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Lesson 6: Magnetic and Gyrocompasses

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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  1. Lesson 6: Magnetic and Gyrocompasses

  2. 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.

  3. 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.

  4. 000 T 270 T 090 T 270 R 180 T 000 R 180 R 090 R

  5. 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.

  6. Advantages of Magnetic Compasses • Little maintenance required • No power source required • Durability

  7. Disadvantages of Magnetic Compasses • Does not seek True North • Affected by surrounding materials • Cannot be used near the poles

  8. 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.

  9. 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.

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

  11. 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

  12. 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.

  13. 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)

  14. Magnetic Compass Error: Deviation

  15. Digital Flux Gate Digital Compass System MV-103AC Junction Box/ Control Unit Remote Display 2.3-26

  16. 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.

  17. “Can Dead Men Vote Twice…at Elections” ADD EAST (SUBRACT WEST) C D M V T ADD WEST (SUBRACT EAST) “Timely Vessels make Distance Count…at War”

  18. “Timely Vessels Make Distance Count…at War” ADD WEST (SUBRACT EAST) T V M D C VARIATION MAGNETIC DEVIATION TRUE COMPASS

  19. “Can Dead Men Vote Twice…at Elections” ADD EAST (SUBRACT WEST) C D M V T DEVIATION MAGNETIC VARIATION COMPASS TRUE

  20. 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

  21. THE GYRO COMPASS USS O'Flaherty DE 340 IC Room

  22. 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

  23. Gyroscope Theory

  24. The Gyrocompass Repeater

  25. 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. Gyrocompass Pro’s and Con’s

  26. 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

  27. 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

  28. 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.

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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”

  34. QUESTIONS?

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