to your question is that it was simply an oversight on the behalf of Bioware. They can't be expected to have every single aspect of their expansive universe make sense. That or they are mainly used as plot devices, so that you have someone to save when they get shot down, or you have an objective to shut down in order to further the mission. The more abstract answer is that with the exception of possibly Tuchanka, all of these cannons are intended as short range small ship deterrents for local or atmospheric defense. For example, the PDCs on Horizon were intended to protect against pirate and slaver attacks. Pirates most likely do not have access to military grade warships (besides Batarian funded slavers) and would rely more heavily on small craft to deploy troops and carry away loot. The cannons on Horizon are more than adequate for this role. Even the Tuchanka cannon was most likely meant for only short distances, as the Cerberus cruiser was in planetary orbit and thus within easy striking distance. Even in Admiral Korris' situation, he was only shot down because he strayed too close to Rannoch and when you enter a planet's gravity well, it takes significantly more energy and time to alter direction to avoid projectiles. Therefore, the most likely fill the role of atmospheric defense and to deny local military dominance by enemy warships within the atmosphere. They are adequate to destroy shuttles and frigates (which couldn't carry mass accelerator weapons before the development of Thanix cannons anyways. The Normandy didn't have any that's for sure.) and the occasional cruiser caught off its guard.

If we take the position that they are meant as more of a cure-all planetary defense, then we need to address your position that it would be easy to snipe them from thousands of miles away. Yes, it would be easy to hit a stationary target, especially with VI-assisted targeting, but I think you forget that these PDCs are stationary at all. While a ship might remain stationary in space, the PDCs are hurtling through space at thousands of miles per hour. On top of that, the planet is rotating at a relatively fast rate and the planet's gravity well and atmosphere would all have adverse effects on the trajectory of a projectile. Therefore, while in theory your logic makes sense, in practice your are shooting at a moving, rotating object with it's own defensive field, greatly reducing the effective range from which you could reliably score a direct hit. You would need to maintain some measure of speed in order to "chase" the planet in it's orbit in order to remain in the same position relative to your target. In order to remain completely within the same relative position to your target, you would have to enter the planet's orbit as well, which would greatly reduce your dodging ability from return fire. Also, all the while you are positioning yourself for a favorable shot, the enemy fleet has the opportunity to favorably engage you. Overall it would be much harder to pull off than you might imagine.

I hope this helped to answer your question, I put a lot of thought into it, haha