Global Positioning System Reference
In-Depth Information
with a lot of it resists changing its spin speed or direction. But what exactly is
the ''direction'' of a spinning object? In the illustration you see how physicists
assign a direction to spin. The top shown is spinning counterclockwise, seen
from above. Curl the fingers of your right hand about the axis, in the same
counterclockwise sense—your extended thumb points in the spin direction.
For a clockwise spin, the direction is into the table.
A top has high angular momentum because it is spinning fast, and so it
resists forces that try to change its spin direction. This is why a spinning top
will not fall over, but one that is not spinning does fall. What about the odd
phenomenon whereby a top moves perpendicular to an applied force? This is
harder to explain; see the appendix if you need to know. It is a consequence of
angular momentum, and it underlies the workings of a gyrocompass.
≤ A ring laser gyro has no moving parts (except the light).
≤ It is small, lightweight, and robust.
≤ It does not resist changes in orientation because it depends on a
principle of optics and not on angular momentum.
4
These characteristics mean that ring laser gyros find applications where
mechanical gyros cannot be used—for example, on board a missile or an
airplane that is maneuvering wildly.
Because a ring laser gyro is sensitive only to rotations in the plane of the
ring, a navigation system must include three ring laser gyros oriented
perpendicular to each other, to enable a navigator to measure rotations
about all three spatial axes. Such an arrangement is employed in modern
inertial navigation systems.
MICROELECTROMECHANICAL SYSTEMS (MEMS)
M
icro
e
lectro
m
echanical
s
ystems, or MEMS, have evolved over the last
20 years as an o√shoot of the already-mature microchip fabrication indus-
try (fig. 8.2). These little machines (less than 1 mm across) are elec-
trically powered mechanical devices and find all kinds of applications.
Of interest to us are the applications as miniature gyroscopes and as ac-
celerometers. The physical principles upon which MEMS act are di√erent
4. The optics principle underlying ring laser gyros, first understood in 1913, is called the
Sagnac e√ect
. It is a relativistic phenomenon, which is to say its full explanation requires an
understanding of Einstein's theory.
