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GRAIN RULES

Any bulk cargo having angle of repose less than 36* known as grain. After completion of loading it has to be secured before commencement of voyage. If it is not effectively secured grain become very dangerous become it liable to shift transversely as v/l rolls. Grain does not act like a liquid due to friction so simple reduction of GM is not sufficient. If the v/l rolls heavily to a large angle grain will shift to one side but with the return roll it may not all shift back?

PRINCIPLES:   The IMO grain rules is based on the fact that the void spaces in filled compartments are bound to occur. This happens because of the difficulty in trimming of cargo and also because of the cargo settling during the voyage. Therefore during calculation an allowance is made for grain shift. So the resulting “TOTAL GRAIN HEELING” is used to determine the reduction in righting levers. The loss of righting arm is called  “HEELING ARM”. The basis of the rules is that after taking into account the grain shift the v/l have sufficient residual stability she will be allowed to load grain.

INTACT STABILITY REQUIREMENT:

Ø  The angle of heel due to grain shift shall not exceed 12 or Q de whichever least.

Ø  The net or residual area between the heeling arm curve and the righting arm curve upto the angle of maximum difference between tow curves, or 40 or the angle of flooding (Of) whichever is least shall not less than 0.075 meter-radius.

Ø  The initial GM, after correction for free surface effect, shall not less than 0.30m.

POINTS TO REMEMBER

Ø  Heeling arm take care of the transverse shift of grain.

Ø  Vertical component allowed for either by the following, (a) If KG of cargo is taking into account then multiply grain heeling moment by 1.06 for full compartment and by 1.12 for partially filled compartment.

Ø  When calculating grain-heeling moments, assume that the grain will shift through 15 in full compartment and 25 in partially full compartments.

Ø  All full compartments should be trimmed, if they are not trimmed, a grain shift of 30 is assumed

IMPROVING CONDITION

 After loading if vessel fails to confirm with the requirement of grain rules. The situation can be handled by either improving vessel’s stability or reducing grain shift.

STABILITY MEASURES:  

Ø  Reducing free surface effect by pressing up employing tanks. This results in increase in fluid GM.

Ø  Increase the solid GM by lowering weights or by adding weight low down (e.g. filling a double bottom tank).

CARGO MEASURES.

The shift can be reduced in full compartment by:

Ø  Fitting of temporary longitudinal subdivision (shifting boards).

Ø  Use of bugged cargo in a saucer.

Ø  Bundling in bulk.

  The shift can be eliminated in partially filled compartment by building a dunnage platform on top level of grain and then:

Ø  Over stowing with other cargo.

Ø  Over stowing with bagged cargo.

Ø  Stropping and lashing using steel strops and bottle screw.

DOCUMENTS OF AUTHORISATION:

This document is issued to any ship intending to carry grain by ship’s national administration. It is the evidence that the ship is capable of carrying grain as per grain regulations. This document should be kept onboard along with ship’s “GRAIN LOADING STABILITY BOOKLET” as guidance for Master to load grain.        

GRAIN LOADING STABILITY BOOKLET:

Grain loading stability booklet includes the following information.

Details of required stability criteria as given by IMO.

General arrangement plan and stability for the vessel.

Curve or table of grain heeling moment for every compartment, filled or partially filled. Effect of temporary filling such as shifting boards.

Tables of maximum permissible heeling moments.

Details of shifting board, saucer and bundling in bulk and overstowing arrangements.

Typical loaded departure and arrival calculation.

Worked example for grain stowing at 1.25, 1.53 and 1.81m/t.

Instruction for maintaining adequate stability throughout the voyage.

Other information supplied under ship’s particular.

            WT HEELING MOMENT=  VOL. HEELING MOMENT

                                                                  STOWAGE FACTOR

            APPROX. ANGLE OF HEEL =  TOTAL HEELING MOMENT          X 12

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Describe the various effects on a ship behavior, which can be expected as a result of entering shallow water.

When there is limited UKC the restriction in the velocity of the water flow which causes a drop in pressure. This reduces the buoyancy force of the v/l. since the weight of the ship unchanged the v/l will tend to sink further thereby increasing draught in order to resolve equilibrium. There is also likely to be a change in trim because the LCB is likely to change thereby creating a trimming moment.

EFFECTS:  

1.      Vessels take longer to answer helm.

2.      Response to engine movements becomes sluggish.

3.      Vibrations will be set up.

4.      Extremely difficult to correct a sheer.

5.      When a ship is nearing an extreme shallow depth of water such as shoal. She is likely to take a sudden sheer, first towards it and then away.

6.      The bow waves and astern waves of ship increase in height.

7.      The trough which normally exist the quarter become deeper and the after of the ship drawn downwards towards the bottom.

8.      Increase of time due to squat.

9.      The increase in the propeller speed, increase efficiency of the rudder but will not increase the ship’s speed.

10. Transverse thrust of the propeller will change.

11. Minimum RPM to maintain steerage is more than normal.

12. Color of water changes.

(a)  Explain the term synchronous rolling and describe the dangers if any associated with it.

This occurs when the natural period of roll is equal to the apparent period of wave. When this occurs the wave gives the ship a push each time she rolls (like a swing) causing her to roll more and more heavily. This effect is known as synchronous rolling.

DANGERS:

1.      Possible danger of capsizes.

2.      Cargo shifting due to heavy rolling.

3.      Possible cargo damage and structural damage, personnel injury.

4.      Dangers of free surface effect.

5.      Possible machinery / Nav. Aids damage.

6.      Ship is more vulnerable if engine break down occurs.

(C) Describe the action which may be taken by the ship’s officer when it becomes apparent that the vessel is experiencing synchronous rolling.

1.      Alter course this will alter the apparent period of the waves, an alteration of course towards the is likely to be particularly effective, as it reduces the apparent period of the wave.

2.      Alter speed (effective if the area not abeam).

3.      Change GM or distribution of weights aboard the vessel by ballasting/deballasting / shifting weights.

a)     What is meant by squat and explain how does it occur.

SQUAT:

This is a term used to define changes in draught and trim which occurs when the depth of water beneath the vessel is less than one and a half time the draught of the vessel when travelling at a significant speed.

CAUSES:

                  When there is a limited clearance under the keel the restriction increases the velocity of water flow which causes a drop in pressure thereby reducing the buoyancy force on the vessel. This effect is increased still further when vessel is in the confined channel since the velocity of water flow must increase due to further restriction.

                Since the weight of the vessel remains unchanged the ship will have to sink further thereby increasing her draught in order to restore equilibrium. There is likely to is a change in trim since the LCB likely to change therefore creating a trimming moment. Where LCF is greater than LCB there will be a trimming moment at astern, where LCF is less than LCB there will be a trimming moment by the head and where LCF = LCB there will be no trimming effect and maximum squat will be of equal value at fwd and aft.

b)     List the factors, which effect the magnitude of squat.

1.      Speed of the ship.

2.      Draught / water ratio.

3.      Propeller revolution.

4.      Form of bow waves.

5.      Length / breadth ratio.

6.      Block co-efficient.

7.      Change width / beam ratio.

8.      Initial trim.

c)     Describe the overall effect of shallow water on the maneuvering characteristics of a vessel.

1.      Speed of the vessel decreases as squat is directly proportional to square of speed.

2.      R.P.M. decreases and high R.P.M. increases astern trim.

3.      Higher the draught to depth of water ratio greater the squat which results in lesser U.K.C.

4.      Vibration may occur.

5.      In shallow water squat causes abnormal bow and stem wave to build up there by the type of bow effects wave making and pressure distribution.

6.      Steering is effected because the water displaced by the hull is not so easily replaced by other water and the propeller and rudder might be working in partially vacuum conditions. The vessel takes long to answer her helm and response to engine movement become sluggish.

7.      It will be extremely difficult to correct a yaw or sheer with any degree of rapidity.

8.      The moving vessels bow wave, stem wave and trough increase in amplitude.

SIGNS OF SQUAT  

1.      Speed decreases.

2.      RPM decreases.

3.      Vibration may occur.

4.      Steering is affected vessel become sluggish to maneuver.

5.      Ship made waves increase in amplitude.

6.      Ship wake changes color and becomes muddy.

a)     Itemizes the contents of an approved ship’s stability book.

1.      General particulars (e.g., ship’s name, port of registry, GT, NRT, LOA. Breadth, DWT, Draft to summer load lines.

2.      General arrangement plan.

3.      Capacities and C.O.G. (cargo spaces, fuel, F.W, Ballast tanks, stores etc.)

4.      Estimated weight and disposition of passengers and crew. 

5.      Estimated weight and disposition of dk cargo (including 15% allowance for timber dk.cargo)

6.      Dead weight scale (displacement, DWT, TCP, MCTC)

7.      Hydrostatic particulars (Displacement, TPC,  MCTC, LCB, LCF, KM)

8.      Free surface information (including an example)

9.      KN tables cross curves (including an example)

10. Pre-worked ship conditions (light ship. Ballast. Arr / Dep, service loaded Arr. / Dep. Homogenous loaded Arr./Dep. Dry Docking etc.). To include for each condition profile diagram indicating disposition of weights, statements of light weights plus disposition pf weight onboard, Metacentric height (GM curve) statical stability (GZ curves). Warning of usage conditions.

11. Special procedures (cautionary notes)

12. Inclining experiment report.

13. Information for longitudinal stresses (For v/ls over 150 m in length).

14. Loading / Discharging / Ballasting sequence for long vessels.

15. Worked KG example of “icing”.

16. Maximum Draught Forward and Aft.

17. Wind heeling moment for high deck cargoes.

18. Maximum height of deck cargoes.

19. Damage stability conditions.

A.    Flooding and damage stability requirements for type A and type B ships.

B.     Flooding and damage stability requirements in the flooded conditions.

C.     Flooding and damage stability information to be presented from flooding conditions.

D.    Flooding and damage stability typical sketches required.

b)     Give example of special cautionary notes for the Master, which may be included in this book.

1.      Required minimum bow height always maintained the Forward draught should not exceed.

2.      Sequence of Ballasting to enable adequate stability throughout the voyage.

3.      Warning against large angle of heel, produced by strong beam wind.

4.      Dangers of icing if the vessel is trading in severe winter conditions.

5.      Incase of Timber deck cargo absorption of water should be considered up to 15% of its own weight.

6.      Special precautions when loading bulk grain.

7.      Recommended minimum draught for heavy weather conditions.

8.      In case of vehicle ferry, the KG of the compartment for carriage of vehicles shall be based on the estimated center of gravity of vehicle and not the volumetric KG of the compartment.

9.      Information’s to enable free surface effect.

10. Any special features regarding the stowage or behavior of cargoes.

If the calculated Metacentric height during Dry Docking is found to be in adequate. Explain clearly the practical measures that can be taken to remedy this, prior to Dry Docking.

1.      Reduces the trim to the minimum so that the critical period reduces significantly.

2.      When the vessel takes the blocks, the “G” will rise due to the “P” force, which acts vertically upwards, from keel blocks.

3.      Therefore, calculate the maximum trim taking into account the virtual loss of GM not more than 0.2 m, so that the vessel can have the adequate GM when she is sitting on the blocks.

4.      Any free surface in the tanks should be removed or reduced to as little as possible either by emptying the tanks or pressing it up to the full conditions.

5.      Sound all the tanks before entering the Dock, to be aware of quantities aboard and note all the soundings in the sounding book.

6.      Empty the wing tanks if possible. Stow derricks, cranes and riggings in stowed position re-arrange the deck cargo, or cargo in between deck if any, to L.H, Ballast the D.B. tks. (press up).

Q. NO. 6  DEC’90

A.     State the surveys required in order that an international load line certificate remains valid.

1. Annual survey.        2. Renewal survey every 5 years.

B.     List the items and state the nature of the exam. Required for each item at these surveys.

Preparation should be commenced three months before the expected date of the surveys.

1.      Check all access openings at ends of enclosed structure are in good condition, all daubs, clamps, and hinges should be free and well greased.  

2.      Check all cargo hatches and access to holds for water tightness, especially battening device such as cleats and wedges.

3.      Securing of portable beams.

4.      Tarpaulins must be in good condition and two for each hold.

5.      Check all machinery space openings on exposed decks.

6.      Check all ventilator openings are provided with water tight closing.

7.      All air pipes must be provided with permanently attached satisfactory means for closing and openings.

8.      Check all manholes and flush scuttles are water tight.

9.      Inspect cargo ports below free board deck for water tightness.

10. Non-return valves on over board discharge are operating satisfactorily.

11. Side scuttles must have internal water tightness.

12. All freeing ports to be in good working condition.

13. All guard rails and bulwarks in satisfactory condition.

14. Rigged lifelines required to be filled in certain areas.

15. De-rust and paint the deck line, load line marks and draft marks.   

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Sextant and it's Errors
	Make sure you use the shades on the sextant when looking at the SUN When taking a bearing in an Oral Exam, make sure you push the shades away so you can see the object (but not the SUN)
Remember the initials (PSI) the same initials thats on Sub-aqua diving bottles (P) Perpendicular Error - (1st Error) (S) Side Error - (2nd Error) (I) Index Error - (3rd error) Now remember the letters that go with it (PI) Perpendicular error - is when the (Index) glass is not perpendicular to the plane of the instrument (SH) Side Error - is when the (Horizon) mirror is not perpendicular to the plane of the instrument (I - H - I) Index Error - is when the (Horizon) mirror and the (Index) glass are not parallel to each other (Q) What does (Perpendicular) mean? (a) 090degrees If asked to find if there is an error in the sextant then do the following (1st Error) Set the Index bar between 30 and 40 degrees, hold the sextant horizontally with the arc furthest away from you, look into the index glass with the sextant tilted to a small angle, check if the true and reflected arcs are in line If they are not in line then adjust the screw on the index glass (The index glass has only one screw) (2nd Error) Set the index bar at ZERO, hold the sextant nearly horizontal, look into the eye piece to see if the true and reflected arcs are in line If they are not in line then adjust the screw nearest the plane of the instrument on the horizon mirror (There are two screws on the horizon mirror, use the one nearest the base of the sextant) (3rd Error) Set the index bar to ZERO, hold the setant vertically, look into the eye piece to see if the true and reflected arcs are in line If they are not in line then adjust it with the screw furthest away from the base of the sextant on the horizon mirror or adjust the vernier wheel If it's ON the ARC then subtract it from your final bearing If it's OFF the ARC then add it to your final bearing most captains will expect you to read the vernier wheel to see what the index error is To find the distance your off a land object (Usually a lighthouse) you would find the height of it on the chart, find the angle from the base to the top of the lighthouse allowing for any errors, and go into "NORRIES TABLES" and find out the distance in the chapter "Distance by vertical angles" A sextant is used to find vertical and horizontal angles, it was first used to find the ships position by aligning up the sun with the horizon and by knowing the time of day you could find the Latitude on a chart To find the longtitude they needed an accurate time-keeping clock, the chronometer was the answer for this as it kept real accurate time, now they can do it using nautical tables and trigonometric sight reduction tables to find the longtitude by using the Sun, moon or any one of 57 visible stars

MAGNETIC COMPASS

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Magnetic Compass

Learning Objectives

Basic Theory of the Magnetic Compass

Understand the use and care of ship's compass.

Describe the construction of a liquid card magnetic compass.

State the composition of the liquid used.

Explain how allowance is made for changes in volume of the liquid in

a liquid card compass.

Describe the marking of the lubber line and it's purpose.

Describe the Binnacle of the compass.

Explain why regular comparisons between the Standard compass,

Steering compass and the Gyrocompass is necessary.

Demonstrate taking bearings of celestial bodies and landmarks.

List the care and precautions for a Magnetic compass.

Magnetism of the Earth and the Ship’s Deviation.

Theory of Magnetism.

The theory sates that all magnetic substances consists of magnetic molecules each being a minute magnet. When a substance is unmagnetised these minute magnets are not arranged in any particular direction. In fact it can be proved that they prefer this arrangement rather than to be lined up in a particular direction.

Once these minute magnets are aligned, the mutual attraction of their poles tend to hold them in position after the removal of the external force used to align them. This alignment can be destroyed by physical vibration such as hammering or by heating. The ease with which it can be destroyed depends on whether the substance is magnetically hard or magnetically soft. In the case of ferrous material the terms hard iron and soft iron refer to this particular property.

In pure soft iron the molecules are entirely free and unless under some external magnetic field the iron will be unmagnetised. This is not the case of hard iron. The molecules are not free to move nor are they easily moved. But once lined up in a particular direction they tend to remain in that direction indefinitely. In this case the iron is said to be magnetised permanently (permanent magnets). Permanent magnets apart from the effects of vibration or heating are not truly permanent, but tend gradually to lose their magnetism during the course of time. Magnets made of magnetically hard substances are normally referred to as permanent magnets.

Ferromagnetic materials are those in which each molecule has a substantial magnetic moment. The molecular fields interact and the crystalline structure of the materials is such that groups of molecules become aligned over regions, which are called domains. If a bar of such material is subjected to an impressed/inducing field, the domains tend to realign themselves with the field.

The degree of alignment depends upon the structure of the material and the strength of the inducing field. When the maximum alignment has occurred the material is said to be magnetically saturated and further increases in the inducing field will evoke no further contribution from the molecular fields.

Ferromagnetism is a strong effect and permeabilities are much greater than 1. Above a certain temperature known as Curie point, thermal agitation of the molecules is sufficient to prevent the formation of domains and ferromagnetic materials at normal temperatures may be made to exhibit ferromagnetic properties if cooled sufficiently.

Soft Iron: A bar of ferromagnetic material placed in a magnetic field becomes induced with magnetism. If the material is easily magnetised, but loses most of its magnetism when removed from the inducing field, it is said to be magnetically soft. Such materials usually, but not necessarily, have high permeabilities and are mechanically soft.

A line drawn through the magnet in the direction of its internal field (joining it poles) is known as the magnetic axis of the magnet. A line at right angles to the magnetic axis midway between the poles is called the neutral axis of the magnet. Magnetic poles exert a force upon one another. Like poles repel and unlike poles attract one another. The force between two poles is dependent upon their distance. The strength of the north and south poles of a magnet are equal.

Magnetic Field of the Earth

The earth as a magnet is obvious from the fact that a freely suspended magnet will come to rest in a direction approximately north and south. In other words the magnet will settle in a direction of the earths field at the place at which the magnet is being used.

It would appear that the earths magnetic field is similar to that of a bar magnet. As a first approximation this is substantially correct. The general magnetic field of the earth is similar to that which could be expected at the surface of a short but strongly magnetised bar magnet were located at the centre.

The above partly explains the fact that the magnetic poles are relatively large areas, due to the spreading out of the lines of force from the magnet. It also gives the reason for the direction of the field being horizontal in the vicinity of the equator. It is most probable that there is such a magnet at the centre of the earth. In actual fact many scientists are investigating the cause of the field. No theory put forward up to the present time has found acceptance.

As far a we are concerned the idea of a magnet at the centre of the earth is useful as it helps us to visualise the general form of the magnetic field, as it is known to be despite the many imperfections. The area termed the North Magnetic Pole is situated approximately 71⁰N, 96⁰W. The South Magnetic Pole is situated in 73⁰S, 156⁰E. These positions are very approximate, but one fact emerges namely, that the South Pole is not diametrically apposite to the North Pole.

Theoretically the maximum strength of the earth’s magnetic field should be at the poles. Actually the field strength in certain other areas in both high north and south latitudes is found to exceed that at the magnetic poles. These are called magnetic foci. In order to determine the direction and force of the earth’s magnetism at any place we require three of four magnetic elements. The four elements are variation, dip, horizontal force and vertical force.

Magnetic Pole

Is the region of a magnetic that exhibits magnetic properties from which the greater part of the magnetic flux emerges or at which it enters. In the case of a bar magnet the longer the bar in comparison with its thickness the more nearly do the poles approach the ends of the magnet.

Magnetic Equator

A line joining all positions on the earth’s surface where the direction of the magnetic field is horizontal is called the magnetic equator.

Angle of Dip

The vertical angle contained between the horizontal and the direction of the earth’s magnetic field at any given place is called the Angle of Dip. Dip is conventionally considered positive when the north end of a freely suspended magnetised needle dips below the horizontal, and negative when the south end dips below the horizontal.  Thus all angles of dip north of the equator will be positive and all angles of dip south of the magnetic equator will be negative.

REGULATIONS (Concerning Safety of Navigation)

In Singapore context, the Merchant Shipping (Safety Convention) Regulations apply to all Singapore flagged vessels engaged in international voyages.

The following regulation refers to navigational aids.

Regulation 12 is quite lengthy and Ws entire interpretation is beyond the scope of this module, however it's salient features concerning Magnetic Compass is being reproduced below.

Ships of 500 GRT and upward need to be equipped with .........

** A Standard Magnetic Compass with a reflector for the use of the helmsman. If without the reflector, than another compass for steering.

COMPASSES (An Introduction)

A Compass is an instrument designed to seek a certain direction (preferably North, in shipboard applications) and to hold that direction permanently.

Magnetic compasses depend for it's directional properties on the magnetism of the earth. Their role in present day navigation is substantially reduced, but because of the compass independence from power failures, it continues to remain an essential element in the ship's overall navigational equipment. In fact, it is legally required to be carried, (Remember Reg. 12 - Shipborne Navigational Equipment) and it's error checked and logged.

Magnetic compasses suffer from:

Magnetic Variation

Magnetic Deviation

But with careful maintenance, service, correction and care, the instrument can be a good back up during emergencies.

The re-entry of Transmitting Magnetic Compass (TMC) and Flux Gate Magnetic Compass is likely to re-kindle interest in the Magnetic Compasses. Understanding the functioning of a TMC and a flux-gate compass is beyond the scope of this module but another type of compass namely the Gyrocompass would be discussed in the next module.

THE MAGNETIC COMPASS

A Magnetic compass is usually fitted on the upper bridge, (also known as the monkey island), more or less on the centre line of the ship. This is referred to as a Standard Compass because it is a primary means of indicating direction on a ship.

There are two (2) basic types:

(a) The dry card Compass

(b) The wet card Compass

The basic compass (whether dry or wet) consists of a card with cardinal graduations, suspended inside a bowl. The suspension should provide a frictionless support.

The directive element in these types of compasses consists of needle magnets attached to the card. Modern compasses use ring magnet as a directive element.

The compass card is enclosed in a cylindrical brass bowl having a transparent top glass. The top glass is retained in position by a brass "verge ring", which is secured to the brass bowl by brass screws along the circumference. A rubber washer between the verge ring and the top glass ensures water-tightness.

The dry card compass is too sensitive for steering purposes, especially in bad weather and even small disturbances causes the dry card to oscillate. This type is not very popular and hence we will not discuss the same in this module.

In the wet card compass, the oscillations are damped, without loss of accuracy, by immersing the card in liquid. The card therefore does not oscillate but has “a dead beat" movement.

THE LIQUID (in the compass bowl)

The bowl is filled with a mixture of distilled water and pure ethyl alcohol thereby making the mixture to have the following properties

(a)                Low freezing point about -30˚C

(b)               Small coefficient of expansion

(c)        Does not discolour the card

(c)                Low relative density about 0.93

The top of the bowl is of transparent glass. The bottom is of frosted glass to diffuse the light coming from an electric bulb below.

ALLOWANCE FOR EXPANSION

Increase and decrease of atmospheric temperature expands and contracts the liquid inside the bowl. Different methods are adopted to cope with this problem. Two types of arrangements are as follows:

(1)        Fitting of corrugated chambers

(2)        Fitting of nut-and-screw expansion chambers

In this module, we shall try and understand the first arrangement i.e. using a small accordion-like expansion chamber attached to tile howl.

This arrangement is similar to the attachment of corrugated bellows in an aneroid barometer. The chamber increases or decreases in volume whenever the liquid inside the bowl expands or contracts due to variations in atmospheric temperature.

THE LUBBER LINE

Forward, inside part of tile bowl, there is usually a small projection with a line marked on it, This line is called the "lubber line", and it represents the direction of the ship's head.

The compass is fixed on the centre line of tile ship, with the lubber line aligned towards forward.

The reading of the compass card, which is in line with the lubber line, is the compass course of the ship at that time.

THE BINNACLE (of a magnetic compass)

The binnacle is a cylindrical container made of teak wood. No magnetic material is used in the construction. The compass bowl is slung inside the top portion of the binnacle. The middle portion is accessible by a door and contains -in electric bulb. Light from this bulb passes upwards through a slot. Key issues bottom of the compass bowl to illuminates the compass card from below.

A mechanical shutter can control the intensity of the light. The number of magnets in the bucket, the bucket's position with reference to the compass card and the number of hard iron magnets depends on the disturbing forces. A qualified “compass adjustor” can calculate this force after conducting certain tests.

Once tile compass has been adjusted, the magnets should not be disturbed and the doors giving access to tile corrector magnets should be kept locked.

Quadrennial Correctors, these are two "Soft iron" spheres, which are fitted in brackets, one on either side of the binnacle. The brackets have a sliding way or slots so that the distance between the spheres can be altered as desired during compass adjustment.

Flinders bar, this is a soft iron corrector, diameter about 7.5 cms, inserted in a 60 cm long brass case, fitted vertically either on the forward or aft part of the binnacle. The position forward or aft depends on where the superstructure is more.

The Helmet, the top of the binnacle is provided with a large brass helmet. This protects the compass bowl from direct sunlight, rain, spray, dew, frost etc. during non-use.

COMPARING COMPASSES (why is it necessary)

The needles of a magnetic compass do not point to the north pole but to a point about 1600 kilometres away (called the magnetic north pole). Mariners using a compass of this type have to make an allowance in their steering, and this allowance differs not only from place to place but also from year to year (because the magnetic poles slowly alter their positions). This is variation. Further, the magnetic compass is affected by the magnetism of the ship itself and this error is the deviation.

The solution to the above problem is through the use of a marine Gyro Compass, which is basically a Point-magnetic compass capable of being made to point true north and using the directional property of a free gyroscope.

Marine gyro Compasses are more complicated than the magnetic compass, reliable and accurate. The directional signals from the gyro Compass can be inputted into an automatic steering system thus allowing the ship to be steered in the required direction without continuous human effort.

In spite of all the above advantages, the basic disadvantage of the gyro systems is the requirement of electric power and that too a 3-phase power supply which is not possible from a back-up battery. This disadvantage creates a need for the carrying of a magnetic compass by all ships as a reliable safety measure in case of an electrical failure.

Since the power failure could be sudden and unexpected, it becomes necessary to compare the magnetic compass and gyro Compass and check the error and the deviation. This is done at least once during the navigational watch as well as after every course change. In case of gyro or power failure, the ship can continue to be navigated since the errors are known.

In case the ship is installed with a Transmitting Magnetic Compass (TMC), the auto steering can be re-connected to the magnetic compass via the transmitting system and the ship can continue on her voyage on the autopilot.

PRECAUTIONS, CARE AND MAINTENANCE

(1)        The doors giving access to the corrector magnets should always be kept locked.

(2)        The wooden parts of the binnacle should be varnished and not painted, as paint may cause the doors to jam.

(3)        The soft iron spheres and their brackets should be painted. This prevents rust.

(4)        All magnetic materials like aerials, electrical wires and equipment etc. should be well away from the compass.

(5)        The binnacle light should be switched off during daytime.

(6)        The helmet should always protect the azimuth mirror and the compass card from sprays, direct sunlight, rain etc. except when bearings are being actually taken.

G P S (GLOBAL POSITIONING SYSTEM)

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Intoduction

GPS is a satellite navigation system designed to provide real time information anywhere, any time and in any weather of receiver’s:

1.      Position

2.      Velocity

3.      Time

GPS comprises  of three segments:

Control Segment

• The Control Segment consists of a system of  one master control station at  Colorado Springs (USA) and 4 tracking and monitoring stations located around the world. (Ascension Island, Diego Garcia, Kwajalein & Hawai)
• The monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites. The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite and sent to the Master Control Station. 
• The Master Control station estimates the satellites orbit and clock performance as well assesses the health of satellites and determines if any repositioning may be required.
• The Master control station then uploads orbital parameters and clock data to the SV’s through 3 uploading stations co located at Ascension Islands, Diego Garcia and Kawajalien.
• The SV’s then send subsets of the orbital ephemeris data to GPS receivers over radio signals.
• Due to the worldwide spread of the control stations, all GPS satellites are tracked 92% of the time.

Space Segment

• The Space Segment of the system consists of the GPS satellites. These space vehicles (SV’s) send radio signals from space.
• The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 sidereal hours. Minimum of 21 operating 98% of the time.
• Orbiting in near circular orbits at a height of 20,200 Kms. above Earth’s surface.
•  6 orbital planes. 4 satellites in each orbit planes.
• Each orbital plane inclined by 55* to the equatorial plane and separated in R.A. by 60*.
• Each satellite visible for about 5hrs. above horizon.
• Constellation of satellites is configured so that any receiver located anywhere on globe will have minimum of four satellites in view having elevation of more than 9.5*.

  User Segment

  • The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. GPS receivers are used for navigation, positioning, time dissemination, and other research.
  • To decipher the GPS signals, the receiver must perform the following tasks:
  1.      Locking to satellites in view
  2.      Measure time interval
  3.      Measuring and tracking of satellites
  4.      Recovering navigational data
  5.      Compute to give receiver position, velocity and GPS time.
  
  
  What is the Principle of  GPS? How is position fix obtained in GPS?
  
  
  ·         The principle used to obtain the position of the ship is by ranging.
        Range = C x T 
        C is speed of electromagnetic waves = 300,000,000 m/s
  T is time taken between transmission and reception.
  ·         The positions of the satellites in space are accurately known (from satellite messages, computed by control station through 4 monitoring station).
  ·         By knowing the positions of the satellites and measuring the time taken for a message to reach the receiver, the receiver gets a range, and thus a position spheroid.
  ·         Intersection of  the position spheroid and earth’s surface is a small circle.
  ·         Simultaneous application of the second range measurement from second satellite gives a second small circle on Earth’s surface.
  ·         The two circles intersect at 2 point but as the two intersection points are significantly away from each other there will be no ambiguity.
  ·         Since the Clocks of the satellite and user clocks are different clocks with differing accuracy. The T time difference between time of transmission (Satellite clock) and reception (user clock) as measured by the receiver gives psuedo ranges.
  ·          3 psuedo ranges from 3 different satellites are measured to obtain fix on earth’s surface to resolve/ calculate user clock error and to get true range and hence the position of the receiver.
  ·         Let the coordinates of the known position of the three satellites be (x’,y’,z’), (x”,y”,z”), (x’’’,y’’’, z’’’) and of the receiver which to be found out be (x,y,z).
   
  Time taken for the satellites message to reach user as measured by the user clock be T’,T’’, T’’’ 
  
  
  Unknown Error of the user clock = Dt
  
  
  Then we have three equations
  
  
  PR’ = C x T’= C x ( t’± Dt) = sqr root  √ (x’- x) ² + (y’-y) ² + (z’-z) ²
  
  
  PR” = C x T”= C x ( t”± Dt) =   √ (x”- x) ² + (y”-y) ² + (z”-z) ²
  
  
  PR”’ = C x T”’= C x ( t”’± Dt) =   √ (x”’- x) ² + (y”’-y) ² + (z”’-z) ²
  
  
  For an observer on Earth’s surface  Radius of earth  r =   √x²  +  y²  +  z ²                          z can be found  in  terms of x and y.
  
  
  As we have three simultaneous equations and three unknowns x, y, and Dt which can be computed and observer/receiver’s position can be computed.
  
  
  For an observer in space 4 psuedo ranges from 4  satellites will be required as then unknowns will be 4( x, y, z, Dt).
  
  
  What do you understand by pseudo range and true range? How the error due to user clock bias obviated (removed).
  
  
  Since the Clocks of the satellite and user clocks are different clocks with differing accuracy. T the time difference between time of transmission (Satellite clock) and reception (user clock) as measured by the receiver gives psuedo range PR =  C x T Pseudo range is false range because it includes range error due to user clock bias.
  
  
  The time delay as measured by the receiver gives psuedo ranges.
  
  
  The user clock bias (error) is computed by the processor of the receiver.
  Since all pseudo ranges to different satellites have the same clock offset during one measurement epoch.
  The receiver measures 4 psuedo ranges from 4 different satellites at the same instant. And we get 4 simultaneous equations and 4 unknowns ( x, y, z, Dt) which are computed to get user clock error.
  
  
    Let the coordinates of the known position of the three satellites be (x’,y’,z’), (x”,y”,z”),  
    (x’’’,y’’’, z’’’), (x””, y””,z””) and of the receiver which to be found out be (x,y,z).
   
  Time taken for the satellites message to reach user as measured by the user clock be T’,T’’, T’’’, T””
  Unknown Error of the user clock = Dt
  
  
  we have four  equations
  PR’ = C x T’= C x ( t’± Dt) = sqr root  √ (x’- x) ² + (y’-y) ² + (z’-z) ²
  
  
  PR” = C x T”= C x ( t”± Dt) =   √ (x”- x) ² + (y”-y) ² + (z”-z) ²
  
  
  PR”’ = C x T”’= C x ( t”’± Dt) =   √ (x”’- x) ² + (y”’-y) ² + (z”’-z) ²
  
  
  
  
  PR”” = C x T””= C x ( t””± Dt) =   √ (x””- x) ² + (y””-y) ² + (z””-z) ²
  
  
  True Range(TR) is the actual range of the satellite from the receiver without any user clock error and ionospheric Dt’ and tropospheric Dt” delay errors.
  
  
  TR = PR(as measured) ± C x  Dt  - C x  Dt’(ionosphere)  - C x Dt” (troposphere)
  
  
  What Factors effect the accuracy of  a GPS position?
  
  
  1. Satellite vehicle clock error : Error of the satellite vehicle clocks is computed by the control stations and sent in the navigation data to be applied by the receiver.
  
  
  
  1.  Satellite Vehicle  clock Error : The atomic clocks used on the satellites  are very,    very precise but they're not perfect. Minute discrepancies can occur, and these translate into travel time measurement errors.
  The satellite vehicle clock error is monitored by monitoring stations and is calculated & transmitted by the control station to the receiver in navigation message.
  Error can be obviated by using DGPS.
  2. Error in predicted ephemeris of satellite vehicles: Though the satellites positions are constantly monitored through monitoring stations but  they can't be watched every second. So slight position or " ephemeris" errors can sneak in between monitoring times.
  Error can be obviated by using DGPS.
  3. Error due to Ionosphere : The ionosphere extends from a height of 70 to 1000 km above the Earth, as the signal propagates through the ionosphere, the carrier experiences a phase advance and the codes experience a group delay.
  In other words, the GPS code information is delayed resulting in the pseudoranges being measured too long as compared to the geometric distance to the satellite.
  The extent to which the measurements are delayed depends on the electron density along the signal path.
  The electron density is dependent on three further factors: the geomagnetic latitude of the receiver, the time of day and the elevation of the satellite. Significantly larger delays occur for signals emitted from low elevation satellites (since they travel through a greater section of the ionosphere), peaking during the daytime and subsiding during the night (due to solar radiation). In regions near the geomagnetic equator or near the poles, the delays are also larger .
  
  The ionospheric delay is frequency dependent and can therefore be eliminated using dual frequency GPS observations, hence the two carrier frequencies in the GPS design. Two frequency are used for transmission of P code.
  
  
  The ionospheric delay can be obviated by using DGPS.
  
  
  1.   Error due to Troposphere: 
  2.      The troposphere which from the ground level to 70 km, the troposphere causes a delay in the transmitted signals. Error due to troposphere is not frequency dependent (within the GPS L band range) it cannot be canceled out by using
  dual frequency measurements.
  This error is maximum when angle of incidence of transmitted waves is high, to avoid large errors in the positions ,the satellite vehicles having angle of elevation of less than 9.5* are not used in computation of user position.
  The ionospheric delay can be obviated by using DGPS.
  

  5. Multipath: Mutipath is the phenomena by which the GPS signal is reflected by some object or surface (mountains, buidings near the coast, funnel near the antenna) before being detected by the antenna. Mutipath is more commonly considered to be the reflections due to surfaces surrounding the antenna and can cause range errors as high as 15 cm for the L1 carrier and of the order of 15-20 m for the pseudoranges. The surface most prone to multipath is water, whilst sandy soil is the least .

  This error can not be obviated by using DGPS.

  This error can be minimized by using special antennas.

  6.Receiver Noise: Errors which are due to the measurement processes used within the receiver are grouped together as receiver noise. These are dependent on the design of the antenna, the method used for the analogue to digital conversion, the correlation processes, and the tracking loops and bandwidths .
  Better the receiver lesser the errors.
  7. Geometric dilution of Precision : Basic geometry itself can magnify these other errors with a principle called "Geometric Dilution of Precision" or GDOP.
  There are usually more satellites available than a receiver needs to fix a position, so the receiver picks a few and ignores the rest.
  If it picks satellites that are close together in the sky the intersecting circles that define a position will cross at very shallow angles. That increases the gray area or error margin around a position.
  If it picks satellites that are widely separated the circles intersect at almost right angles and that minimizes the error region.
  Good receivers determine which satellites will give the lowest GDOP.
  8. Error due to Selective Availability: GPS is funded and controlled by the U.S department of defense. Therefore they can intentionally degrade the gps signals  and deny gps signals on regional basis. (selective availability discontinued since may 1, 2000)
  9. Error due to perturbing forces on the GPS satellite.
  Kepler's laws are for an idealised satellite orbit where the only attracting force is a spherical gravity field. For any satellite orbiting the Earth this is not the case and its Keplerian position will be affected by the following perturbing forces:
  Earth not being true sphere and mass not equally distributed therefore gravitational forces different at equator and at pole.
  
  Ø  Gravitational forces of sun, moon and other planets.
  Ø  Solar radiation
  Ø  Atmospheric drag.
   10. User Clock Bias: Explained above.
  BLUNDERS
  
  1.      Control segment mistakes due to computer or human error can cause errors from one metre to hundreds of kms.
  
  
  2.   Datum Shift:Incorrect geodetic datum selection can cause errors from 1 to hundreds of meters.
  3.     
• ·         Receiver errors from software or hardware failures can cause blunder errors of any size.
  ·         Interference of other radio signals with GPS signals.
  ·         User mistakes like plotting a previous frozen position or not reading if the gps is giving DR or fix.
  
  
  
  
  What are the various componenets of GPS signals and what is there significance?
  
  
  Components of GPS signals are:
  
  
  ·         Carrier Signal (L1 & L2)
  ·         Code (P or C/A)
  ·         Navigation Data Message
  
  
  Carrier Signal (L1 & L2)
  GPS satellites transmit two L-Band signals that can be used for carrying the code and navigation data message from GPS satellites to the receiver.
  The signals, which are generated from a standard frequency of 10.23 MHz, are L1 at 1575.42 MHz (10.23 * 154) and L2 at 1227.60 MHz(10.23 * 120) and are called the carriers. The frequencies are generated from the fundamental satellite clock frequency of fo =10.23 MHz.
  The reason for using high frequency is because higher the frequency lower is the interference or attenuation thru the ionosphere.
  The reason for using two different frequencies is so that errors introduced by ionospheric refraction can be eliminated.
  
  

  Signal	Frequency (MHz)	Wavelength (cm)
  L1	154fo = 1575.42	~19
  L2	120fo =1227.60	~24

  
  
  
  
  Code (P or C/A)
  
  
  Since the carriers are pure sinusoids, they cannot be used easily for instantaneous recognition/identification of GPS satellites.
  Therefore two binary codes are modulated onto them: the C/A (coarse acquisition) code and P (precise) code.
  Also it is necessary to know the coordinates of the satellites and this information is sent within the broadcast data message which is also modulated onto the carriers.
  For purposes of imposing the binary data onto the carriers, all of the codes are transferred from the 0 and 1 states to the -1 and 1 factors respectively.
  The broadcast data message is then modulo-2 added to both the C/A code and the P code. This inverts the code and has the effect of also inverting the autocorrelation function.
  Modulo-2 Addition
  
  Binary biphase modulation (also known as binary phase shift keying [BPSK]) is the technique that is used to modulate the codes onto the initial carrier waves.
  Binary Biphase Modulation
  
  The codes are now directly multiplied with the carrier, which results in a 180 degree phase shift of the carrier every time the state of the code changes.
  The modulation techniques also have the properties of widening the transmitted signal over a much wider frequency band than the minimum bandwidth required to transmit the information which is being sent (Pratt, 1992). This is known as spread spectrum modulation and has the benefits of developing processing gain in the de-spreading operation within the receiver, and it helps prevent possible signal jamming.
  The L1 signal is modulated by both the C/A code and the P code, in such a way that the two codes do not interfere with each other. This is done by modulating one code in phase and the other in quadrature (ie they are at 90 degrees to each other).
  L1 Signal Structure
  
  The C/A code is also amplified so that it is between 3 and 6 dB stronger than the P code (Spilker, 1980).
  For L2, it is stated that the signal is modulated by P code or the C/A code (Spilker, 1980) although normal operation has seen the P code being used. It should be noted that the precision obtained from P code measurements is thought not to be in the interests of US national security and therefore will be restricted for civilian users. This is the same for both the L1 and L2 frequencies.

  ---

  C/A Code
  The C/A code is a pseudo random (PN) binary code (states of 0 and 1) consisting of 1,023 elements, or chips, that repeats itself every millisecond.
  The term pseudo random is used since the code is apparently random although it has been generated by means of a known process, hence the repeatability.
  Due to the chipping rate (the rate at which each chip is modulated onto the carrier) of 1.023Mbps, the chip length corresponds to approximately 300m in length and due to the code length, the ambiguity is approximately 300km - ie the complete C/A code pattern repeats itself every 300km between the receiver and the satellite.
  CA Code represenation

  The C/A code can be thought of as a number of rulers extending from the satellite to the receiver. The length of each ruler is approximately 300km, and each graduation is 300m apart.

  The code is generated by means of a linear feedback register which is a hardware device representing a mathematical PN algorithm.
  The sequences that are used are known as Gold codes which have particularly good autocorrelation and cross correlation properties. The cross correlation properties of the Gold codes are such that the correlation function between two different sequences is low - this is how GPS receivers distinguish between signals transmitted from different satellites.

  ---

  P Code
  The P code, or precise code, is a long binary code that would repeat only every 38 weeks (Pratt, 1992).
  Despite the code being shortened to a one week repeatability because each satellite transmits a different weekly section of the code, there is still no ambiguity between the satellite and receiver.
  P code representation

  The P code can be thought of as a ruler extending from the satellite to the receiver. The length of the ruler is approximately one week multiplied by the speed of light, and each graduation is 30m apart.

  Rapid access to the relevant part of the code for a particular satellite is carried out by means of a hand-over-word obtained from the broadcast data message.
  The chipping rate is at 10.23 MHz resulting in a chip length of approximately 30 m.

  ---

  Navigation / Broadcast Data Message

  The data message includes information describing the positions of the satellites, their health status, and the hand-over-word.
  Each satellite sends a full description of its own orbit and clock data (within the ephemeris information) and an approximate guide to the orbits of the other satellites (contained within the almanac information).

  The data is modulated at a much slower rate of 50 bps and thus it takes 12.5 minutes to transmit all of the information. To reduce the time it takes to obtain an initial position, the ephemeris and clock data is repeated every 30 seconds (Langley, 1990).

  Parameters representing the delay caused by signal propagation through the ionosphere are also included within the data message.
  
  
  
  
  
  
  Differential Global Positioning System (DGPS)
  DGPS works by placing a high-performance GPS receiver (reference station) at a known location. Since the receiver knows its exact location, it can determine the errors in the satellite signals. It does this by measuring the ranges to each satellite using the signals received and comparing these measured ranges to the actual ranges calculated from its known position. The difference between the measured and calculated range is the total error. The error data for each tracked satellite is formatted into a correction message and transmitted to GPS users. The correction message format follows the standard established by the Radio Technical Commission for Maritime Services, Special Committee 104 (RTCM-SC104) These differential corrections are then applied to the GPS calculations, thus removing most of the satellite signal error and improving accuracy. The level of accuracy obtained is a function of the GPS receiver. Sophisticated receivers like the Starlink DNAV-212 and INVICTA 210 series can achieve accuracy on the order of 1 meter or less.
  
  REFERENCE STATION:
  
  • The reference station GPS receiver knows exactly the position of its antenna, therefore it knows what each satellite range measurement should be. It measures the ranges to each satellite using the received signals just as if it was going to calculate position. The measured ranges are subtracted from the known ranges and the result is range error. The range error values for each satellite are formatted into messages in the RTCM SC104 format and transmitted continuously.
  MODULATOR:
  • Depending on the transmission format, the modulator encodes the data as necessary for transmission. For example, in the free USCG system, the modulator creates a carrier signal which varies using MSK modulation. A "one" data bit is represented by a advancing carrier phase and a "zero" bit by a retarding carrier phase. The modulated carrier output from the modulator is connected to the transmitter.
  TRANSMITTER:
  • The transmitter is basically a power amplifier which is connected to an antenna system. The modulated carrier is amplified and driven to the antenna. In the USCG system, the transmitter is 250-1000 Watts and operates in the 300KHz frequency range. The amplified signal is radiated via the antenna to remote DGPS receivers for real-time position correction.
  DGPS CORRECTION RECEIVER:
  • A DGPS correction receiver decodes the signals received from a reference site. Data is formatted into a data stream and provided to the remote GPS receiver. There are many types of DGPS correction receivers.
  GPS RECEIVER:
  • The GPS receiver measures ranges to each satellite, but before the measurements are used to calculate position, corrections received from the DGPS receiver are applied to the measurements. The position is then calculated using the corrected range measurements providing vastly increased accuracy.
  NOTES:
  • GPS with SA has an accuracy of about 100 meters. GPS with corrections (DGPS) has an accuracy of between 1 and 5 meters depending on the quality (price) of the GPS receiver.

  Accuracies that are guaranteed to the SPS user are better than, or equal to ... (DoD/DoT, 1995b)

  100 m in horizontal position,
  95% of the time
  
  
  156 m in the vertical component
  95% of the time
  
  
  300 m in horizontal position
  99.99% of the time
  
  
  500 m in the vertical component
  99.99% of the time
  
  
  340 nanoseconds timing accuracy
  
  
  95% of the time