<s>
A	O
fibre-optic	B-Device
gyroscope	I-Device
(	O
FOG	O
)	O
senses	O
changes	O
in	O
orientation	O
using	O
the	O
Sagnac	O
effect	O
,	O
thus	O
performing	O
the	O
function	O
of	O
a	O
mechanical	O
gyroscope	B-Application
.	O
</s>
<s>
However	O
its	O
principle	O
of	O
operation	O
is	O
instead	O
based	O
on	O
the	O
interference	O
of	O
light	O
which	O
has	O
passed	O
through	O
a	O
coil	O
of	O
optical	B-Architecture
fibre	I-Architecture
,	O
which	O
can	O
be	O
as	O
long	O
as	O
.	O
</s>
<s>
Beam	O
splitting	O
optics	O
split	O
light	O
from	O
a	O
laser	O
diode	O
(	O
or	O
other	O
laser	O
light	O
source	O
)	O
into	O
two	O
waves	O
propagating	O
in	O
both	O
clockwise	O
and	O
anticlockwise	O
directions	O
through	O
a	O
coil	O
consisting	O
of	O
many	O
turns	O
of	O
optical	B-Architecture
fibre	I-Architecture
.	O
</s>
<s>
Unlike	O
the	O
classic	O
spinning-mass	O
gyroscope	B-Application
or	O
resonant/mechanical	O
gyroscopes	B-Application
,	O
the	O
FOG	O
has	O
no	O
moving	O
parts	O
and	O
does	O
n't	O
rely	O
on	O
inertial	O
resistance	O
to	O
movement	O
.	O
</s>
<s>
Hence	O
,	O
the	O
FOG	O
is	O
an	O
excellent	O
alternative	O
to	O
a	O
mechanical	O
gyroscope	B-Application
.	O
</s>
<s>
The	O
FOG	O
typically	O
shows	O
a	O
higher	O
resolution	O
than	O
a	O
ring	O
laser	O
gyroscope	B-Application
.	O
</s>
<s>
Like	O
all	O
other	O
gyroscope	B-Application
technologies	O
and	O
depending	O
on	O
detailed	O
FOG	O
design	O
,	O
FOGs	O
may	O
require	O
initial	O
calibration	O
(	O
determining	O
which	O
indication	O
corresponds	O
to	O
zero	O
angular	O
velocity	O
)	O
.	O
</s>
