Basic PDC?

[TheDeadlyShoe]

Well, not all inhabited planets have handy moons, especially if they are moons themselves.  Other than that, there seem to be basically three downsides.  One, the distance (~400k for earth-luna) makes beam defence impractical and gives your countermissiles increased travel time. You may also have lockon issues depending on enemy ECM. Two, in the case of a ground invasion there is no PDC to resist enemy attack.  Three, most times Earth has been bombarded its because the enemy ships instantly appeared in earth orbit because I mashed way too long of a time interval.  Lunar PDCs are problematically slow in that case.

[metalax]

The distance between Luna and Earth is only really a problem for beam defence at low tech levels. Although you shouldn't rely on luna based defences entirely.

[TheDeadlyShoe]

My thought if you have enough tech to mount beam defenses on luna (reasonably high up there!) earth is unlikely to be threatened xD

[Din182]

My thought if you have enough tech to mount beam defenses on luna (reasonably high up there!) earth is unlikely to be threatened xD

One word: Invaders.

[Yonder]

I think that it's relatively common for people to put Active Sensor PDCs on the moon, as they are the most likely to be attacked and the least sensitive to range.

I generally don't use PDCs that much at all though. I prefer Orbital Bases, not so much for the beam weapons (although that's not true anymore in my "no missiles" game) but for their mobility. It's nice to be able to tow them around as the front lines change, and they aren't all that expensive to refit given their lack of engines, so once you build an Orbital Base they can remain useful in combat for a long time.