The Electronic Age - The Science of Navigation - page 198

FIGURE 8.1. The six degrees of
freedom. An unconstrained
moving object can rotate
about three mutually perpen-
dicular axes. Roll is rotation
about the heading direction
(here assumed to be horizon-
tal), pitch is rotation about the
second horizontal axis, and
yaw is rotation about the ver-
tical axis. There are three
corresponding linear accel-
erations along these axes,
known as surge , sway , and
heave .
device, it produces a signal that can be fed into inertial navigation systems
and (in a military context) fire control systems. On the other hand, it is
a precision-engineered machine—hence its expense—and thus is high-
maintenance. Also, it needs a constant source of electrical power.
The gyrocompass is a gyroscope, except that it is motorized so that it
never runs down but instead maintains a constant high spin rate—typically
about 20,000 rpm (330 Hz). The surprising physics of gyroscopes, based
upon precession caused by gravitational torque, ensures that a gyrocom-
pass will try to orient its spin axis so that it aligns with the earth's spin
axis—north-south, in other words. The physical explanation of gyrocom-
passes is often garbled, I have found, so I have made a second exception to
my rule about avoiding technicalities and have explained (but still without
math) the workings of gyrocompasses in the technical appendix, to which I
refer the interested reader.
Gyrocompasses have largely supplanted magnetic compasses as the
main shipboard instruments for spatial orientation. (Magnetic compasses
remain, as inexpensive backup devices.) Like most other navigational in-
struments, a gyrocompass is subjected to unwanted forces caused by ship
movement. To minimize the e√ects of unwanted roll, pitch, and yaw
torques, it is placed in the central part of a ship; to minimize unwanted
surge and sway accelerations, there is some fine-tuning of the instrument.
(See fig. 8.1 for an explanation of these terms.) To minimize friction e√ects,
some gyros are suspended in pools of mercury. This arrangement keeps the
gyro axis in a horizontal plane, which is important for correct operation
(see the appendix for an explanation). It also harks back to the very early