The gyrocompass is normally the main reference for direction for the
surface navigator. When properly used, serviced, and maintained, the
modern gyrocompass is extremely accurate. However, as is the case
with all electronic instruments, it is subject to error and damage.
One power failure or other casualty can render the entire system useless.
All naval ships are equipped with gyro failure alarms. The alarms sound
when a loss of power is experienced. It is during this time that the
magnetic compass comes into play. As you learned earlier, the magnetic
compass does not require electricity to operate. Its always ready for
use by the navigator.
Most normally functioning gyrocompasses will not have an error of
more than 2.0° . More often than not, the error is between 0.0° and
Rule: When at sea, the Quartermaster must determine the gyrocompass
error at least once a day. However, the prudent navigator will take
advantage of every opportunity to check the accuracy of a gyro.
There are many methods of checking the accuracy of a gyrocompass.
The following methods are commonly used on U.S. Navy ships:
Trial and error (Franklin technique)
Azimuth of the Sun
Amplitude of the Sun
The first two methods are used only when a ship is near land. They use
aids to navigation and geographic locations shown on a chart for
reference. The last two methods are used when the ship is at sea, and
they use the Sun as a reference.
Before we learn these methods, we have to learn how to use the bearing
circle, alidade, and PMP. They play a large part in the first two
methods. The last two methods use celestial navigation methods to
determine error and will be discussed in length in the Celestial