<s>
Gyro	B-Device
rate	I-Device
unit	I-Device
refers	O
to	O
a	O
fire-control	B-Application
computer	I-Application
developed	O
by	O
the	O
Royal	O
Navy	O
of	O
the	O
United	O
Kingdom	O
in	O
1937	O
,	O
and	O
which	O
was	O
used	O
extensively	O
on	O
British	O
warships	O
in	O
World	O
War	O
II	O
.	O
</s>
<s>
In	O
the	O
1930s	O
the	O
Royal	O
Navy	O
began	O
to	O
investigate	O
the	O
possibility	O
of	O
combining	O
gyroscopes	B-Application
with	O
optical	O
sights	O
to	O
directly	O
and	O
accurately	O
measure	O
target	O
aircraft	O
speed	O
and	O
direction	O
and	O
began	O
development	O
of	O
the	O
GRU	O
in	O
1937	O
.	O
</s>
<s>
A	O
gyroscope	B-Application
was	O
attached	O
,	O
via	O
mechanical	O
linkage	O
,	O
to	O
an	O
optical	O
monocular	O
sight	O
to	O
form	O
the	O
gyro	B-Device
rate	I-Device
unit	I-Device
or	O
GRU	O
.	O
</s>
<s>
Gyroscopes	B-Application
,	O
when	O
spinning	O
,	O
keep	O
their	O
spin	O
axes	O
pointed	O
in	O
a	O
given	O
direction	O
if	O
they	O
are	O
undisturbed	O
.	O
</s>
<s>
The	O
basic	O
premise	O
of	O
the	O
GRU	O
was	O
that	O
as	O
the	O
cross-hairs	O
of	O
the	O
optical	O
sight	O
were	O
kept	O
centred	O
on	O
the	O
moving	O
target	O
aircraft	O
,	O
the	O
mechanical	O
linkage	O
would	O
pull	O
the	O
gyroscope	B-Application
in	O
the	O
direction	O
of	O
the	O
aircraft	O
movement	O
.	O
</s>
<s>
The	O
force	O
required	O
to	O
move	O
the	O
gyroscope	B-Application
is	O
proportional	O
to	O
the	O
observed	O
target	O
movement	O
across	O
the	O
line	O
of	O
sight	O
.	O
</s>
<s>
This	O
force	O
was	O
measured	O
by	O
the	O
deflection	O
of	O
a	O
spring-loaded	O
device	O
and	O
the	O
deflection	O
measurement	O
was	O
combined	O
with	O
rangefinder	O
,	O
and/or	O
,	O
radar	O
measured	O
target	O
range	O
and	O
altitude	O
in	O
a	O
specialized	O
computer	O
,	O
the	O
gyro	B-Device
rate	I-Device
unit	I-Device
box	O
(	O
GRUB	O
)	O
.	O
</s>
<s>
In	O
1930	O
the	O
RN	O
began	O
equipping	O
ships	O
with	O
the	O
High	B-Device
Angle	I-Device
Control	I-Device
System	I-Device
,	O
a	O
non-tachymetric	O
anti-aircraft	O
fire	O
control	O
system	O
,	O
that	O
would	O
compute	O
the	O
gun	O
laying	O
orders	O
and	O
the	O
time	O
fuze	O
setting	O
of	O
the	O
anti-aircraft	O
guns	O
,	O
to	O
hit	O
the	O
target	O
.	O
</s>
<s>
The	O
HACS	B-Device
marks	O
I	O
through	O
IV	O
depended	O
upon	O
the	O
control	O
officer	O
inputting	O
to	O
the	O
computer	O
an	O
estimated	O
aircraft	O
direction	O
,	O
and	O
speed	O
,	O
which	O
was	O
combined	O
with	O
range	O
and	O
height	O
measurement	O
from	O
an	O
optical	O
coincidence	O
rangefinder	O
to	O
permit	O
the	O
computer	O
to	O
form	O
a	O
solution	O
.	O
</s>
<s>
Unfortunately	O
,	O
these	O
estimates	O
of	O
target	O
speed	O
and	O
direction	O
were	O
often	O
in	O
error	O
,	O
and	O
it	O
took	O
time	O
for	O
the	O
HACS	B-Device
to	O
correct	O
these	O
estimates	O
through	O
a	O
feedback	O
loop	O
from	O
the	O
director	O
to	O
the	O
computer	O
,	O
thus	O
delaying	O
the	O
generation	O
of	O
a	O
correct	O
fire	B-Application
control	I-Application
solution	O
and	O
reducing	O
the	O
accuracy	O
of	O
the	O
resulting	O
gunfire	O
.	O
</s>
<s>
In	O
the	O
1930s	O
the	O
Royal	O
Navy	O
began	O
to	O
investigate	O
the	O
possibility	O
of	O
combining	O
gyroscopes	B-Application
with	O
optical	O
sights	O
to	O
directly	O
and	O
accurately	O
measure	O
target	O
aircraft	O
speed	O
and	O
direction	O
and	O
began	O
development	O
of	O
the	O
GRU	O
in	O
1937	O
.	O
</s>
<s>
[ [ 	O
File:The	O
Royal	O
Navy	O
during	O
the	O
Second	O
World	O
War	O
A3651.jpg	O
|360px|thumbnail|left|	O
Pom-pom	B-Device
directors	I-Device
,	O
Mk	O
IV	O
on	O
HMS	O
King	O
George	O
V	O
.	O
The	O
large	O
rectangular	O
box	O
centered	O
above	O
the	O
director	O
contains	O
the	O
Gyro	B-Device
Rate	I-Device
Unit	I-Device
.	O
</s>
<s>
The	O
gyro	B-Device
rate	I-Device
unit	I-Device
box''	O
used	O
the	O
measured	O
target	O
motion	O
,	O
range	O
and	O
height	O
,	O
to	O
accurately	O
determine	O
the	O
true	O
direction	O
of	O
movement	O
of	O
the	O
target	O
,	O
including	O
its	O
rate	O
of	O
altitude	O
change	O
,	O
and	O
passed	O
this	O
information	O
to	O
the	O
HACS	B-Device
computer	O
,	O
which	O
then	O
generated	O
the	O
gun	O
laying	O
orders	O
and	O
the	O
correct	O
time	O
fuze	O
setting	O
.	O
</s>
<s>
The	O
HACS	B-Device
computer	O
could	O
not	O
directly	O
use	O
rate	O
of	O
altitude	O
change	O
information	O
,	O
so	O
the	O
GRUB	O
would	O
calculate	O
the	O
target	O
altitude	O
,	O
direction	O
and	O
apparent	O
speed	O
for	O
a	O
short	O
interval	O
of	O
time	O
,	O
equal	O
to	O
the	O
loading	O
cycle	O
of	O
the	O
guns	O
,	O
in	O
advance	O
of	O
the	O
actual	O
time	O
and	O
feed	O
this	O
to	O
the	O
HACS	B-Device
computer	O
allowing	O
it	O
to	O
generate	O
correct	O
gunlaying	O
and	O
fuze	O
setting	O
orders	O
.	O
</s>
<s>
The	O
GRUB	O
thus	O
converted	O
the	O
HACS	B-Device
into	O
a	O
tachymetric	B-Device
fire	B-Application
control	I-Application
system	I-Application
.	O
</s>
<s>
GRU/GRUB	O
was	O
also	O
used	O
on	O
The	O
Pom-Pom	B-Device
Director	I-Device
,	O
Mark	O
IV	O
,	O
and	O
with	O
the	O
Fuze	B-Device
Keeping	I-Device
Clock	I-Device
.	O
</s>
