<s>
A	O
plan	B-Algorithm
position	I-Algorithm
indicator	I-Algorithm
(	O
PPI	O
)	O
is	O
a	O
type	O
of	O
radar	B-Algorithm
display	I-Algorithm
that	O
represents	O
the	O
radar	O
antenna	O
in	O
the	O
center	O
of	O
the	O
display	O
,	O
with	O
the	O
distance	O
from	O
it	O
and	O
height	O
above	O
ground	O
drawn	O
as	O
concentric	O
circles	O
.	O
</s>
<s>
It	O
is	O
the	O
most	O
common	O
type	O
of	O
radar	B-Algorithm
display	I-Algorithm
.	O
</s>
<s>
This	O
change	O
is	O
not	O
a	O
straight	O
line	O
but	O
a	O
curve	O
as	O
the	O
surface	O
of	O
the	O
Earth	O
is	O
curved	O
and	O
sinks	O
below	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
.	O
</s>
<s>
Originally	O
,	O
data	O
was	O
displayed	O
in	O
real	O
time	O
on	O
a	O
cathode	B-Device
ray	I-Device
tube	I-Device
,	O
and	O
thus	O
the	O
only	O
way	O
to	O
store	O
the	O
information	O
received	O
was	O
by	O
taking	O
a	O
photograph	O
of	O
the	O
screen	O
.	O
</s>
<s>
Farnsworth	O
refined	O
a	O
version	O
of	O
his	O
picture	B-Device
tube	I-Device
(	O
cathode	B-Device
ray	I-Device
tube	I-Device
,	O
or	O
CRT	O
)	O
and	O
called	O
it	O
an	O
"	O
Iatron	O
;	O
"	O
generically	O
known	O
as	O
a	O
storage	B-General_Concept
tube	I-General_Concept
.	O
</s>
<s>
This	O
slow-to-fade	O
display	O
tube	O
was	O
used	O
by	O
air	B-Application
traffic	I-Application
controllers	I-Application
from	O
the	O
very	O
beginning	O
of	O
radar	O
usage	O
.	O
</s>
<s>
The	O
PPI	O
is	O
used	O
in	O
many	O
domains	O
involving	O
display	O
of	O
range	O
and	O
positioning	O
,	O
especially	O
in	O
radars	O
,	O
including	O
air	B-Application
traffic	I-Application
control	I-Application
,	O
ship	O
navigation	O
,	O
meteorology	B-Application
,	O
on	O
board	O
ships	O
and	O
aircraft	O
etc	O
.	O
</s>
<s>
PPI	O
displays	O
are	O
also	O
used	O
to	O
display	O
sonar	B-Application
data	O
,	O
especially	O
in	O
underwater	O
warfare	O
.	O
</s>
<s>
However	O
,	O
because	O
the	O
speed	O
of	O
sound	O
in	O
water	O
is	O
very	O
slow	O
compared	O
to	O
microwaves	O
in	O
air	O
,	O
a	O
sonar	B-Application
PPI	O
has	O
an	O
expanding	O
circle	O
that	O
starts	O
with	O
each	O
transmitted	O
"	O
ping	O
"	O
of	O
sound	O
.	O
</s>
<s>
In	O
meteorology	B-Application
,	O
a	O
competing	O
display	O
system	O
is	O
the	O
CAPPI	B-Algorithm
(	O
Constant	B-Algorithm
Altitude	I-Algorithm
Plan	I-Algorithm
Position	I-Algorithm
Indicator	I-Algorithm
)	O
when	O
a	O
multi-angle	O
scan	O
is	O
available	O
.	O
</s>
<s>
Using	O
computers	O
to	O
process	O
data	O
,	O
modern	O
sonar	B-Application
and	O
lidar	O
installations	O
can	O
mimic	O
radar	O
PPI	O
displays	O
too	O
.	O
</s>
