# Early warning detection system for ground troops, against orbital strikes

A ground based army wishes to be able to defend against orbital strikes by unguided hypervelocity projectiles. Handling orbital laser platforms and guided orbital projectiles is out of scope for this design. Since the best defense against these kinds of attacks is to not be where the projectile impacts, the early warning system is supposed to facilitate getting out of the way.

The design intent is to develop an early warning system that will allow ground troops to function as autonomously as possible. Army doctrine is to not rely on the Space Navy anymore than absolutely required. Having a constellation of satellites would solve this problem but is unacceptable to Army High Command. High Command wishes to have fallback early detection services should the Navy decide they need to be elsewhere.

## Early Warning System Requirements

• Self-contained
• Able to run off 100KW power source
• Mobile (static early warning measures are huge targets)
• Carry enough processing power to make the signals gathered into something useful.
• Only for use on Earth-like planets.
• This system can be tied into the global battle information network but should be able to function without this connection/functionality.

## World Constraints

• No FTL travel or comms
• Laws of orbital mechanics are no different than real life
• There are space fleets in orbit.
• Troops are delivered and retrieved from the surface by large shuttles.
• Tracked and wheeled vehicles only. Legged vehicles have been shown to be less effective and harder to maintain.
• All infantry is mechanized. Every infantryman has a truck, tank or APC that they can use. Troops are trained then drilled frequently to facilitate rapid egress from an area.

Is ground based detection of hypervelocity projectiles from space possible? Can the detection method be mounted on mobile platform? Is radar good enough or are there other/better approaches?

• Is this question about "rod from god"-like weapons (unguided, 1kton weapon does 1kton damage), or about any hypervelocity weapon (guided, nuclear) that could be launched from orbit? The Rods may be fleeable on truck but a guided missile certainly is not. – bukwyrm May 22 '18 at 15:56
• @bukwyrm unguided rods from the gods style weapons. Guided weapons require far more than early warning. – Green May 22 '18 at 16:12
• Is the enemy capable of fielding black paint? If yes, then you have no chance. – Aron May 22 '18 at 17:15
• @Green it's not an early warning system then is it. Assuming the black paint is removed at the karman line of earth (100km), and the projectile is coming in at escape velocity (11km/s). You have about 10 seconds to get everyone to a safe distance from the impact site...I'm thinking at least a few km... – Aron May 22 '18 at 21:24
• @Renan Military SF almost always refers to military spacecraft as a "navy", and uses naval-style ranks. Some have "space marines" who boarding enemy ships and stations, landing on the ground to seize key enemy buildings, etc. – Monty Harder May 22 '18 at 22:35

Is ground based detection of hypervelocity projectiles from space possible?

No. If you are detecting potential projectiles from ground stations only, without being able to trace their trajectories in space, you won't be able to differentiate a missile from a meteorite until it is too late.

A hypervelocity projectile may have the same mass and profile as a meteorite, but with an ablator to keep it from disintegrating in the atmosphere.

All an enemy needs to do to fool your system is throwing a lot of rocks against the planet in different locations. The amount of false positives this will trigger on the systems should be enough to render it useless - you will either be constantly and pointlessly moving your forces around, or you will stop trusting the system.

Once a projectile has gone deep enough into the atmosphere that it won't be burning anymore (about 70 kilometers away from sea level), it can discard ablators, turn on thrusters and hit the ground in a matter of seconds. I did a quick research on the world's fastest missiles nowadays... I don't know if this is the fastets one, but The Indian Armed Forces's Shaurya are able to reach Mach 7.5, even on low altitudes. They could clear the distance between ablator discarding altitude and sea level in less than twenty-nine seconds. If the payload is a nuke, you will never get far enough from the blast. Food for thought: in the future, missiles may become even faster.

Can the detection method be mounted on mobile platform?

With our current technology, we use things like these to search for NEO's (near Earth objects):

And it can only detect a portion of of the sky at a time. In order to replicate that with mobile bases, you would need a very large array of mobile antennas covering about the same area, and working as a network. And you will only be looking at a very narrow piece of the sky. Not efficient enough for your needs.

Is radar good enough or are there other/better approaches?

Radar is the method favored nowadays for Ground-Based Midcourse Defense (GMD), so at least here you're good.

• I would imagine you could use spectroscopy to tell the material differences between a natural rock meteorite and a manufactured ablator shield. – Josh King May 22 '18 at 22:05
• Why turn on thrusters? If you are a "hypervelocity" projectile, mach 7.5 is slow... – Dietrich Epp May 22 '18 at 23:25
• @DietrichEpp without engines of its own, most projectiles would decelerate to terminal speed. That will give plenty of time - in the order of many minutes - for a ground force to destroy it with a ground-to-air missile. – Renan May 23 '18 at 1:15
• From the point of view of the men on the ground, it's the same being killed by a meteorite than being killed by a projectile, so a system which detects both is a feature, not a bug. If the enemy is able to launch swarms of meteorites they would do so, since rocks are cheaper than high-tech projectiles. – Rekesoft May 23 '18 at 9:33
• @Rekesoft I beg to differ. If you are only detecting a meteorite through ground based radar and optics, you will have a hard time determining whether it will: A) disintegrate in the upper atmosphere B) take fifteen minutes to hit the ground and open a five-meters wide crater, killing at most a handful people or C) turn out to be a nuke inside an ablator shield, taking half a minute to reach detonation altitude with a blast range of a few kilometers, killing all of your platoon. Now imagine an enemy bombarding a 100-kilometers wide circle with one true nuke and a thousand cheap duds. – Renan May 23 '18 at 22:41

## Sensors

IR and Radar: A projectile entering atmosphere is going to generate a lot of heat. Thus, infrared imaging is how you first find your target. Then a powerful radar lights up the target to get a more accurate trajectory.

## Platforms

Air or Ground: You're going to mount these sensors on different platforms depending on your specific conditions. Airborne drones would be nice - there's less atmosphere getting in the way between you and your targets. Also, there's some distance between you and your radar in case someone detects it's activation and fires on it. But if you don't have air superiority, a ground based asset may be more survivable / defensible.

## Actions

Shoot it Down: Getting out of dodge probably isn't a good strategy - the time from detection to impact will be low. Instead, think of birdshot. Put a lot of flak in the air, and the hyper-velocity round will make contact with some of it. At those speeds, any change in flight profile is going to result in a miss. A hit that's direct enough to induce tumbling is going to bleed off a huge amount of energy all by itself.

## Putting it Together

This may all happen fast enough that person cannot be in the loop. It's a system that you turn on, and it alerts you as it's shooting down the target.

• Put a lot of flak in the air, and the hyper-velocity round will make contact with some of it If that was viable, current missile shield systems would work like that. – Renan May 22 '18 at 20:41
• @renan Currently, only missiles have the range to kill other missiles. I have every expectation that the "put a lot of flak" in the air tactic will take over once the Navy gets their railgun working - missiles are dreadfully expensive. – codeMonkey May 22 '18 at 20:49
• Also see - Navy CIWS. – codeMonkey May 22 '18 at 21:58
• By the time the flak can intercept the round it's going to be pretty close to the ground anyway. If it's of any size you didn't do any good. – Loren Pechtel May 23 '18 at 1:51
• As for the CIWS--it's got one big advantage, the missile almost certainly has lift surfaces. It doesn't take much to make it lose lift and fall into the ocean. Even then, though, a successful intercept very well might result in missile bits hitting the ship. When you're talking KEWs, missile bits hitting are just as bad as the whole thing hitting. – Loren Pechtel May 23 '18 at 1:52

A rod from god drops at a bit less than orbital speed (because it is "braked" out of orbit, in which it had, by definition, orbital speed). By m*v² we know that is a lot of energy, and we shudder in awe. But. The atmosphere will make sure, for any reasonably small object, that it hits ground with terminal velocity. For a heavy rod 10 meters long that may be Mach 10, but 3km/s is still much less than orbital velocity. This does not improve for rods that are somehow "shot" down, i.e. accelerated into an more eccentric orbit that intersects the atmosphere - that would just mean more energy to bleed. It is this atmospheric braking that essentially confines the rods to being sleek masses of tungsten - most other materials are either not dense enough (the rod needs to be sleek yet heavy), or not heat resistant enough (remember the awe-inspiring amount of energy fresh out of orbit? 80% or more of that will need to go away...). So you can be pretty sure there are no fins or guiding mechanisms on that thing. The trajectory will thus be a parabola. Know three points on a parabola, and you know the parabola. Of course there are winds and non-uniform air-densities and bla bla, but the travel time from intersect with atmosphere to ground will be on the order of 60 seconds, so not much time to interfere. All the shed energy is discarded in those 60 seconds, which means a huge value for energy/time, which is Power So there should be no real obstacle in detecting it. Allow for 2 seconds to discern from micro-meteorite. Blare alarm. Allow ten seconds for every soldier to lash themselves to the nearest truck by means of a specially designed leash. Everyone else just ducks and covers. The trucks have a (probably electric) sprint capability, and a guidance system that allows for "be quick or be dead" kind of path-choosing (better not be in front of one...) the trucks now accelerate away from impact, and eight seconds later they are at 100km/h, meaning they have covered about 100 meters. Every second more (and there should be about 40) will gain another 25 meters. Is that enough?

Wikipedia has a 10 ton rod at about 10 ton TNT impact energy. That is a lot, until you realize it's not a bomb, its a tungsten rod travelling really fast. No air burst, not even ground burst. It's going to penetrate. It's a bunker-buster. You'd probably not like to be around (and even more probably not like to be around in a truck careening at 100km/h), but it's a lot less dangerous for ground troops than it is for bunkers. Assuming the impact-forecast was correct +-100m, and everybody was scattering radially from that point, the most out-of-luck crew will be 1000 meters from impact - it will suffice. It's not high-explosive, and it's not shedding that energy into the air or structures - it's shedding into the ground.

About detection: you'll have to detect a glaring object, from a few hundred kilometers away, there should be no problem. You need a splash-down forecast 12 seconds into the atmosphere (Think, blare, lash), which should be enough for a rough estimate if you have your detection gear lateral from the flight-path. This means the detection gear travelling with your troops is actually protecting the people a few hundred km away, and theirs is protecting you. Good for morale... If your detection gear needs to be with the troops it's protecting, i.e. near ground zero, the best it will be able to do is predict a very long impact-ellipse, which can help to flee the area perpendicularly.

Note that it is impossible to discern between a tungsten rod with, say steel, weak spots, and a usual rod from god. So the enemy now has a means of clearing a km²-sized area by sending a fake rod that breaks up and fireballs harmlessly 10km above target, while their ground troops move into the area unopposed.... Perhaps better to adopt a general duck and cover strategy, maybe splurge on some inflatable cocoons to mitigate the shock wave.

Not really much point, signal return time is not your friend, orbital observation platforms, AKA your troop ships, are better as they can see rounds launching in something like realtime. They also carry enough computing power, it's call an orbital navigation bridge station or their own planetary bombardment systems, to work out their point and time of arrival fast and accurate. Since they're closer to the launch platform they can relay good targeting data to the troops on the ground quicker than the round trip time of any lightspeed ping from ground to orbit and back. Also orbital assets are able to see rounds launching from positions that are "over the horizon" from ground positions.

If the army really wants an independent system they can build one but it's going be less effective, possibly terminally so. Radar or microwave are going to be your most practical lightspeed radiations to resolve incoming rounds but they're also prone to severe pollution in the form of unit communication networks, cell towers, radio stations, etc... and from the ground an orbital round coming straight at you is a very small reflective object too. In short your first warning at ground level is probably going to be when it punches through the cloud layer.

Do note that most orbital bombardment weapons have damage outputs measured in, at minimum, kilotons. As such even mechanised troops are in trouble, as in completely done for, if they're at or close to ground zero.

• Maybe they can have a big dish that spins and goes beep, that way when the press at home ask about early warning systems, its easier to point at the spinner beeping thing then it is to explain its just not feasible? – Blade Wraith May 22 '18 at 15:04
• @BladeWraith My cynical says they'd definitely do that almost regardless of whether they had a working system or not. – Ash May 22 '18 at 15:16

I assume that the people doing the orbital bombardment dont have the option to suckerpunch (get in and only be detected after launching the attack). So perhaps you could use sinple observatories that litterally look at the ships in orbit. They check their bearing and where their weapons are aimed at, those rods from God/nuke weapon mounts shouldnt be too hard to spot.

Taking from someone elses post recently: http://tvtropes.org/pmwiki/pmwiki.php/UsefulNotes/Airships

Using some high-altitude Airships with a large visual range of the space including the sides of the planet and placing observation equipment on them might do the Job. Its relatively cheap, you dont need radar only a direct line of sight and low dust particles for vision and the visual goes at lightspeed so you have one of the fastest mechanics to determine the ships weapon positions. Bonus for having more than 3 Airships with a line of sight so you can without a doubt triangulate the weapons direction and notify troops BEFORE the shot is fired. Although to prevent fake shots the first notification is more of a "get ready you might need to run".

I am going to make some assumptions. The fact that there are ground forces means that they are fighting other ground forces. This means that the enemy will avoid using orbital bombardment methods with extremely high yields. The higher the yield, the more of an early warning you need. This means small impactors.

I am also going to assume that these projectiles are mostly unpowered and unshielded. A clever enemy could make a very deadly orbital strike weapon, resistant to standard detection techniques and the ability to course correct. However, you have to ask yourself if it would be cost effective. The price of a single advanced warhead could pay to fling dozens if not more small "dumb" rocks to the surface.

I do think there is one method that may work, but first I want to dismiss some of the methods that could possibly work.

Visual Detection

A series of cameras on your vehicles pointed skyward would be able to spot the heat of an incoming orbital attack hitting the atmosphere. Decent enough computer systems could calculate the impact point.

The period of time it will be visible can very quite a lot based on the speed and angle of entry. However, on Earth like planets, the atmosphere will be about 60 miles thick. At speeds of around Mach 1, it would take only about 6 minutes if directly incident (what is going to happen to its speed will depend a lot on the aerodynamics, ect, though it will likely slow down). Likely our projectiles will initially be going much faster. Geostationary orbit is close to Mach 4 for example. The Russian meteor a few years back was going close to mach 100.

So a warning time of about a minute might be possible in some situations. That doesn't really seem like enough time to me.

Outside of the Earth's atmosphere they will likely be to dim to detect with small mobile optics.

NEAR IR

One way that we track NEO (Near Earth Objects) is Near IR. Things in space will absorb heat from the sun. They then will reradiate that heat.

You may wish to read up on WISE and NEOCAM. WISE is an orbital satellite with a 40cm diameter lens (about 1 and a third feet). However, there is no reason you couldn't have similar detection on the surface of the Earth. You will need to kick up the sensitivity a lot, but adaptive optics have gotten a lot better.

Based on its field of view, if something similar was mounted on a surface based vehicle it would be able to image the entire hemisphere it was in with about 2,000 images (If I did my math right, 47 arcminute FoV). At 11 seconds an image that is 6 hours to image a hemisphere . Way TO SLOW. You are really likely to miss a threat.

However, if a group of soldiers had 3-4 vehicles, with an array of like 3 of these on top and they were all working in unison? 30 minutes to image a hemisphere.

This detection technique can be used to track things as far out to the keiper belt (they are to cold past there). Not atmospheric interference would greatly reduce the range.

The problem becomes one of resolution. An object about 15m in diameter (the Russian impactor) would need to only be about 2km away to be seen (far too late). In practice it can only practically detect things 150m or larger in diameter.

NEOCam is a new satellite that has been designed that is only slightly larger and uses much more modern technology (though same basic idea) It has a much better resolution, designed to detect things down to 30m. Hypothetically, it could spot a 15m impactor about at about a third the distance to the moon based on its resolution. NEOCam also has a much wider field of view (almost ten times), but I believe takes pictures much slower.

We can assume the future will see continued development on this front.

There is a lot of story potential with this type of detection. They require cooling (WISE requires Significant cooling). A detachment unable to resupply would risk running out of cooling. The ability of a detachment's ability to detect would also decrease if they lost vehicles, degrading their knowledge over time. Without communication to forces in the other hemisphere, they would also be blind to half of space. An impactor could be hiding just around the corner.

During the day impactors would be more visible (they would be heated more), but there would be a blind spot where the sun is.

You can check out This link if you want to do the radar calculations yourself. However, with a power source of only 100kw as stipulated in your question, the system would be able to detect a small 15m impactor at about 50km away. You would honestly need a terrible amount of power to detect an orbital impactor at a useful distance with the type of radar that would be mobile. So radar is out.

LASER

This one is a bit more out there, if you don't want people calling you on getting any of the engineering wrong. Here you can read about using magnetic fields to detect asteroid collisions. Here you can read about using lasers to measure magnetic fields. enter link description here.

It may be theoretically possible to have some sort of LIDAR array looking for magnetic perturbations caused by these impactors. A system like this would have similar limitations as the IR solution.

# I'm going to challenge the basic premise on the grounds that you've already lost the war.

Air superiority is key, if you control the airspace you can control the ground. If you ground troops are worrying about rods from god, you've lost air superiority and probably lost the war. Your air/space superiority forces should be handling this by keeping the appropriate orbits clear of vessels capable of launching such weapons.

# Don't track the projectiles, track the launchers.

The launchers are going to be large, slow moving and fairly obvious. There's no point trying to track a bullet, even one fired from such a range that you've got 5mins or so to impact. By the time you've detected the launch, located the projectile and identified the course, determined and communicated a safe direction for the troops to move in, it's probably too late to get out of the impact zone, especially if they've fired a spread centred on your location.

Let the flyboys handle the flying stuff, the grunts should be worrying only about what's on (or near) the ground.

Noting the recent event of the falling Chinese Tiangong space station, and the inability for anyone to calculate where it would fall due to the unpredictability of the station passing through the atmosphere, it would be virtually impossible to determine with precision (enough precision to move your troops in time) where a projectile might land even if detected with a good fix on position and velocity.

The detection can of course be performed by radar, or even optical observation, but the difficulty lies in calculation, and doing so in time to establish a course of action.

I should add it would be unlikely for your troops to survive a near hit even if mechanised, the scope of an attack would simply be to soon and too large by the time you know where to move to.

• Yes but that station wasn't specifically designed to punch through the atmosphere with minimum deviation a "rock" is. – Ash May 22 '18 at 15:09
• The problem in this case was that it was extremely shallow reentry where the energy loss depended on the very weak aerobraking while orbiting in the highest layers of atmosphere. However a kinetic missile (or ballistic warhead) will be guided on a trajectory intersecting the Earth surface and it will deviate off it only very slightly due to the aerodynamic effects. After all, touch/splash-down location of intentionally landing reentry vehicles is usually calculated quite precisely. – Jan Hudec May 22 '18 at 20:55
• @JanHudec This is the obvious point and why flox hasn't answered the question (i.e. hypervelocity projectile, not just hypervelocity), but poses the realistic problem that a projectile could decide to orbit before it finds its target. In fact, my understanding is space stations orbit the globe in about 90 minutes, so if there were "hypervelocity stations" rapidly deployed ... well, this could be a serious problem. – theREALyumdub May 22 '18 at 21:17
• @theREALyumdub, yes, you can't calculate the impact point until the projectile ends its retrograde burn and that's not much time. – Jan Hudec May 22 '18 at 21:48

The most likely type of unguided orbital strike would be a Thor type railgun strike from an orbiting weapons platform.

A sensitive magnetic sensor could, in theory, pickup the charging of the weapon in space using triangulation and give a warning that it's about to be fired.

It won't tell you what the target is but would make a useful warning to scatter and spread out.

By detecting the charge up, you might have a couple of minutes warning as opposed to seconds if you wait for the weapon to fire.

You cannot see a bullet coming at you. But you can see a guy with a rifle getting set up, and seek cover.

So too with projectiles dropped from orbit. There must be something in orbit to drop them. You will be able to see the something in orbit when it comes over the horizon.

There already exist automated systems for tracking objects in the air and in orbit.

OpticTracker:
Pre-guiding for Satellites, Comets, and More!

You may know exactly where all the interesting celestial objects are, but how about a satellite, a comet or your drone? OpticTracker can use a variety of information to point the telescope to where your target should be, helping you establish a visual lock.

OpticTracker can download satellite orbit data and report to you the exact time they will pass over head. You can follow it rising up from the horizon. When it becomes visible, OpticTracker will get the visual lock.

OpticTracker can also read comet and dwarf planet orbit elements. Use them to pre-guild your telescope, turn on the built in stacking feature, and see if your telescope is powerful enough to pick up the image.

One more thing, for advanced users, you can feed target GPS locations into OpticTracker’s REST API, and all those targets become pre-guide-able.

Your system will be a telescope and a computer that you will set up. It has a database with known satellites. If some of these are known orbital platforms it can warn you when one comes over. If something new shows up it can warn you there is something new and you can take a look at it. If it looks like an orbital weapons platform, take cover. Once it is out of range you can come out.

I should add in this context that the best defense is a good offense. An orbiting platform is a sitting duck. You can predict when and where it will show up. You might even be able to light it up with a laser from the ground. Send a moderately large missile up along that laser.