<s>
Identification	B-Device
,	I-Device
friend	I-Device
or	I-Device
foe	I-Device
(	O
IFF	O
)	O
is	O
an	O
identification	O
system	O
designed	O
for	O
command	O
and	O
control	O
.	O
</s>
<s>
It	O
uses	O
a	O
transponder	B-Architecture
that	O
listens	O
for	O
an	O
interrogation	O
signal	O
and	O
then	O
sends	O
a	O
response	O
that	O
identifies	O
the	O
broadcaster	O
.	O
</s>
<s>
IFF	O
systems	O
usually	O
use	O
radar	B-Application
frequencies	O
,	O
but	O
other	O
electromagnetic	O
frequencies	O
,	O
radio	O
or	O
infrared	O
,	O
may	O
be	O
used	O
.	O
</s>
<s>
It	O
enables	O
military	O
and	O
civilian	O
air	B-Application
traffic	I-Application
control	I-Application
interrogation	O
systems	O
to	O
identify	O
aircraft	O
,	O
vehicles	O
or	O
forces	O
as	O
friendly	O
,	O
as	O
opposed	O
to	O
neutral	O
or	O
hostile	O
,	O
and	O
to	O
determine	O
their	O
bearing	O
and	O
range	O
from	O
the	O
interrogator	O
.	O
</s>
<s>
IFF	O
was	O
first	O
developed	O
during	O
World	O
War	O
II	O
,	O
with	O
the	O
arrival	O
of	O
radar	B-Application
,	O
and	O
several	O
friendly	O
fire	O
incidents	O
.	O
</s>
<s>
IFF	O
is	O
a	O
tool	O
within	O
the	O
broader	O
military	O
action	O
of	O
Combat	B-Device
Identification	I-Device
(	O
CID	O
)	O
,	O
the	O
characterization	O
of	O
objects	O
detected	O
in	O
the	O
field	O
of	O
combat	O
sufficiently	O
accurately	O
to	O
support	O
operational	O
decisions	O
.	O
</s>
<s>
With	O
the	O
successful	O
deployment	O
of	O
radar	B-Application
systems	I-Application
for	O
air	O
defence	O
during	O
World	O
War	O
II	O
,	O
combatants	O
were	O
immediately	O
confronted	O
with	O
the	O
difficulty	O
of	O
distinguishing	O
friendly	O
aircraft	O
from	O
hostile	O
ones	O
;	O
by	O
that	O
time	O
,	O
aircraft	O
were	O
flown	O
at	O
high	O
speed	O
and	O
altitude	O
,	O
making	O
visual	O
identification	O
impossible	O
,	O
and	O
the	O
targets	O
showed	O
up	O
as	O
featureless	O
blips	O
on	O
the	O
radar	B-Application
screen	O
.	O
</s>
<s>
Already	O
before	O
the	O
deployment	O
of	O
their	O
Chain	O
Home	O
radar	B-Application
system	I-Application
(	O
CH	O
)	O
,	O
the	O
RAF	O
had	O
considered	O
the	O
problem	O
of	O
IFF	O
.	O
</s>
<s>
By	O
1938	O
,	O
researchers	O
at	O
Bawdsey	O
Manor	O
began	O
experiments	O
with	O
"	O
reflectors	O
"	O
consisting	O
of	O
dipole	O
antennas	O
tuned	O
to	O
resonate	O
to	O
the	O
primary	O
frequency	O
of	O
the	O
CH	O
radars	B-Application
.	O
</s>
<s>
This	O
consisted	O
of	O
a	O
set	O
of	O
tracking	O
stations	O
using	O
HF/DF	O
radio	B-Device
direction	I-Device
finders	I-Device
.	O
</s>
<s>
Several	O
such	O
stations	O
were	O
assigned	O
to	O
each	O
"	O
sector	O
"	O
of	O
the	O
air	O
defence	O
system	O
,	O
and	O
sent	O
their	O
measurements	O
to	O
a	O
plotting	O
station	O
at	O
sector	O
headquarters	O
,	O
who	O
used	O
triangulation	B-Algorithm
to	O
determine	O
the	O
aircraft	O
's	O
location	O
.	O
</s>
<s>
Known	O
as	O
"	O
pip-squeak	O
"	O
,	O
the	O
system	O
worked	O
,	O
but	O
was	O
labour-intensive	O
and	O
did	O
not	O
display	O
its	O
information	O
directly	O
to	O
the	O
radar	B-Application
operators	O
.	O
</s>
<s>
A	O
system	O
that	O
worked	O
directly	O
with	O
the	O
radar	B-Application
was	O
clearly	O
desirable	O
.	O
</s>
<s>
The	O
first	O
active	O
IFF	B-Device
transponder	I-Device
(	O
transmitter/responder	O
)	O
was	O
the	O
IFF	O
Mark	O
I	O
which	O
was	O
used	O
experimentally	O
in	O
1939	O
.	O
</s>
<s>
They	O
were	O
tuned	O
to	O
the	O
signal	O
from	O
the	O
CH	O
radar	B-Application
(	O
20	O
–	O
30MHz	O
)	O
,	O
amplifying	O
it	O
so	O
strongly	O
that	O
it	O
was	O
broadcast	O
back	O
out	O
the	O
aircraft	O
's	O
antenna	O
.	O
</s>
<s>
In	O
testing	O
,	O
it	O
was	O
found	O
that	O
the	O
unit	O
would	O
often	O
overpower	O
the	O
radar	B-Application
or	O
produce	O
too	O
little	O
signal	O
to	O
be	O
seen	O
,	O
and	O
at	O
the	O
same	O
time	O
,	O
new	O
radars	B-Application
were	O
being	O
introduced	O
using	O
new	O
frequencies	O
.	O
</s>
<s>
Mark	O
II	O
had	O
a	O
series	O
of	O
separate	O
tuners	O
inside	O
tuned	O
to	O
different	O
radar	B-Application
bands	O
that	O
it	O
stepped	O
through	O
using	O
a	O
motorized	O
switch	O
,	O
while	O
an	O
automatic	O
gain	O
control	O
solved	O
the	O
problem	O
of	O
it	O
sending	O
out	O
too	O
much	O
signal	O
.	O
</s>
<s>
Mark	O
II	O
was	O
technically	O
complete	O
as	O
the	O
war	O
began	O
,	O
but	O
a	O
lack	O
of	O
sets	O
meant	O
it	O
was	O
not	O
available	O
in	O
quantity	O
and	O
only	O
a	O
small	O
number	O
of	O
RAF	O
aircraft	O
carried	O
it	O
by	O
the	O
time	O
of	O
the	O
Battle	B-Application
of	I-Application
Britain	I-Application
.	O
</s>
<s>
Even	O
by	O
1940	O
the	O
complex	O
system	O
of	O
Mark	O
II	O
was	O
reaching	O
its	O
limits	O
while	O
new	O
radars	B-Application
were	O
being	O
constantly	O
introduced	O
.	O
</s>
<s>
By	O
1941	O
,	O
a	O
number	O
of	O
sub-models	O
were	O
introduced	O
that	O
covered	O
different	O
combinations	O
of	O
radars	B-Application
,	O
common	O
naval	O
ones	O
for	O
instance	O
,	O
or	O
those	O
used	O
by	O
the	O
RAF	O
.	O
</s>
<s>
But	O
the	O
introduction	O
of	O
radars	B-Application
based	O
on	O
the	O
microwave-frequency	O
cavity	B-Architecture
magnetron	I-Architecture
rendered	O
this	O
obsolete	O
;	O
there	O
was	O
simply	O
no	O
way	O
to	O
make	O
a	O
responder	O
operating	O
in	O
this	O
band	O
using	O
contemporary	O
electronics	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
the	O
magnetron	B-Architecture
,	O
work	O
on	O
this	O
concept	O
began	O
at	O
the	O
Telecommunications	O
Research	O
Establishment	O
as	O
the	O
IFF	O
Mark	O
III	O
.	O
</s>
<s>
Mark	O
III	O
transponders	B-Architecture
were	O
designed	O
to	O
respond	O
to	O
specific	O
'	O
interrogators	O
 '	O
,	O
rather	O
than	O
replying	O
directly	O
to	O
received	O
radar	B-Application
signals	O
.	O
</s>
<s>
These	O
interrogators	O
worked	O
on	O
a	O
limited	O
selection	O
of	O
frequencies	O
,	O
no	O
matter	O
what	O
radar	B-Application
they	O
were	O
paired	O
with	O
.	O
</s>
<s>
Equivalent	O
sets	O
were	O
manufactured	O
in	O
the	O
US	O
,	O
initially	O
as	O
copies	O
of	O
British	O
sets	O
,	O
so	O
that	O
allied	O
aircraft	O
would	O
be	O
identified	O
upon	O
interrogation	O
by	O
each	O
other	O
's	O
radar	B-Application
.	O
</s>
<s>
It	O
was	O
tuned	O
to	O
the	O
low-VHF	O
band	O
at	O
125MHz	O
used	O
by	O
the	O
Freya	O
radar	B-Application
,	O
and	O
an	O
adaptor	O
was	O
used	O
with	O
the	O
low-UHF-banded	O
550	O
–	O
580MHz	O
used	O
by	O
Würzburg	O
.	O
</s>
<s>
To	O
start	O
the	O
identification	O
procedure	O
,	O
the	O
ground	O
operator	O
switched	O
the	O
pulse	O
frequency	O
of	O
his	O
radar	B-Application
from	O
3,750	O
Hz	O
to	O
5,000	O
Hz	O
.	O
</s>
<s>
The	O
radar	B-Application
operator	O
would	O
then	O
see	O
the	O
blip	O
lengthen	O
and	O
shorten	O
in	O
the	O
given	O
code	O
.	O
</s>
<s>
The	O
system	O
included	O
a	O
way	O
for	O
ground	O
controllers	O
to	O
determine	O
whether	O
an	O
aircraft	O
had	O
the	O
right	O
code	O
or	O
not	O
but	O
it	O
did	O
not	O
include	O
a	O
way	O
for	O
the	O
transponder	B-Architecture
to	O
reject	O
signals	O
from	O
other	O
sources	O
.	O
</s>
<s>
IV	O
radar	B-Application
,	O
which	O
originally	O
operated	O
at	O
212MHz	O
.	O
</s>
<s>
This	O
technique	O
is	O
now	O
known	O
as	O
a	O
cross-band	O
transponder	B-Architecture
.	O
</s>
<s>
However	O
,	O
this	O
also	O
turned	O
out	O
to	O
be	O
close	O
to	O
the	O
frequencies	O
used	O
by	O
the	O
German	O
Würzburg	O
radar	B-Application
and	O
there	O
were	O
concerns	O
that	O
it	O
would	O
be	O
triggered	O
by	O
that	O
radar	B-Application
and	O
the	O
transponder	B-Architecture
responses	O
would	O
be	O
picked	O
on	O
its	O
radar	B-Application
display	O
.	O
</s>
<s>
As	O
development	O
continued	O
it	O
was	O
decided	O
to	O
introduce	O
an	O
encoding	O
system	O
known	O
as	O
the	O
"	O
Selective	B-Device
Identification	I-Device
Feature	I-Device
"	O
,	O
or	O
SIF	O
.	O
</s>
<s>
These	O
sets	O
included	O
a	O
new	O
military	O
Mode	O
3	O
which	O
was	O
essentially	O
identical	O
to	O
Mode	O
2	O
,	O
returning	O
a	O
four-digit	O
code	O
,	O
but	O
used	O
a	O
different	O
interrogation	O
pulse	O
,	O
allowing	O
the	O
aircraft	O
to	O
identify	O
if	O
the	O
query	O
was	O
from	O
a	O
military	O
or	O
civilian	O
radar	B-Application
.	O
</s>
<s>
Mode	B-Architecture
C	I-Architecture
responded	O
with	O
a	O
12-bit	O
number	O
encoded	O
using	O
Gillham	B-Device
code	I-Device
,	O
which	O
represented	O
the	O
altitude	O
as	O
(	O
that	O
number	O
)	O
x	O
100	O
feet	O
-	O
1200	O
.	O
</s>
<s>
Radar	B-Application
systems	I-Application
can	O
easily	O
locate	O
an	O
aircraft	O
in	O
two	O
dimensions	O
,	O
but	O
measuring	O
altitude	O
is	O
a	O
more	O
complex	O
problem	O
and	O
,	O
especially	O
in	O
the	O
1950s	O
,	O
added	O
significantly	O
to	O
the	O
cost	O
of	O
the	O
radar	B-Application
system	I-Application
.	O
</s>
<s>
Modern	O
interrogators	O
generally	O
send	O
out	O
a	O
series	O
of	O
challenges	O
on	O
Mode	O
3/A	O
and	O
then	O
Mode	B-Architecture
C	I-Architecture
,	O
allowing	O
the	O
system	O
to	O
combine	O
the	O
identity	O
of	O
the	O
aircraft	O
with	O
its	O
altitude	O
and	O
location	O
from	O
the	O
radar	B-Application
.	O
</s>
<s>
This	O
allowed	O
enemy	O
transmitters	O
to	O
trigger	O
the	O
response	O
,	O
and	O
using	O
triangulation	B-Algorithm
,	O
an	O
enemy	O
could	O
determine	O
the	O
location	O
of	O
the	O
transponder	B-Architecture
.	O
</s>
<s>
One	O
is	O
that	O
the	O
Interrogation	O
pulse	O
is	O
followed	O
by	O
a	O
12-bit	O
code	O
similar	O
to	O
the	O
ones	O
sent	O
back	O
by	O
the	O
Mark	O
3	O
transponders	B-Architecture
.	O
</s>
<s>
When	O
the	O
number	O
is	O
received	O
and	O
decoded	O
in	O
the	O
aircraft	O
transponder	B-Architecture
,	O
a	O
further	O
cryptographic	O
encoding	O
is	O
applied	O
.	O
</s>
<s>
If	O
the	O
result	O
of	O
that	O
operation	O
matches	O
the	O
value	O
dialled	O
into	O
the	O
IFF	O
in	O
the	O
aircraft	O
,	O
the	O
transponder	B-Architecture
replies	O
with	O
a	O
Mode	O
3	O
response	O
as	O
before	O
.	O
</s>
<s>
This	O
solves	O
the	O
problem	O
of	O
the	O
aircraft	O
transponder	B-Architecture
replying	O
to	O
false	O
interrogations	O
,	O
but	O
does	O
not	O
completely	O
solve	O
the	O
problem	O
of	O
locating	O
the	O
aircraft	O
through	O
triangulation	B-Algorithm
.	O
</s>
<s>
When	O
received	O
by	O
an	O
enemy	O
that	O
does	O
not	O
see	O
the	O
interrogation	O
pulse	O
,	O
which	O
is	O
generally	O
the	O
case	O
as	O
they	O
are	O
often	O
below	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
,	O
this	O
causes	O
a	O
random	O
displacement	O
of	O
the	O
return	O
signal	O
with	O
every	O
pulse	O
.	O
</s>
<s>
The	O
IFF	O
of	O
World	O
War	O
II	O
and	O
Soviet	O
military	O
systems	O
(	O
1946	O
to	O
1991	O
)	O
used	O
coded	O
radar	B-Application
signals	O
(	O
called	O
Cross-Band	O
Interrogation	O
,	O
or	O
CBI	O
)	O
to	O
automatically	O
trigger	O
the	O
aircraft	O
's	O
transponder	B-Architecture
in	O
an	O
aircraft	O
illuminated	O
by	O
the	O
radar	B-Application
.	O
</s>
<s>
Radar-based	O
aircraft	O
identification	O
is	O
also	O
called	O
secondary	O
surveillance	O
radar	B-Application
in	O
both	O
military	O
and	O
civil	O
usage	O
,	O
with	O
primary	O
radar	B-Application
bouncing	O
an	O
RF	O
pulse	O
off	O
of	O
the	O
aircraft	O
to	O
determine	O
position	O
.	O
</s>
<s>
It	O
required	O
the	O
operator	O
to	O
perform	O
several	O
adjustments	O
to	O
the	O
radar	B-Application
receiver	O
to	O
suppress	O
the	O
image	O
of	O
the	O
natural	O
echo	O
on	O
the	O
radar	B-Application
receiver	O
,	O
so	O
that	O
visual	O
examination	O
of	O
the	O
IFF	O
signal	O
would	O
be	O
possible	O
.	O
</s>
<s>
By	O
1943	O
,	O
Donald	O
Barchok	O
filed	O
a	O
patent	O
for	O
a	O
radar	B-Application
system	I-Application
using	O
the	O
abbreviation	O
IFF	O
in	O
his	O
text	O
with	O
only	O
parenthetic	O
explanation	O
,	O
indicating	O
that	O
this	O
acronym	O
had	O
become	O
an	O
accepted	O
term	O
.	O
</s>
<s>
In	O
1945	O
,	O
Emile	O
Labin	O
and	O
Edwin	O
Turner	O
filed	O
patents	O
for	O
radar	B-Application
IFF	O
systems	O
where	O
the	O
outgoing	O
radar	B-Application
signal	O
and	O
the	O
transponder	B-Architecture
's	O
reply	O
signal	O
could	O
each	O
be	O
independently	O
programmed	O
with	O
a	O
binary	O
codes	O
by	O
setting	O
arrays	O
of	O
toggle	O
switches	O
;	O
this	O
allowed	O
the	O
IFF	B-Device
code	I-Device
to	O
be	O
varied	O
from	O
day	O
to	O
day	O
or	O
even	O
hour	O
to	O
hour	O
.	O
</s>
<s>
Mode	O
3/A	O
–	O
military/civilian	O
;	O
provides	O
a	O
4-digit	O
octal	O
(	O
12	O
bit	O
)	O
identification	O
code	O
for	O
the	O
aircraft	O
,	O
assigned	O
by	O
the	O
air	B-Application
traffic	I-Application
controller	O
.	O
</s>
<s>
Mode	O
5	O
–	O
military	O
only	O
;	O
provides	O
a	O
cryptographically	O
secured	O
version	O
of	O
Mode	O
S	O
and	O
ADS-B	B-Application
GPS	O
position	O
.	O
</s>
<s>
Still	O
,	O
Identification	B-Device
of	I-Device
friend	I-Device
or	I-Device
foe	I-Device
(	O
IFF	O
)	O
has	O
not	O
been	O
regarded	O
a	O
high	O
concern	O
before	O
the	O
1990s	O
by	O
the	O
US	O
military	O
as	O
not	O
many	O
other	O
countries	O
possess	O
submarines	O
.	O
</s>
<s>
Painted	O
colours	O
or	O
bands	O
have	O
been	O
used	O
as	O
an	O
easy	O
and	O
low-tech	O
combat	B-Device
identification	I-Device
system	O
.	O
</s>
<s>
On	O
twin-engine	O
aircraft	O
the	O
stripes	O
were	O
wide	O
,	O
placed	O
outboard	O
of	O
the	O
engine	O
nacelles	B-Application
on	O
the	O
wings	O
,	O
and	O
forward	O
of	O
the	O
leading	O
edge	O
of	O
the	O
tailplane	O
around	O
the	O
fuselage	O
.	O
</s>
