Sensors and Countermeasures

There are numerous ways to guide a missile or drone to the target. But for every potential way to guide an autonomous payload to the target, there is a countermeasure, and possible counter-countermeasures, and so on.

Close up of photodetectors of a telescope.

These techniques remain relatively unchanged from Earth, but there are a few significant differences in space. One is that there is no horizon.

Two, there is no significant intervening medium (air or water). This first makes sonar and acoustic homing irrelevant, but it also vastly reduces the effectiveness of any gaseous countermeasure like smoke. Another consequence is that exhaust plumes fade out very quickly. However, the biggest change this makes is that it reduces the noise floor heavily, making targets stand out much more against the background.

Three, there is no GPS or satellite network for additional guidance help, and if there is one, only the defender would have access to it.

Before we discuss guidance techniques, a quick primer on sensors. Sensors are built as Photodetectors for a specific wavelength likely with a telescope lens. While they can be expensive, especially if you want them diffraction limited, they require so little power that power use is assumed negligible, especially in comparison to all the high power systems of a spacecraft, missile, or drone. Sensors actually give you power, but this will be less than any rotators or computing system utilizing the sensor.

Photodetectors being installed on the HESS telescope.

Unlike lasers, sensors can be made very close to being diffraction limited because they are so low power (if kept very cold).

What can they see? As mentioned in Stealth in Space, you can detect the radiators and the exhaust plume from far away, so there’s no doubt you will see them up close. At combat ranges, the ship itself will be visible from light reflecting off the hull. Less sensitive sensors will be needed at that close of ranges, lest you burn out your sky-scanning sensors.

But what is the visual resolution of these sensors? Although you can see the exhaust plume billions of kilometers away, it will show up as a single pixel, which is not helpful. Note that you can not achieve stealth in this way. You can’t hide multiple identical ships in a single fleet via pixel resolution. Careful study of the spectrum of this single pixel over time will reveal multiple overlapped exhaust plumes.

plume spectra.png
Data produced from a single pixel can be studied and differing emitters can be determined.

Visual resolution of a diffraction limited optic can be easily calculated using the angular diameter (follow the links for the relevant equations). Here are a few example calculations to give sense of range:

Given a detector 10 cm in diameter (reasonable for a drone, missile, or capital ship) looking for visible (550 nm, green) wavelengths, what’s the visual resolution?

At 1000 km away (orbital distance, close missile launching distance), each pixel is about 7 m in size.

At 100 km away (very long range projectile combat), each pixel is about 70 cm in size.

At 10 km away (close range projectile, drone, and laser combat), each pixel is about 7 cm in size.

At 1 km away (only missiles about to hit and maybe close ranged drones would ever get this close), each pixel is about 7 mm in size.

In Children of a Dead Earth, capital ships tend to be at least 50 m in length, often around 100 m long, or twice that for the flagships. Drones and missiles tend to be 10 m long at most, and usually much shorter.

This means that at missile launching distance, enemy ships are a tiny blob of a pixels while missiles are single pixels on screen. At very long range projectile combat, the enemy ship might be distinguishable as a shape with radiators, while missiles will be a few blurry pixels. At close projectile range, you’ll get a nice view of the enemy and their missiles as your shots tear them apart.

Of course, taking into account enemy countermeasures, your view screen will likely look as clear as this.

Larger sensors will give you better feedback though larger sizes tends to be a problem for missiles and drones. Additionally, infrared sensors will have half the resolution stated above at best (since pixel resolution decreases as wavelength increases).

Visual resolution is rather important for missiles and drones, however, as their guidance systems need to be able to distinguish what they need to hit versus decoys. The better the visual resolution, the further away decoys need to be launched to fool the missiles.

Let’s go into the guidance techniques available in space.

Infrared (IR) Homing is a passive technique where the missile or drone chases infrared wavelengths. Often it would chase the greatest heat source, though this is easily fooled with decoys. Infrared Homing is the most important of all guidance techniques in space, principally because all ships give off tremendous amounts of heat from their engine and their radiators.

The eye of an IR homing missile.

Ultraviolet Homing chases after the ultraviolet part of the spectrum instead of the IR portion of the spectrum. This is beneficial because UV decoys are much more difficult to produce and field. On the other hand, low heat radiators like life support radiators emit very little UV light, which means turning engines off and retracting powerplant radiators is an effective countermeasure.

Spectral Seeking is a little more sophisticated, where a specific spectra is targeted. For instance, the spectra of methane at 3000 K is unique among spectra, and only exhaust plumes of methane at 3000 K would be targeted. This means in order for decoys to throw this type of guidance off, the decoy needs to match the spectra exactly (which is difficult). On the other hand, the target needs only change the spectra of the engine or the radiators to throw off the guidance system.

Passive Radar is another passive technique where the missile chases after radio signals bouncing off of the target from third party sources (likely civilian). In space, due to distance, the signals are significantly weaker than on Earth.

Active Radar Homing is the same technique, but with an active radar system specifically illuminating the target. This is much more effective, and doesn’t rely on weak third party civilian signals. However, all radar systems can be fooled very easily by chaff. Additionally, Radar Absorbent Material is another effective countermeasure.

A radar display affected by chaff. The left side is jammed by chaff, and produces no usable data.

Semi-active Radar Homing is also the same technique, but with the radar illuminator being separate from the radar homing device. In this case, the capital ships would have the illuminator, while the missiles would chase the illuminated targets. This yields a somewhat cheaper option, but it still has the same limitations as previously mentioned.

Laser Guidance swaps out radar for lidar, radio waves for Visible, IR, or UV light. The target is illuminated by a laser either with a separate device (Semi-Active Laser Homing) or with the homing device itself (Active Laser Homing). It can be fooled by paint absorbent to the particular wavelength, and such paint is much cheaper to produce than Radar Absorbent Materials. On Earth, this sort of countermeasure can be fooled by simply aiming near the target instead of directly at the target, but in space, no such solution is possible, since there is no nearby “terrain” to aim at.

Schematic of a laser guidance system. The laser is “painted” on the moving target, and the missile chases the dot like a cat.

Beam Riding is a technique which uses either a laser beam or a radar beam to illuminate the target. Then, the missile “rides” the beam down, using the beam as a guide. On Earth, this restricts the missile to line-of-sight attacks, which is problematic, but in space, this is not a problem at all. Unlike laser guidance, it is immune to absorbent materials. The main issue, however, is diffraction. Any laser or radio beam will diffract, meaning the beam gets increasingly inaccurate as range increases. This restricts missile usage to very close combat, similar to lasers themselves.

Diagram of a missile riding a beam to the target.

Neutron Homing is a guidance technique that does not currently exist on Earth, but is likely to gain prominence in space. The nuclear reactors on a capital ship dump out an extraordinary amount of neutron radiation (both fast and thermal) in every direction, and these neutrons more or less pass through any material short of radiation shielding. Trying to shield this radiation in every direction requires a huge amount of mass, so weak unidirectional shielding in a single direction (towards the crew compartment) is used instead in addition to long distance.

Shielding against neutron radiation is rather costly in terms of mass.

This means that the neutron radiator of the reactor will be dumped out in a sphere shape, attenuated with the inverse square law. The background neutron of space is essentially zero in the absence of an atmosphere. This means very even trace quantities of neutrons can be picked up and used by homing missiles. The main countermeasure besides tons of expensive shielding would be launching neutron decoys: cheap, highly radioactive waste materials spewing off plenty of neutron radiation. Alternatively, a highly directional beam of neutrons can be generated as a way to throw off such missiles.

Schematic of a Neutron Generator.

Command Guidance dispenses with all those sophisticated tracking methods, and relies solely on the launching ship to guide the missile with manual controls. It is immune to all countermeasures except for communications jamming and spoofing. This is a common technique for drones, since they do not need to collide with the target, they only need to aim at the target. In Children of a Dead Earth, this is the go-to method for aiming missiles if the enemy has countermeasures for every other technique tried.

Inertial Guidance is the ultimate “dumb” missile method, which rejects all attempts at actual guidance and homing, and simply follows a preset trajectory based on the target’s initial velocity and position. It is the least accurate of all homing techniques, but it is completely immune to all countermeasures. It can be defeated by simple acceleration or dodging, however.

Whew! That’s a lot of different guidance techniques and countermeasures and counter-countermeasures. So what is employed? All are employed to a small degree, but the primary homing technique used is one of the simplest: Infrared Homing.

decoys 2.png
A salvo of missiles and a fleet of three ships in the background dropping numerous decoy flares.

This is because of the heat radiators on ships, and the extremely bright exhaust plumes. Countermeasures can be developed for every other homing technique, but for IR Homing, countermeasures are much more expensive.

First off, smoke (including thermal smoke) dissipates rapidly in space. Without an atmosphere, smoke expands at a constant velocity, required a huge amount of mass to provide a smokescreen for any extended period of time.

Second, deploying thermal decoys is expensive in terms of mass. Concealing hundreds of megawatts of radiator heat against a black background requires a similar amount of power emitted by the decoys. And these decoys have to be burning for a decent amount of time (10 seconds seems to be the minimum). This means the decoys will be rather massive pyrotechnic payloads.

decoys 3.png
These three ships really don’t want to get hit.

Extremely massive payloads are difficult to launch at high velocities away from ones own spacecraft. If they aren’t far enough away by the time the missiles hit, significant damage can still result even if the missiles hit the decoys. Launching very massive decoys at high speeds requires significant amounts of power, which requires even larger decoys, which requires even larger launchers, and so on.

Third, using a high powered IR laser to blind the incoming missiles works great, except these missiles are insensitive to all but the brightest signals. This means the laser needs to produce comparable power to the radiators they decoy, which is costly.

This is exacerbated by the fact that missiles are launched in salvos, not one by one. As a result, the laser needs to widen its beam in encompass an entire formation of missiles, vastly reducing the power. And missile salvo formations can easily reach hundreds of meters wide, which means your beam needs to be hundreds of meters wide too, which is pathetically weak. Either that, or you need hundreds of high power lasers, each focused on tracking and blinding each individual missile, which is prohibitively expensive.

100 missiles.png
A salvo of 100 missiles on its way to the enemy defense force around Ceres. Enemy force is roughly 40 km away.

Generally speaking, IR homing is the most effective guidance system, and IR decoys are the most commonly used counter-measure. In particular, dropping radiators and shutting off the engine while launching decoy flares is a common survival technique against missiles, though it is not foolproof. Things are especially difficult for large laser crafts with enormous amounts of waste heat.

In the end, no exact technique trumps all other techniques, and most electronic warfare focuses around IR homing and counter-measures. And when counter-measures are effective, Command Guidance is the usual response.


23 thoughts on “Sensors and Countermeasures

    1. Capital ships have sensor fusion and lots of expensive countermeasures so jamming/spoofing them is generally not an option (so having fleet communication always on is considered a given). Missiles and drones are much smaller and cheaper and so can’t field as many sensors, so they are much more easily disrupted.


  1. If the launching ship has sensor fusion (it has every sensor in the game onboard and computers integrate them to defeat basically all spoofing techniques), the launching ship could send the command updates as laser pulses. Small light sensitive patches on the back on the missiles/drones would receive the laser pulses, and the algorithms would be cryptographically secure and unjammable.

    This is state of the art, circa 2016. If you picked up a phone and had the right authorization and enough money, a defense contractor could build a system like this and deliver it in a decade. It’s a straightforward extension of the state of the art.

    Why didn’t you realize this?


    1. There seems to be two different suggestions here:
      1.Sensor fusion. This seems to be easy to add, actually-you ask each individual sensor if there’s a ship there, and add the results together to get a certainty rating(“70% sure there’s a ship there.”)

      2.Laser communications.
      Could be good for short range, encrypted communications

      The problems with using this on a missile are:
      >Your laser has to track each missile exactly, throughout the flight.
      >The laser receivers have to be sensitive enough to receive said laser despite diffraction and the interference from the missile’s exhaust plume. Which means they can be blinded very easily.
      >And, it’s one way unless you invest in a laser comms suite for each missile. Which amounts to a tinny, tiny little laser turret and sensor suite, kinda expensive for a expendable munition.

      This is something I’d rather have for drones or other capital ships. Hidden communications are only good if the receiver can actually do something the target doesn’t expect, and missiles in this game don’t seem to have the deltaV to do anything fancier than charge down the enemy’s throats.


      1. The laser doesn’t have to track each individual missile. The needs of an intercept course mean that missile’s can only burn a limited amount of dV “diverging” from the optimal path between the launching ship and the target. So you just defocus the laser and broadcast over several whole degrees. It doesn’t have to be all that bright, either – as I mentioned, the missile’s receivers would be sensor “patches” located over the missile body. The missile distinguishes between actual messages and stray bursts of light and enemy jamming using various codes and 1 time pads. (the one time pad being an RNG data file loaded into the missile at launch, shared with only the launching ship, so the enemy cannot spoof it)


        1. To address each in order:
          1.I’ll allow that defocusing the laser could work over short ranges, and that you would refocus the laser to cover the same area further out. However, see 3.

          2.If the optimal course for a missile is predetermined, is there really a need for communications secrecy between a ship and a missile? There’s not much you can glean from communications to a missile in flight that you wouldn’t get by observing the missile itself. Maybe which ship in a fleet you’re aiming for, but is that much of a help?

          3.You have to balance the sensitivity of the receivers with the strength of the comms laser-the farther out you have to operate, the stronger the comms laser needs to be, or the more sensitive the receivers have to be.

          The benefits of comms laser strength would be limited by beam waist, as detailed in The Photon Lance. Defocusing to cover an area the missile MIGHT be in wouldn’t help, as detailed in this article about blinding infrared homers.

          I dunno what the limitation for upping the sensitivity of the receivers would be, but I’m willing to bet more sensitive receivers burn out or get blinded more easily. Remember that the reason blinding doesn’t work on infrared homing is that the receivers don’t need to be super sensitive-the ships put out a LOT of heat.

          4.You could just as easily encrypt radio messages the same way, and use FHSS governed by same RNG to evade jamming. There isn’t enough time over the missile’s flight to decode the messages you’re sending to it before it impacts. Also, see 2.


          1. The reason behind OTP or some other method of encryption isn’t to prevent the enemy from decoding the messages. The messages are course corrections – the launching ship’s sensor array sees both the missile and the target ship, using sensor fusion as mentioned, and informs the missile what it needs to do to correct for any errors in it’s flight and to counteract any courses changes since launch by the target ship.

            These “patches” are very low mass and it’s fine if some of them burn out – the ones on the “back”, facing the launching ship’s laser transmitter are the only ones that need to survive.

            The encryption/OTP is so the missile can distinguish from false messages and jamming the real course correction packet. Only a tiny number of the bits need to get through to do this, making jamming pointless.


            1. I’m not contesting how these are supposed to operate. What you’re describing is basically Command guidance with laser comms.
              What I’m contesting is the laser comms part. See 4 on my previous post-it still holds mostly true, even if the primary aim is to cut through jamming.

              “These “patches” are very low mass and it’s fine if some of them burn out – the ones on the “back”, facing the launching ship’s laser transmitter are the only ones that need to survive. ”

              I’m presuming this means they’re essentially nothing but the photoreceptors themselves and the computers required to interpret them.
              In which case, the ability of any one patch to resolve the transmitting laser at range gets reduced substantially. You’d need multiple surviving patches to resolve the laser, and if you want to up the range you’d need either more patches or a stronger laser-see 3 for why this is a problem.

              “Only a tiny number of the bits need to get through to do this, making jamming pointless.”

              How so? I’m not sure how an incomplete message can be distinguishable from static even with encryption, assuming enough data survived to be useful.
              Unless you mean the message is being repeated a couple hundred times per course correction?

              And if you do find a solution to this, chances are it can be applied to radio comms as well.


              1. Laser comms are real. At these ranges you don’t need lens. They are much better than radio, we just don’t use them on earth because they don’t propagate through air and obstructions very well.

                The reason to use them instead of radio is they can’t be jammed or affected by EMP. The light from the launching ship comes from an angle that enemy fire and jamming is not coming from, and laser is extremely directional, much more so than RF. There is also a lot more bandwidth, not that you need it.


                1. “At these ranges you don’t need lens.”
                  Not saying that. What I’m saying is, you need to be able to see the laser and distinguish it from other light sources, at range.
                  At minimum, you need to resolve the laser to see which direction the laser’s coming from. Otherwise the directional advantages are lost and jamming becomes effective.

                  Also, there’s nothing preventing the enemy from sending out drones with comms lasers a hundred kilometers out, with the idea of letting the missiles pass, then blinding the rear ‘patches’ with the comm laser. Again, unlike blinding infrared seekers, not a whole lot of power is required-just enough to match or exceed the power of the controlling ship, which will be at a range disadvantage when firing at a thousand kilometers out.


                  1. Vim, the “drones behind” might work. Maybe. You obviously have a lot of control out of what you make the laser patches out of it.

                    My background is computer engineering. So I know this works. The detail I omitted – because I thought it was trivially obvious – was that you have a dedicated scrap of circuitry assigned to every single receptive patch, always trying to translate any signals back into a valid message. Technically every patch would feed to a common digital bus, probably just serial, and there would be an FPGA or custom ASIC for this on the missile’s main PCB that is wired to all patches.

                    The reason jamming doesn’t work is that no pattern of pulses from the enemy will match up with the OTP or other encrypted header for each message. Omnidirectional jamming, with drone spacecraft behind, would work, however those drone spacecraft would be kind of a target.


                    1. Your competence in anything was not in question, Mr. Monroe, and I admit to not being an expert in anything.
                      That being said, can partial messages(without the header) still be decoded and make sense? An earlier reply seemed to imply this.
                      If they can be, that would reduce the effectiveness of jamming, like you said-If not, continuous jamming is not needed, as rapid pulses can ‘chop up’ incoming messages instead of having to completely blind the patches.


                    2. Seem to have hit reply depth limits – TLDR there are codes that let you recover N bits of message data, out of an M bit long message (with M>N), so long as at least N bits were received successfully. So you have an update packet for the missile, you send it with 100 or 1000 times as much data as the length of the packet, and only 1% or so of the message data need be received to recover the message. Enemy jamming must be better than 99% or 99.9% or the whole message gets through undamaged.


    2. The major problem with command guidance is light lag. Reaction time is double the distance in light seconds. May or may not be an issue here.


  2. Another good read. I didn’t see any mention of Anti-Radiation or Home on Jam type guidance. Which basically directs a missile at a radar and/or jamming source. It is another take on passive detection that uses the enemies own active emissions against them. Some missile in the real world include them as a back up guidance.

    Also have you considered penetration aides and Electronic Warfare missiles? Both have real world use on various systems.

    Will there be an ability to mount multiple sensor or tracking systems on a single missile? I am assuming so since it is done on quite a few weapons already.


    1. Missiles and drones use multiple sensors, though cost and mass limits them heavily, which is why IR homing remains the primary method of tracking and countermeasuring.

      Missiles which act at target decoys can be built and fielded in game.

      As mentioned by Kelly Von Kibble, lidar is preferred to radar in space, so radar homing like ARM is less emphasized.


  3. “Note that you can not achieve stealth in this way. You can’t hide multiple identical ships in a single fleet via pixel resolution.”

    I have a feeling this is directed at me… 😀

    Question: How does the player interact with the detection/countermeasures mechanics? Can a player realistically create enough decoys and such to counter the vast range of sensors an opponent can be equipped with?


    1. Generally, capital ships have sensor fusion as well as a number of counter-countermeasures and trying to decoy or fool them is more or less fruitless. Decoying and spoofing is only really important for missiles and drones to a lesser degree, which have a much tighter mass budget and cost budget.


  4. Just to clarify: by ‘100 or 1000 times as much data as the length of the packet’ do you mean repeating the packet by that many times?


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