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  Applicable reading: Hobbs pg

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 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 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.  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 PoleMagnetic 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. 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: 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 o M and degaussing on or off (DG ON/DG OFF)

Magnetic Compass Error: Deviation Magnetic Compass Error: Deviation

Digital Flux Gate Digital Compass System MV-103AC Junction Box/ Control Unit Remote Display

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 T ADD EAST (SUBRACT WEST) “Can Dead Men Vote Twice…at Elections” ADD WEST (SUBRACT EAST) “Timely Vessels make Distance Count…at War”

T V M D C ADD WEST (SUBRACT EAST) “Timely Vessels Make Distance Count…at War” TRUETRUETRUETRUE VARIATIONVARIATIONVARIATIONVARIATION MAGNETICMAGNETICMAGNETICMAGNETIC DEVIATIONDEVIATIONDEVIATIONDEVIATION COMPASSCOMPASSCOMPASSCOMPASS

C D M V T ADD EAST (SUBRACT WEST) “Can Dead Men Vote Twice…at Elections” COMPASSCOMPASSCOMPASSCOMPASS DEVIATIONDEVIATIONDEVIATIONDEVIATION MAGNETICMAGNETICMAGNETICMAGNETIC VARIATIONVARIATIONVARIATIONVARIATION TRUETRUETRUETRUE

056.5 Magnetic/Gyro Compass Practical Problems #1 #2 #3 #4 #5 G E T V M D C E 0 1W 12E W 1W 12E W 1.5E E 1E E E E E E 018 1W 12E 12E 1.5E 1W Compass Error for #1= Compass Error for #2= 12E 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

Gyroscope Theory

The Gyrocompass Repeater The Gyrocompass Repeater

Gyrocompass Pro’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.

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 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 1 o )  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 –Heading while pierside –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

G E T E E E E Correcting (Gyro Compass to True) E / -W #1 # #3 1.0E Applying Gyro Error Gyro Compass best = Error West Gyro Compass least = Error East

G E T W W W W Correcting (Gyro Compass to True) E / -W #4 # #6 1.5W Applying Gyro Error 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”

QUESTIONS?