<s>
The	O
Foster	B-Algorithm
scanner	I-Algorithm
,	O
or	O
Variable	O
Path	O
scanner	O
,	O
is	O
a	O
type	O
of	O
radar	B-Application
system	I-Application
that	O
produces	O
a	O
narrow	O
beam	O
that	O
rapidly	O
scans	O
an	O
area	O
in	O
front	O
of	O
it	O
.	O
</s>
<s>
Foster	B-Algorithm
scanners	I-Algorithm
were	O
widely	O
used	O
in	O
post-World	O
War	O
II	O
radar	B-Application
systems	I-Application
used	O
for	O
artillery	O
and	O
mortar	O
spotting	O
.	O
</s>
<s>
Modern	O
radars	B-Application
in	O
this	O
role	O
normally	O
use	O
electronic	B-Algorithm
scanning	I-Algorithm
in	O
place	O
of	O
a	O
Foster	B-Algorithm
scanner	I-Algorithm
for	O
this	O
purpose	O
.	O
</s>
<s>
The	O
Foster	B-Algorithm
scanner	I-Algorithm
consists	O
of	O
two	O
parts	O
;	O
a	O
box-shaped	O
antenna	O
and	O
the	O
"	O
scanner	O
"	O
itself	O
.	O
</s>
<s>
The	O
resulting	O
system	O
has	O
the	O
basic	O
size	O
and	O
shape	O
of	O
a	O
large	O
pizza	B-General_Concept
box	I-General_Concept
with	O
the	O
rear	O
side	O
rounded	O
.	O
</s>
<s>
Small	O
metal	O
"	O
teeth	O
"	O
at	O
the	O
point	O
where	O
the	O
input	O
waveguide	O
meets	O
the	O
cone	O
causes	O
the	O
radar	B-Application
signal	O
to	O
be	O
reflected	O
90	O
degrees	O
into	O
the	O
slot	O
between	O
the	O
two	O
cones	O
.	O
</s>
<s>
Used	O
alone	O
,	O
this	O
system	O
causes	O
the	O
radar	B-Application
signal	O
to	O
travel	O
a	O
different	O
total	O
path	O
length	O
at	O
the	O
narrow	O
end	O
of	O
the	O
cone	O
as	O
opposed	O
to	O
the	O
wide	O
end	O
.	O
</s>
<s>
The	O
Foster	B-Algorithm
scanner	I-Algorithm
was	O
used	O
in	O
a	O
number	O
of	O
post-World	O
War	O
II	O
counterbattery	O
radar	B-Application
systems	I-Application
,	O
the	O
first	O
examples	O
reaching	O
service	O
in	O
the	O
late	O
1950s	O
and	O
early	O
1960s	O
.	O
</s>
<s>
In	O
these	O
systems	O
the	O
scanner	O
was	O
mounted	O
horizontally	O
,	O
in	O
order	O
to	O
produce	O
a	O
signal	O
that	O
scanned	O
back	O
and	O
forth	O
horizontally	O
in	O
front	O
of	O
the	O
radar	B-Application
set	O
.	O
</s>
<s>
Early	O
systems	O
included	O
the	O
US	O
AN/MPQ	O
-4	O
,	O
which	O
used	O
lobe	B-Algorithm
switching	I-Algorithm
to	O
switch	O
between	O
two	O
angles	O
about	O
35	O
mils	O
apart	O
(	O
about	O
2	O
degrees	O
)	O
.	O
</s>
<s>
The	O
UK	O
Green	O
Archer	O
radar	B-Application
used	O
a	O
manual	O
switch	O
to	O
flip	O
between	O
the	O
vertical	O
angles	O
.	O
</s>
<s>
The	O
radar	B-Application
was	O
normally	O
left	O
in	O
the	O
"	O
lower	O
"	O
angle	O
setting	O
,	O
while	O
the	O
operator	O
waited	O
for	O
returns	O
.	O
</s>
<s>
The	O
Foster	B-Algorithm
scanner	I-Algorithm
was	O
originally	O
designed	O
by	O
John	O
Stuart	O
Foster	O
while	O
working	O
at	O
the	O
MIT	O
Radiation	O
Laboratory	O
as	O
part	O
of	O
a	O
technical	O
exchange	O
from	O
the	O
Canadian	O
National	O
Research	O
Council	O
.	O
</s>
