Compass- Chapter 9 Flashcards
The FOG (fibre-optic gyrocompass) has no what
Moving parts
What is gyroscopic inertia
refers to the tendency of a spinning object to maintain its orientation
In short how does a FOG work
It simply uses a number of sensors to compute the direction of rotation of the earth (East), and from this derives the direction of geographical North.
What is a gimbal system
a mechanism that uses a set of pivoted support rings to allow an object to rotate freely about a single axis, or even multiple axes
Does a FOG have a gimbal system
no- it has not moving parts
Why are FOGs more suitable for high speed vessels
As the ship’s movement is computed and corrections applied directly, removing the time lag that may occur before a conventional gyrocompass settles following changes in course and/or speed.
What happens to the beam of light entering the fibreoptic coil
A beam of light is split before the two paths are sent opposite directions around the fibreoptic coil.
What happens when the beams return from the fibreoptic coil
the wavelengths are compared using interferometry.
What is a phase shift
refers to a change in the phase of light waves as they travel through the fiber.
Sagnac effect
The phase shift created by the rotation of the coil
When a fiber optic coil rotates, light waves traveling in opposite directions around the coil experience different path lengths due to the rotation. This difference in path length results in a phase shift between the two beams.
What does phase difference show
How far the coil has rotated
Fibre Optic cable is Quartz glass and __m long. Total diameter of the coil is _cm
500
8
What splits the light beam
Polarizer
What converts the light to an electronic signal
photodetector
What does the fibre optic coil use the sagnac effect to do
to measure the angular velocity and thus determine the movement and attitude of the ship.
What does an accelerometer do at rest on the earths surface
detect the acceleration caused by the earths gravity
the accelerometer will also measure linear acceleration of the ship itself due to earths rotation, movement in a seaway, or propulsion.
x5 advantages of FOGs
No moving parts thus time between failures and service intervals is increased
Smaller, lighter units
Settling time is smaller
Continuous computations avoid errors in rapid or large course changes, meaning the units are particularly suited to high latitude or high speed operations
No ballistic or rolling errors
x3 disadvantages of FOGs
High cost of manufacture
Limited ability to service or repair onboard
No increase in accuracy over a conventional system
What is a ballistic error
when a sudden change in velocity or heading (such as rapid acceleration, deceleration, or turning) causes the compass to give inaccurate readings. This
What is a rolling error
refers to inaccuracies in compass readings caused by the rolling motion of a vessel or aircraft.
What is inertia
describes an object’s resistance to changes in its state of motion.
How many meters of fibre optic cable do FOGs have in each coil
up to 5000 metres
