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.

It really wouldn't work the same way. Firing uphill and firing downhill are quite different. Firing at ships in orbit is firing up a mega hill, i.e. fighting gravity. Firing down into the gravity well makes things much less accurate, but adds potency. (I.e. you can do an Orky/Tau trick and simply asteroids on folks, despite them not being free.)

That said, in absract terms, I'd just fluff the range and strength of weapons in the planet. -1 Damage per 'VU' for lances (representing dispersal from atmosphere?) and half-range for equivalent macrocannon batteries. Seems nice and simple, I'd say. Perhaps not terribly accurate, but it's just dealing with gravity and atmosphere. Gravity hurts the macros, atmosphere more for the lances.

EDIT: Actually, I'd presume that planetary-based devices have enhanced targetter things. Also, as it'd be planet based, you can probably pour a ton of power into the weapons, extending the range. They didn't do this in BFG (not even for Orbital Defences), but thinking about it, they'd be of much more massive range than starship weapons, for the most part. Why? Because they can afford to be!

But that doesn't really work out either. I think the damage reduction/half-range trick works neatly. Possibly add into that a -15 BS modifier in addition? (And certainly the idea of doing precision targetting would be massively reduced...)

I wouldn't call it a mega-hill, exactly. The higher the initial velocity of the macrocannon shell, the smaller will the relative effect of gravity be - the quicker it gets away, the shorter a time will gravity have to exert a force on the shell. And it would have to have quite an initial velocity, if it is actually supposed to be able to hit anything, even when leading the target. Mind you, gravity decreases proportionally to the square of the distance to the centre of the planet. A typical inhabited planet would be at most 1 VU in radius, and shells fired at ranges up to a dozen VUs. The "mega-hill" would be a steep slope for a little while, but quickly get gentler.

Sure, a beam weapon fired from the planet would dissipate some of its energy into the atmosphere, but so would a beam weapon fired from space to a target on the surface. And when it comes to the atmosphere, plausibly solid projectiles flying through it at spacey velocities would heat up and melt in the atmosphere just like, well, anything in the real world making an atmosphere entry without being specifically shielded.

The best planetary defense is a fleet of ships to face the enemy where ever he might appear. A stationary planet based gun cannot shoot through the planet and thus you will need lots of those installations to even cover a small portion of a planet.

Played a session last night where my players faced off against a pair of dark Eldar raiders.

They were helped by a collection of 24 defence lasers freshly installed on the world they were defending. I ruled that due to them being spread out over the planets surface, only 6 could fire at a time.

The defence lasers themselves were lance batteries (i.e. the biggest, nastiest lances in the RT book). They were able to focus fire as a salvo to get through void shields.

Nasty indeed, although the DE shadowfield made them of limited use.

I wouldn't allow projectile ground based weapons, atmosphere and gravity would gimp them horribly, but huge defence lasers seem practical.

If we grab the old BFG rules planetary defenses are a varied lot.

Depending on the sophitication of the defenses a good defense would be:

For deep system coverage system ships can be used. to hunt and scare away pirates. Not much use against true warships though as they are slow, badly armored and with limited weaponry. Or they can use Defense Monitors which are nasty little ships with decent armor and good firepower. Front lance and tons of dorsal macrobatteries. They should prove more then a match to any destroyer and frigate. Luckily they are very slow and easy to outrun.

When you get into orbit you can face dedicated weapon platforms armed witn macro batteries, lances or torpedoes. Some parts of the orbit can be closed of with a screen of orbital mines. Then there are defernses like the simple orbital dock or the larger space stations. Both with good defenses against attackers. Finally they can send in so called fire ships. Rigged to explode or ram any vessel that gets to close to them and filled with volatiles and explosives.

Once you come close enough to target the planet itself you will face the planetary based defences like Lances, Torpedoes and waves of Fighter/Bombers.

To bring down such a defense, I suggest to bring a fleet.

lasers too would suffer horribly from being ground based the atmosphere would cause the laser to lose cohesion, though the amount is different depending on the wavelength.

Projectiles would not be espeically more gimped than energy weapons. Both would simply need more power to get the same striking power, which is not really an issue planetside.

I just scaled down the small space station in the RT main book, increased it's armor, and gave it a single keel mount. (Note though, this was to represent a single lance turret mounted on an asteroid. The armor increase was to simulate shooting through a solid chunk of metal wider then a crusier.)