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Comment by next_xibalba | Hacker News Reader

> every large aircraft in the world is equipped with a device called an IFF -- "identification friend or foe."

This is weird, I'm sure I don't have all the facts. In a conflict, why would a military aircraft ever want to identify itself?

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So you don't get shot down by your own air defense

- as demonstrated by more than one aircraft since the war in Ukraine began.

Why would it need to be interoperable then? Presumably it tells adversaries that you’re friend OR foe though. The friend bit is obvious. The foe… still not seeing the logic.

There's a difference between civilian transponders which generally squawk all the time and military IFF, which typically only responds to a coded challenge (called an IFF interrogation). If a military IFF doesn't receive a valid interrogation, it remains silent.

It is Very Bad if an enemy can either spoof a valid "friend" IFF response or cause IFFs to respond and give up one's position. The latter was done with great effect in both WW2 and the Vietnam War.

> spoof a valid "friend" IFF

Tricking an enemy plane to respond seems "fair", but would a plane disguising itself as friendly count as perfidy?

That's easy. On many civilian planes, beacon turned on automatically, but on military must be turned on intentionally.

Because of this, extremely typical, when you see military plane, but have not got alarm about it's approach - just pilot forgot to turn on beacon.

FoF system is totally another measure, in real dogfight they do not consider it much, it is mostly like additional spare fuse, may save your life in some case. Rumors said, pilots of U-2 was pray, to avoid cases, where need to use these radio tricks.

As already pointed out, civilian transponders are not really "IFF", they're a tool for ATC to keep track of you.

Military IFF transponders don't emit unless they get interrogated by a valid code, and then only briefly.

The interrogating plane would typically only interrogate if you show up as a radar contact, and at that point, being able to say "I'm friendly" is very useful. You're not hidden anyway.

Note that the name "Identification Friend or Foe" is misleading; IFF can only positively identify friendlies. A nonresponsive bogey (unknown) may still be a friendly with bent IFF, wrong codes, etc.

I have tried to find the sentence you quoted in the article and was not able to. But it is not true that "every large aircraft (...) carries an IFF". IFF is strictly military. What civilian aircraft do have (large and small) is a transponder. The article talks about this but in a confusing way.

Eh. Military aircraft would also have a transponder, they just wouldn't necessarily have active broadcasts.

Civilian aircraft do broadcast actively (ADS-B). But they also respond to secondary radar for Mode A/C, which are basically cases of IFF Mode III (okay, maybe not exact term, but the idea applies.) So it's still a challenge-response/IFF, just in this case always responding.

Military aircraft use different modes and presumably don't respond unless interrogated with an appropriate challenge, but the principles are the same.

The quote is from the 2nd or 3rd post in the thread. The ui is pretty bad so it’s not obvious at first that it is a thread.

More interestingly why not fake it and claim to be civilian?

Logically speaking:

- no identification: risk being shot down by both sides

- correct identification: risk being shot down by the enemy

- fake civilian identification: no risk of being shot down?

It’s a war crime. If you start doing it the other side won’t shoot at nothing - they’ll shoot at everything, civilians included.

It’s only a war crime if you lose.

You tell your enemies where you are so they wont get close. Its sabre rattling. A favorite engagement for soldiers is firing at other soldiers somewhat out of practical range.

Pretending to not be the enemy will get you close before the enemy realizes and everybody loses.

Civilian flights get recorded from takeoff to landing. If you start pulling this stunt a lot, you'll get caught really quickly.

It's like wearing the Uniform of your enemy as a Soldier, sure you can do it but it's a war crime...

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