A Question About Fighter Aircraft

[gumboot]

Hi, I have a question for anyone who is familiar with fighter aircraft, in particular American ones. I apologise for starting an entire thread to get an answer to a simple question, but I didn't know another way.

The question relates to the IFF transponder and how it works. As I understand it civilian airliner transponders have some sort of IFF function as well, although it is not used for this purpose.

Am I right in thinking that military aircraft have some sort of IFF "interrogator" that allows them to "ping" the IFF transponder in other aircraft? If this is correct, would all military aircraft have one, or would only combat aircraft? If this is not the case, how is the identity of friendly aircraft determined via the IFF transponder?

Finally, assuming the above is correct, is a military aircraft able to "ping" the transponder of a civilian aircraft?

If they are, would this be similar to the way the FAA's Air Traffic Control Radar Beacon System (ATCRBS) "pings" civilian Mode 3/A aircraft transponders? If it is dissimilar, how is it dissimilar? Would the military aircraft receive similar information to what the FAA does? (Altitude, transponder code, heading, airspeed, etc.)?

Thank you in advance to anyone who can provide insight into these questions.

[Hafast]

Well, let's see if I can answer this one for you. I used to work on air traffic control radar systems before I became a computer geek for the military.

The transponder in an aircraft responds to interrogations from the ground-based radar system. This is actually considered "syntetic" radar, since you're not actually bouncing an RF signal off of an aircraft. Also, there is data in the aircraft's response to the interrogation, which consists of altitude and any emergency codes that the pilot inputs (hijack, in-flight emergency, those kinds of things.

Essentially, military and civilian aircraft have the same transponders. The difference is that military aircraft have an extra mode that they can use. This is used for IFF, or Identify Friend or Foe purposes. This mode allows military controllers to identify friendly aircraft. Air traffic controllers in the states (military or civilian) do not identify friendly aircraft.

The one area I'm not completely sure about is whether or not aircraft can interrogate each other. I don't think that they can, only the controllers on the ground have the receivers. I could very well be wrong on that. The exception that I do know of would be the AWACS aircraft, which is essentially an airborne version of the ground radar. At this point we're moving beyond air traffic control, and getting into the air control arena, which is a completely different animal, but works in the same basic way.

This is a very basic answer, but essentially the answer is that it's up to the controllers on the ground to identify aircraft and point them in the right direction. Hope this helps, and I'm sure someone here can correct any mistakes that I've made.

[Schneibster]

Originally Posted by gumboot

Hi, I have a question for anyone who is familiar with fighter aircraft, in particular American ones. I apologise for starting an entire thread to get an answer to a simple question, but I didn't know another way.

That's how it's done here. You're in plenty of good company. It's not like we're going to run out of threads anytime soon.

Originally Posted by gumboot

The question relates to the IFF transponder and how it works. As I understand it civilian airliner transponders have some sort of IFF function as well, although it is not used for this purpose.

That's generally SIF; you're approximately correct, and most modern IFF interrogation equipment also has the capability to query SIF transponders.

I think the big difference is, the transponders on military aircraft can be put into IFF mode. In this mode, the transponder DOES NOT RESPOND unless it authenticates the interrogator; that is, recognizes the encryption key that interrogator is using.

I'm pretty sure that military transponders aren't put into this mode unless the aircraft is on a mission, attack or reconnaissance; I don't know the precise rules, but I would expect that, for example, US military aircraft moving from one base to another inside the US, would be put in a mode that would respond to SIF interrogation with SIF data, and to IFF interrogation that is authenticated with IFF data, but not respond to unauthenticated IFF interrogation.

I'm also pretty sure there is IFF transponder gear out there that can "spoof;" that's obviously going to be much more complicated. One thing would be to pretend you're someone else on SIF. But if you want to get really, really sneaky, you might try to spoof an IFF interrogator. Obviously, if you screw it up, you can expect to find a lot of really negative attention coming your way in extremely short order; and given the way modern encryption systems work, it's probably not worthwhile unless you're doing it against a technologically inferior interrogator. If I were designing them, they'd start simple so you couldn't tell what you were getting into, then get complicated, to catch anyone who thinks they can spoof in order to eliminate them from the battlefield. And given I can dream that up and don't do it for a living, I expect you could get a lot nastier and more baroque than that.

Originally Posted by gumboot

Am I right in thinking that military aircraft have some sort of IFF "interrogator" that allows them to "ping" the IFF transponder in other aircraft?

I believe that most US military combat aircraft have this capability, to reduce fratricide. I'm certain AWACS have it, since they were designed for ATC on the battlefield.

Originally Posted by gumboot

If this is correct, would all military aircraft have one, or would only combat aircraft? If this is not the case, how is the identity of friendly aircraft determined via the IFF transponder?

I believe all military aircraft have the IFF transponder, all commercial aircraft have the SIF transponder, and private pilots who can afford it have the SIF transponder too. I believe the interrogator is a separate piece of equipment, and only the combat aircraft have it, and AWACS, since they're the ones who would need it.

Originally Posted by gumboot

Finally, assuming the above is correct, is a military aircraft able to "ping" the transponder of a civilian aircraft?

US military aircraft probably also have the capability to interrogate SIF, since it would in general be our policy not to attack airliners that might get caught in the wrong place at the wrong time. Of course, that would open the opportunity for a "spoof," since SIF is not authenticated. Still, I expect it's set up that way.

Originally Posted by gumboot

If they are, would this be similar to the way the FAA's Air Traffic Control Radar Beacon System (ATCRBS) "pings" civilian Mode 3/A aircraft transponders? If it is dissimilar, how is it dissimilar? Would the military aircraft receive similar information to what the FAA does? (Altitude, transponder code, heading, airspeed, etc.)?

You've gotten very technical here, and I'm going to let someone with more expertise than me have the opportunity to respond; if you don't hear anything for a while, ping me with a PM and I'll see what I can scare up on google. Some of this may already have been answered by Hafast, whose answers look better in terms of this level of detail than mine.

Originally Posted by gumboot

Thank you in advance to anyone who can provide insight into these questions.

You're quite welcome.