User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130128042915/@comment-5267262-20130129192221

scanning a ground target and space target are intrinsically different, for a space target, you need to know something is there on the order of magnitude that you would expect a ship to have, i.e. get a ping on your space radar that is about a kilometre long an a few hundred metres in other directions and you have found yourself a dreadnought/carrier. Finding a ground target requires splitting your scan signal into the difference between terrain and ground based targets, trying to tell the difference between a hill and a turret at long range. Besides in space battles you probably hold some ships back at long range and triangulate off the sensors of allied ships closer to your target (like cortez giving you coordinates for where to fire the PDC on tuchunka and quarian tactics mentioned in the codex entry on the quarian war to retake rannoch in ME3)