You might ask on Space Exploration about how orbits were tracked before. And note that ICBM’s predate GPS!

@JDługosz That's great and all, but they used (I think some still do) star orientation to plot trajectories. If we assume this is an arbitrary system, there's no guarantee the offensive ship(s) will have an appropriate star chart immediately available.

A star chart would give the craft's orientation in space, not its position near the Earth (nor its first derivitive with respect to time). For a vessel not in free fall star sightings (along wit a clock) give an approximate position.

But that comment is still based on a guess… find out on SX how they really worked in some detail, and then adapt that. I think such systems for attack really did exist before GPS.

Just something to keep in mind: If you make the shell too light you'll probably degrade your accuracy due to wind. If you make it too heavy you'll end up with a WMD.

How far are you from the planet when you launch these things? That will give a sense of the sort of tech you're wielding. If you can accelerate something to 0.1%c and can hit something on Earth from Mars, there are very few limits as to what you can do.

At 300,000 m/s your projectile will not survive the re-entry, it will melt and disintegrate. @CortAmmon I assumed the ships are in GSO, 42,000 km from Earth.

@ventsyv I agree it wont survive, though I don't think that's all that important for the projectile wielders. It certainly wont have time to melt, though disintegration may be a concern. Relativistic weapons work very differently than the kind of weapons we were looking at before the clarification. Suddenly you actually care about things like the diameter of nucleii because air molecules could actually pass quite far into such a projectile before finally making contact with something inside the projectile. Everything is a fluid at those speeds.

How GPS is relevant with orbital artillery?

@ventsyv do meteorites reach the surface? their speeds are only 1/10th of the one given here. 300 km/s is a normal speed for interstellar objects. whether it'll melt or not will depend on its total mass, and shape. tungsten rod might melt a tip, but there won't be enough time for all of it to melt. so the real question is, can the non-WMD constraint be fulfilled ?

@WillNess Sorry I had my units mixed up, it's 1000 degree K for every 1 km/s, or 1K per 1 m/s. Space shuttle re-entry speed is ~ 8000 m/s. Meteorites vary between 11,000 and 72,000, still well short of 300,000 m/s. I also calculated that 50kg projectile will yield 2kt, Hiroshima was about 15kt, so it's safe to say that's your WMD limit right there.

I don't know what you're discounting the fleet ships as being a GPS network, because they would be. All you need is a good clock and a radio transmitter (and some processing power to do the math), which it's hard imagining a spaceship not having. For that matter, if you've got three ships, you could use simple triangulation to accurately pinpoint any point on the surface of the planet. Or laser designators, which humans have had for decades. We use them for guided munitions, but there's no reason you couldn't fire a dumb projectile really fast back along the path of the laser reflection.

@ventsyv 50kg at 300000 m/s is 0.5 kt TNT equivalent of kinetic energy found by (E = m v^2/2). which translates to 25m radius fireball, by extrapolation.

Uhm you really mean 0.1%of $c$? or 0.1$c$? As the way you have written it, its probably stills relatively fast, but nothing I would consider as "really really fast" when speaking of an launched projectile I mean 0.1% of C is like.... 1080000 Kilometre Per Hour what is to speak in meters pers second.... "just" 300 KM per second. If you aimed with that canon trying to shoot down Juno just near Jupiter, it would take the projectile a few weeks to reaching it. Nothing I really would consider "really really fast"

@Zaibis When it hits you in the face, 0.1% of c is still pretty fast.

@AmiralPatate: When something hits me in the face even mach1 is pretty fast. As this is a thing of relativity to other things I just asked for calrification and your last comment didn't help to calrify at all.

@Zaibis I purposefully mentioned 0.1% of c. We're talking surface warfare here, the next fastest thing on the battlefield would be something like supersonic cruise missiles which is three orders of magnitude below. It is really really fast.

If you can move a fleet of capitol, destroyer and transporter ships through inter-stellar space you are going to know where they are relative to each other. If you put your bombardment ships around the planet in in known positions you have a constellation of satellites. If you have a constellation of satellites that can broadcast signals to units on the ground you have the ability to define a Global Navigation Satellite System all of your own. If you did need GPS your fleet and marines already have it. Mapping of targets and topography is your hard task here, not automatic location.

Obligatory xkcd "what if" link delving into impact damage on a planet with higher and higher speeds...