Model XAE IFF kit, the first radio
recognition IFF system in the U.S.
In
telecommunications,
identification, friend or foe (IFF) is a
cryptographic identification
system designed for command and control. It is a
system that enables military, and national (civilian-located ATC)
interrogation systems to distinguish friendly aircraft, vehicles,
or forces, and to determine their bearing and range from the
interrogator.
IFF was first developed during
World War
II. The term is a bit of a misnomer, as IFF can only positively
identify friendly targets but not hostile ones. If an IFF
interrogation receives no reply, the object can only be treated as
suspicious but not as a positively identified foe.
There are many reasons for not replying to IFF by friendly
aircraft: battle damage, loss of encryption keys, wrong encryption
keys, or equipment failure. Aircraft hugging terrain are very often
poor candidates for microwave line-of-sight systems such as the IFF
system. Microwaves can't penetrate mountains, and very often
atmosphere effects (referred to as
anomalous propagation) cause timing,
range, and
azimuth issues.
Method of operation
IFF is still in use by both military and civilian aircraft. Modes
1, 2, 3, 4 and 5 are for military use only. Modes 1, 2 and 3 are
collectively known as Selective Identification Feature (SIF) modes.
Civilian aircraft use modes A, C and S. Mode C which includes
barometric pressure altitude information is often used in
conjunction with mode A. Mode A is often referred to as mode 3/A
due to the similarity to the military mode 3. Mode S is a new
civilian mode developed to replace both mode A and C.
IFF is also called secondary radar, with primary radar bouncing an
RF pulse off of the aircraft to determine position. Position with
IFF is determined by comparing antenna dish angle and the delay
from the interrogator (1,030 MHz) pulse to the received IFF
pulses on (1,090 MHz).
The IFF of
WW II and Soviet military systems
(1946 to 1991) used coded
radar signals
(called Cross-Band Interrogation, or CBI) to automatically trigger
the aircraft's transponder in an aircraft "painted" by the radar.
Modern IFF systems use a separate specialized transponder beacon
which can operate without radar. They are referred to as
cross-band beacon or transponders.
An IFF
transponder responds:
- in a military aircraft, vehicle, or unit by returning a coded
reply signal only when the incoming interrogation is identified as
part of the friendly forces network;
- if no IFF response is generated, a civil (Selective
Identification Feature, SIF) interrogation may then be generated
and the aircraft, by returning various mode replies can then be
identified or sorted.
In an IFF network both the interrogation and the reply are verified
as friendly.
Each IFF transponder also has a KIR or KIT cryptography computer
associated with it. The KIR (designed for interrogators) and the
KIT (designed for transponders) have an access port where the
encryption keys are inserted. The military IFF system will not
function without a valid key. Civilian SIF systems and
mode S do not require encryption keys.
An IFF transponder receives interrogation pulses at one frequency
(1,030 MHz), and sends the reply pulses at a different
frequency (1,090 MHz). Just the opposite of a SIF
interrogation, which is composed of two pulses spaced apart by a
different amount for each mode, with the transponder reply being a
long series of bits; the IFF interrogation is instead a long series
of bits that contains the encrypted message and parity, and the
reply is just three pulses.
The IFF message is encrypted with a secret key. IFF transponders
with the same secret key will be able to decode the IFF message.
Once decoded, the IFF transponder will execute the message and send
back a 3 pulse reply. The interrogator then compares each reply to
the challenge messages, and marks these targets friendly while also
storing their azimuth and range.
A second possibility is a target being marked as a spoof target.
That is, the target replies, but fails to process the IFF message
correctly on a significant number of challenges. Targets marked as
a spoofer can be declared hostile and, inside a battle-space, are
often destroyed when able.
If no reply is received from the IFF transponder, the target
continues to be an unknown. The IFF system is not used to declare a
target hostile if they do not reply. Very often the pilot can have
the wrong code (encryption key) selected, or the code is expired,
and they will have an audible and visual alarm every time they are
interrogated by IFF. If they can't clear the alarm they follow the
pre-briefed safe passage procedures.
The major military benefits of IFF include preventing "
friendly fire" and being able to positively
identify friendly forces.
IFF modes
There are
two military modes of operation designated for use by NATO
forces:
- Mode 4 is crypto-secure mode.
- Mode 5, Levels 1 and 2 are crypto-secure with Enhanced
encryption, Spread Spectrum Modulation, and Time of Day
Authentication.
- Mode 5, Level 1 is similar to Mode 4 information but enhanced
with an Aircraft Unique PIN number.
- Mode 5, Level 2 is the same as Mode 5 level one but includes
additional information such as Aircraft Position and Other
Attributes.
See also
References
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