Magnetic And Gyro Compass
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Transcript of Magnetic And Gyro Compass
Lesson 9: Magnetic and Gyro
Compasses
Lesson 9: Magnetic and
Gyrocompasses
AGENDA:
– Directional reference systems
– The Magnetic Compass
– Magnetic Error (Variation and Deviation)
– Compass Conversion (T M)
– The Gyrocompass
– Gyrocompass Error
Applicable reading: Hobbs chapter 9.
Learning Objectives
1. The student will comprehend the basic principles of the operation of the gyrocompass and its advantages and disadvantages.
2. The student will apply correct procedures in determining and correcting for gyrocompass and magnetic error.
3. The student will comprehend the differences between true, magnetic, gyrocompass, and relative direction reference systems, and apply proper procedures to make direction conversions from any one system to any other.
Learning Objectives
4. The student will comprehend the basic principles of operation of the magnetic compass and its advantages and disadvantages.
5. The student will comprehend the reasons for variation and deviation and how these affect the magnetic compass.
6. The student will apply proper procedures in converting from true direction to compass direction and vice versa.
7. The student will apply correct procedures to determine variation using navigation charts.
Directional Reference
Systems
Directional References
– Relative Bearings (R) = bearings measured with
reference to the ship’s longitudinal axis
– Magnetic Bearings (M) = bearings measured with
respect to magnetic north.
– True Bearings (T) = bearings measured with
respect to true of geographic north.
Ship’s Head ( or heading) – a special bearing denoting the direction in which the
ship is pointing.
000 R
090 R
270 R
180 R
270 T
000 T
090 T
180 T
Magnetic Compasses
Operation -
– Magnets within the compass tend to align
themselves with the earth’s magnetic lines of force.
Construction -
– The compass consists of a circular card, graduated
with 360 degrees around the face.
– A pair of magnets is attached to the underside of the
card, beneath the north-south axis.
– The card floats in the fluid to reduce friction and
dampen the vibrations caused by ship’s movement.
Magnetic Compasses
Advantages of Magnetic
Compasses
Little maintenance required
No power source required
Durability
Disadvantages of Magnetic
Compasses
Does not seek True North
Affected by surrounding materials
Cannot be used near the poles
Magnetic Compass Error:
Variation
Variation: the angle between a magnetic
line of force and a geographic (true)
meridian at any location on the earth.
Variation exists because the earth’s
magnetic and geographic poles are not in
the same location.
Magnetic anomalies in the earth’s crust
also contribute to variation.
True North Pole Magnetic North Pole
Notice that the
two poles
aren’t together.
The magnetic
compass points
to the magnetic
pole, and this
gives us
VARIATION.
Magnetic Compass Error:
Variation
Variation also changes from year to year
as the earth’s magnetic poles tend to
wander.
Variation is printed inside compass roses
on all navigation charts.
Always use the compass rose nearest
your current DR position.
Magnetic Compass Error:
Variation
Magnetic Compass Error:
Deviation
Deviation: the angle between the
magnetic meridian and the north line on
the compass card.
Deviation is caused by the interaction of
the ship’s metallic structure and
electrical systems with the earth’s
magnetic field.
DEVIATION
A ship’s compass also
must deal with
magnetic forces from
the ship itself. The sum
total of these forces
pulls the compass
slightly away from
magnetic north,
producing
DEVIATION.
Magnetic Compass Error:
Deviation
Deviation can be compensated for but
never eliminated.
A compass table is used, which provides
the value of deviation for every 15
degrees of ship’s head.
Entering argument for the table is oM
and degaussing on or off (DG ON/DG
OFF)
P. 154 in Hobbs
Swinging Ship
Swinging ship is the process used to
determine the ship’s magnetic table.
– Swing ship around 15 degrees to measure
the magnetic deviation with each ship’s
heading.
– Done when the deviation on any heading
exceeds 3 degrees.
Compass Conversions
Converting Compass to True
C D M V T (AE)
[Can dead men vote twice (at elections)?]
Converting True to Compass
T V M D C (AW)
[True virgins make dull companions (at weddings)]
Gyrocompasses
A north-seeking gyroscope
The gyro spins at a very high velocity,
and its spin axis remains aligned with
terrestrial meridians.
The gyro has three axes:
– spin axis
– torque axis
– precession axis
Gyroscope Theory Page 160 in Hobbs
Gyrocompass
Advantages
Seeks geographic (true) north instead of
magnetic north.
Can be used near the earth’s magnetic
poles, where magnetic compass is useless.
Unaffected by surrounding material.
Signal can be fed to other systems (INS,
fire control, automatic steering)
Gyrocompass
Disadvantages
Intricate electronic instrument.
Requires a constant source of electrical
power and is sensitive to power
fluctuations.
Requires periodic maintenance by highly
skilled technicians.
Gyrocompass Error
Although the gyrocompass is a very
accurate instrument, it normally has a
small error associated with its readings.
(normally less than 1o)
Like the magnetic compass, this error is
expressed as east or west.
Page 163 in Hobbs
Gyrocompass Error
To remember how to use the gyro error,
two memory aids are commonly used:
If Gyro is best (higher), error is west;
If Gyro is least (lower), error is east.
Also,
G.E.T. -- Gyro + East = True
Determining Gyro
Error
Methods of determining gyro error:
– Celestial Methods (to be discussed later)
– Observing a visual range.
– Observing bearing to an object while at a
known location.
– Heading while pierside
– Trial and error adjustment of three or more
simultaneous LOPs.
– Compare to gyrocompass of known error.
Questions?