Navigation Compasses

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Navigation Compasses
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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

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Gyrocompass

• Gyroscope
• 3 axes
• Spin
• Torque
• Precession
• Gyrorepeaters

Precession axis
Spin axis
Torque axis
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Compasses - Abbreviations

• Gyrocompass PGC
• Magnetic Compass
• Steering Compass PSTC
• Standard Compass PSC

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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.

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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

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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

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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
• Ships motion
• Electronic malfunctions
• Power Fluctuations

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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

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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

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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

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Shipboard Compasses

• Three principle references of direction
• Ships longitudinal axis RELATIVE BEARING (R)
• Local magnetic meridian MAGNETIC BEARING (M)
• True meridian TRUE BEARING (T)

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Relative Vs. True

• Relative bearings Can NOT be plotted so must
  converted
• Relative Bearing Ships Head True Bearing
• 090 090 180

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Magnetic

• The Earth is a core of Iron which acts as a large
  magnet with its poles aligned with the earths
  axis
• NOT located at 090 North Latitude
• Difference is VARIATION
• Magnetic Meridians - Skewed due to
  irregularities in the Earths
• core

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Applying Variation

• Variation is found by referencing the compass
  rose closest to the ships position

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Deviation

• Each ship is made of Steel and Iron
• It has its own magnetic field that can effect
  changes in the ships 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 ships heading

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Degaussing

• Degaussing system - electrical installation
  designed to protect ships against magnetic mines
  and torpedoes
• Purpose counteract the ships 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

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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

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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

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Example

• Truly True Course 125
• Valiant Variation 8 W
• Marines - Magnetic Course 133
• Dont - Deviation 2.5 E
• Cry Compass 133 2.5 130.5
• Because of ADD WEST

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Application of Compass Error
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
Can Dead Men Vote Twice At Elections
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Sample Problem 1

• Degaussing is ON
• Ships Magnetic Compass reads 030 PSTC
• Var is 11 W
• Find ships true head

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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?

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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.

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Sample Problem 2

• C D M V T AE

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Sample Problem 2
C D M V T AE 030
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Sample Problem 2
C D M V T AE 030 (-3)
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Sample Problem 2
C D M V T AE 030 (-3) 027
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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
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Sample Problem 2
Reinterpolate - 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
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Sample Problem 2
030 - 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)
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Sample Problem 2
C D M V T AE 030 (-3) 027 Reinterpolate
027
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Sample Problem 2
C D M V T AE 030 (3) 027 Reinterpolate
027 (11)
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Sample Problem 2
C D M V T AE 030 (3) 027 Reinterpolate
027 (11) 016
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Questions?

• Homework Read Chapter 8 pg. 129-147
• 1st Homework due Tuesday
• 2nd due Next Thurs