IFF stands for Interchange File Format. It was created by Electronic Arts in the mid-80s for the Commodore Amiga. IFF is an extendable file format that you can use for almost everything, and IMHO is the greatest format ever, but it has never been really successfull on other platforms. On PCs some programs (like Deluxe Paint) saved images as LBMs, which is basically an IFF file with an image inside.

IFF, or Identification - Friend or Foe, (affectionately nicknamed "Reply or Die") is a civilian and military co-operative Mk XII system. It was originally designed during World War II for the purposes of identifying U.S. Aircraft from enemy aircraft. It's come a long way since then, and continues to evolve.

IFF Operation

IFF works on a two channel system, with one frequency (1030 megahertz) being the uplink, or the interrogator signals, and the other (1090 megahertz) being the downlink, or the reply signal. This system is further broken down into four SIF modes of operation (two for Military and Civilian use, and two for Military use only) and one secure mode (Mode 4). Each mode of operation gathers specific information from the aircraft being interrogated.

  • Mode 1: Mode 1 has a possible total of 64 reply codes. It is used by the military Air Traffic Control (ATC) to determine what kind of aircraft is being interrogated, and what type of mission it is on.

  • Mode 2: Mode 2 is also for military use. It requests the tail number of the aircraft. (The plane's ID number). There are a total possible 4096 codes available for Mode 2.

  • Mode 3/A: Mode 3/A (three-alpha) is for use by military and civilians internationally, and is the standard Air Traffic Control mode. Along with identification, Mode 3/A can be used to signal emergencies and/or radio failures.

  • Mode C: Mode C is, quite simply, the altitude of the aircraft. If you interrogate an aircraft using Mode C, the response will be the altitude of the aircraft to the nearest 100 feet.

  • Mode 4: Mode 4 is the only true IFF mode which returns a Hostile or Friendly reply. This is not to say that every time a hostile reply is located a Stinger missile is launched at the offending target. In fact, Mode 4 does not even interrogate an aircraft in the same way that the SIF modes do. Mode 4 looks at the environment in a specific location, and determines hostile or friendly based on information which I'm not entirely sure isn't classified. So I'll just leave it at that. Anyway, hostile doesn't necessarily mean an enemy is there - only that something is there that needs to be looked at with some scrutiny.

The History of IFF

As I said above, IFF came into use around the time that World War II began - when the use of aircraft caused a sudden and drastic change in warfare. Before the widespread use of aircraft in battle, it was relatively easy to determine whom it was you were to kill. If their uniform looked different than yours you blew them away. This is not quite as easily accomplished when dealing with planes and jets. Especially today with supersonic aircraft and missiles.

It was around this time that radar was being developed, and although radar was extremely helpful, it could only detect; it could not tell if an incoming craft was hostile, or friendly. The the incoming attack on Pearl Harbor was noticed by the radar station at Diamond Head, however they were mistaken for an American aircraft company. No IFF.

The first example of IFF can be attributed to the Germans. They realized that identifying themselves to each other was crucial. So the German fighter pilots began to roll their planes over when they detected radar from the ground. This would change the polarization of the radar reflections and create a blip on the radar screen that was different from the rest, and the German radar operators could easily identify their own planes.

Meanwhile, the British and the Americans came up with their own workable identification system as well. In 1940, MK I (pronounced "Mark One") was introduced and activated. MK I transmitted a signal when it received a radar signal. MK III was a bit more sophisticated, and could be programmed to respond to six different interrogation codes.

Modern IFF uses MK XII which I described above.

The Future of IFF

Actually the present... MK XII technology is roughly 40 years old. When it was designed we couldn't possibly dream of doing some of the things that are taken for granted now. NATO is integrating Mode 5 into MK XII to make up for many of the technological shortcomings, which have been solved between 1963 and today. Mode 5 uses much more sophisticated communications technology which improves message quality, and security. Mode 5 eliminates a lot of Garble or FRUIT (interference), and now provides Error Detection and Correction (EDAC).

The mathematical expression of "logically equivalent". In mathematics, logical equivalence is of utmost importance; every logical equivalence is another tool for a mathematician to use in any given problem. "If and only if" is commonly abbreviated iff, because many mathematicians are lazy. For statements a and b, a is logically equivalent to b exactly when a iff b.

Here is a truth table showing the truth values of the statements a, b, a -> b (a implies b), b -> a and a <-> b (a iff b):

 a | b | a -> b | b -> a | a <-> b
 T | T |   T    |   T    |    T
 T | F |   F    |   T    |    F
 F | T |   T    |   F    |    F
 F | F |   T    |   T    |    T

Mathematicians like iff statements when they are true. For example, let k be a positive integer. k is prime iff k has no divisors other than itself and one (one not being prime). Implications are fine and dandy, but iff is better because then two statements can be interchanged exactly. In some cases, whole classes of mathematical thought can be exchanged directly for each other (see Fermat's Last Theorem, modular forms and elliptic curves).

Some "common" logical equivalences:

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