Published on November 13, 2007
Lesson 9: Magnetic and Gyro Compasses: Lesson 9: Magnetic and Gyro Compasses Lesson 9: Magnetic and Gyrocompasses: Lesson 9: Magnetic and Gyrocompasses AGENDA: Directional reference systems The Magnetic Compass Magnetic Error (Variation and Deviation) Compass Conversion (T M) The Gyrocompass Gyrocompass Error Applicable reading: Hobbs chapter 9. Learning Objectives: Learning Objectives 1. The student will comprehend the basic principles of the operation of the gyrocompass and its advantages and disadvantages. 2. The student will apply correct procedures in determining and correcting for gyrocompass and magnetic error. 3. The student will comprehend the differences between true, magnetic, gyrocompass, and relative direction reference systems, and apply proper procedures to make direction conversions from any one system to any other. Learning Objectives: Learning Objectives 4. The student will comprehend the basic principles of operation of the magnetic compass and its advantages and disadvantages. 5. The student will comprehend the reasons for variation and deviation and how these affect the magnetic compass. 6. The student will apply proper procedures in converting from true direction to compass direction and vice versa. 7. The student will apply correct procedures to determine variation using navigation charts. Directional Reference Systems: 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. Slide6: 000 R 090 R 270 R 180 R 270 T 000 T 090 T 180 T Magnetic Compasses: 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 degrees 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. Magnetic Compasses: Magnetic Compasses Advantages of Magnetic Compasses: Advantages of Magnetic Compasses Little maintenance required No power source required Durability Disadvantages of Magnetic Compasses: Disadvantages of Magnetic Compasses Does not seek True North Affected by surrounding materials Cannot be used near the poles Magnetic Compass Error: Variation: Magnetic Compass Error: Variation Variation: the angle between a magnetic line of force and a geographic (true) meridian at any location on the earth. Variation exists because the earth’s magnetic and geographic poles are not in the same location. Magnetic anomalies in the earth’s crust also contribute to variation. Slide12: 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. Magnetic Compass Error: Variation: Magnetic Compass Error: Variation Variation also changes from year to year as the earth’s magnetic poles tend to wander. Variation is printed inside compass roses on all navigation charts. Always use the compass rose nearest your current DR position. Magnetic Compass Error: Variation: Magnetic Compass Error: Variation Magnetic Compass Error: Deviation: Magnetic Compass Error: Deviation Deviation: the angle between the magnetic meridian and the north line on the compass card. Deviation is caused by the interaction of the ship’s metallic structure and electrical systems with the earth’s magnetic field. Slide16: DEVIATION A ship’s compass also must deal with magnetic forces from the ship itself. The sum total of these forces pulls the compass slightly away from magnetic north, producing DEVIATION. Magnetic Compass Error: Deviation: 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) Magnetic Compass Error: Deviation: Magnetic Compass Error: Deviation P. 154 in Hobbs Swinging Ship: Swinging Ship Swinging ship is the process used to determine the ship’s magnetic table. Swing ship around 15 degrees to measure the magnetic deviation with each ship’s heading. Done when the deviation on any heading exceeds 3 degrees. Compass Conversions: Compass Conversions Converting Compass to True C D M V T (AE) [Can dead men vote twice (at elections)?] Converting True to Compass T V M D C (AW) [True virgins make dull companions (at weddings)] Gyrocompasses: Gyrocompasses A 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: Gyroscope Theory Page 160 in Hobbs The Gyrocompass Repeater: The Gyrocompass Repeater Gyrocompass Advantages: Gyrocompass Advantages Seeks geographic (true) north instead of magnetic north. Can be used near the earth’s magnetic poles, where magnetic compass is useless. Unaffected by surrounding material. Signal can be fed to other systems (INS, fire control, automatic steering) Gyrocompass Disadvantages: Gyrocompass Disadvantages Intricate electronic instrument. Requires a constant source of electrical power and is sensitive to power fluctuations. Requires periodic maintenance by highly skilled technicians. Gyrocompass Error: 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. Slide27: Page 163 in Hobbs Gyrocompass 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. Also, G.E.T. -- Gyro + East = True Determining Gyro Error: 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. Questions?: Questions?