Lesson 9: Magnetic and Gyro Compasses

Lesson 9: Magnetic and Lesson 9: Magnetic and GyrocompassesGyrocompasses

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 ObjectivesLearning 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 ObjectivesLearning 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 Directional Reference SystemsSystems

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 CompassesMagnetic 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 CompassesMagnetic Compasses

Advantages of Magnetic Advantages of Magnetic CompassesCompasses

Little maintenance required No power source required Durability

Disadvantages of Magnetic Disadvantages of Magnetic CompassesCompasses

Does not seek True North Affected by surrounding materials Cannot be used near the poles

Magnetic Compass Error: Magnetic Compass Error: VariationVariation

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 PoleNotice that the two poles aren’t together. The magnetic compass points to the magnetic pole, and this gives us VARIATION.

Magnetic Compass Error: Magnetic Compass Error: VariationVariation

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: Magnetic Compass Error: VariationVariation

Magnetic Compass Error: Magnetic Compass Error: DeviationDeviation

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.

DEVIATIONA 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: Magnetic Compass Error: DeviationDeviation

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)

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P. 154 in Hobbs

Swinging ShipSwinging 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 ConversionsCompass 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)]

GyrocompassesGyrocompasses

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 TheoryGyroscope TheoryPage 160 in Hobbs

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Gyrocompass Gyrocompass AdvantagesAdvantages

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 Gyrocompass DisadvantagesDisadvantages

Intricate electronic instrument. Requires a constant source of electrical

power and is sensitive to power fluctuations.

Requires periodic maintenance by highly skilled technicians.

Gyrocompass ErrorGyrocompass 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 ErrorGyrocompass 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 Determining Gyro ErrorError

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?Questions?