<s>
The	O
radar	B-Algorithm
horizon	I-Algorithm
is	O
a	O
critical	O
area	O
of	O
performance	O
for	O
aircraft	B-Application
detection	I-Application
systems	O
that	O
is	O
defined	O
by	O
the	O
distance	O
at	O
which	O
the	O
radar	B-Application
beam	O
rises	O
enough	O
above	O
the	O
Earth	O
's	O
surface	O
to	O
make	O
detection	O
of	O
a	O
target	O
at	O
low	O
level	O
possible	O
.	O
</s>
<s>
It	O
is	O
associated	O
with	O
the	O
low	O
elevation	O
region	O
of	O
performance	O
,	O
and	O
its	O
geometry	O
depends	O
on	O
terrain	O
,	O
radar	B-Application
height	O
,	O
and	O
signal	O
processing	O
.	O
</s>
<s>
This	O
is	O
associated	O
with	O
the	O
notions	O
of	O
radar	B-Application
shadow	O
,	O
the	O
clutter	O
zone	O
,	O
and	O
the	O
clear	O
zone	O
.	O
</s>
<s>
Airborne	O
objects	O
can	O
exploit	O
the	O
radar	B-Application
shadow	O
zone	O
and	O
clutter	O
zone	O
to	O
avoid	O
radar	B-Application
detection	O
by	O
using	O
a	O
technique	O
called	O
nap-of-the-earth	O
navigation	O
.	O
</s>
<s>
Without	O
taking	O
into	O
account	O
the	O
refraction	O
through	O
the	O
atmosphere	O
,	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
would	O
be	O
the	O
geometrical	O
distance	O
from	O
the	O
radar	B-Application
to	O
the	O
horizon	O
only	O
taking	O
into	O
account	O
the	O
height	O
of	O
the	O
radar	B-Application
above	O
sea-level	O
,	O
and	O
the	O
radius	O
of	O
the	O
earth	O
(	O
approximately	O
6.4	O
·103km	O
)	O
:	O
</s>
<s>
With	O
this	O
calculation	O
,	O
the	O
horizon	O
for	O
a	O
radar	B-Application
at	O
a	O
altitude	O
is	O
.	O
</s>
<s>
The	O
radar	B-Algorithm
horizon	I-Algorithm
with	O
an	O
antenna	O
height	O
of	O
over	O
the	O
ocean	O
is	O
.	O
</s>
<s>
However	O
,	O
since	O
the	O
pressure	O
and	O
water	O
vapor	O
content	O
of	O
the	O
atmosphere	O
varies	O
with	O
height	O
,	O
the	O
path	O
used	O
by	O
the	O
radar	B-Application
beam	O
is	O
refracted	O
by	O
the	O
change	O
in	O
density	O
.	O
</s>
<s>
And	O
for	O
the	O
same	O
examples	O
:	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
for	O
the	O
radar	B-Application
at	O
a	O
altitude	O
will	O
be	O
and	O
the	O
one	O
at	O
will	O
be	O
.	O
</s>
<s>
Below	O
3MHz	O
,	O
the	O
whole	O
volume	O
of	O
the	O
air	O
acts	O
as	O
a	O
waveguide	O
to	O
fill	O
in	O
the	O
radar	B-Application
shadow	O
and	O
also	O
reduces	O
radar	B-Application
sensitivity	O
above	O
the	O
duct	O
zone	O
.	O
</s>
<s>
Ducting	O
fills	O
in	O
the	O
shadow	O
zone	O
,	O
extends	O
the	O
distance	O
of	O
the	O
clutter	O
zone	O
,	O
and	O
can	O
create	O
reflections	O
for	O
low	O
PRF	O
radar	B-Application
that	O
are	O
beyond	O
the	O
instrumented	O
range	O
.	O
</s>
<s>
Objects	O
below	O
this	O
height	O
are	O
in	O
the	O
radar	B-Application
shadow	O
.	O
</s>
<s>
The	O
Clutter	O
Zone	O
is	O
where	O
radar	B-Application
energy	O
is	O
in	O
the	O
lowest	O
several	O
thousand	O
feet	O
of	O
air	O
.	O
</s>
<s>
This	O
extends	O
to	O
a	O
distance	O
of	O
about	O
120%	O
of	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
.	O
</s>
<s>
A	O
beam	O
wide	O
will	O
illuminate	O
millions	O
of	O
square	O
feet	O
of	O
surface	O
by	O
the	O
time	O
the	O
radar	B-Application
pulse	O
reaches	O
.	O
</s>
<s>
The	O
antenna	O
for	O
radar	B-Application
with	O
no	O
signal	O
processing	O
clutter-reduction	O
improvement	O
is	O
not	O
normally	O
aimed	O
near	O
the	O
ground	O
to	O
avoid	O
overwhelming	O
computers	O
and	O
users	O
.	O
</s>
<s>
Prevailing	O
wind	O
and	O
weather	O
can	O
degrade	O
MTI	O
performance	O
,	O
and	O
MTI	O
introduces	O
blind	B-Algorithm
velocities	I-Algorithm
.	O
</s>
<s>
Pulse-Doppler	B-Algorithm
radar	I-Algorithm
can	O
reduce	O
clutter	O
by	O
over	O
60dB	O
,	O
which	O
can	O
allow	O
objects	O
smaller	O
than	O
to	O
be	O
detected	O
without	O
overloading	O
computers	O
and	O
users	O
.	O
</s>
<s>
Systems	O
using	O
pulse-Doppler	B-Algorithm
signal	I-Algorithm
processing	I-Algorithm
with	O
speed	O
rejection	O
set	O
above	O
the	O
wind	O
speed	O
have	O
no	O
clutter	O
zone	O
.	O
</s>
<s>
The	O
Clear	O
Region	O
is	O
the	O
zone	O
that	O
begins	O
several	O
kilometers	O
beyond	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
at	O
low	O
elevation	O
angles	O
.	O
</s>
<s>
A	O
number	O
of	O
radar	B-Application
systems	I-Application
have	O
been	O
developed	O
that	O
allow	O
detection	O
of	O
targets	O
in	O
the	O
shadow	O
zone	O
.	O
</s>
<s>
These	O
systems	O
are	O
collectively	O
known	O
as	O
over-the-horizon	B-Architecture
radars	I-Architecture
.	O
</s>
