Compass- Chapter 9

Question 1

The FOG (fibre-optic gyrocompass) has no what

Answer 1

Moving parts

Question 2

What is gyroscopic inertia

Answer 2

refers to the tendency of a spinning object to maintain its orientation

Question 3

In short how does a FOG work

Answer 3

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.

Question 4

What is a gimbal system

Answer 4

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

Question 5

Does a FOG have a gimbal system

Answer 5

no- it has not moving parts

Question 6

Why are FOGs more suitable for high speed vessels

Answer 6

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.

Question 7

What happens to the beam of light entering the fibreoptic coil

Answer 7

A beam of light is split before the two paths are sent opposite directions around the fibreoptic coil.

Question 8

What happens when the beams return from the fibreoptic coil

Answer 8

the wavelengths are compared using interferometry.

Question 9

What is a phase shift

Answer 9

refers to a change in the phase of light waves as they travel through the fiber.

Question 10

Sagnac effect

Answer 10

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.

Question 11

What does phase difference show

Answer 11

How far the coil has rotated

Question 12

Fibre Optic cable is Quartz glass and __m long. Total diameter of the coil is _cm

Answer 12

500
8

Question 13

What splits the light beam

Answer 13

Polarizer

Question 14

What converts the light to an electronic signal

Answer 14

photodetector

Question 15

What does the fibre optic coil use the sagnac effect to do

Answer 15

to measure the angular velocity and thus determine the movement and attitude of the ship.

Question 16

What does an accelerometer do at rest on the earths surface

Answer 16

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.

Question 17

x5 advantages of FOGs

Answer 17

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

Question 18

x3 disadvantages of FOGs

Answer 18

High cost of manufacture

Limited ability to service or repair onboard

No increase in accuracy over a conventional system

Question 19

What is a ballistic error

Answer 19

when a sudden change in velocity or heading (such as rapid acceleration, deceleration, or turning) causes the compass to give inaccurate readings. This

Question 20

What is a rolling error

Answer 20

refers to inaccuracies in compass readings caused by the rolling motion of a vessel or aircraft.

Question 21

What is inertia

Answer 21

describes an object’s resistance to changes in its state of motion.

Question 22

How many meters of fibre optic cable do FOGs have in each coil

Answer 22

up to 5000 metres