17 the gryo compass

8/8/2019 17 the Gryo Compass

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Navigation

NAU 102

Lesson 17


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Interpolation

Much of navigation uses tables.

e.g. What is the deviation when heading 300M?

Ans: 3E

DEVIATION TABLE

MAG HDG DEV300 3E

315 0

330 1W


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Interpolation

What if what we are looking for

falls between table values?

We must interpolate.

Definition: to estimate the value of a

function between two known values.


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Interpolation


300M = 3E

DEVIATION TABLE

MAG HDG DEV

300 3E

315 0330 1W

315M = 0

305M = ?


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Interpolation

S

tep 1: Determine intervals between knowntable values

300 3E

315 0

305 ?15 3



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Interpolation

Step 2: Determine interval between base

table values and desired values

5 X


300 3E

315 0

305 ?15 3


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Interpolation

Step 3: Compute the ratio to solve for X.

=X

X = 1


5 X300 3E

315 0

305 ?15 3

15

5

3


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Interpolation

Step 4: Apply resulting interval (X) to base

value.


5 1300 3E

315 0

30515 32E


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Interpolation


Magnetic

Heading

Deviation Magnetic

Heading

Deviation Magnetic

Heading

Deviation

0 4.0 W 120 2.0 W 240 6.0 E

15 4.0 W 135 1.5 W 255 4.5 E

30 3.5 W 150 0.5 W 270 3.0 E

45 3.0 W 165 1.5 E 285 0.5 E

60 2.5 W 180 4.5 E 300 1.0 W

75 2.5 W 195 5.5 E 315 2.5 W

90 2.0 W 210 6.5 E 330 3.5 W

105 2.0 W 225 6.5 E 345 4.0 W


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Gyrocompass

An Electronic

Compass

Points to true North

Works on theprinciple of the

gyroscope.


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Gyroscope

Components

Wheel (or Rotor)

Mounted on a spin axis.

Supported by gimbals:Torque axis

Precession axis


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Gyroscope

Gyroscopic Inertia

When the wheel spins at

high rpm, it will tend to

maintain its orientationin space.


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Gyroscope

Gyroscopic Inertia

Unfortunately, for

our purposes, itdoesnt maintain

its orientation

relative to theEarth.


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Precession

The Gyroscope must be forced to point

to true north.

When a force is applied to a gyroscope,the movement is 90 from the direction of

the applied force.

The force is called torque.

The movement is called precession.


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Precession

The torque is provided by the rotating

Earth.

Since the Earth rotates East-West, the gyro

is precessed 90 to the North-South axis.

Weights are added to the axes to keep the

rotor horizontal to the Earth and to cause

the gyrocompass to seek true north.


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Errors

Latitude Error torque caused by Earths

rotation is greatest at the equator.

Higher the latitude = less precession. Therefore,the gyro tends to fall off of true north.

Gyrocompasses need to be adjusted for latitude

changes. Manual or automatic adjustment,depending on the model.

Accuracy decreases above 75 latitude.


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Errors

Speed Error torque (weights, etc) are applied

based on Earths East-West rotation.

When the ships heading has North-South

components, the compass settles off of true north.

The faster the North-South change, the larger

the error.


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Gyrocompass

One or more north-

seeking gyroscopes.

Housing, electrical

supply and control

elements.


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Gyrocompass

Must spin up and

settle on the

meridian.Takes up to four hours to

settle.

Usually left on unless

in port for long

periods.


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Advantages Over Magnetic

Seeks true meridian. No need to apply

variation.

Can be used near the magnetic poles.

Not affected by ships magnetism.

Deviation corrections are unnecessary.

Output can be sent to other electronic

devices (e.g. automatic pilot, radar, etc.)


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Gyrocompass Repeaters

A remote indication of

the reading of the master

gyrocompass.

Usually on bridge wings

and helm console.


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Disadvantages

Expensive

Maintenance and repairs are more

complicated.

Needs electricity

If operation is interrupted, requires

several hours to settle.


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Gyros Can Fail!

Regularly compare the gyro and

magnetic compasses.

Insert Sea Story here.

the failure of a ships gyro went undetected for

a period of over twelve hours, with the resultthat, at the time of grounding the vessel was

more than 110 off course and more than 200

miles out of position Duttons, pg. 82


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Introduction to Navigation

Questions?

17 the gryo compass

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