<s>
The	O
constant	B-Algorithm
altitude	I-Algorithm
plan	I-Algorithm
position	I-Algorithm
indicator	I-Algorithm
,	O
better	O
known	O
as	O
CAPPI	B-Algorithm
,	O
is	O
a	O
radar	B-Application
display	O
which	O
gives	O
a	O
horizontal	O
cross-section	O
of	O
data	O
at	O
constant	O
altitude	O
.	O
</s>
<s>
It	O
has	O
been	O
developed	O
by	O
McGill	O
University	O
in	O
Montreal	O
by	O
the	O
Stormy	O
Weather	O
Group	O
to	O
circumvent	O
some	O
problems	O
with	O
the	O
PPI	B-Algorithm
:	O
</s>
<s>
Altitude	O
changing	O
with	O
distance	O
to	O
the	O
radar	B-Application
.	O
</s>
<s>
Ground	O
echoes	O
problems	O
near	O
the	O
radar	B-Application
.	O
</s>
<s>
In	O
1954	O
,	O
McGill	O
University	O
obtained	O
a	O
new	O
radar	B-Application
(	O
CPS-9	O
)	O
which	O
had	O
a	O
better	O
resolution	O
and	O
used	O
FASE	O
(	O
Fast	O
Azimuth	O
Slow	O
Elevation	O
)	O
to	O
program	O
multi-angle	O
soundings	O
of	O
the	O
atmosphere	O
.	O
</s>
<s>
If	O
we	O
look	O
at	O
the	O
diagram	O
,	O
each	O
angle	O
of	O
elevation	O
or	O
PPI	B-Algorithm
has	O
data	O
at	O
height	O
X	O
at	O
a	O
certain	O
distance	O
from	O
the	O
radar	B-Application
.	O
</s>
<s>
Assembling	O
all	O
the	O
rings	O
coming	O
from	O
the	O
different	O
angles	O
gives	O
you	O
the	O
CAPPI	B-Algorithm
.	O
</s>
<s>
The	O
CAPPI	B-Algorithm
is	O
composed	O
of	O
data	O
from	O
each	O
angle	O
that	O
is	O
at	O
the	O
height	O
requested	O
for	O
the	O
cross-section	O
(	O
bold	O
lines	O
in	O
zig-zag	O
on	O
the	O
left	O
diagram	O
)	O
.	O
</s>
<s>
By	O
the	O
mid-1970s	O
,	O
computer	O
developments	O
made	O
it	O
possible	O
to	O
gather	O
data	O
in	O
electronic	O
form	O
and	O
make	O
CAPPIs	B-Algorithm
more	O
easily	O
.	O
</s>
<s>
Today	O
,	O
weather	B-Application
radars	I-Application
collect	O
in	O
real-time	O
data	O
on	O
a	O
large	O
number	O
of	O
angles	O
.	O
</s>
<s>
Many	O
countries	O
such	O
as	O
Canada	O
,	O
UK	O
and	O
Australia	O
,	O
scan	O
a	O
large	O
enough	O
number	O
of	O
angles	O
with	O
their	O
radars	B-Application
to	O
have	O
an	O
almost	O
continuous	O
vertical	O
view	O
(	O
taking	O
into	O
account	O
the	O
radar	B-Application
beam	O
width	O
)	O
and	O
produce	O
CAPPIs	B-Algorithm
.	O
</s>
<s>
Above	O
right	O
is	O
an	O
example	O
of	O
a	O
CAPPI	B-Algorithm
at	O
1.5km	O
altitude	O
.	O
</s>
<s>
Looking	O
at	O
the	O
diagram	O
of	O
angles	O
,	O
that	O
depending	O
on	O
the	O
height	O
of	O
the	O
CAPPI	B-Algorithm
,	O
there	O
comes	O
a	O
distance	O
where	O
no	O
data	O
is	O
available	O
.	O
</s>
<s>
The	O
portion	O
beyond	O
this	O
distance	O
on	O
a	O
CAPPI	B-Algorithm
is	O
then	O
showing	O
data	O
from	O
the	O
lowest	O
PPI	B-Algorithm
.	O
</s>
<s>
The	O
higher	O
is	O
the	O
CAPPI	B-Algorithm
above	O
ground	O
,	O
the	O
smaller	O
is	O
that	O
PPI	B-Algorithm
zone	O
.	O
</s>
