At various odd places in the fluff, planet-based orbital defences are mentioned in passing, and when you think about it, such defences make sense within the context of the game. If a macrocannon on a ship can shoot at a planetary target from space, an identical macrocannon would be able to shoot back. Realistically, the atmosphere would be bit of an impediment, but it would pretty much work identically both ways.
Has anyone else made up rules for planetary defences? Thus far I've come up with a fairly simple system, though I haven't put it to use yet. Here's:
-Any weapon that can be mounted on a starship can be built as a planetary defence weapon, using the same stats and being fired in the same way as a ship-based weapon.
-In space combat, a planetary defence weapon has a fixed 180 degree firing arc due to being otherwise freely turnable but not being able to fire "below" the horizon (well, except for a tiny amount due to planetary gravity). Also, for a ship to be able to fire on the planetary defence weapon, it must be in this arc, otherwise the planet is in the way blocking line of fire.
-Firing at a planetary defence weapon from a ship is at -30 to hit, in addition to possible range modifiers. A planetary defence weapon is only a single weapon battery, not a kilometres-long spaceship! If, however, the firing ship is aided by a spotter on the ground providing target guidance, the penalty is only -10.
-If a planetary defence weapon is hit by a ship weapon, it is destroyed without any need for a damage roll. It is, after all, just an array of big guns, a target with damage resistance in an entirely different order of magnitude than a big spaceship enclosed in steel.
-Purchasing a planetary defence weapon is done at the same acquisition difficulty as the corresponding ship weapon. In addition, a planet that intends to make use of one or more planetary defence weapons needs auger arrays to be able to spot targets; a land-based auger station is an identical acquisition to a ship-based auger array.