lesson 6: magnetic and gyrocompasses. lesson 9: magnetic and gyrocompasses agenda: –directional...
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Lesson 6: Magnetic and Gyrocompasses
Lesson 9: Magnetic and Lesson 9: Magnetic and GyrocompassesGyrocompasses
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.
Directional Reference SystemsDirectional 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 CompassesMagnetic 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 Advantages of Magnetic CompassesCompasses
Little maintenance required No power source required Durability
Disadvantages of Magnetic Disadvantages of Magnetic CompassesCompasses
Does not seek True North Affected by surrounding materials Cannot be used near the poles
Magnetic Compass Error: Magnetic Compass Error: VARIATIONVARIATION
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 Pole Magnetic North PoleNotice that the two poles aren’t together. The magnetic compass points to the magnetic pole, and this gives us VARIATION.
COMPASS ROSECOMPASS 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: Magnetic Compass Error: DEVIATIONDEVIATION
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: DeviationMagnetic 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)
Mag
net
ic
Mag
net
ic
Co
mp
ass
Err
or:
C
om
pas
s E
rro
r:
Dev
iati
on
Dev
iati
on
Digital Flux GateDigital Flux Gate
Digital Compass System MV-103AC
Junction Box/Control Unit
RemoteDisplay
2.3-26
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 TC D M V T
ADD EAST (SUBRACT WEST)ADD EAST (SUBRACT WEST)
““Can Dead Men Vote Twice…at Elections”Can Dead Men Vote Twice…at Elections”
ADD WEST (SUBRACT EAST)ADD WEST (SUBRACT EAST)
““Timely Vessels make Distance Count…at War”Timely Vessels make Distance Count…at War”
T V M D CT V M D CADD WEST (SUBRACT EAST)ADD WEST (SUBRACT EAST)
““Timely Vessels Make Distance Count…at War”Timely Vessels Make Distance Count…at War”
TTRRUUEE
VVAARRIIAATTIIOONN
MMAAGGNNEETTIICC
DDEEVVIIAATTIIOONN
CCOOMMPPAASSSS
C D M V TC D M V TADD EAST (SUBRACT WEST)ADD EAST (SUBRACT WEST)
““Can Dead Men Vote Twice…at Elections”Can Dead Men Vote Twice…at Elections”
CCOOMMPPAASSSS
DDEEVVIIAATTIIOONN
MMAAGGNNEETTIICC
VVAARRIIAATTIIOONN
TTRRUUEE
056.5056.5
Magnetic/Gyro Compass Practical Problems
#1
#2
#3
#4
#5
G E T V M D C
170.5 170.5 172172 12E 12E 0 0
1W 12E 1W 12E 056 056 0 0.5W.5W
1.5E1.5E 160160 160160
069069 068068
339339 12E 1E12E 1E 325325
347.5347.5 12E 12E 337 337 338338
1.5E 1.5E 031.5031.5 1.5E1.5E 018018
1W1W
12E12E
1.5E1.5E
1W1W
349349
326326338338
030030 019.5019.5
Compass Error for #1=Compass Error for #2=
12E11.5E
Compass Error for #3= 13ECompass Error for #4= 11ECompass Error for #5= 13.5E
THE GYRO COMPASSTHE GYRO COMPASS
USS O'Flaherty DE 340 IC Room
GyrocompassesGyrocompasses
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 TheoryGyroscope Theory
Th
e T
he
Gyr
oco
mp
ass
Gyr
oco
mp
ass
Rep
eate
rR
epea
ter
Gyrocompass Gyrocompass Pro’s and Con’sPro’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.
TermsTerms 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 Terms Associated With the Gyro CompassGyro Compass
True North:True North:– Direction referenced to True North,
measured clockwise from 000° to 359° Gyro Error (East or West):Gyro Error (East or West):
– The Difference between True North and the Gyro Compass Reading
Gyro Repeater Error:Gyro Repeater Error:– The Difference between the Gyro Compass
and Gyro Repeater
Gyrocompass ErrorGyrocompass 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.
Determining Gyro ErrorDetermining 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 ErrorGyrocompass 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
015015 1.0E1.0E
335335 1.0E1.0E
Correcting (Gyro Compass to True)Correcting (Gyro Compass to True)
016016
336336
+E / -W
#1
#2
265265 266 266#3 1.0E1.0E
Applying Gyro ErrorApplying Gyro Error
Gyro Compass Gyro Compass bestbest = Error = Error WestWest
Gyro Compass Gyro Compass leastleast = Error = Error EastEast
G E T
015015 1.5W1.5W
335335 1.5W1.5W
Correcting (Gyro Compass to True)Correcting (Gyro Compass to True)
013.5013.5
333.5333.5
+E / -W
#4
#5
265265 263.5 263.5#6 1.5W1.5W
Applying Gyro ErrorApplying Gyro Error
Gyro Compass Gyro Compass bestbest = Error = Error WestWest
Gyro Compass Gyro Compass leastleast = Error = Error EastEast
REVIEWREVIEW
– 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?QUESTIONS?