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Navigation CompassesNavigation Compasses
Gyro Theory
• A Gryro compass is a North Seeking Gyroscope
• A spinning wheel held by two GIMBALS
• Has 3 axies of angular freedom– Spin Axis– Horizontal Axis– Vertical Axis
Gyrocompass
• Gyroscope
• 3 axes– Spin– Torque– Precession
• Gyrorepeaters
Spin axisSpin axis
Precession axisPrecession axis
Torque axisTorque axis
Compasses - Abbreviations
• Gyrocompass PGC
• Magnetic Compass– Steering Compass PSTC
– Standard Compass PSC
Gyro Theory
• Physics – A Gyro Scope will always point in one direction (in this case TRUE NORTH)
• Precision – Apply force to the spin axis and the gyro scope will rotate, not in the direction in which the force was applied but 90 degrees from that force vector.
Gyro Compasses
• Ships normally have 2 gyro compasses– Main – Orientated N/S used for Navigation– Auxiliary – Oriented E/W for combat systems or as
a backup for Navigation.
• Located deep within the ship
• On the center line
Gyro Uses
• Gyro Repeaters– Located on the bridge wings, Navigation Table,
Dead reckoning plot– Are used for Fire Control for gunnery systems –
Pitch, yaw, and Roll– Centerline Pelorus – Center of Bridge used as the
most accurate reference
Gyro Error
• Mechanical Device – has some inherent error – THE KEY IS TO KNOW WHAT IT IS AND HOW TO CORRECT FOR IT
• Sources of error– Mechanical Friction– Ship’s motion– Electronic malfunctions– Power Fluctuations
Determining Error
• Visual Range
• Triangulation– Adjusting three or more lines of position to find
location and bearing .
• Compare course with a known true course– Entering and exiting port or in a channel
• Celestial Azmiuths and Amplitudes
• Comparison with a gyro of known error
Gyro Error
• Error – the difference between the true bearing and the Gyro reading
• Expressed in degrees East or degrees West
Gyro Least, error East
Gyro Best, error West
Gyro Bearing + East Error = True Bearing
Error Example
You are dead center on a Range Bearing 173 Degrees True and your Gyro reads 175 Degrees PGC what is your Error?
GYRO = 175 True = 173
GYRO is BEST ERROR is WEST
175 – 173 = 2 degrees West
Shipboard Compasses
• Three principle references of direction– Ships longitudinal axis RELATIVE BEARING (R)– Local magnetic meridian MAGNETIC BEARING (M)– True meridian TRUE BEARING (T)
Relative Vs. True
• Relative bearings Can NOT be plotted so must converted
Relative Bearing + Ship’s Head = True Bearing
090 + 090 = 180
Magnetic• The Earth is a core of Iron which acts as a large
magnet with its poles aligned with the earth’s axis
• NOT located at 090 North Latitude– Difference is VARIATION
• Magnetic Meridians - Skewed due to irregularities in the Earth’s
core
Applying Variation
• Variation is found by referencing the compass rose closest to the ship’s position
Deviation
• Each ship is made of Steel and Iron• It has its own magnetic field that can effect changes in
the ship’s magnetic compass• Determined by angle in which the Keel is laid during
initial construction• Equipment – electronics can also cause deviation• Expressed in Degrees East
and West (rounded to nearest
.5 Degree)• Changes with the ship’s heading
Degaussing• Degaussing system - electrical installation designed to
protect ships against magnetic mines and torpedoes• Purpose – counteract the ship’s magnetic field and
establish a condition such that the magnetic field near the ship is, as nearly as possible, just the same as if the ship were not there
• Degaussing installation consists of permanently installed degaussing coils wrapped around ship on underside of hull, control unit to control the coil current, and compass compensating equipment to prevent disturbances to mag compasses – Coil is a large diameter electrical wire– A, F, L, M, Q Coils
Compass Error
• Deviation + Variation = Compass Error
Observed Bearing Least = Error East
Observed Bearing Best = Error West
Ex. Deviation = 3 West
Variation = 13 West
3 + 13 = 16 West
Example
• USS Princeton is on course 125 degrees true when you lose your gyro scope. The Navigation gives you the following:
• Variation = 8 degrees West
• Deviation = 2.5 degrees East
Example
• Truly – True Course = 125
• Valiant – Variation = 8 W
• Marines - Magnetic Course = 133
• Don’t - Deviation = 2.5 E
• Cry – Compass – 133 –2.5 = 130.5
• Because of ADD WEST
Compass Head What the mag compass reads
Deviation
Magnetic Head What the magnetic heading is
Variation
True Head What the true heading is
Add
East
CanCan
DeadDead
MenMen
VoteVote
TwiceTwice
AtAt
ElectionsElections
Application of Compass Error
Sample Problem #1
• Degaussing is ON
• Ship’s Magnetic Compass reads 030° PSTC
• Var is 11° W
• Find ship’s true head
Interpolation
• Bill and Jorge decide to split a pizza.
• The pizza costs $9.00 and has 6 slices.
• Bill eats 5 slices
• Jorge eats 1 slice
• How much should Bill pay?
Interpolation• Eating all the pizza slices costs $9.00
• Eating none of the pizza costs $0.00
• So Bill has eaten 5/6 of $9.00
• 5/6 times $9.00 = $7.50
This is interpolation - calculation of an internal value by assuming a linear relationship with surrounding data.
Sample Problem #2
C D M V T AE
Sample Problem #2
C D M V T AE
030
Sample Problem #2
C D M V T AE
030 (-3)
Sample Problem #2
C D M V T AE
030 (-3)
027
Sample Problem #2
C D M V T AE
030 (-3)
027
Reinterpolate - We assumed that the magnetic head was 030, we now see it was 027
Sample Problem #2Reinterpolate - We assumed that the magnetic head was 030, we now see it was 027.
Value for 030 = 3.0W
Value for 015 = 3.5W
3.0W-3.5W = -0.5W
Sample Problem #2030 - 015 = 15 degrees
027 - 015 = 12 degrees
So we are 12/15 of the way from 015-030
12/15 times -0.5 = -0.4
BUT we round to the nearest .5 so it is -0.5
3.5W - 0.5W = 3.0W
Or no change (in this case)
Sample Problem #2
C D M V T AE
030 (-3)
027
Reinterpolate
027
Sample Problem #2
C D M V T AE
030 (3)
027
Reinterpolate
027
(11)
Sample Problem #2
C D M V T AE
030 (3)
027
Reinterpolate
027
(11)
016
Questions?• Homework – Read Chapter 8 pg. 129-147
• 1st Homework due Tuesday
• 2nd due Next Thurs