Forum: F-16 Design & Construction

I have a question. On the F-16 MLU there are IFF 'bird slicers'. I know that's to identify friend or foe. But how does it work, I mean how do other planes know which frequency is from a friend and wich from an enemy. How does the interpretation work of signals send by IFF?

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All F-16's carry an IFF system, friend or foe is detedrmined by a preset frequency or identification number changed daily, the when another plane is spotted it is hailed by the IFF system, a response is sent if it's friendly, if it's not the other guy never even knows he's been lased. All of this without the pilot moving a finger, all can be observed of course.

This is all just a basic explanation though, can't get into too much detail about that system for obvious reasons.

s_ellebaut wrote:

I have a question. On the F-16 MLU there are IFF 'bird slicers'. I know that's to identify friend or foe. But how does it work, I mean how do other planes know which frequency is from a friend and wich from an enemy. How does the interpretation work of signals send by IFF?

There are actually two different versions of IFF on F-16 aircraft. The one with the "bird slicer" antennae is Advanced IFF. AIFF differs from IFF in that the aircraft has the capability of interrogating other aircraft. Only the ADF variant of the USAF F-16 has this capability (to date). The MLU, Taiwan Block 20, and several other FMS countries have AIFF.

Conventional IFF has 5 modes, 1, 2, 3, C, and 4.

Mode 1 is set by maintenance, when the IFF R/T is installed. It's assigned by the wing and corresponds to the tail number of the aircraft.

Mode 2 is pilot settable. It's referred to as a Pilot Identifier. The pilot sets this prior to strapping in. The control is on the front of the R/T.

Mode 3/A works in the same manner as civilian aircraft transponder. ATC directs aircraft to "squawk" a certain 4-digit octal code. On A-models, this was done via a panel in the cockpit. On C-models (as well as MLU), it's done via up front controls.

Mode C is encoding altitude. The IFF R/T receives a signal from the CADC, and reports pressure altitude, in thousands of feet.

Mode 4 is where Identify Friend or Foe gets its name. I don't want to stumble into classified material, but you can probably find some good info elsewhere on the internet.

The functionality of IFF isn't too difficult to understand. The aircraft will respond appropriately to any interrogation it can "hear". If 10 aircraft are flying around, and can hear a mode C (altitude) request, they'll all respond. The interrogating IFF sorts out this confusion by tying IFF to the radar.

Somewhere on your base, there's a big, red Air Traffic Contraol radar antenna turning in a circle. You'll notice that bolted to the top of this antenna is a much smaller antenna. That one's for IFF. So even though more than one aircraft may hear (and respond) to IFF interrogations, the ground (or AIFF) system keeps track of them all by displaying IFF information next to radar skin paints on the scope.

If you need more detail, let me know.

Quote:

Only the ADF variant of the USAF F-16 has this capability (to date).

USAF jets that have been through CCIP also have AIFF.

15 s have it too. and have had it for years!!

Forum Veteran Joined: Nov 07, 2003 - 09:12 PM Posts: 901

I believe the need for interrogation (AIFF) cap onboard the Viper is motivated by the integration of the BVR kill capability. No? Rolling Eyes

Pumpkin wrote:

I believe the need for interrogation (AIFF) cap onboard the Viper is motivated by the integration of the BVR kill capability. No? Rolling Eyes

That's definately one reason. If you're in a hot zone with fire permission you'd want to know who's who before you hose 'em, obviously. Another use is to determine which bogey or bogies to VID (visually identify) if that's called for. If your scramble order says to ID a non-squawking aircraft, you can sweep the area declared by the Sector Controller with the radar and the IFF Interrogator will tell you which radar hit to run the intercept on. And you don't have to have a radar lock to run the intercept, either. There are procedures in place, which I won't elaborate on, to come up on the contact smartly without tipping your hand with a radar lock.

Zarband wrote:

the other guy never even knows he's been lased.

Lased? This isn't an IR system.

Pumpkin wrote:

I believe the need for interrogation (AIFF) cap onboard the Viper is motivated by the integration of the BVR kill capability. No? Rolling Eyes

Indirectly - I think it is more motivated by the operations in conflicted airspace where you can't visually tell the good guys from the bad guys by the type of jet they are flying.

Hypothetical example: let's say one country with F-16s in their air force went to war with another country with F-16s in their air force. AIFF would allow each side to tell who was who, and (hopefully) avoid friendly fire incidents.

Forum Veteran Joined: Nov 07, 2003 - 09:12 PM Posts: 901

thanks habu2, LinkF-16SimDude

I guess AIFF is just another essential mechanism in a networked fighting umbrella in the future war. Wink

One of the main reasons for the AIFF interrogation capability onboard the F-16 itself is to allow it to find out who's out there (BVR or otherwise), without needing anyone, or anything else. That's why countries using the F-16 as an Air Defense Fighter need this capability. Without AIFF, the Viper driver needs someone else to do the IFF interrogation for him/her (AWACs, ground-based radar, an F-15, etc.). An F-16 with AIFF (APX-109 et al) can send out an encrypted code, that a friendly will recognize, and reply to with another encrypted code.

And to answer s_ellebaut's original question:

Quote:

how do other planes know which frequency is from a friend and which from an enemy. How does the interpretation work of signals send by IFF?

It isn't done by different frequencies, but by the code encrypted on a particular frequency. Think of me sending a Morse code message to you while you're out on a mission. You know (actually the aircraft knows if the C-Shoppers kicked the jet's Mode IV IFF correctly) what that Morse code message should be for that particular day (and it changes every day at midnight, Greenwich Mean time), and you reply with your own special code for the day. I see it, recognize it as the only reply I will accept, and let you live another day, cause you?re my buddy. Screw it up, or have a system malfunction, and things get ugly real fast (BVR or otherwise).

And just when you thought it was safe to go back in the water (or back into the air as it were):

Mode IV IFF is being replaced, with Mode 5 IFF (IAW NATO STANAG 4193). One reason is to give much better interrogation capability beyond visual range, so that BVR weapons can be used to their full capacity (and range).

Mode 5 IFF is the replacement for encrypted Mode 4. It is a major upgrade to IFF. The Mode 5 Mark XIIA system will not only encrypt and pass identity information indicating that a platform is friendly, but will transmit encrypted Mode 1, 2, 3, 4 and C data and additional operational combat ID information related to a platform (i.e. encrypted GPS position data).

The actual Mode 5 upgrade is a major DoD and international military effort that touches every military platform with Mode 4 equipment and every unit that can interrogate Mode 4. Almost all these platforms and units will be upgraded to Mode 5 over the next few years beginning in 2005.

Well that's both good news and bad news. Mode IV is a Navy program that they didn't even use that well---a majority of our scrambles before 9-11 were for Navy aircraft ou over the water that weren't squwking Mode IV!!! As far as upgrades go, that's bad news for us---we still have the old analog HQII UHF radios! We're in the business of Air Defense and if that changes before we get new airframes I'm afraid we will finally become too antiquated to be of any use.

Oh well, I guess we'll just have to wait and see how everything shakes out with this next BRAC.

Was wondering,

With the IFF is on the nose as "bird slicers". What happened to the IFF cans on the leading edge of the previous F-16C/D's? Are they still there?

Thanks

Those "Beer can" antennae on the LEFs were never for IFF at all. Those sense incoming RF energy for the RTWS (radar threat warning system), either from ground emitters, or other aircraft, in front of the F-16 (there are others in the tail for scanning the rear quadrants). They are all passive receivers only, and the beer cans replace the antennae that used to be beneath the teardrops on either side of the nose. The teardrop covers are still there, but the antenna beneath each is gone.

Forum: F-16 Design & Construction

IFF 'bird slicers' - How does it work?