<s>
Precision	B-Algorithm
approach	I-Algorithm
radar	I-Algorithm
(	O
PAR	O
)	O
is	O
a	O
type	O
of	O
radar	B-Application
guidance	O
system	O
designed	O
to	O
provide	O
lateral	O
and	O
vertical	O
guidance	O
to	O
an	O
aircraft	O
pilot	O
for	O
landing	O
,	O
until	O
the	O
landing	O
threshold	O
is	O
reached	O
.	O
</s>
<s>
The	O
overall	O
concept	O
is	O
known	O
as	O
ground-controlled	O
approach	O
(	O
GCA	O
)	O
,	O
and	O
this	O
name	O
was	O
also	O
used	O
to	O
refer	O
to	O
the	O
radar	B-Application
systems	I-Application
in	O
the	O
early	O
days	O
of	O
its	O
development	O
.	O
</s>
<s>
PAR	O
radars	B-Application
use	O
a	O
unique	O
type	O
of	O
radar	B-Algorithm
display	I-Algorithm
with	O
two	O
separate	O
"	O
traces	O
"	O
,	O
separated	O
vertically	O
.	O
</s>
<s>
Precision	B-Algorithm
approach	I-Algorithm
radars	I-Algorithm
are	O
most	O
frequently	O
used	O
at	O
military	O
air	B-Application
traffic	I-Application
control	I-Application
facilities	O
.	O
</s>
<s>
These	O
radars	B-Application
can	O
provide	O
precision	O
guidance	O
to	O
a	O
distance	O
of	O
10	O
to	O
20	O
miles	O
.	O
</s>
<s>
There	O
are	O
systems	O
that	O
provide	O
PAR	O
functionality	O
without	O
using	O
primary	O
radar	B-Application
.	O
</s>
<s>
These	O
non-traditional	O
PAR	O
systems	O
use	O
transponder	B-Architecture
multilateration	O
,	O
triangulation	B-Algorithm
and/or	O
trilateration	O
.	O
</s>
<s>
One	O
such	O
system	O
,	O
Transponder	B-Architecture
Landing	O
System	O
(	O
TLS	O
)	O
precisely	O
tracks	O
aircraft	O
using	O
the	O
mode	O
3/A	O
transponder	B-Architecture
response	O
received	O
by	O
antenna	O
arrays	O
located	O
near	O
the	O
runway	O
.	O
</s>
<s>
The	O
signal	O
strength	O
for	O
the	O
secondary	O
surveillance	O
radar	B-Application
subsystem	O
of	O
a	O
non-traditional	O
PAR	O
is	O
not	O
attenuated	O
by	O
rain	O
since	O
the	O
frequency	O
is	O
within	O
the	O
long	O
range	O
band	O
,	O
L-band	B-Algorithm
.	O
</s>
<s>
This	O
system	O
is	O
co-operative	O
depending	O
,	O
it	O
means	O
that	O
in	O
the	O
case	O
of	O
transponder	B-Architecture
failure	O
no	O
aircraft	O
detection	O
will	O
be	O
provided	O
.	O
</s>
<s>
A	O
traditional	O
PAR	O
is	O
flight	O
inspected	O
by	O
comparing	O
written	O
notes	O
between	O
two	O
observers	O
,	O
one	O
taking	O
notes	O
at	O
a	O
truth	O
reference	O
system	O
such	O
as	O
a	O
theodolite	O
and	O
the	O
other	O
observer	O
taking	O
notes	O
while	O
observing	O
the	O
radar	B-Application
console	O
;	O
see	O
ICAO	O
Document	O
8071	O
.	O
</s>
<s>
The	O
Transponder	B-Architecture
Landing	O
System	O
(	O
TLS	O
)	O
non-traditional	O
PAR	O
can	O
transmit	O
an	O
ILS	O
signal	O
that	O
corresponds	O
to	O
the	O
aircraft	O
position	O
relative	O
to	O
the	O
precision	O
approach	O
.	O
</s>
