<s>
Fluctuation	B-Algorithm
loss	I-Algorithm
is	O
an	O
effect	O
seen	O
in	O
radar	B-Application
systems	I-Application
as	O
the	O
target	O
object	O
moves	O
or	O
changes	O
its	O
orientation	O
relative	O
to	O
the	O
radar	B-Application
system	I-Application
.	O
</s>
<s>
The	O
effect	O
occurs	O
when	O
the	O
target	O
's	O
physical	O
size	O
is	O
within	O
a	O
key	O
range	O
of	O
values	O
relative	O
to	O
the	O
wavelength	O
of	O
the	O
radar	B-Application
signal	O
.	O
</s>
<s>
As	O
the	O
signal	O
reflects	O
off	O
various	O
parts	O
of	O
the	O
target	O
,	O
they	O
may	O
interfere	O
as	O
they	O
return	O
to	O
the	O
radar	B-Application
receiver	O
.	O
</s>
<s>
At	O
any	O
single	O
distance	O
from	O
the	O
station	O
,	O
this	O
will	O
cause	O
the	O
signal	O
to	O
be	O
amplified	O
or	O
diminished	O
compared	O
to	O
the	O
baseline	O
signal	O
one	O
calculates	O
from	O
the	O
radar	B-Application
equation	O
.	O
</s>
<s>
The	O
effect	O
can	O
be	O
reduced	O
or	O
eliminated	O
by	O
operating	O
on	O
more	O
than	O
one	O
frequency	O
or	O
using	O
modulation	O
techniques	O
like	O
pulse	B-Algorithm
compression	I-Algorithm
that	O
change	O
the	O
frequency	O
over	O
the	O
period	O
of	O
a	O
pulse	O
.	O
</s>
<s>
This	O
offered	O
the	O
ability	O
to	O
model	O
real-world	O
objects	O
like	O
aircraft	O
to	O
understand	O
the	O
expected	O
fluctuation	B-Algorithm
loss	I-Algorithm
effects	O
.	O
</s>
<s>
For	O
basic	O
considerations	O
of	O
the	O
strength	O
of	O
a	O
signal	O
returned	O
by	O
a	O
given	O
target	O
,	O
the	O
radar	B-Application
equation	O
models	O
the	O
target	O
as	O
a	O
single	O
point	O
in	O
space	O
with	O
a	O
given	O
radar	B-Application
cross-section	O
(	O
RCS	O
)	O
.	O
</s>
<s>
Such	O
models	O
fail	O
to	O
account	O
for	O
real-world	O
effects	O
due	O
to	O
the	O
radar	B-Application
signal	O
reflecting	O
off	O
multiple	O
points	O
on	O
the	O
target	O
.	O
</s>
<s>
If	O
the	O
distance	O
between	O
these	O
points	O
is	O
on	O
the	O
order	O
of	O
the	O
wavelength	O
of	O
the	O
radar	B-Application
signal	O
,	O
the	O
reflections	O
are	O
subject	O
to	O
wave	O
interference	O
effects	O
that	O
can	O
cause	O
the	O
signal	O
to	O
be	O
amplified	O
or	O
diminished	O
depending	O
on	O
the	O
exact	O
path	O
lengths	O
.	O
</s>
<s>
As	O
the	O
target	O
moves	O
in	O
relation	O
to	O
the	O
radar	B-Application
,	O
these	O
distances	O
change	O
and	O
create	O
a	O
constantly	O
changing	O
signal	O
.	O
</s>
<s>
On	O
the	O
radar	B-Algorithm
display	I-Algorithm
,	O
this	O
causes	O
the	O
signal	O
to	O
fade	O
in	O
and	O
out	O
,	O
making	O
target	O
tracking	O
difficult	O
.	O
</s>
<s>
One	O
way	O
to	O
reduce	O
or	O
eliminate	O
this	O
effect	O
is	O
to	O
have	O
two	O
or	O
more	O
frequencies	O
in	O
the	O
radar	B-Application
signal	O
.	O
</s>
<s>
This	O
was	O
used	O
in	O
the	O
AN/FPS	O
-24	O
radar	B-Application
,	O
for	O
instance	O
.	O
</s>
<s>
Multi-frequency	O
signals	O
of	O
this	O
sort	O
also	O
give	O
the	O
radar	B-Application
system	I-Application
frequency	B-Algorithm
agility	I-Algorithm
,	O
which	O
is	O
useful	O
for	O
avoiding	O
jamming	O
from	O
a	O
carcinotron	B-Algorithm
,	O
so	O
most	O
radars	B-Application
of	O
the	O
1960s	O
had	O
some	O
capability	O
to	O
avoid	O
fluctuation	B-Algorithm
loss	I-Algorithm
even	O
if	O
this	O
was	O
not	O
an	O
explicit	O
design	O
goal	O
.	O
</s>
<s>
The	O
Swerling	B-Algorithm
target	I-Algorithm
models	I-Algorithm
address	O
these	O
issues	O
by	O
modeling	O
the	O
target	O
as	O
a	O
number	O
of	O
individual	O
radiators	O
and	O
considering	O
the	O
result	O
using	O
the	O
chi-squared	O
distribution	O
:	O
</s>
<s>
For	O
instance	O
,	O
a	O
sea-based	O
radar	B-Application
system	I-Application
is	O
most	O
likely	O
to	O
view	O
a	O
ship	O
from	O
the	O
side	O
,	O
the	O
front	O
,	O
and	O
the	O
back	O
,	O
but	O
never	O
the	O
top	O
or	O
the	O
bottom	O
.	O
</s>
<s>
This	O
is	O
the	O
case	O
for	O
a	O
scanning	O
radar	B-Application
,	O
which	O
sweeps	O
its	O
signal	O
past	O
the	O
target	O
in	O
a	O
relatively	O
short	O
time	O
,	O
often	O
on	O
the	O
order	O
of	O
milliseconds	O
.	O
</s>
<s>
This	O
is	O
the	O
case	O
for	O
very	O
high-speed	O
targets	O
,	O
or	O
,	O
more	O
commonly	O
,	O
"	O
staring	O
"	O
radars	B-Application
like	O
fire-control	O
radars	B-Application
or	O
search	O
radars	B-Application
that	O
are	O
locked-on	O
to	O
a	O
single	O
target	O
.	O
</s>
