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One of the main difficulties that have harped militaries from prehistory to the modern day has been identifying who's your friend and who's the foe. Many solutions have been used to solve this conundrum, but with metamaterials promising the ability to, partially, blend into the foreground the issue of identifying friendly forces once more rears it head.

So what are some possible ways to prevent friendly fire?

Some assumptions:

  • The cloaking device comes in the form of either a trench coat or poncho like cloth that refracts that light.
  • The cloak does not need much power to function.
  • The cloak works by having a metamaterial bend and refract the light around the person. The cloaking is not perfect, if you look hard enough you can see a distortion. Similar to this depiction in the clip.
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    $\begingroup$ IFF. We already have this problem with aircraft, which usually present as featureless dots on a radar screen, and we need to classify them as Friends or Foes. $\endgroup$
    – AlexP
    Commented Apr 5, 2022 at 8:08
  • $\begingroup$ I was going to suggest the same thing but was too busy to reply and figured someone else would suggest it. $\endgroup$
    – Mon
    Commented Apr 5, 2022 at 10:46
  • $\begingroup$ IFF is automatic $\endgroup$
    – Faito Dayo
    Commented Apr 6, 2022 at 6:15

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So what are some possible ways to prevent friendly fire?

There is no substitute for good organization, training and leadership. IFF transponders have existed for decades, and yet "blue on blue" (and its various non-NATO counterparts) incidents persist.

Even where super advanced high-tech cryptographically secured IFF systems exist, the transponders still get turned off in various circumstances, and the interrogation systems may not always be used all the time in all locations.

  • Emitting an IFF query reveals your location.
  • The nature of the IFF query signal may reveal to an intruder that they have been spotted, which may in turn prevent them from revealing their intentions or from being intercepted.
  • Emitting an IFF response signal reveals your location. Perfect pinpoint non-back-scattering tight-beam communications are basically impractical in the real world.
  • You cannot guarantee that an IFF signal (query or response) has not been spoofed by an enemy.
  • An IFF system, emitter or receiver, may have been damaged and unable to function.
  • IFF signals, either queries or responses, may be rendered unintelligible by various kinds of electronic countermeasures or environmental conditions.

And so on.

There are enough reasons why IFF can't or won't be used under various circumstances, that you have to train and plan for the eventuality that it isn't available. Gating all firing decisions behind an IFF challenge-response is very risky, and will likely end up getting people killed when their smart-guns suddenly refuse to fire, or everyone just lights up with a big "shoot here!" radio beacon due to a software exploit.

(also, you're probably all gonna get murdered by automated weapon systems and drones using passive thermal and audio and chemical sensors, because the battlefield is no place for things which move, act and react with the speed of a meat glacier, but that's a different topic)

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Directional IFF transponder.

Your active camouflage doesn't send any information in order to remain hidden. It does passively pick up the IFF transponders of your allies. Should these allies come within a certain range, the directional IFF transponder sends updates to that ally. Using networking this signal is then disseminated between all the other allies in order to minimize the electronic footprint of the cloaked people.

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    $\begingroup$ This of course leads to a cyber war behind the shooting war, where you try to replicate your enemies' IFF signals and find their invisitroopers. But sure in the modern era the cyber war is already happening. $\endgroup$
    – Daron
    Commented Apr 5, 2022 at 14:37
  • $\begingroup$ @Daron Just another layer of cryptography. The Caesars wheel, the enigma machine, ... active camo transponders. $\endgroup$
    – user458
    Commented Apr 5, 2022 at 14:41
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    $\begingroup$ With our current day understanding of cryptography and cryptoanalysis, unbreakable encryption systems are already a reality. But the Achilles heel of every cryptographic system is keeping the keys secret. When every soldier needs to have a device with the key, then it's difficult to prevent it from falling into enemy hands. $\endgroup$
    – Philipp
    Commented Apr 5, 2022 at 14:57
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As per Demigans answer re: IFF. Although I would specifically suggest IFF be built into the sights/targeting pods of friendly combatants. A soldier detects motion and acquires a potential target in his sights. Moving his finger to the trigger (not pulling it) activates an electro/optical or similar circuit which sends out a coded 'ping', a radio or laser light pulse in the direction of the target. If the target is a 'friendly' his or her suit/helmet/IFF pod automatically sends back an answering 'ping' confirmation of their identity. This appears in the users sight as blue (don't shoot') digital overlay on the targets image in the soldiers sight. The circuits involved work close enough to light speed to make human reflexes glacial in comparison so the soldier will usually never have a chance to pull the trigger which locks itself anyway. And on top of that the soldier has been trained from day in making shoot/don't shoot decision based on the info displayed in his targeting system.

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  • $\begingroup$ As per the above... Above what? $\endgroup$ Commented Apr 5, 2022 at 11:35
  • $\begingroup$ I would be afraid that people wouldnt wait for identification. Imagine spotting the signs of cloak of an unannounced* person near you. If you dont shoot first they might notice you aiming at them, giving them time to retaliate. If you didnt spot the cloak well enough you might also miss while sighting in, letting you think its an enemy so you just hoze down the area with bullets since its hard to aim specifically at the target. *perhaps simply announcing yourself to allies could work? $\endgroup$
    – Demigan
    Commented Apr 5, 2022 at 11:37
  • $\begingroup$ Tortliena: the first answer which specified the use of IFF. $\endgroup$
    – Mon
    Commented Apr 5, 2022 at 11:48
  • $\begingroup$ Demigan: The question doesn't state but does implies that both 'sides' have access to the stealth technology in question. (Otherwise there wouldn't be any problem telling friend from foe.) Besides which its pretty much a universal rule that once someone invests a weapon others soon reproduce it. That said I did consider the issue that concerns you but I'm assuming a gun wont fire until its received speed of light confirmation that the target is not friendly. At 300,000 kps a target even 1000 meters away could be ID'ed before human reaction times could engage a trigger. $\endgroup$
    – Mon
    Commented Apr 5, 2022 at 11:56
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    $\begingroup$ @Mon it is the answer above you ,,, for now, I would recommend paraphrasing the post (with credit) as each answer should be complete without any outside help. If your post was rated higher, it would move to the top completely changing the answer order. $\endgroup$
    – IT Alex
    Commented Apr 5, 2022 at 12:12
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Your active camouflage does not extend around you in a complete circle. It's only active on the side that's facing the enemy. When advancing towards an enemy position, you'll be invisible from the front but your squadmates behind you can still see you (there's no benefit in hiding from them anyway). The opposite will be true when fleeing. The soldier would have manual control over the angle of effectiveness, but this could also be somewhat automated so that the system reorients itself as you (for example) flank your enemy and come up behind them.

You won't be able to see the friendly units behind you, but that shouldn't matter. The important part is that you can see the friendly troops that are between you and the enemy since that's the direction that you should be firing in. You would still have a problem in the case where you've surrounded the enemy and are attacking them from opposite sides at the same time, but cross-fire would still be a problem if you had 100% perfect visibility.

The only way for this type of system to be intercepted and used by your enemy is if they were behind you. If that's the case, then you've got bigger problems than your camo.

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As others have pointed out, you need an IFF signal of some kind. But an active IFF signal will ruin the stealth provided by your optical camouflage... unless the signal cannot be recognized as such without the proper key.

The IFF feature of the cloak will induce very slight variations in the distortion caused by the camouflage, on very specific wavelengths of light that get changed over time, according to an algorithm that uses a clock and a secret key. The principle would be essentially the same as when hiding text into a JPG photo image file using steganographic tools.

Without the proper equipment and key, the induced variations are extremely hard or impossible to detect, so the camouflage effect will be essentially the same as without the IFF feature.

But when the same algorithm + key that was used to generate the IFF effect is used to tune the pass-band of a very narrow-band filter so that the detector is restricted to "looking" only at exactly the right wavelengths of light at the right time, a detectable pattern can be seen on the cloaked figure or object. Only those with the correct key (and a clock that is in sync with the clock of the camouflaged person/thing) in their IFF detector unit can see it.

Complication: since the cloak works by manipulating existing light, the IFF will be less reliable when there is very little light available.

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