<s>
A	O
height	B-Algorithm
finder	I-Algorithm
is	O
a	O
ground-based	O
aircraft	O
altitude	B-Application
measuring	O
device	O
.	O
</s>
<s>
Early	O
height	B-Algorithm
finders	I-Algorithm
were	O
optical	O
range	O
finder	O
devices	O
combined	O
with	O
simple	O
mechanical	B-Device
computers	I-Device
,	O
while	O
later	O
systems	O
migrated	O
to	O
radar	B-Application
devices	O
.	O
</s>
<s>
The	O
unique	O
vertical	O
oscillating	O
motion	O
of	O
height	B-Algorithm
finder	I-Algorithm
radars	I-Algorithm
led	O
to	O
them	O
also	O
being	O
known	O
as	O
nodding	O
radar	B-Application
.	O
</s>
<s>
Devices	O
combining	O
both	O
optics	O
and	O
radar	B-Application
were	O
deployed	O
by	O
the	O
U.S.	O
Military	O
.	O
</s>
<s>
In	O
World	O
War	O
II	O
,	O
a	O
height	B-Algorithm
finder	I-Algorithm
was	O
an	O
optical	O
rangefinder	O
used	O
to	O
determine	O
the	O
altitude	B-Application
of	O
an	O
aircraft	O
(	O
actually	O
the	O
slant	O
range	O
from	O
the	O
emplacement	O
which	O
was	O
combined	O
with	O
the	O
angle	O
of	O
sight	O
,	O
in	O
a	O
mechanical	B-Device
computer	I-Device
,	O
to	O
produce	O
altitude	B-Application
)	O
,	O
used	O
to	O
direct	O
anti-aircraft	O
guns	O
.	O
</s>
<s>
A	O
height	B-Algorithm
finder	I-Algorithm
radar	I-Algorithm
is	O
a	O
type	O
of	O
2-dimensional	O
radar	B-Application
that	O
measures	O
altitude	B-Application
of	O
a	O
target	O
.	O
</s>
<s>
The	O
operator	O
slews	O
the	O
antenna	O
toward	O
a	O
desired	O
bearing	O
,	O
identifies	O
a	O
target	O
echo	O
at	O
a	O
desired	O
range	O
on	O
the	O
RHI	O
display	O
(	O
RHI	O
=	O
range	O
height	O
indicator	O
)	O
,	O
then	O
bisects	O
the	O
target	O
with	O
a	O
cursor	O
that	O
is	O
scaled	O
to	O
indicate	O
the	O
approximate	O
altitude	B-Application
of	O
the	O
target	O
.	O
</s>
<s>
Such	O
systems	O
often	O
complement	O
2-dimensional	O
radars	B-Application
which	O
find	O
distance	O
and	O
direction	O
(	O
search	O
radar	B-Application
)	O
;	O
thus	O
using	O
two	O
2-dimensional	O
systems	O
to	O
obtain	O
a	O
3-dimensional	O
aerial	O
picture	O
.	O
</s>
<s>
Height	B-Algorithm
finding	I-Algorithm
radars	I-Algorithm
of	O
the	O
1960s	O
and	O
70s	O
were	O
distinguished	O
by	O
their	O
antenna	O
being	O
tall	O
,	O
but	O
narrow	O
.	O
</s>
<s>
As	O
beam	O
shape	O
is	O
a	O
function	O
of	O
antenna	O
shape	O
,	O
the	O
height	B-Algorithm
finder	I-Algorithm
beam	O
was	O
flat	O
and	O
wide	O
horizontally	O
(	O
i.e.	O
,	O
not	O
very	O
good	O
at	O
determining	O
bearing	O
to	O
the	O
target	O
)	O
,	O
but	O
very	O
thin	O
vertically	O
,	O
allowing	O
accurate	O
measurement	O
of	O
elevation	O
angle	O
,	O
thus	O
altitude	B-Application
.	O
</s>
<s>
Modern	O
3D	B-Algorithm
radar	I-Algorithm
sets	O
find	O
both	O
azimuth	O
and	O
elevation	O
,	O
making	O
separate	O
height	B-Algorithm
finder	I-Algorithm
radars	I-Algorithm
largely	O
obsolete	O
.	O
</s>
