Compasses

Compasses:Magnetic compasses consist of a small, lightweight magnet balanced on a nearly frictionless pivot point. Magnetic compasses have several problems when used on moving platforms like ships. They must be level, they correct rather slowly when the platform turns, they are influenced by ferrous metals and they
indicates magnetic north rather than true north . For these reasons, most ships and airplanes use gyroscopic compasses.
A spinning mass gyrocompass has a spinning body with three axes of angular freedom constituting a gyroscope. They can take up to several hours to spin up and stabilize.
A flux gate compass consists of a field sensor, usually an inductor, mounted to a gimbaled platform that is intended to sense the horizontal component of the earth’s magnetic field.
Fiber Optic Gyrocompasses (FOGs) and Ring Laser Gyros (RLGs) work by shining light through two coils of optical fiber. The input light beam is split into two beams that travel the same path but in opposite directions: one clockwise and the other counter-clockwise. The beams are recombined and sent to the output detector. In the absence of rotation, the path lengths will be the same and the output will be the total constructive interferences of the two beams. If the apparatus rotates, there will be a difference in the path lengths traveled by the two beams, resulting in a net phase difference and destructive interference. The net signal will vary in amplitude, depending on the phase shift; therefore, the resulting amplitude is a measurement of the phase shift, and consequently, the rotation rate. Because there are no moving parts, there is no friction, and hence no drift.
Flux gates are small, light weight and relatively inexpensive. Spinning mass gyros can be more accurate but are large and heavy by comparison. FOGS are intermediate in size and offer the highest accuracy, along with the highest price tag. For more information, see http://www.autonav.com/html/anav4009.htm