Ok, are you building orbital defense stations or are you building PDC's? Because you seem to be swapping those two terms around like they mean the same thing.
Something in orbit wouldn't have any limitations on it's weapons.
Something in a PDC does, unless you are building it on a zero atmosphere body, since the more atmosphere you have, the more interference you get.
A orbital defense station would be a engine-less (at least in my mind) space station with lots of pew pew's on it.
A PDC is a large armoured base dug into the ground of whatever body it is built (or assembled) on.