Lesson 6: Magnetic and Gyrocompasses

Getting Acquainted LectureInstructor Bio: Commissioned May, 1993; NROTC Unit, IIT Reported to Hue City (CG-66) homeported in Mayport, FL for a 42 mos tour24 mos - Engineering Dept (Auxiliaries Officer & Damage Control Assistant)18 mos - Operations Dept (Air Warfare Officer/BW Flag Liaison Officer) Command Employment (while aboard Hue City)1995 Med Deployment 95-2 with USS Theodore Roosevelt1996 Baltops 96 (Scandinavian countries)1997 Med Deployment 97-2 with USS John F. Kennedy Midshipmen Cruises (emphasize student contribution to course) CV (CV 63) - Secrest Ticos (LKE, VLG) -Waterston, Pope, Sullivan Burkes (DDG 61) - Kuckel OH Perry (FFG 61) - Phillips LHA/LHD (LSD 44) - Fink, Sutherland AOE (AOE-8) - Hertel Review Syllabus: Course objectives Course resources (webpage, Textbook, Workbook) Case studies - provide vivid and intriguing examples of the the fundamental principles taught in this course.

Lesson 9: Magnetic and GyrocompassesAGENDA:

Directional reference systemsThe Magnetic CompassDigital Flux Gate Magnetic CompassMagnetic Error (Variation and Deviation)Compass Conversion (T M)The GyrocompassGyrocompass Error Applicable reading: Hobbs pg. 148-165.

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Directional Reference SystemsDirectional References

Relative Bearings (R) = bearings measured with reference to the ships longitudinal axisMagnetic 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 R090 R270 R180 R270 T000 T090 T180 T

Magnetic CompassesOperation -

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 CompassesLittle maintenance requiredNo power source requiredDurability

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Disadvantages of Magnetic CompassesDoes not seek True NorthAffected by surrounding materialsCannot be used near the poles

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Magnetic Compass Error: VARIATIONVariation: 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 PoleMagnetic North PoleNotice that the two poles arent 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 changes from year to year as the earths magnetic poles tend to wander.

ALWAYS use the compass rose nearest your current DR position.

2.2-8Variation 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.

Because Variation changes by location due to magnetic signature of the earths surface, you must look on the chart you are working with to determine what the variation is.

Magnetic Compass Error: DEVIATIONDeviation: 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: DeviationDeviation 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 GateDigital Compass System MV-103ACJunction Box/Control UnitRemoteDisplay2.3-26

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-compensationGuides to Magnetic NorthVariation may be manually or automatically entered.

C D M V TADD EAST (SUBRACT WEST)Can Dead Men Vote Twiceat ElectionsADD WEST (SUBRACT EAST)Timely Vessels make Distance Countat War

T V M D CADD WEST (SUBRACT EAST)Timely Vessels Make Distance Countat WarTRUEVARIATIONMAGNETICDEVIATIONCOMPASS

C D M V TADD EAST (SUBRACT WEST)Can Dead Men Vote Twiceat ElectionsCOMPASSDEVIATIONMAGNETICVARIATIONTRUE

056.5Magnetic/Gyro Compass Practical Problems#1#2#3#4#5 G E T V M D C 170.5 172 12E 0 1W 12E 056 0.5W 1.5E160160069068339 12E 1E 325 347.5 12E 337 338 1.5E 031.5 1.5E 0181W 12E1.5E1W349326338030019.5Compass Error for #1=Compass Error for #2=12E11.5ECompass Error for #3=13ECompass Error for #4=11ECompass Error for #5=13.5E

THE GYRO COMPASSUSS O'Flaherty DE 340 IC Room

GyrocompassesA true north-seeking gyroscopeThe gyro spins at a very high velocity, and its spin axis remains aligned with terrestrial meridians.The gyro has three axes:

spin axistorque axisprecession axis

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Gyroscope Theory

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The Gyrocompass Repeater

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Gyrocompass Pros and ConsAdvantages: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).

DisadvantagesIntricate 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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TermsCompass 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

Terms Associated With the Gyro CompassTrue North:

Direction referenced to True North, measured clockwise from 000 to 359Gyro Error (East or West):

The Difference between True North and the Gyro Compass ReadingGyro Repeater Error:

The Difference between the Gyro Compass and Gyro Repeater

Gyrocompass ErrorAlthough 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 ErrorMethods of determining gyro error:

Celestial Methods (to be discussed later)Observing a visual rangeObserving bearing to an object while at a known locationHeading while piersideTrial and error adjustment of three or more simultaneous LOPsCompare to gyrocompass of known error

Use a reference point by placing yourself on a known bearing from that reference point and then observe your gyro error.

Gyrocompass ErrorTo 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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G E T 015 1.0E 335 1.0E Correcting (Gyro Compass to True)016336+E / -W#1#2 265 266#31.0EApplying Gyro ErrorGyro Compass best = Error West

Gyro Compass least = Error East

G E T 015 1.5W 335 1.5W Correcting (Gyro Compass to True)013.5333.5+E / -W#4#5 265 263.5#61.5WApplying Gyro ErrorGyro Compass best = Error West

Gyro Compass least = Error East

REVIEWRelative Bearings (R) = bearings measured with reference to the ships longitudinal axisMagnetic 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 ErrorBe able to compensate for variation and deviation using C D M V T

QUESTIONS?

Getting Acquainted LectureInstructor Bio: Commissioned May, 1993; NROTC Unit, IIT Reported to Hue City (CG-66) homeported in Mayport, FL for a 42 mos tour24 mos - Engineering Dept (Auxiliaries Officer & Damage Control Assistant)18 mos - Operations Dept (Air Warfare Officer/BW Flag Liaison Officer) Command Employment (while aboard Hue City)1995 Med Deployment 95-2 with USS Theodore Roosevelt1996 Baltops 96 (Scandinavian countries)1997 Med Deployment 97-2 with USS John F. Kennedy Midshipmen Cruises (emphasize student contribution to course) CV (CV 63) - Secrest Ticos (LKE, VLG) -Waterston, Pope, Sullivan Burkes (DDG 61) - Kuckel OH Perry (FFG 61) - Phillips LHA/LHD (LSD 44) - Fink, Sutherland AOE (AOE-8) - Hertel Review Syllabus: Course objectives Course resources (webpage, Textbook, Workbook) Case studies - provide vivid and intriguing examples of the the fundamental principles taught in this course.1

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Because Variation changes by location due to magnetic signature of the earths surface, you must look on the chart you are working with to determine what the variation is. 1

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Use a reference point by placing yourself on a known bearing from that reference point and then observe your gyro error.1