View Full Version : IFF systems in british bombers in ww2 ?

02-27-2005, 10:44 AM
I read somewere that british bombers ( and propably and american too ) had IFF ( Indidafication Friend or Foe - a targeting system that can recognize if a target is friendly or enemy ). Is that true? And if it is true, how such a system worked in ww2 since computers were not invented yet?

I know that the Germans had developed Infrared systems, and propably so had the allies. Maybe that was the "IFF" ? ...

02-27-2005, 11:34 AM
You don't need a computer to read MODE 1, 2, 0r 3 IFF; they are based on a simple automatic response to an IFF interrogation signal.

This will be all off the top of my head, because it's been almost 25 years since I last worked on an IFF system in the US Navy, but I remember the basics clearly enough:

IFF is a system 'piggybacked' on all radar systems with only two assigned frequencies: one for interrogation, and one for an automated response. Postwar, the two frequencies have been an international standard (1060 and 1090 MHz, I believe, and I don't recall which is which).

The 'Mode' is determined by the length of time between two interrogation pulses, the shortest interval between pulses indicating Mode 1, the longest indicating Mode 3 (there is now a Mode 4, which is more complicated, and a coded mode for military applications, but these were developed long after the war).

The radar operator sends this interrogation pulse along with the radar signal (the radar is always going to be a different frequency, to avoid confusion). Onboard an aircraft, there is a transponder, a radio device that automatically receives & recognizes these specific interrogations, and responds in a preset way. The response can be translated as 1, 2, or 3 (according to mode) rows of blips seen on the radar scope behind the interrogated target that can be translated as numbers 1 through 7, with 7 always denoting an emergency situation (7 providing the largest video representation on the old analog displays, making it easily recognizable). The receive antenna of the IFF interrogator is usually mounted on the radar antenna, allowing a close association with the video return on the operator's display (IFF timing has to be closely tied to the radar's in order for this to work).

This would be received by the receiver section of the interrogating unit as a series of pulses somewhat 'behind' the video return of the radar target, unless the IFF was set to only display certain coded responses, like '3' in Mode 1, or '22' in Mode 2, or '615' in Mode 3 (these are only examples: the idea is that the response had to be in the right code for a given Mode).

So, if the IFF response was incorrect, ther would be no 'extra' video behind the basic radar video on the scope, and that would identify the video target as 'Foe.'

That was the system as it was used by the 1960s, but the principle was established during the Second World War, and used by both sides: a seperate send and receive system at a different frequency from your radar that can send and receive a preset coded response that will be displayed 'behind' the normal radar video of the target.

The early British system used a manual response; the ground controller would order a pilot to 'flash his device (I think that was the term used),' and the pilot would then turn on his IFF transponder, sending out that 'extra' signal at a timed interval after the radar strobed him. The use of the system was carefully limited at first, to keep the enemy from figuring it out and using it against them.

Once a coded response could be worked out and changed on a daily basis, the automated response sytem came into play. I'm pretty sure that this occurred fairly early in the war, because the 'flash your device' incident is conspicuously missing from the memoirs of USAAF pilots who served in the ETO and used the RAF's control system.

Obviously, you would only turn on your IFF transponder while in the immediate vicinity of friendly territory, because the 'extra' signal might serve to identify your location to the enemy's radio direction finders - a number of nightfighters on both sides used systems to track enemy emissions to augment their airborn radar systems.

Sorry for the lengthy response, and hope this hasn't totally confused you.



02-27-2005, 12:16 PM
Thanks for an interesting read!

02-27-2005, 01:56 PM
I take the liberty to summarize Horseback's story a bit http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

IFF is essentially a radio systen wich emits a certain code whenever the aircraft is 'asked' by it from any ground based radar. in WWII, this IFF system was probably not integrated with radar and the crew on the ground would have seperate equipment for that. The code that the aircraft would transmit, is a certain, previously agreed upon code wich changes often so that the enemy cannot use the same could as you would.

The Brits allready had this technology very early in the war, since it used radar in BoB to intercept the German bomber formations.

02-27-2005, 03:45 PM
In addition to PlatyPus' post:

"Give me a canary" ground crew / radar operators would ask a pilot via radio, the pilot would 'answer' by flipping a switch and thus send a signal. This to check if the radar 'blip' the operators were viewing really was the one they asked a canary for. When switched on the IFF would alter the radar blip in a distinct way.
I saw this explained sometime in some documentary on Discovery Channel, if I remember correctly, it was about night fighters....

Have Fun!

02-27-2005, 04:37 PM
think theres some infa red reflectors on the nose, the bomb aiming bubble, two small circles. think they where for gun laying radar which some lancs had fitted, to avoid blue on blue....

heard of b29 using IFF on a mayday setting so they get assistance...

02-28-2005, 01:24 AM
Thanks for the answers guys http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

02-28-2005, 08:58 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by p1ngu666:
think theres some infa red reflectors on the nose, the bomb aiming bubble, two small circles. think they where for gun laying radar which some lancs had fitted, to avoid blue on blue....

heard of b29 using IFF on a mayday setting so they get assistance... <HR></BLOCKQUOTE>

If you see a Lancaster with two small circles on the bomb aimer's bubble it means it is fitted with 'Z Equipment' for IFF identification. These were transponders carried by aircraft which had radar-controlled rear turrets. The idea was to tell the rear gunner of the machine in front that you weren't the enemy... This system operated independently of ground control.