Lesson 6: Magnetic and Gyrocompasses

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Lesson 6 Magnetic and Gyrocompasses
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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.

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Directional Reference Systems

• Directional References
• Relative Bearings (?R) bearings measured with
  reference to the ships longitudinal axis
• Magnetic Bearings (?M) bearings measured with
  respect to magnetic north.
• True Bearings (?T) bearings measured with
  respect to true of geographic north.
• Ships Head (or heading)
• a special bearing denoting the direction in which
  the ship is pointing.

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000 T
270 T
090 T
270 R
180 T
000 R
180 R
090 R
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Magnetic Compasses

• Operation -
• Magnets within the compass tend to align
  themselves with the earths 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 ships
  movement.

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Advantages of Magnetic Compasses

• Little maintenance required
• No power source required
• Durability

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Disadvantages of Magnetic Compasses

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

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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 earths magnetic and
  geographic poles are not co-located.
• Magnetic anomalies in the earths crust also
  contribute to variation.

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True North Pole
Magnetic North Pole
Notice that the two poles arent together. The
magnetic compass points to the magnetic pole, and
this gives us VARIATION.
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COMPASS ROSE

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

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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
  earths magnetic poles tend to wander.
• ALWAYS use the compass rose nearest your current
  DR position.

2.2-8
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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
  ships metallic structure and electrical systems
  with the earths magnetic field.

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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 ships head.
• Entering argument for the table is oM and
  degaussing on or off (DG ON/DG OFF)

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Magnetic Compass Error Deviation
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Digital Flux Gate
Digital Compass System MV-103AC
Junction Box/ Control Unit
Remote Display
2.3-26
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• 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.

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Can Dead Men Vote Twiceat Elections
ADD EAST (SUBRACT WEST)
C D M V T
ADD WEST (SUBRACT EAST)
Timely Vessels make Distance Countat War
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Timely Vessels Make Distance Countat War
ADD WEST (SUBRACT EAST)
T V M D C
VARIATION
MAGNETIC
DEVIATION
TRUE
COMPASS
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Can Dead Men Vote Twiceat Elections
ADD EAST (SUBRACT WEST)
C D M V T
DEVIATION
MAGNETIC
VARIATION
COMPASS
TRUE
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Magnetic/Gyro Compass Practical Problems
G E T V M
D C
170.5 172 12E
0
1.5E
160
160
1
1W 12E 056
0.5W
068
056.5
069
2
1W
326
338
339 12E
1E 325
3
1W
1.5E
349
347.5 12E 337
338
4
12E
030
019.5
1.5E 031.5 1.5E 018
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Compass Error for 1
12E
Compass Error for 2
11.5E
Compass Error for 3
13E
Compass Error for 4
11E
Compass Error for 5
13.5E
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THE GYRO COMPASS
USS O'Flaherty DE 340 IC Room
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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

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Gyroscope Theory
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The Gyrocompass Repeater
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Gyrocompass Pros and Cons

• Advantages
• Seeks geographic (true) north instead of
  magnetic.
• Can be used near the earths 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.

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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
• Compass Heading
• The Course That the Ship Is Steering, Using the
  Ships Magnetic Compass
• - This Heading Includes the Compass Error

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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

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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.

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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

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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

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Applying Gyro Error
Correcting (Gyro Compass to True)
E / -W
G E T
015 1.0E
016
1
335 1.0E
336
2
265 266
1.0E
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Gyro Compass best Error West Gyro Compass
least Error East
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Applying Gyro Error
Correcting (Gyro Compass to True)
E / -W
G E T
015 1.5W
013.5
4
335 1.5W
333.5
5
265 263.5
1.5W
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Gyro Compass best Error West Gyro Compass
least Error East
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REVIEW

• Relative Bearings (?R) bearings measured with
  reference to the ships 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

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• QUESTIONS?