First, you only respond to correctly encrypted/signed requests. Everything else you ignore.
So, second, once you have your valid request, you respond to it, also in an encrypted fashion. IIRC F-35 can use their radars, with their bean-forming abilities, for stealthy comms, sending a very narrow beam of radio signals to the recipient.
And if this is fantasy, I suppose by the time you receive an IFF query, you know you're about to be shot at. So you might as well respond, sacrificing stealth. Hence why you want to really be sure you're really asked by your side, rather than receiving a spoof request.
Airlines just broadcast a ton of public information about themselves, so they don't really need an IFF system. They put their hands up and say "don't shoot, I'm a civilian!".
I don’t think that’s the most common case for an IFF query. AWACS oversight aircraft likely issue more IFF queries than fighter intercepts do. You still want to respond to a friendly AWACS inquiry, even though that aircraft is 100nm away and no direct/immediate threat to you.
It always gets me when someone abbreviates nautical miles in lowercase even though context is (almost) always clear it can't be nanometers.
TIL that nm isn’t actually accepted as an abbreviation for nautical miles. (It’s used so commonly in aviation that I assumed it was correct and it took me 3 or 4 searches to accept that this thing I knew was correct actually wasn’t. Thanks [seriously]!)
I know that there are unclassified textbooks on this very topic, but I've long forgotten their names.
> If you think about it, it sounds like a pain of a problem, the plane must be as stealthy as possible, but somehow, during those moments, communicate in a non-spoofable way that it's a friendly. No idea how they do this.
Well, part of this problem is pretty trivial to solve: if you're coming back from a mission and are well in friendly airspace, you don't need to be stealthy at all, so you turn on your radio transmitters.
It's my understanding that when you're on a mission where you need to be as hard to spot as possible, you turn off all unnecessary transmitters and rely on Command telling Air Traffic Control and other interested parties where you're operating so as to reduce the chance that you collide with another aircraft and/or get shot down by friendlies.
That being said in the Red Sea incident they likely were not stealthy at all. For one F-18 has a rather big cross section, it's a 4th generation jet. For another the Houthis are not known to operate any sensible air defense (I may be wrong here, someone please correct me if I'm wrong).
They could use something similar for communication. Send the first byte on a frequency, the second byte on another one, etc.... The frequencies could be calculated to be un-distinguishable from the background radio radiation. Of course, both the sender and the receiver have to agree on this mechanism.
https://en.wikipedia.org/wiki/Frequency-hopping_spread_spect...
https://en.wikipedia.org/wiki/Hedy_Lamarr
The really clever counterpart is how to transmit through your own spread-spectrum jamming. Quiet slots in the jamming must be manipulated in the same way as the active broadcast. For example, this allows communication to a ground convoy that is jamming all frequencies to prevent wireless remote-control IED detonation.