<s>
Radar	O
,	O
Air-to-Surface	O
Vessel	O
,	O
or	O
ASV	B-Algorithm
radar	I-Algorithm
for	O
short	O
,	O
is	O
a	O
classification	O
used	O
by	O
the	O
Royal	O
Air	O
Force	O
(	O
RAF	O
)	O
to	O
refer	O
to	O
a	O
series	O
of	O
aircraft-mounted	O
radar	O
systems	O
used	O
to	O
scan	O
the	O
surface	O
of	O
the	O
ocean	O
to	O
locate	O
ships	O
and	O
surfaced	O
submarines	O
.	O
</s>
<s>
It	O
is	O
part	O
of	O
the	O
wider	O
surface	B-Algorithm
search	I-Algorithm
radar	I-Algorithm
classification	O
,	O
which	O
includes	O
similar	O
radars	O
in	O
ground	O
and	O
ship	O
mountings	O
.	O
</s>
<s>
The	O
first	O
ASV	O
was	O
developed	O
after	O
the	O
accidental	O
detection	O
of	O
wharves	O
and	O
cranes	O
while	O
testing	O
an	O
air-to-air	B-Device
radar	I-Device
in	O
1937	O
.	O
</s>
<s>
ASV	O
was	O
useful	O
for	O
detecting	O
U-boats	B-Operating_System
at	O
night	O
,	O
but	O
the	O
target	O
had	O
to	O
be	O
seen	O
to	O
be	O
attacked	O
,	O
a	O
problem	O
that	O
was	O
addressed	O
with	O
the	O
Leigh	O
light	O
with	O
rapidly	O
increasing	O
success	O
.	O
</s>
<s>
As	O
German	O
U-boat	B-Operating_System
losses	O
shot	O
up	O
in	O
1942	O
,	O
they	O
concluded	O
the	O
RAF	O
was	O
using	O
radar	O
to	O
detect	O
them	O
and	O
responded	O
with	O
the	O
Metox	O
radar	O
detector	O
.	O
</s>
<s>
The	O
RAF	O
responded	O
by	O
deploying	O
the	O
microwave-frequency	O
ASV	O
Mark	O
III	O
,	O
which	O
the	O
Germans	O
were	O
unable	O
to	O
detect	O
until	O
the	O
U-boat	B-Operating_System
fleet	O
had	O
already	O
been	O
decimated	O
.	O
</s>
<s>
In	O
the	O
post-war	O
era	O
,	O
several	O
new	O
ASV	B-Algorithm
radars	I-Algorithm
were	O
developed	O
,	O
notably	O
ASV	O
Mark	O
7A	O
,	O
ASV	O
Mark	O
13	O
and	O
ASV	O
Mark	O
21	O
.	O
</s>
<s>
By	O
the	O
late	O
1960s	O
the	O
original	O
terminology	O
was	O
no	O
longer	O
being	O
used	O
,	O
and	O
the	O
last	O
major	O
entries	O
in	O
the	O
series	O
were	O
known	O
simply	O
as	O
Searchwater	O
and	O
Seaspray	B-Algorithm
.	O
</s>
<s>
Development	O
of	O
the	O
original	O
ASV	O
systems	O
started	O
in	O
1937	O
after	O
the	O
team	O
testing	O
an	O
experimental	O
air-to-air	B-Device
radar	I-Device
noticed	O
odd	O
returns	O
while	O
flying	O
near	O
the	O
shore	O
of	O
the	O
North	O
Sea	O
.	O
</s>
<s>
Their	O
effect	O
was	O
dramatic	O
;	O
German	O
U-boats	B-Operating_System
had	O
previously	O
been	O
almost	O
completely	O
safe	O
at	O
night	O
,	O
and	O
could	O
operate	O
out	O
of	O
the	O
Bay	O
of	O
Biscay	O
in	O
spite	O
of	O
it	O
being	O
close	O
to	O
British	O
shores	O
.	O
</s>
<s>
By	O
the	O
spring	O
of	O
1942	O
,	O
Biscay	O
was	O
increasingly	O
dangerous	O
,	O
with	O
aircraft	O
appearing	O
out	O
of	O
nowhere	O
in	O
the	O
middle	O
of	O
the	O
night	O
,	O
dropping	O
bombs	O
and	O
depth	B-Device
charges	I-Device
,	O
and	O
then	O
disappearing	O
again	O
in	O
moments	O
.	O
</s>
<s>
With	O
experience	O
,	O
the	O
operators	O
could	O
tell	O
whether	O
the	O
aircraft	O
was	O
approaching	O
or	O
just	O
flying	O
by	O
,	O
allowing	O
the	O
U-boat	B-Operating_System
to	O
dive	O
and	O
escape	O
detection	O
.	O
</s>
<s>
The	O
introduction	O
of	O
the	O
cavity	B-Architecture
magnetron	I-Architecture
in	O
early	O
1940	O
led	O
to	O
efforts	O
to	O
develop	O
microwave-frequency	O
versions	O
of	O
the	O
various	O
radars	O
then	O
in	O
use	O
,	O
including	O
a	O
new	O
ASV	O
under	O
the	O
name	O
ASVS	O
for	O
"	O
Sentimetric	O
"	O
.	O
</s>
<s>
In	O
August	O
,	O
shipping	O
losses	O
to	O
submarines	O
was	O
the	O
lowest	O
since	O
November	O
1941	O
,	O
and	O
more	O
U-boats	B-Operating_System
were	O
sunk	O
than	O
cargo	O
ships	O
.	O
</s>
<s>
The	O
reason	O
for	O
the	O
long	O
delay	O
in	O
discovering	O
Mark	O
III	O
is	O
somewhat	O
surprising	O
given	O
that	O
a	O
magnetron	B-Device
from	O
H2S	O
fell	O
into	O
German	O
hands	O
almost	O
immediately	O
after	O
it	O
was	O
first	O
used	O
in	O
February	O
1943	O
.	O
</s>
<s>
Sources	O
disagree	O
on	O
the	O
reason	O
;	O
the	O
magnetron	B-Device
was	O
either	O
unknown	O
to	O
the	O
German	O
Navy	O
,	O
or	O
they	O
did	O
not	O
believe	O
it	O
could	O
be	O
used	O
against	O
U-boats	B-Operating_System
.	O
</s>
<s>
Better	O
detectors	O
arrived	O
very	O
late	O
in	O
the	O
war	O
,	O
but	O
by	O
that	O
time	O
the	O
U-boat	B-Operating_System
force	O
had	O
largely	O
been	O
destroyed	O
.	O
</s>
<s>
The	O
magnetron	B-Device
was	O
revealed	O
to	O
the	O
United	O
States	O
in	O
1940	O
during	O
the	O
Tizard	O
Mission	O
,	O
and	O
local	O
development	O
began	O
at	O
the	O
MIT	O
Radiation	O
Laboratory	O
in	O
a	O
matter	O
of	O
weeks	O
.	O
</s>
<s>
The	O
key	O
trick	O
to	O
Mark	O
VI	O
was	O
the	O
"	O
Vixen	O
"	O
device	O
that	O
allowed	O
the	O
operator	O
to	O
progressively	O
mute	O
the	O
output	O
as	O
they	O
approached	O
the	O
U-boat	B-Operating_System
,	O
hopefully	O
fooling	O
the	O
radio	O
operator	O
into	O
believing	O
they	O
were	O
flying	O
away	O
.	O
</s>
<s>
Mark	O
VI	O
never	O
fully	O
replaced	O
Mark	O
III	O
in	O
service	O
,	O
as	O
truly	O
effective	O
detectors	O
did	O
not	O
become	O
available	O
until	O
the	O
U-boat	B-Operating_System
fleet	O
had	O
largely	O
been	O
destroyed	O
.	O
</s>
<s>
The	O
failure	O
of	O
Naxos	O
and	O
later	O
devices	O
led	O
to	O
morale	O
problems	O
in	O
the	O
U-boat	B-Operating_System
force	O
.	O
</s>
<s>
From	O
1943	O
,	O
both	O
the	O
UK	O
and	O
US	O
began	O
developing	O
magnetrons	B-Device
that	O
worked	O
on	O
even	O
shorter	O
wavelengths	O
,	O
first	O
in	O
the	O
X-band	B-Algorithm
at	O
3cm	O
wavelength	O
,	O
and	O
later	O
in	O
the	O
K-band	B-Algorithm
at	O
1.25cm	O
.	O
</s>
<s>
One	O
was	O
the	O
introduction	O
of	O
new	O
classes	O
of	O
U-boats	B-Operating_System
with	O
much	O
higher	O
performance	O
,	O
and	O
the	O
other	O
was	O
the	O
use	O
of	O
the	O
schnorkel	O
,	O
allowing	O
even	O
older	O
types	O
to	O
spend	O
most	O
of	O
their	O
time	O
submerged	O
.	O
</s>
<s>
This	O
made	O
the	O
X-band	B-Algorithm
versions	O
of	O
ASV	O
a	O
requirement	O
,	O
as	O
they	O
had	O
the	O
resolution	O
needed	O
to	O
detect	O
the	O
schnorkel	O
.	O
</s>
<s>
In	O
the	O
immediate	O
post-war	O
era	O
,	O
development	O
of	O
the	O
system	O
continued	O
as	O
an	O
air-sea	O
rescue	O
system	O
,	O
as	O
it	O
could	O
detect	O
life	O
rafts	O
even	O
if	O
they	O
did	O
not	O
carry	O
a	O
transponder	B-Architecture
.	O
</s>
<s>
The	O
radome	O
's	O
location	O
made	O
the	O
carriage	O
of	O
a	O
torpedo	O
impossible	O
,	O
so	O
these	O
aircraft	O
were	O
fit	O
with	O
eight	O
RP-3	O
rockets	O
with	O
armor-piercing	O
warheads	O
to	O
damage	O
or	O
puncture	O
the	O
U-boat	B-Operating_System
making	O
it	O
impossible	O
to	O
dive	O
,	O
and	O
flares	B-Application
to	O
mark	O
the	O
location	O
for	O
follow-up	O
attacks	O
by	O
other	O
aircraft	O
carrying	O
depth	B-Device
charges	I-Device
.	O
</s>
<s>
The	O
Beaufighter	O
,	O
which	O
became	O
one	O
of	O
Coastal	O
Command	O
's	O
primary	O
strike	B-Application
fighters	I-Application
,	O
had	O
the	O
problem	O
that	O
the	O
fitting	O
of	O
ASV	O
required	O
the	O
removal	O
of	O
some	O
other	O
devices	O
to	O
make	O
room	O
.	O
</s>
<s>
The	O
opening	O
of	O
the	O
Cold	B-Application
War	I-Application
led	O
to	O
a	O
rapid	O
re-evaluation	O
of	O
this	O
stance	O
,	O
especially	O
as	O
the	O
Soviets	O
were	O
known	O
to	O
be	O
introducing	O
new	O
submarines	O
surpassing	O
even	O
the	O
late-war	O
German	O
designs	O
.	O
</s>
<s>
ASV21	O
was	O
also	O
selected	O
for	O
the	O
Mark	O
II	O
models	O
of	O
the	O
Canadair	O
CP-107	O
Argus	O
,	O
replacing	O
the	O
American	O
AN/APS	B-Device
-20	I-Device
of	O
the	O
Argus	O
Mark	O
I	O
's	O
.	O
</s>
<s>
The	O
first	O
such	O
example	O
is	O
the	O
Seaspray	B-Algorithm
,	O
a	O
small	O
unit	O
designed	O
to	O
be	O
mounted	O
on	O
the	O
Westland	O
Lynx	O
.	O
</s>
<s>
The	O
latest	O
versions	O
,	O
Seaspray	B-Algorithm
7000	O
,	O
are	O
completely	O
rebuilt	O
and	O
share	O
only	O
the	O
name	O
with	O
the	O
original	O
models	O
.	O
</s>
<s>
ASV13	O
and	O
ASV21	O
used	O
magnetrons	B-Device
,	O
which	O
was	O
a	O
technology	O
developed	O
during	O
World	O
War	O
II	O
.	O
</s>
<s>
"	O
Searchwater	O
was	O
a	O
completely	O
new	O
concept	O
,	O
having	O
a	O
high	O
power	O
wideband	O
TWT	O
transmitter	O
and	O
being	O
the	O
first	O
generation	O
of	O
ASV	B-Algorithm
radars	I-Algorithm
to	O
include	O
modern	O
signal	O
and	O
data	O
processing	O
(	O
digital	O
as	O
well	O
as	O
analogue	O
)	O
"	O
.	O
</s>
<s>
III	O
but	O
operating	O
in	O
the	O
X-band	B-Algorithm
at	O
3cm	O
.	O
</s>
<s>
ASV	O
Mark	O
X	O
-	O
RAF	O
name	O
for	O
US	O
ASD-1/APS	O
-15	O
X-band	B-Algorithm
system	O
.	O
</s>
