It's war on other planets! A century old feud devolved into full scale interstellar war as the Federal Fleet goes on the offensive for the first time in its history.

In an attempt to end the war swiftly, Federal Strategic Fleet Command devised a daring plan, a plan that hinges on the success of military operations on the surface.

With the battle still raging in space, battleships move into position to land their troops on the planet below, and stand by to deliver orbital artillery support....

There are two problems here packed in one inconvenient package. The first problem is to locate targets on the surface. The second problem is to locate the ship relative to the surface.

You might be thinking GPS, which would make things trivial. However, this scenario can happen on a planet with a GPS system controlled by the enemy, or no GPS at all, which is effectively the same thing. You cannot rely on infrastructures the planet may or may not already have, and that you may or more likely may not get access to.

So the question is:

How can you accurately and reliably hit targets with orbital artillery without using GPS?

I was thinking triangulation using the ships, however they aren't really built to be GPS satellites. I'm not quite sure what you need for an accurate geolocation besides a good clock. Also there's the fact that the ship themselves don't really know where they are themselves, which may or may not be a problem.

You can use anything you can bring with your own spaceships, preferably something that doesn't need to be deployed (like satellites) since those can be shot down. Troops on the ground can use any sort of beacon to designate targets, though the beacon has to be man-portable (and also woman-portable), and preferably launchable through a 40mm grenade launcher.

We are talking about precision strike, give or take a couple meters. Targets are not mobile. The goal we strive for is zero collateral damage.

I should also specify, the artillery we are talking about launches dumb (i.e. not smart, i.e. not guided) projectiles. It launches them really really fast, like 0.1% of c fast. They are also stupidly light to limit kinetic energy at non-WMD levels.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Commented Jul 12, 2016 at 19:00

9 Answers 9


You don't need GPS if you can detect the target from orbit via radar or using high quality optics. Once you select your target your computers will calculate what you need to do to hit it. The calculations are pretty standard, you convert from spacecraft reference frame coordinate system to ECEF coordinate system taking into account the pointing angle. That requires line of sight but if you position a handful of ships in GSO you can pretty much cover the whole planet.

I don't know why you put restriction on smart/guided ammunition.They've been around for a while., there is no good reason for space going civilization not to have them.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Commented Jul 12, 2016 at 19:00

There are some very deeply rooted, age-old issues behind your question. In a nutshell they all boil down to: How do interstellar travelers navigate among the stars? Let's take a look at this question in depth, and then we can see about answering your specific question.

Terrestrial Navigation

So how do people on EARTH navigate, anyway? Well, thanks to generations of people who were either very bored or very curious, humanity has a strong collective grasp of exactly where the Earth is located within the solar system, and exactly what we can expect the night sky (and day sky) to look like at any given time on any given day. This is key to navigation. Using a sextant and any one of a set of Navigational Stars (or the sun, moon, or planets) a navigator can determine his/her exact location on the surface of the Earth. The positions of satellites in orbit are monitored by tracking stations at fixed, known location all over the Earth whose positions are accurately determined using stellar navigation. The GPS would not be able to function without these tracking stations, since their positions would decay over time. The tracking stations are able to monitor and adjust the orbits of the GPS satellites, since the positions of the tracking stations never vary, but remember that their position could never have been determined without first using the stars to get a bearing. No matter how advanced the navigation technique, a map of the sky is needed to take bearings.

Something to remember is that the mapping of the sky around the Earth has taken literally thousands of years. Methods have become more sophisticated and technology more advanced, but the core of knowledge was still gained through prolonged observation and experience. This is presumably something our intrepid interstellar explorers will not have.

Extraterrestrial Maps

Luckily, thanks to a nifty phenomenon called Stellar Parallax, we humans have managed to map the stars beyond our solar system to a startling degree of accuracy. Unfortunately, this method has a limited range. Presumably as science marches on we will produce more and more accurate maps and longer and longer ranges, but as it stands we can currently only take accurate parallax measurements to a distance of ~1600 ly. This might sound like a lot (and it really is pretty damn far) but the galaxy is a very vast place. Our maps cover about 0.07% of the Milky Way Galaxy. That's not terribly encouraging, but for now let's just focus on what we do have mapped.

Extraterrestrial Navigation

So let's say we have someone who is really bored, really rich, and really sick of people. They hop in star ship and blast off at FTL speeds (#magic) and makes it to Polaris, which is about 433 ly away. But what now? Our explorer is too far away from Sol to use accepted sky maps. Stars will be in different positions in the sky thanks to parallax, since our explorer is so far from home. Luckily, our star charts cover this area, and with a bit of extrapolation our explorer can predict what the sky will look like in the Polaris system just from our knowledge of where the stars lay with respect to the Sun. If the ship has a powerful enough telescope, our explorer can use his current navigational data to orient himself in Polaris and start mapping more distant stars using more parallax. This might take some time, but it will both increase the accuracy of current star map and sky map and extend the current star map past what is currently achievable from Earth.

Next our explorer will want to start mapping the planet (I assume he found a planet upon which he wants to settle). In an empty vacuum it would be very tough to chart a planet's orbit, but our explorer has luckily already mapped out the sky using his extrapolated parallax data. Using these stars as a reference, our explorer can watch the planet(s) and sun(s) and moon(s) and get a pretty accurate picture of the system of orbits, including critical data like eccentricity, inclination, etc. Combining this new data with the sky map can produce an almanac of useable navigational points in the sky including stars, planets, moons, suns, and possibly other alien objects. It is imperative, however, that this process goes in this order. Trying to map the planetary orbits without first mapping the sky will result in failure.

Now that we have an almanac of navigation points our explorer can navigate the star system to his heart's content. Tracking stations can be placed on the surface of the planet using stellar navigation similar to that used on Earth, only with an alien sky and alien navigation points. These tracking stations, being permanent references, can be used to further deploy more navigation equipment like GPS satellites or the like. Now our explorer can move on to another star system and repeat the process for as long as there is predictable parallax data from which to pull.

Orbital Bombardment

Now for the fun part: bombing stuff. In the best case scenario (for bombing, that is) the planet over which the war is being fought is populated, the sky map is well documented, tracking stations are in place, and orbit data is very accurate. In this case orbital strikes can be calculated down to the tens of centimeter (that is not an exaggeration; we can almost do that right now on Earth with missiles). This is, of course, utilizing every available resource from ground station data to GPS satellites to visual rangefinding. In less optimistic situations the battleships will likely have at least some combination of above resources from which to draw. This will limit accuracy somewhat, but if there is a sky map and available orbital data then strikes are possible no matter what. In the worst case scenario where a battlefleet must fight in an unmapped system there will be considerable difficulty even hitting the correct continent. Sky mapping and orbital analyses will be fairly critical for accurate strikes against ground targets.

That being said, all of the above assumes the use of fairly modern navigational methods. It is entirely possible that as science marches on technology will allow for accurate machine vision enabled computers that can orient a ship in orbit without any sort of sky map. A sky map would still help, of course, but in a pinch such a computer might be able to put the hurt on a target even without in-depth star mapping and orbit analysis. High power telescopes and accurate laser weaponry would also need no navigational data to hit a target in visual range. Just point and shoot so to speak. Technology may also allow for very fast mapping of the sky and orbit analysis. Current methods would take months to accomplish this with any guarantee of accuracy, but delicate enough sensors could perhaps give accurate data in minutes or even seconds, making the entire process effectively instantaneous.

Final Thoughts

I would say that given any reliably effective method of travel between star systems, there will also be an equally effective method of navigation to ensure star ships can get from place to place. Mapping is also the first thing anyone will likely do upon entering a star system for the first time, so unless there is literally no time at all to get one's bearings, it will probably be simple enough to warp into a system (or whatever method of FTL you prefer), map out the sky and any planetary orbits, and then commence with the mission, whatever it is. If mapping is truly daunting, then I would personally sent unmanned mapping probes far ahead of my colonists and military forces to ensure some data is gathered and waiting if I enter an unknown system.

I hope I've helped some. Good luck!

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    $\begingroup$ "thanks to generations of people who were either very bored or very curious" I love your answer. +1. $\endgroup$ Commented Jul 11, 2016 at 16:51
  • 2
    $\begingroup$ "now for the fun part: bombing stuff." By the way, an excellent answer. $\endgroup$
    – Wick
    Commented Jul 12, 2016 at 0:57
  • $\begingroup$ GPS measures most everything relative to known, carefully documented datum of the planet. You don't mention this, but come within a hair's breadth by mentioning all the ways in which those surface datum are gathered. Well done. +1. But look up datum publications anyways. $\endgroup$
    – Lord Dust
    Commented Sep 4, 2016 at 7:51

When you have at least four battleships in orbit above you (three when you got a very good clock), you have GPS.

To determine your position on the planet surface, you need

  • the exact positions of three objects in orbit (orbits are very deterministic, so you only need to know the exact orbital elements of a satellite to extrapolate its position at any point in time)
  • your distance to them.

That allows you to triangulate your position. GPS determines the distance by having the GPS senders in orbit send their current time accurately to the nanosecond. The GPS receiver can then calculate the distance to the sender based on the time-lag due to the speed of light. A fourth GPS sender is required as a time reference, unless the GPS receiver also has a clock which is as accurate as that of the GPS senders. Present day GPS receivers do not have clocks which are accurate enough, but in a science fiction scenario where you have FTL travel, atomic clocks might be small enough to fit into a handheld device.

That means you just need your ships in orbit to send their current position in orbit and their current time to the troops on the ground (normal GPS satellites do not send their position regularly. They don't need to because they maintain predictable orbits. But a ship in battle will likely maneuver a lot). Note that you must use some form of communication which is bound to the speed of light for this to work. The troops can then use these messages to triangulate their position. Orbital reconissance can also be provided by the ships in orbit.

When you want ground troops to assign targets to ships in orbit, they can do so by sending the GPS coordinates to the ships. Should a poor soldier have lost their GPS receiver and only have their radio available to shout "blow up that thing a mile southwest from me", the ships in orbit can triangulate the source of the radio signal. You just need three ships in orbit to receive the radio signal. They can then do the GPS triangulation in reverse: They compare the times where they received the radio signal to calculate their individual distances to it and thus find the exact location from which the signal originates.


One existing tool which has been used for year is laser guidance. You have someone on the ground "designate" a target, and the bomb seeks in on that point. Its basically a super-fancy semi-active seeker where the target is lit up by a human who "knows" where the shot needs to go.

If you don't have eyes on the ground or some comparative ability from orbit (like from satellites), then it may be wise to call into question why you need a precision strike against something you cannot see. What are you trying to hit so precisely anyway?

  • $\begingroup$ I added a clarification, the projectiles aren't guided, though I do have laser-guidance in my general arsenal. $\endgroup$ Commented Jul 11, 2016 at 15:29

It seems to me that any properly motivated attacking force would have many, many ways in which to do this.

First and foremost, deploy your own GPS sattelites, or hijack the enemy's. After all, you control the orbitals, so what's to stop you from hacking, or destroying their own network, and then launching new ones?

If the enemy destroys them you can always simply launch more, or deploy smaller support ships with defensive weaponry at key points such that they act as guidance and triangulation beacons (GPS ships, as it were).

This is absolutely vital, and you have no real excuse for not doing so. After all, your armada will be surrounding and attacking the planet, how are you planning to maintain communications if you don't establish a network of satellites or ships?

The other major variable is the type of projectile you're launching.

Guided missiles are very accurate, can hit mobile targets, and are generally easy to guide. All the troops need is to paint the target with a laser, or deploy a radio beacon, and the missile does the rest. However, they can be interefered with via electronic warfare, and if you're launching them from orbit they must first safely enter the atmosphere before striking their targets. You're better off deploying them on the ground and launching them from within the atmosphere IMO.

When you want to take out enemy bunkers and strong points, however, your best bet is "dumb" artillery - aka dropping rocks on them. And these must be carefully aimed, because they're not going to discriminate against who they're squishing.

The problem with these projectiles (rocks) is that you can't possibly aim them that accurately. Not only will you have a difficult time calculating the trajectory from orbit relative to planetary rotation (although this is doable), but you can't possibly account for all the trajectory adjustments needed for atmospheric entry, winds, etc.

However, you can take the concept of dropping rocks, and come up with something a little smarter. Your projectile can be a massive chunk of metal with both a shaped heat shield, deployable fins, stabilizing thrusters for fine tunning the trajectory, and possiblly a gyroscopic stabilizer at its core. Once it has entered the atmosphere it can start tracking in on its target much like a missile, but without suffering from the same ... fragility. The damage it would inflict on its target would be based on the force of its impact, not due to any explosive payload it might carry (although antimatter, or nuclear warheads could be deployed in this fashion).

  • $\begingroup$ The idea was to throw rocks straight down, to limit atmosphere interference. The smart rock option is interesting though. $\endgroup$ Commented Jul 11, 2016 at 15:44
  • $\begingroup$ @AmiralPatate - "The idea was to throw rocks straight down, to limit atmosphere interference." <- that's like flapping your arms so that you won't hit the ground as hard. A "dumb" projectile will experience significant trajectory degradation when entering the atmosphere. Also, if it enters the atmosphere too agressively (aka "straight down") it's going to probably disintegrate unless it's large enough, at which point it's no longer a "precision strike", and will level anything for miles and miles around the point of impact. $\endgroup$
    – AndreiROM
    Commented Jul 11, 2016 at 15:47

GPS is irrelevant. As the Battery Commander of a collection of orbital artillery pieces you need to know where to point your guns and how fast the projectiles should leave your guns' barrels.

GPS is designed for troopers to report their position in a common frame of reference so that a battery without line-of-sight on the target can calculate the appropriate angles and weapon charges to deliver munitions to target.

If your spacecraft are in low orbit (300-400Km above the surface) the flight time of your projectiles are ~1s and with "good enough" optics you could potentially take hand signals from the troops and aim manually at their targets.

If your spacecraft are in a stationary orbit (~30 000Km for an Earth-sized planet), the flight times of your projectiles are ~100s. In this case, if you position your ships above the target, the trajectories will still be essentially straight lines (down) and with the crosshairs adjusted for the planetary rotation while the projectile is in flight, you will still be able to aim manually and hit your targets.

Different orbital periods will complicate the trajectories that need to be calculated but the mathematics that allow a GPS receiver to locate itself on the surface of the Earth with timestamps broadcast from three different satellites on deterministic paths over that surface is called Trilateration (not Triangulation, Grr).

The same mathematics are equally applicable to a collection of spacecraft that know their relative positions and each receive the same timestamp broadcast by a trooper on the ground.


While the WW2 they hit target out of sight with shells from a ship over the curve of the planet.

With advanced tech their is so much way of doing it I don't know where to start. For triangulation you just need 3 points. Your ship, your troops, your target thats 3. You can keep track of your troops with a radar tracking a specific frequency that there communicator transmit, then the troop use material like a laser coupled with a compas or even simple instruction like : "500m north" and you calculate where to shoot.

You can add precision with a high altitude drone that keep an eye on the action. Or even create your own mobile GPS with a net of drones.


You can use mountain peaks and ranges as your beacons. Triangulate three points and get your bearings that way.

If there are no visible features for your ground troops, have the orbiting ship quickly 'scan' the planet and create a digital elevation model, DEM map(as detailed or undetailed as your require for your setting). Your ship then transmits this map to your groundtroops who use standard navigation techniques to figure out where they are and where they are going.

Your troops can either transmit their calculated co-ordinates. If they are not accurate enough, your ship could use location beacons attached to your troops. These beacons would not be tied to a planetary GPS system but rather use the ship itself as an origin point. The ship can then determine the troops x,y,z distance from the ship.

If you don't want to bomb your own troops, they can say that the target is 3 degrees, 14km from them and your ship takes the necessary reading from them and works out where the target is. Or they could just 'light them up' with those portable distance reading laser things.

  • $\begingroup$ What "standard navigation techniques" do you propose to use on the surface of an alien planet? $\endgroup$ Commented Jul 11, 2016 at 15:20
  • $\begingroup$ Triangulation off known bearings, taking a star (any star) and seeing which way it appears to move from your changing position. The length of a shadow at high noon, the direction of that particular sun's setting and rising, counting steps, using that laser distance thingy to read the terrain. Droping a beacon at a certain point and then using that as a starting reference frame. Stuff like that. If the planet happens to be full of swamp grass and has three moons, it shouldn't affect how you would use your brain to figure out were you were. Groundtroops should be trained in how to navigate... $\endgroup$ Commented Jul 11, 2016 at 15:27

The simplest thing if you have spacial advantage and lots of time - park in stationary orbit. Then you can do whatever you want and the result will land in pretty much the same location. Even if you had nothing else, you could do things like small drop at high speed to inhabited location nearby the target, and then approximate from it the changes for the final bombardment.


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