Global Positioning System Reference
In-Depth Information
days of the first magnetic compass, which, you may recall, consisted of a
magnetized needle floating in a bowl of water.
Increasingly, gyrocompasses are being replaced by nonmechanical de-
vices, thus cutting down on precision-engineering production and main-
tenance costs. There are ring laser gyros and MEMS—of which more
shortly—fiber-optic gyros, and fluxgate compasses (electromagnetic ver-
sions of the old marine compass, sensitive to the geomagnetic field).
All these instruments, being electronic at least in part, are better suited
for inertial navigation (IN) applications than the purely mechanical mag-
netic compass. We will look at inertial navigation in a later section of the
chapter.
Gyros are particularly useful in airplanes because of their relatively free
movements in all directions. An airplane can rotate (spin) about all three
axes and translate (move linearly) along all three axes. It is thus easy for a
pilot to become disorientated. The propensity of gyros to maintain a fixed
spin axis direction is a great boon for airborne navigators.
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RING LASER GYROS
Ring laser gyroscopes are replacing mechanical gyroscopes in many ap-
plications because they exhibit a number of important advantages. The
physical principle upon which ring laser gyros operate is di√erent from
that of mechanical gyros. A ring laser gyro consists of a laser that sends a
coherent beam of light around a circular track.
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The beam is split so that
half the light circulates in one direction and half in the other direction. The
two beams produce interference patterns. When the ring apparatus is
rotating (it is easiest to imagine it rotating about the center of the ring,
though the gyro works even if the axis of rotation is outside the ring), the
two light beams require di√erent intervals of time to complete a loop, and
so the interference pattern changes. The change is proportional to the
rotation rate and can be measured with great accuracy. This measurement
accuracy makes ring laser gyros very sensitive to rotations. The advantages
of this system are significant:
2. This propensity is also employed by firearms manufacturers, who rifle the barrels of
their weapons to cause bullets to spin fast. A spinning bullet flies straighter than one with
no spin.
3. The track that is followed by the laser light does not have to be circular, but it does
need to enclose an area (because the e√ectiveness of the ring laser gyro increases with the
size of the enclosed area). Laser light is made to circulate around a closed track via mirrors
or optical fiber.
