Section 9: Gyrocompass Flashcards

1
Q

What are the SOLAS carriage requirements for gyrocompasses?

A

As per SOLAS Chap V Reg 19 and enforced by M.O. 27 Safety of navigation and radio equipment.

The requirement for ships >/= 500 GT.

A gyrocompass, or other means, to determine and display their heading by shipborne non-magnetic means, being clearly readable by the helmsman at the main steering position. These means shall also transmit heading information for input to the X-Band RADAR, AIS and ARPA.

A gyrocompass heading repeater, or other means, to supply heading information visually at the emergency steering position if provided.

A gyrocompass bearing repeater, or other means, to take bearings over an arc of the horizon of 360° using the gyrocompass. However, ships of less than 1,600 gross tonnages shall be fitted with such means as far as possible.

Also, all ships > 50,000 GT are required to have a rate of turn indicator, which is not possible without having a gyro.

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2
Q

Basic principle of gyrocompass.

A

It is a navigational instrument which makes use of a continuously driven gyroscope to accurately seek the direction of true north. It operates by seeking an equilibrium direction under the combined effects of the properties of the gyroscpe and the daily rotation of Earth.

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3
Q

Define gyroscope.

A

A rapidly rotating mass free to move about one or both axes perpendicular to the axis of rotation and to each other. It is characterised by gyroscopic inertia and precession. Usually shortened to a ‘gyro’, the term also refers colloquially to the gyrocompass.

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4
Q

List some gyroscopic properties.

A
  1. Gyroscopic Inertia (Rigidity in Space).
  2. Gyroscopic Precession.
  3. Tilt
  4. Drift.
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5
Q

Define the gyroscopic inertia. (Rigidity in Space)

A

The axis of rotation of the gyro wheel tends to remain in a fixed direction in space if no force is applied to it.

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6
Q

Define gyroscopic precession.

A

The axis of rotation has a tendency to turn at a right angle to the direction of an applied force.

The horizontal component of precession is called drift, and the vertical component is called topple. Also called ‘induced precession’ or ‘real precession’.

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7
Q

Define apparent precession

A

The apparent change in the direction of the axis of rotation of a a gyroscope due to rotation of the earth. As a result of gyroscopic inertia, to an observer on the rotating earth, a gyroscope appears to turn or precess.

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8
Q

Define gyroscopic drift

A

The horizontal rotation of the spin axis of a gyroscope about the vertical axis.

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9
Q

Define true heading

A

The ‘‘true heading’ is the horizontal angle between the vertical plane passing through the true meridian and the vertical plane passing through the ship’s fore and aft datum line. It is measured from true north (000°) clockwise through 360°.

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10
Q

When is the compass considered to be settled? What is settle point heading?

A

The compass is said to be ‘settled’ if any three readings taken at intervals of thirty minutes when the compass is on a level and stationary base are within a band of 0.7°.

The ‘settle point heading’ is the mean value of ten readings taken at twenty-minute intervals after the compass has settled.

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11
Q

What is settle point error?

A

The ‘settle point error’ is the difference between settle point heading and true heading.

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12
Q

Explain why a gyrocompass has less directional force in higher latitudes than it does near the equator.

A

The rotational speed of the earth’s surface varies with increasing latitude from around 900 knots at the equator to zero at the poles – the higher the latitude, the slower the speed of the earth’s surface and the less directional force can be applied to the free gyroscope.

As the directional force reduces, the gyro finds it more and more difficult to settle on the meridian and the spin axis follows an elliptical path, swinging to either side of the meridian. The effects of alterations of vessel course and speed are more noticeable in higher latitudes.

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13
Q

What are the two categories of IMO performance standards for gyrocompass?

A

The two categories of IMO performance standards for gyrocompass are:

a. For Vessels of < 30 knots.
b. For Vessels of Speeds between 30 and 70 knots.

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14
Q

What is the allowed time period for settling of gyro?

A

When switched on in accordance with the manufacturer’s instructions, the compass should settle within six hours in latitudes of up to 60°.

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15
Q

What are the performance requirements regarding reliability and power for a gyro?

A

The equipment should be capable of operating continuously in accordance with the requirements of this recommendation in the presence of such variations of the power supply as are normally expected in a ship.

Means should be incorporated for the protection of the equipment from excessive currents and voltages, transients and accidental reversal of power supply polarity.

If the provision is made for operating the equipment from more than one source of electrical energy, arrangements for rapidly changing from one source of supply to the other should be incorporated.

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16
Q

Make a brief comparison between gyro and magnetic compass for the purpose of navigation?

A

Advantages of gyrocompass:

  • Not influenced by variation or deviation.
  • Points to (almost) true north.
  • Can provide directional information to many other bridge instruments.
  • Errors are generally small.
  • Steadier than magnetic compass.
  • Not affected by the type of cargo onboard.
  • No need to swing the vessel and prepare the dev card.

Disadvantages of gyrocompass:

  • Entirely power-dependent.
  • Requires a higher frequency of periodic maintenance.
17
Q

Describe damping and damping error.

A

Damping an oscillator involves changing its energy state by opposing the velocity of the body. To allow the unit to settle on the meridian and not keep passing through it.

Two principal methods for damping have been used:

  1. Used in all gyrocompasses except for Sperry.
  2. Used in the Sperry gyrocompass by supporting a wire suspension with a power-driven follow-up system, known as a phantom ring.

Damping error is also known as latitude error. It is constant on all headings.

Sin error = tan lat / 40

Modern gyrocompass has latitude input to correct.

18
Q

Describe speed error in gyrocompass.

A
  • Gyrocompass only moves east or west when it is stationary or placed on a vessel moving east or west.
  • The gyro compass settles in the N/S direction by sensing the Earth’s spinning motion and the axis of the gyrocompass settles in a direction perpendicular to the Earth’s surface speed and the ship’s velocity. The direction in which the compass settles is not directly at True North.
  • This causes the compass to tend to settle a bit off True North. This error is westerly if the vessel’s course is northerly and easterly if the course is southerly.
  • Its magnitude depends on the vessel’s speed, course, and latitude. It is maximum in courses of 0° and 180° and null in courses of 90° and 270°.
  • This error can be corrected internally using a special corrector known as a cosine cam mounted on the underside of the azimuth gear, which removes most of the error.
19
Q

What are the minor errors affecting the gyrocompass?

A

Vessel Motions-Rolling errors in heavy weather will cause the gyro to wander about True North. It is reduced by gimbals. Though the gimbals greatly dampen the effect caused by the vessel motions, there is always some hysteresis (lagging effect).

Inconsistent power supply: Inconsistencies of the ship’s power supply cause minor errors in gyro. Modern power supply systems and the use of UPS minimise these fluctuations.

Ballistic Damping: Ballistic damping error is a temporary oscillation introduced by changes in course or speed. During a change in course or speed, the mercury in the ballistic is subjected to centrifugal and acceleration/deceleration forces. This causes a torquing of the spin axis and subsequent error in the compass reading.

Ballistic Deflection: Ballistic deflection error occurs when there is a marked change in the north-south component of the speed. East-west accelerations have no effect. A change of course or speed also results in speed error in the opposite direction, and the two tend to cancel each other if the compass is properly designed.

20
Q

How does the gyrocompass point towards the North?

A

The gyrocompass works based on the gyroscopic effect, which is a spinning object’s tendency to maintain its rotation axis.

A gyrocompass consists of a fast-spinning electric motor, mounted on gimbals to allow free movement in all three axes.

Initialization, to manually align the instrument using a magnetic compass or a GPS system.

Gyroscopic Stability is achieved once the spinning rotor maintains its orientation in space and resists any external forces including the rotation of the Earth.

Precession is a phenomenon when the force of gravity is acting on the tilted axis of the spinning rotor due to the Earth’s rotation.

Earth’s Rotation causes the gyrocompass to precess slowly. The axis of the spinning rotor gradually aligns itself with the Earth’s axis of rotation, which is parallel to the Earth’s geographic north-south axis.

North-seeking happens whent the gyrocompass aligns its axis with the Earth’s axis of rotation.

As a result, it can accurately determine the true north regardless of any local magnetic anomalies or disturbances.

21
Q

Properties of a Free turning Gyroscope.

A
  • Freedom to rotate about the spin axis.
  • Freedom to tilt about the horizontal axis.
  • Freedom to drift about the vertical axis.
22
Q

What are the systems connected to the gyro?

A
  1. Steering console.
  2. ECDIS.
  3. Course recorder.
  4. ARPA.
  5. Electronic Chart Plotters.
  6. SATCOM Antennas.
  7. ROTI.
  8. IBS.
  9. GPS.
  10. DP System.