Navigation

 

The vast expanse of the ocean can be a lonely and daunting place. It becomes much safer and familiar when you know where you are. In the past there were many explorers who voyaged out with their only references being the stars and the sea. Yet they managed to cross many oceans and find many unexplored islands and continents. The modern day explorers often still return to the stars for navigational aids. The North Star and the Southern Cross are often used as a reference. In times gone by sailors would also look to the water and be able to determine the location of land over the horizon simply by the way the water moved. This practice is still used by explores today in order to be able to locate shoals that they approach by observing wave patterns. Two other aspects to navigation that are fundamental to today’s voyaging are piloting and electronic navigation. Navigation allows you to determine the position of a ship, and chart a course for the vessel to move safely from one point to another. The practice of navigation requires not only a thorough knowledge of the subject, but also considerable experience and judgment.

In the Beginning, Celestial Navigation

For celestial navigation the navigator uses a sextant and a chronometer. Both the English mathematician John Hadley and the American inventor Thomas Godfrey, invented the sextant independently.1 It enables a navigator to measure the angular elevation of the Sun and other celestial bodies ie. stars, and from this information the navigator's latitude and longitude (location on the Earth’s surface) can be determined.

The sextant is a double-reflecting instrument that measures the angle between two objects by bringing into coincidence rays of light received directly from one object and by reflection from the other. Its principal use is to determine the altitude of celestial bodies above the horizon. The chronometer is a accurate timepiece with a nearly constant rate of daily gain or loss. It is set to the time of a standard meridian and it makes possible the fortitude of longitude at sea. Its daily rate of gain or loss is checked by radio time signals broadcast from various countries.1

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Various sextants

The sextant's operation depends upon superimposition of the images of the two objects whose distance is being measured. This is achieved by means of an optical system consisting of a telescope and two mirrors, one fixed and one movable.

The actual observed horizon can be used for measuring altitudes. On land this method of observation is seldom possible because of the irregularity of terrain. In this case a so-called artificial horizon is employed, consisting of a pool of mercury or some other horizontal reflecting surface. For example making an observation of a star itself and the image of the star in the mercury, a sextant reading can be obtained that is equal to double the actual altitude of the star.

The Origins of Charts

The surface of the Earth is represented, as a plane surface upon which lines of latitude and longitude are superimposed as well as the desired features of surface and underwater topography. Charts are maps that draw attention to the strength of position, direction, and distance, and stress points of interest to a navigator. The basic problems of navigating any craft involve the determination of its position, direction and the measurement of speed, distance, and time. Position on the Earth's surface can be defined in terms of an accepted set of coordinates, such as latitude and longitude. The direction of one place relative to another is measured in degrees, from the direction of true north. Speed is often expressed in knots, or nautical miles per hour (1 knot = 1.853 km/hr or 1.15 mph).2 The initial planning and the end results of navigation are plotted on maps and charts.

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Tools of the Navigator

The most widely used form of navigation charts are Mercator projections, named after the 16th-century Flemish mathematician and geographer Gerardus Mercator, who devised it. These charts display the Earth's surface projected on a cylinder tangential to the surface of the Earth at the equator. When this cylinder is flattened out, the lines of longitude appear as equally spaced vertical lines and the latitude appear as parallel horizontal lines.1 The majority of coastal charts are Mercator projections with the longitude and latitude crossing at right angles. Most of the navigable waters of the world have been surveyed accurately by the hydrographic services of maritime nations so that reliable charts of the waters are usually available to the navigator. The equipment used by the navigator resembles to an extent the tools used in technical drawing. Compasses for drawing circles, dividers for measuring distances, plotters and protractors, are all basic tools commonly found on the chart table of a ship.

Navigation instruments are designed to measure direction and distance, determine speed, measure the depth of water, and observe the elements of the weather. Sometimes various instruments are used simultaneously to yield the required information.

Navigational Tools

The magnetic compass is a directional device and works by aligning itself in the direction of the Earth's magnetic poles. It is one of the oldest yet most useful instruments onboard a ship. The compass retains its original role as the basic navigational instrument because it is not subject to electromechanical faults, and for this reason, on most ships it is an essential backup instrument.

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Lou under the dash with the gyro compass.

Because of the location of the magnetic poles, the needle of a compass will point to the geographical North Pole only in a few places. Elsewhere it will point east or west of north. The difference in degrees between the direction of the compass needle and that of true north is called variation. To make things easier for navigators, the variation in many parts of the world has been measured, and charts have been prepared that show by isogonic lines (curves connecting points of equal variation, the fairly accurate east or west variation for a given place). On such charts, the line of zero variation, along which the compass points true north, is called an agonic line. In the liquid compass, which is the most stable type of mariners’ compass, the bowl is filled with a liquid, usually a mixture of alcohol and water. The liquid helps to support the graduated card, which, in this type of compass, pivots about its centre and floats in the liquid, thereby reducing pivot friction and lessening the vibrations of the card caused by the motion of the vessel. Because of these advantages the liquid compass is used more often than the dry compass.

There are also gyrocompasses, which use a gyroscope as its directive element, indicating true north. The gyroscope in this compass is a rapidly rotating body, free to move about one or two axes, perpendicular to the axis of rotation and to each other. Control elements are added to the gyroscope to convert it to a true direction marker.1 To look at they are a big black box nothing near as aesthetically pleasing as a traditional compass.

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STARSHIP's sonar display indicating a depth of 414 metres.

To determine water depth a navigator uses either a lead shot or an echo sounder. The lead, which consists essentially of a lead weight at the end of a suitably marked line, is used in coastal or shallow waters under conditions of low visibility. The echo sounder, which is found on almost all seagoing vessels, indicates the depth of water by measuring the time period between the release of a sonic or ultrasonic signal and the return of its echo from the bottom. A navigator can then cross reference with a chart.

What is piloting?

Piloting is the most exacting form of navigation because it entails the movement of ships under many potentially dangerous conditions. The greatest care is necessary for success in piloting, especially in poorly charted waters or under inauspicious weather and visibility conditions. One of the principal concerns of the navigator is to avoid collision with other ships.

A basic concept in piloting is known as the line of position, a line indicating a series of possible positions of a craft and determined usually by observation. The line may be straight or curved, and is produced by plotting a series of soundings taken over a period. One line of position is not sufficient to determine the exact position of a ship. The point of intersection of two or more lines of position, taken simultaneously, can give an accurate position.

A line of position may be derived from several methods. These are just a few: a range within which two known fixed objects appear in line, and the ship is placed somewhere on this line; a compass bearing of an object observed visually or by radar; a horizontal angle, measured by a sextant, between two known objects; and the most common device used today GPS’s (electronic systems). Navigational aids come in various forms be they buoys, beacons and light vessels; their distinguishing shapes and colours provide at least partial daytime identification, and colours of lights provide identification at night.


Global Positioning System picture

Global Positioning System (GPS) is a space-based navigation system, with 24 satellites, that provides accurate, three-dimensional position, velocity, and time, 24 hours a day, everywhere in the world, and in all weather conditions. Because the user does not communicate to the satellite, GPS serves an unlimited number of users at any given time.

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STARSHIP's TRANSAS GPS system with our location superimposed 
on an electronic chart of the area.

The system is maintained by the United States Department of Defense, the Navstar GPS began in 1973 to reduce the proliferation of navigational aids. By creating a system that overcame the limitations of many existing navigation systems, GPS became attractive to a broad spectrum of users commercial and private. Since the earliest satellites, it has successfully proven itself in navigation applications. The greatest thing is it’s capabilities are obtainable in small, inexpensive equipment.

GPS satellites carry atomic clocks that measure time to a high degree of accuracy. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the range to the satellite. The receiver must account for propagation delays caused by the ionosphere and the troposphere. With three ranges to three satellites and knowing the location of the satellite when the signal was sent, the receiver can compute its three-dimensional position.

To compute ranges directly, however, the user must have an atomic clock synchronized to the global positioning system. By taking a measurement from an additional satellite, the receiver avoids the need for an atomic clock. The result is that the receiver uses four satellites to compute latitude, longitude, altitude, and time.

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The dash of STARSHIP may seem daunting at first but everything 
has a purpose. The screens from left to right; alarm monitoring system, 
TRANSAS GPS system and the sonar.

On STARSHIP we use a variety of different navigational aids. The most predominant being the GPS and a computer chart system. We also have the aid of two radars and sonar. Most if not all navigation is referred back to paper charts related to our areas of travel.

Although there have been major changes in navigation over the past century the fundamentals have remained the same. Navigation by practice is simply getting the vessel from one point to the next be it by stars or satellites. With all these different technologies we can only expect and hope for safe passage at sea.

Louise Oliver

NOTES :
[1]Microsoft Encarta Encyclopedia Deluxe 2000
[2]Maloney. E.S. (1996) Chapman Piloting seamanship & Small Boat Handling 62ND Edition, Motorboating and Sailing, Hearst Marine Books, New York.

REFERENCES :
Rousmaniere. J. (1989) The Annapolis Book of Seamanship. Simon and Schuster, New York.
Maloney. E.S. (1996) Chapman Piloting seamanship & Small Boat Handling 62ND Edition, Motorboating and Sailing, Hearst Marine Books, New York.
Queensland Transport (1998) Small Ships Manual, 13th Edition, Published by Queensland Dept Transport, Brisbane
Microsoft Encarta Encyclopedia Deluxe 2000