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