<s>
In	O
radar	B-Application
systems	I-Application
,	O
the	O
blip-to-scan	B-Algorithm
ratio	I-Algorithm
,	O
or	O
blip/scan	O
,	O
is	O
the	O
ratio	O
of	O
the	O
number	O
of	O
times	O
a	O
target	O
appears	O
on	O
a	O
radar	B-Application
display	O
to	O
the	O
number	O
of	O
times	O
it	O
theoretically	O
could	O
be	O
displayed	O
.	O
</s>
<s>
"	O
Blip	O
"	O
refers	O
to	O
the	O
dots	O
drawn	O
on	O
early	B-Algorithm
warning	I-Algorithm
radars	I-Algorithm
based	O
on	O
plan	B-Algorithm
position	I-Algorithm
indicator	I-Algorithm
(	O
PPI	O
)	O
displays	O
.	O
</s>
<s>
A	O
radar	B-Application
with	O
a	O
low	O
blip-to-scan	B-Algorithm
ratio	I-Algorithm
draws	O
only	O
a	O
few	O
reflections	O
from	O
an	O
object	O
(	O
mainly	O
aircraft	O
)	O
,	O
making	O
it	O
more	O
difficult	O
to	O
detect	O
.	O
</s>
<s>
For	O
an	O
aircraft	O
flying	O
at	O
high	O
speed	O
and	O
altitude	O
the	O
ratio	O
is	O
further	O
reduced	O
,	O
rendering	O
the	O
aircraft	O
almost	O
invisible	O
to	O
radar	B-Application
.	O
</s>
<s>
This	O
change	O
in	O
radar	B-Application
signature	O
is	O
also	O
known	O
as	O
the	O
Rodgers	O
effect	O
after	O
its	O
proponent	O
in	O
the	O
US	O
,	O
Franklin	O
Rodgers	O
.	O
</s>
<s>
However	O
,	O
upgrades	O
to	O
Soviet	O
radar	B-Application
systems	I-Application
increased	O
their	O
blip-to-scan	O
ratios	O
,	O
rendering	O
the	O
A-12	O
obsolete	O
before	O
it	O
could	O
be	O
deployed	O
.	O
</s>
<s>
Classic	O
radars	B-Application
measure	O
range	O
by	O
timing	O
the	O
delay	O
between	O
sending	O
and	O
receiving	O
pulses	O
of	O
radio	O
signals	O
,	O
and	O
determine	O
the	O
angular	O
location	O
by	O
the	O
mechanical	O
position	O
of	O
the	O
antenna	O
at	O
the	O
instant	O
the	O
signal	O
is	O
received	O
.	O
</s>
<s>
The	O
returned	O
signal	O
is	O
displayed	O
on	O
a	O
circular	O
cathode	B-Device
ray	I-Device
tube	I-Device
that	O
produces	O
dots	O
at	O
the	O
same	O
angle	O
as	O
the	O
antenna	O
and	O
displaced	O
from	O
the	O
center	O
by	O
the	O
time	O
delay	O
.	O
</s>
<s>
Such	O
a	O
display	O
is	O
called	O
a	O
Plan	B-Algorithm
Position	I-Algorithm
Indicator	I-Algorithm
,	O
usually	O
simply	O
a	O
"	O
PPI	O
"	O
.	O
</s>
<s>
Under	O
optimal	O
conditions	O
,	O
every	O
pulse	O
sent	O
out	O
by	O
the	O
radar	B-Application
will	O
be	O
returned	O
and	O
cause	O
a	O
blip	O
to	O
be	O
displayed	O
on	O
the	O
screen	O
.	O
</s>
<s>
On	O
Cold	B-Application
War	I-Application
era	O
radar	B-Application
displays	O
,	O
the	O
phosphor	O
coatings	O
on	O
the	O
CRTs	O
were	O
mixed	O
so	O
that	O
they	O
would	O
have	O
a	O
half-life	B-Application
on	O
the	O
order	O
of	O
the	O
rotational	O
speed	O
of	O
the	O
antenna	O
.	O
</s>
<s>
One	O
key	O
characteristic	O
in	O
all	O
radars	B-Application
is	O
the	O
pulse	O
repetition	O
frequency	O
(	O
PRF	O
)	O
,	O
which	O
determines	O
the	O
maximum	O
effective	O
range	O
.	O
</s>
<s>
For	O
instance	O
,	O
a	O
radar	B-Application
designed	O
to	O
have	O
a	O
range	O
of	O
needs	O
to	O
wait	O
2milliseconds	O
for	O
a	O
pulse	O
to	O
travel	O
the	O
maximum	O
range	O
and	O
back	O
at	O
the	O
speed	O
of	O
light	O
(	O
300,000	O
km/s	O
)	O
.	O
</s>
<s>
This	O
means	O
that	O
such	O
a	O
radar	B-Application
can	O
send	O
out	O
,	O
at	O
most	O
,	O
500	O
pulses	O
per	O
second	O
(	O
PRF	O
)	O
.	O
</s>
<s>
If	O
the	O
radar	B-Application
sent	O
out	O
1000	O
pulses	O
per	O
second	O
,	O
it	O
would	O
be	O
impossible	O
to	O
determine	O
whether	O
a	O
particular	O
reflection	O
came	O
from	O
an	O
object	O
at	O
150km	O
from	O
the	O
pulse	O
just	O
sent	O
out	O
,	O
or	O
an	O
object	O
at	O
300km	O
reflecting	O
the	O
previous	O
pulse	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
a	O
150km	O
radar	B-Application
requires	O
only	O
1millisecond	O
;	O
this	O
makes	O
a	O
PRF	O
of	O
1000possible	O
.	O
</s>
<s>
However	O
,	O
the	O
radar	B-Application
system	I-Application
cannot	O
detect	O
reflections	O
while	O
it	O
is	O
sending	O
a	O
pulse	O
.	O
</s>
<s>
To	O
have	O
a	O
30km	O
minimum	O
range	O
,	O
for	O
instance	O
,	O
a	O
radar	B-Application
can	O
have	O
pulses	O
no	O
longer	O
than	O
0.1	O
ms	O
in	O
duration	O
.	O
</s>
<s>
For	O
an	O
early	B-Algorithm
warning	I-Algorithm
radar	I-Algorithm
the	O
minimum	O
range	O
is	O
generally	O
not	O
important	O
,	O
so	O
longer	O
pulses	O
are	O
used	O
to	O
maximize	O
the	O
returns	O
,	O
but	O
the	O
duty	O
cycle	O
was	O
nonetheless	O
an	O
important	O
design	O
consideration	O
.	O
</s>
<s>
This	O
means	O
it	O
is	O
inherently	O
difficult	O
to	O
produce	O
a	O
radar	B-Application
system	I-Application
able	O
to	O
see	O
small	O
objects	O
at	O
long	O
distances	O
,	O
especially	O
one	O
that	O
can	O
also	O
detect	O
those	O
objects	O
at	O
shorter	O
distances	O
.	O
</s>
<s>
With	O
modern	O
electronics	O
it	O
is	O
relatively	O
simple	O
to	O
arrange	O
for	O
a	O
radar	B-Application
to	O
have	O
different	O
PRFs	O
and	O
duty	O
cycles	O
to	O
allow	O
operation	O
over	O
a	O
wide	O
variety	O
of	O
ranges	O
,	O
but	O
with	O
tube-based	O
electronics	O
from	O
the	O
1950s	O
this	O
was	O
extremely	O
expensive	O
.	O
</s>
<s>
A	O
radar	B-Application
antenna	O
is	O
normally	O
arranged	O
to	O
produce	O
a	O
very	O
narrow	O
beam	O
,	O
in	O
order	O
to	O
improve	O
angular	O
resolution	O
.	O
</s>
<s>
Beam	O
widths	O
of	O
2	O
to	O
5	O
degrees	O
are	O
common	O
for	O
long-range	O
radars	B-Application
.	O
</s>
<s>
Intertwined	O
with	O
beam	O
width	O
is	O
the	O
speed	O
of	O
the	O
antenna	O
’s	O
rotation	O
,	O
because	O
it	O
also	O
determines	O
the	O
amount	O
of	O
time	O
that	O
a	O
spinning	O
radar	B-Application
will	O
spend	O
painting	O
a	O
given	O
object	O
on	O
every	O
scan	O
.	O
</s>
<s>
For	O
example	O
,	O
consider	O
a	O
radar	B-Application
with	O
a	O
beam	O
width	O
of	O
one	O
degree	O
and	O
an	O
antenna	O
that	O
rotates	O
once	O
every	O
ten	O
seconds	O
,	O
or	O
36	O
degrees	O
per	O
second	O
.	O
</s>
<s>
If	O
the	O
radar	B-Application
has	O
a	O
PRF	O
of	O
500	O
,	O
the	O
object	O
will	O
be	O
painted	O
with	O
14	O
pulses	O
per	O
scan	O
at	O
most	O
.	O
</s>
<s>
Moreover	O
,	O
Cold-War	O
radar	B-Application
systems	I-Application
were	O
far	O
from	O
perfect	O
.	O
</s>
<s>
These	O
design	O
characteristics	O
and	O
susceptibility	O
to	O
glitches	O
combine	O
to	O
determine	O
a	O
radar	B-Application
's	O
blip/scan	O
.	O
</s>
<s>
During	O
the	O
time	O
period	O
being	O
discussed	O
,	O
one	O
of	O
the	O
most	O
common	O
radars	B-Application
in	O
the	O
Soviet	O
fleet	O
was	O
the	O
P-20	O
radar	B-Application
and	O
its	O
various	O
modified	O
versions	O
.	O
</s>
<s>
Assuming	O
a	O
high-speed	O
aircraft	O
flying	O
at	O
90,000	O
feet	O
,	O
or	O
27km	O
,	O
this	O
means	O
the	O
aircraft	O
will	O
be	O
above	O
the	O
maximum	O
angle	O
of	O
the	O
radar	B-Application
when	O
it	O
approaches	O
within	O
about	O
of	O
the	O
station	O
.	O
</s>
<s>
A	O
high-speed	O
,	O
high-altitude	O
aircraft	O
might	O
fly	O
over	O
early	B-Algorithm
warning	I-Algorithm
radars	I-Algorithm
without	O
being	O
recognized	O
.	O
</s>
<s>
Originally	O
these	O
studies	O
focused	O
entirely	O
on	O
the	O
reduction	O
of	O
the	O
radar	B-Application
cross	O
section	O
(	O
RCS	O
)	O
,	O
but	O
after	O
Franklin	O
Rodgers	O
introduced	O
the	O
idea	O
of	O
spoofing	O
the	O
blip/scan	O
in	O
1957	O
,	O
the	O
plans	O
were	O
changed	O
to	O
research	O
high-speed	O
,	O
high-altitude	O
designs	O
instead	O
.	O
</s>
<s>
Lockheed	O
calculated	O
that	O
in	O
order	O
to	O
be	O
effective	O
against	O
known	O
Soviet	O
radars	B-Application
,	O
an	O
aircraft	O
would	O
have	O
to	O
travel	O
between	O
Mach	O
2	O
and	O
Mach	O
3	O
at	O
90,000	O
ft	O
and	O
have	O
an	O
RCS	O
of	O
about	O
10	O
square	O
meters	O
.	O
</s>
<s>
It	O
was	O
discovered	O
that	O
the	O
high-temperature	O
exhaust	O
of	O
these	O
aircraft	O
engines	O
reflected	O
radar	B-Application
energy	O
at	O
certain	O
wavelengths	O
,	O
and	O
persisted	O
in	O
the	O
atmosphere	O
for	O
some	O
time	O
.	O
</s>
<s>
It	O
would	O
be	O
possible	O
for	O
the	O
Soviets	O
to	O
modify	O
their	O
radars	B-Application
to	O
use	O
these	O
frequencies	O
,	O
and	O
thereby	O
track	O
the	O
targets	O
indirectly	O
but	O
reliably	O
.	O
</s>
<s>
It	O
was	O
also	O
realized	O
that	O
since	O
blip/scan	O
avoidance	O
relied	O
more	O
heavily	O
on	O
a	O
problem	O
in	O
Soviet	O
displays	O
rather	O
than	O
in	O
the	O
principles	O
of	O
radar	B-Application
,	O
changing	O
these	O
displays	O
could	O
render	O
the	O
technique	O
moot	O
.	O
</s>
<s>
A	O
system	O
that	O
recorded	O
the	O
radar	B-Application
returns	O
in	O
a	O
computer	O
and	O
then	O
drew	O
the	O
targets	O
on	O
the	O
display	O
as	O
an	O
icon	O
whose	O
brightness	O
was	O
independent	O
of	O
the	O
physical	O
return	O
(	O
a	O
system	O
in	O
which	O
returns	O
did	O
not	O
have	O
to	O
"	O
add	O
up	O
"	O
in	O
order	O
to	O
appear	O
on	O
the	O
display	O
)	O
eliminated	O
the	O
potential	O
for	O
operator	O
confusion	O
.	O
</s>
<s>
This	O
was	O
particularly	O
worrying	O
,	O
because	O
the	O
USAF	O
was	O
itself	O
in	O
the	O
process	O
of	O
introducing	O
precisely	O
this	O
sort	O
of	O
display	O
as	O
part	O
of	O
their	O
SAGE	B-Device
project	I-Device
.	O
</s>
<s>
Radars	B-Application
for	O
plotting	O
an	O
air	O
intercept	O
were	O
generally	O
made	O
as	O
long-range	O
as	O
possible	O
in	O
order	O
to	O
give	O
the	O
operators	O
ample	O
time	O
to	O
guide	O
intercept	O
aircraft	O
onto	O
the	O
target	O
as	O
it	O
moved	O
across	O
the	O
display	O
.	O
</s>
<s>
Missiles	O
stations	O
guided	O
their	O
missiles	O
with	O
their	O
own	O
radar	B-Application
systems	I-Application
,	O
which	O
had	O
maximum	O
ranges	O
only	O
slightly	O
longer	O
than	O
the	O
missile	O
's	O
own	O
flight	O
range	O
,	O
about	O
40km	O
in	O
the	O
case	O
of	O
the	O
SA-2	O
Guideline	O
;	O
therefore	O
they	O
had	O
much	O
higher	O
PRFs	O
,	O
and	O
as	O
a	O
result	O
the	O
blip/scan	O
problems	O
were	O
greatly	O
reduced	O
.	O
</s>
<s>
Defenders	O
would	O
still	O
have	O
the	O
problem	O
of	O
finding	O
the	O
target	O
in	O
time	O
to	O
prepare	O
for	O
a	O
missile	O
counterattack	O
,	O
but	O
this	O
was	O
by	O
no	O
means	O
as	O
difficult	O
or	O
as	O
time	O
consuming	O
as	O
scrambling	O
manned	O
aircraft	O
and	O
relying	O
on	O
the	O
radar	B-Application
operator	O
to	O
guide	O
them	O
onto	O
the	O
target	O
before	O
the	O
aircraft	O
left	O
radar	B-Application
range	O
.	O
</s>
