Lesson 6: Magnetic and Gyrocompasses

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# Lesson 6: Magnetic and Gyrocompasses - PowerPoint PPT Presentation

Lesson 6: Magnetic and Gyrocompasses. Lesson 9: Magnetic and Gyrocompasses. 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

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## Lesson 6: Magnetic and Gyrocompasses

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Presentation Transcript
Lesson 9: Magnetic and Gyrocompasses
• 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 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.
• a special bearing denoting the direction in which the ship is pointing.

000 T

270 T

090 T

270 R

180 T

000 R

180 R

090 R

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 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.
• Little maintenance required
• No power source required
• Durability
• 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.
• 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 Pole

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

COMPASS ROSE
• Used to Determine Magnetic and True Courses.
• Outer ring is in True.
• Inner Ring is in Magnetic.

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.

2.2-8

Magnetic Compass Error: DEVIATION
• 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: 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)
Digital Flux Gate

Digital Compass System

MV-103AC

Junction Box/

Control Unit

Remote

Display

2.3-26

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.

“Can Dead Men Vote Twice…at Elections”

C D M V T

“Timely Vessels make Distance Count…at War”

“Timely Vessels Make Distance Count…at War”

T V M D C

VARIATION

MAGNETIC

DEVIATION

TRUE

COMPASS

“Can Dead Men Vote Twice…at Elections”

C D M V T

DEVIATION

MAGNETIC

VARIATION

COMPASS

TRUE

Magnetic/Gyro Compass

Practical Problems

G E T V M D C

1.5E

160

170.5 172 12E 0

160

#1

1W 12E 056 0.5W

068

069

056.5

#2

1W

338

326

33912E 1E325

#3

1.5E

1W

349

347.5 12E 337 338

#4

030

12E

019.5

1.5E 031.51.5E018

#5

12E

Compass Error for #1=

Compass Error for #2=

11.5E

Compass Error for #3=

13E

Compass Error for #4=

11E

Compass Error for #5=

13.5E

THE GYRO COMPASS

USS O'Flaherty DE 340 IC Room

Gyrocompasses
• 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

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).

Intricate electronic instrument.

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

Requires periodic maintenance by qualified technicians.

Gyrocompass Pro’s and Con’s
Terms
• Compass course:

The heading indicated by the magnetic compass toward where the ship is traveling.

• Compass Error (CE):

The Sum of Variation and Deviation

The Course That the Ship Is Steering, Using the Ship’s Magnetic Compass

- This Heading Includes the Compass Error

Terms Associated With the Gyro Compass
• True North:
• Direction referenced to True North, measured clockwise from 000° to 359°
• Gyro Error (East or West):
• The Difference between True North and the Gyro Compass Reading
• Gyro Repeater Error:
• The Difference between the Gyro Compass and Gyro Repeater
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.
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
• Trial and error adjustment of three or more simultaneous LOPs
• Compare to gyrocompass of known error
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.

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

Applying Gyro Error

Correcting (Gyro Compass to True)

+E / -W

GE T

0151.0E

016

#1

3351.0E

336

#2

265 266

1.0E

#3

Gyro Compass best = Error West

Gyro Compass least = Error East

Applying Gyro Error

Correcting (Gyro Compass to True)

+E / -W

GET

0151.5W

013.5

#4

3351.5W

333.5

#5

265 263.5

1.5W

#6

Gyro Compass best = Error West

Gyro Compass least = Error East

REVIEW
• 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”