<s>
Constant	B-Algorithm
false	I-Algorithm
alarm	I-Algorithm
rate	I-Algorithm
(	O
CFAR	O
)	O
detection	O
refers	O
to	O
a	O
common	O
form	O
of	O
adaptive	B-Algorithm
algorithm	I-Algorithm
used	O
in	O
radar	B-Application
systems	I-Application
to	O
detect	O
target	O
returns	O
against	O
a	O
background	O
of	O
noise	O
,	O
clutter	O
and	O
interference	O
.	O
</s>
<s>
In	O
the	O
radar	B-Application
receiver	O
,	O
the	O
returning	O
echoes	O
are	O
typically	O
received	O
by	O
the	O
antenna	O
,	O
amplified	O
,	O
down-converted	O
to	O
an	O
intermediate	O
frequency	O
,	O
and	O
then	O
passed	O
through	O
detector	O
circuitry	O
that	O
extracts	O
the	O
envelope	O
of	O
the	O
signal	O
,	O
known	O
as	O
the	O
video	O
signal	O
.	O
</s>
<s>
The	O
term	O
video	O
refers	O
to	O
the	O
resulting	O
signal	O
being	O
appropriate	O
for	O
display	O
on	O
a	O
cathode	B-Device
ray	I-Device
tube	I-Device
,	O
or	O
"	O
video	O
screen	O
"	O
.	O
</s>
<s>
The	O
role	O
of	O
the	O
constant	B-Algorithm
false	I-Algorithm
alarm	I-Algorithm
rate	I-Algorithm
circuitry	O
is	O
to	O
determine	O
the	O
power	O
threshold	O
above	O
which	O
any	O
return	O
can	O
be	O
considered	O
to	O
probably	O
originate	O
from	O
a	O
target	O
as	O
opposed	O
to	O
one	O
of	O
the	O
spurious	O
sources	O
.	O
</s>
<s>
If	O
this	O
threshold	O
is	O
too	O
low	O
,	O
more	O
real	O
targets	O
will	O
be	O
detected	O
,	O
but	O
at	O
the	O
expense	O
of	O
increased	O
numbers	O
of	O
false	B-Error_Name
alarms	I-Error_Name
.	O
</s>
<s>
Conversely	O
,	O
fewer	O
targets	O
will	O
be	O
detected	O
if	O
the	O
threshold	O
is	O
too	O
high	O
,	O
but	O
the	O
number	O
of	O
false	B-Error_Name
alarms	I-Error_Name
will	O
also	O
be	O
low	O
.	O
</s>
<s>
In	O
most	O
radar	B-Application
detectors	O
,	O
the	O
threshold	O
is	O
set	O
to	O
achieve	O
a	O
required	O
probability	O
of	O
false	B-Error_Name
alarm	I-Error_Name
(	O
equivalently	O
,	O
false	B-Error_Name
alarm	I-Error_Name
rate	O
or	O
time	O
between	O
false	B-Error_Name
alarms	I-Error_Name
)	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
a	O
fixed	O
threshold	O
level	O
can	O
be	O
chosen	O
that	O
provides	O
a	O
specified	O
probability	O
of	O
false	B-Error_Name
alarm	I-Error_Name
,	O
governed	O
by	O
the	O
probability	O
density	O
function	O
of	O
the	O
noise	O
,	O
which	O
is	O
usually	O
assumed	O
to	O
be	O
Gaussian	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
a	O
changing	O
threshold	O
can	O
be	O
used	O
,	O
where	O
the	O
threshold	O
level	O
is	O
raised	O
and	O
lowered	O
to	O
maintain	O
a	O
constant	O
probability	O
of	O
false	B-Error_Name
alarm	I-Error_Name
.	O
</s>
<s>
This	O
is	O
known	O
as	O
constant	B-Algorithm
false	I-Algorithm
alarm	I-Algorithm
rate	I-Algorithm
(	O
CFAR	O
)	O
detection	O
.	O
</s>
<s>
In	O
most	O
simple	O
CFAR	B-Algorithm
detection	I-Algorithm
schemes	O
,	O
the	O
threshold	O
level	O
is	O
calculated	O
by	O
estimating	O
the	O
noise	O
floor	O
level	O
around	O
the	O
cell	O
under	O
test	O
(	O
CUT	O
)	O
.	O
</s>
<s>
This	O
is	O
particularly	O
common	O
in	O
maritime	O
surveillance	O
(	O
radar	B-Application
)	O
applications	O
,	O
where	O
the	O
background	O
of	O
sea	O
clutter	O
is	O
particularly	O
spikey	O
and	O
not	O
well	O
approximated	O
by	O
additive	O
white	O
Gaussian	O
noise	O
.	O
</s>
<s>
This	O
is	O
a	O
difficult	O
detection	O
problem	O
,	O
as	O
it	O
is	O
difficult	O
to	O
differentiate	O
between	O
spikes	O
due	O
to	O
the	O
sea	O
surface	O
returns	O
and	O
spikes	O
due	O
to	O
valid	O
returns	O
from	O
,	O
for	O
example	O
,	O
submarine	O
periscopes	B-Language
.	O
</s>
<s>
The	O
K-distribution	B-Algorithm
is	O
a	O
popular	O
distribution	O
for	O
modelling	O
sea	O
clutter	O
characteristics	O
.	O
</s>
