Lesson 6: Magnetic and Gyrocompasses

Lesson 9: Magnetic and Lesson 9: Magnetic and GyrocompassesGyrocompasses

AGENDA:– Directional reference systems– The Magnetic Compass– Digital Flux Gate Magnetic Compass– Magnetic Error (Variation and Deviation)– Compass Conversion (T M)– The Gyrocompass– Gyrocompass Error

Applicable reading: Hobbs pg. 148-165.

Directional Reference SystemsDirectional 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 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 degreed 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.

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.

Causes:– Variation exists because the earth’s magnetic

and geographic poles are not co-located.– 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.

COMPASS ROSECOMPASS ROSE• Used to Determine

Magnetic and True Courses.

• Outer ring is in True.

• Inner Ring is in Magnetic.

• Used to Determine Magnetic and True Courses.

• Outer ring is in True.

• Inner Ring is in Magnetic.

2.2-8

Variation for any given locality is shown on the compass rose of the chart for that area, together with the amount of annual increase or decrease.

Variation changes from year to year as the earth’s magnetic poles tend to wander.

ALWAYS use the compass rose nearest your current DR position.

Magnetic Compass Error: Magnetic Compass Error: DEVIATIONDEVIATION

Deviation: the angle between the actual magnetic meridian and the north line on the compass card.

Causes:– Deviation is caused by the

interaction of the ship’s metallic structure and electrical systems with the earth’s magnetic field.

Magnetic Compass Error: DeviationMagnetic 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)

Mag

net

ic

Mag

net

ic

Co

mp

ass

Err

or:

C

om

pas

s E

rro

r:

Dev

iati

on

Dev

iati

on

Digital Flux GateDigital Flux Gate

Digital Compass System MV-103AC

Junction Box/Control Unit

RemoteDisplay

2.3-26

Digital Flux Gate Compass:Digital Flux Gate Compass:–Antennae is located at the top of the mast to remove metallic effects of the hull.

–Allows Deviation to be “Zeroed” by auto-compensation

–Guides to Magnetic North–Variation may be manually or automatically entered.

C D M V TC D M V T

ADD EAST (SUBRACT WEST)ADD EAST (SUBRACT WEST)

““Can Dead Men Vote Twice…at Elections”Can Dead Men Vote Twice…at Elections”

ADD WEST (SUBRACT EAST)ADD WEST (SUBRACT EAST)

““Timely Vessels make Distance Count…at War”Timely Vessels make Distance Count…at War”

T V M D CT V M D CADD WEST (SUBRACT EAST)ADD WEST (SUBRACT EAST)

““Timely Vessels Make Distance Count…at War”Timely Vessels Make Distance Count…at War”

TTRRUUEE

VVAARRIIAATTIIOONN

MMAAGGNNEETTIICC

DDEEVVIIAATTIIOONN

CCOOMMPPAASSSS

C D M V TC D M V TADD EAST (SUBRACT WEST)ADD EAST (SUBRACT WEST)

““Can Dead Men Vote Twice…at Elections”Can Dead Men Vote Twice…at Elections”

CCOOMMPPAASSSS

DDEEVVIIAATTIIOONN

MMAAGGNNEETTIICC

VVAARRIIAATTIIOONN

TTRRUUEE

056.5056.5

Magnetic/Gyro Compass Practical Problems

#1

#2

#3

#4

#5

G E T V M D C

170.5 170.5 172172 12E 12E 0 0

1W 12E 1W 12E 056 056 0 0.5W.5W

1.5E1.5E 160160 160160

069069 068068

339339 12E 1E12E 1E 325325

347.5347.5 12E 12E 337 337 338338

1.5E 1.5E 031.5031.5 1.5E1.5E 018018

1W1W

12E12E

1.5E1.5E

1W1W

349349

326326338338

030030 019.5019.5

Compass Error for #1=Compass Error for #2=

12E11.5E

Compass Error for #3= 13ECompass Error for #4= 11ECompass Error for #5= 13.5E

THE GYRO COMPASSTHE GYRO COMPASS

USS O'Flaherty DE 340 IC Room

GyrocompassesGyrocompasses

A true 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 Theory

Th

e T

he

Gyr

oco

mp

ass

Gyr

oco

mp

ass

Rep

eate

rR

epea

ter

Gyrocompass Gyrocompass Pro’s and Con’sPro’s and Con’s

Advantages: Seeks geographic (true) north

instead of magnetic.

Can be used near the earth’s magnetic poles, where magnetic compass is useless.

Unaffected by surrounding metals.

Signal can be fed to other systems (weapons, nav).

Disadvantages Intricate electronic

instrument.

Requires a constant source of electrical power and is sensitive to power fluctuations.

Requires periodic maintenance by qualified technicians.

TermsTerms Compass course: The heading indicated by the

magnetic compass toward where the ship is traveling.

Compass Error (CE): The Sum of Variation and Deviation

Compass Heading: The Course That the Ship Is Steering,

Using the Ship’s Magnetic Compass- This Heading Includes the Compass Error

Terms Associated With the Terms Associated With the Gyro CompassGyro Compass

True North:True North:– Direction referenced to True North,

measured clockwise from 000° to 359° Gyro Error (East or West):Gyro Error (East or West):

– The Difference between True North and the Gyro Compass Reading

Gyro Repeater Error:Gyro Repeater Error:– The Difference between the Gyro Compass

and Gyro Repeater

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.

Determining Gyro ErrorDetermining 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

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.

G.E.T. -- Gyro + East = True

G E T

015015 1.0E1.0E

335335 1.0E1.0E

Correcting (Gyro Compass to True)Correcting (Gyro Compass to True)

016016

336336

+E / -W

#1

#2

265265 266 266#3 1.0E1.0E

Applying Gyro ErrorApplying Gyro Error

Gyro Compass Gyro Compass bestbest = Error = Error WestWest

Gyro Compass Gyro Compass leastleast = Error = Error EastEast

G E T

015015 1.5W1.5W

335335 1.5W1.5W

Correcting (Gyro Compass to True)Correcting (Gyro Compass to True)

013.5013.5

333.5333.5

+E / -W

#4

#5

265265 263.5 263.5#6 1.5W1.5W

Applying Gyro ErrorApplying Gyro Error

Gyro Compass Gyro Compass bestbest = Error = Error WestWest

Gyro Compass Gyro Compass leastleast = Error = Error EastEast

REVIEWREVIEW

– 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.

– Be able to apply Gyro Error– Be able to compensate for variation and

deviation using “C D M V T”

QUESTIONS?QUESTIONS?