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?