NS100

Gyro and Magnetic Compass

Objectives:1. Discuss gyrocompass theory and shipboard application.

2. Describe the differences and relationships of true bearings, relative bearings and “per gyro compass” (PGC) bearings.

3. Discuss and describe gyro error and the methods used to determine gyro error.

4. Demonstrate how to determine and correct for gyro error.

Objectives:

5. Describe the purpose and operation of the magnetic compass.

6. Discuss magnetic theory and application.

7. Define, identify, and solve for variation, deviation, compass error, magnetic bearings and true bearings.

Gyro Compass Theory •North-seeking gyroscope •two gimbals Electronically Stabilized

Gyroscope

Gyro Compass Theory

•North-seeking GyroscopeGyroscopic InertiaGravityPrecession

Gyro Applications Shipboard Use - two gyros placed deep in the ship on the centerline.Primary- (Master gyro) oriented to north/south, used for navigation &

shiphandling.

Auxiliary- (Back up gyro) oriented east/west, used for combat

systems.

Gyro ApplicationsBridge - gyro repeaters are located

on the bridge wings, navigation table, captain’s chair & center line which is referred to as “centerline

pelorus.”

CIC - Command and Control System.

Weapons - gyro inputs (i.e.. pitch/roll), are used by fire control

computers

Gyro repeater

Gyro Error in Navigation

Causes of Error• Friction

• Ship’s Motion• Electronic Malfunctions

• Power Fluctuations.

A gyro compass is an electronic/mechanical device with inherent error…. the key is knowing the error and adjusting for it

Gyro Error in Navigation Determination of Error

• Visual Range ( The most accurate).

• Adjusting Three LOP’s (triangulation).

Used before getting underway.

• Comparing a “PGC” bearing with a known true

bearing.

• Celestial Azimuths and Amplitudes.

• Comparison with a Gyro of known error.

A 5º westerly error

VA bears 090ºT.

With reference to the Gyro compass it would bear 90º+5º=95º pgc

VAVA

A 5º Easterly error

VA bears 090ºT.

With reference to the Gyro compass it would bear 90º-5º=85º pgc

VAVA

Gyro error

Gyro Error - is the difference between the gyro reading and the true bearing.

We can have a West or East Gyro error.If Gyro is Least, Error is EastIf Gyro is Best, Error is West

Gyro bearing +Easterly Error =True bearing

Gyro errorIf you are on a range bearing 089

oT and the

centerline pelorus reads 090

opgc,what is the gyro

error? Gyro = 090o and True = 089o,

Gyro = Best Error = West

090o - 089

o = 1

o

Answer: 1o Westerly Error

Summary

pgc (per gyro compass)Gyro error diff. True and Gyro norths

Gyro Failure

In the event of gyro failure LOP’s are taken in relative bearings.

The magnetic compass can be used to know your ship’s head.

Rel. Bearing+Mag. Ship’s Head=Mag. BearingThen the magnetic heading can be converted

to true bearing.

The variation information is contained in the compass rose closest to the ship’s DR position

True Bearing = Magnetic Bearing + Variation(add East Variation or subtract West Variation)

Summary

pgc (per gyro compass)Gyro error diff. True and Gyro norths

M (magnetic)Variation diff. True and Magnetic norths

Deviation Ship’s Magnetic Field

Deviation Degaussing

Compass Deviation

table

Magnetic Compass Error

Deviation + Variation = Compass ErrorIf observed bearing is Least, Error is East.If observed bearing is Best, Error is West.

Example: Deviation is 2o West & Variation is 13o East, what is compass error?

Answer: 13o - 2o = 11o East Error

Applications of Compass Error

Scenario: The USS Boxer just lost it’s gyro compass while steering 125o T. The conning

officer needs to give a magnetic compass course. What course do you recommend?

Given: Variation is 8o West from the compass rose on the chart, annual increase was added to

the variation.Deviation is 2.5o East from the ship’s

compass card for course 133o M.

Applications of Compass Error Truly True Course Valiant Variation Marines Mag. Mer. Don’t Deviation Cry Compass @ AddWeddings West

Applications of Compass Error

T = 125o T V = 8o West M = 125 + 8 = 133 D = 2.5o East C = 133 + (- 2.5) Ans = 130.5o pstgc

Summary

pgc (per gyro compass)Gyro error diff. True and Gyro norths

M (magnetic)Variation diff. True and Magnetic norths

pstgc (per steering compass) Deviation diff. Mag and steering compass norths