User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130128042915/@comment-5267262-20130129171126

Scanning range being limited source:

"The Argus Advanced Mineral Scanner delivers superior long-range topographical scanning resolution and rendering speed. By deploying an orbital multistatic grid of 100 radar-emitting micro-satellites, the Argus quickly delivers a global Digital Elevation Model (DEM) at 15 meters per pixel (mpp) resolution, vastly outclassing the Normandy's previous scanner peak performance of 27 mpp. Such imaging quality provides superior defense intelligence, and at a speed warranted by the dangers of combat. At slower scanning and rendering speed, the Argus can resolve down to an astonishing 0.001 millimeter per pixel—ideal for geological and biological prospecting, archaeological research, and long-term security surveillance. By employing such a massive multistatic grid of nearly-untraceable micro-emitters, the resilient Argus is virtually invulnerable to electronic countermeasures. The spherical geometry of the Argus grid also allows superior cross-sectioning of targets."

From the description/codex entry from the scanner upgrade in ME2. Obviously the o.oo1 mm per pixel is going to spot a PDC at scanning altitude (which is a height at which it is quite quick and easy to fly around a planet but also to deploy and retrieve probes, i.e. within PDC effective range). But how far away are you going to be before you want to sit tight and wait out that slow scan just to get a target, and will you be close enough for good resolution of whats on the ground at that range (they say 0.001 mm pp, but when you are far away, that number will increase almost exponentially).