郑嘉颖新浪微博:Navis.gr - Marine Navigation

Marine Navigation PILOTING DEAD RECKONING ELECTRONIC NAVIGATION CELESTIAL NAVIGATION  The word "navigate" comes from theLatin navis, meaning"ship," and agere,meaning "to move or direct." The Latin word naviscomes from the ancient Greek nafs,meaning "ship".

There are four basic methods of navigation at sea:

Piloting,

Dead Reckoning,

Electronic Navigation,and

Celestial Navigation.

In piloting, the navigatordirects a vessel from one place to another by observingsuch landmarks on the Earth'ssurface as lighthouses, beacons,buoys, and prominent rocks andcliffs, and by measurements, called soundings, of waterdepths.

In dead reckoning, the navigator determines a ship'sposition by keeping a careful account, or reckoning, ofthe distance and direction of travel from a knownposition called the point of departure.

In electronic navigation, the navigator determines aship's position with the aid of such devices as radar. These instruments variouslymake use of the directional properties of radio waves, ofdifferences in the times of arrival of radio signals sentsimultaneously from different locations, or occasionallyof the difference in speed between radio waves and soundwaves.

In celestial navigation, the navigator finds a ship'sposition by observing the sun, moon, planets, and stars.

 Seealso: 'ExaminationQuestions for Merchant Mariners'  Course, Heading, andTrackTheterms course, heading, and track are often loosely used.They should, however, be considered to have the meaningsthat follow. The course is the intended direction of theship's travel. The heading is the direction in which theship is pointed at any given time. The track, or coursemade good, is the direction of a straight line between apoint of departure and a present position.
The factors that together result in failure to make goodan intended course are termed drift. The flow of oceanwater, however, is only one of the factors involved.  Direction and DistanceOn the Earth'ssurface each meridian, or line of longitude, is half of agreat circle which passes through the geographical poles of the Earth andlies in a true north-and-south direction (Latitude and Longitude). Thecenter of a great circle on the Earth's surface lies atthe center of the Earth. The shortest distance betweentwo points on the Earth's surface is the shorter arc of agreat circle passing through the points.
The track of a ship that sails along a great circle willcross each meridian at a different angle unless the shipis sailing directly along a meridian or the equator. Theship's direction, then, would usually have to be alteredconstantly in order to maintain a perfect great-circlecourse.
In practice, however, a ship's course is changed atregular intervals of perhaps several hours so that theship follows a series of rhumb lines which approximates agreat circle. A rhumb line is a line on the Earth's surface that crosses allmeridians at the same angle. A ship sailing a steady,true course is usually following a rhumb line, and thedistance it covers is greater than that of a great-circlecourse.  The Instruments ofNavigationOne of the basic tools of the marinenavigator is the nauticalchart. This is a representation, drawn to scale, ofthe water and land areas of aparticular region of the Earth'ssurface.
On the chart the navigator keeps a graphic record of theship's progress. Such a record is kept regardless of themethod or combination of methods of navigation that isbeing used. Lines drawn between successive positionsmarked on the chart indicate at a glance the courses thatthe ship has followed. From scales on the chart thenavigator can measure directly, withoutcomputation, the distance that the ship has traveled.
Traditionally, Mercator chartshave been used at sea. Lambert charts may also be usedfor long sea voyages, though they were designed for airnavigation.
A navigator needs other basic instruments to determine acourse and plot it on a chart. A compassindicates direction. Dividers areuseful in measuring distances on charts. Parallel rulersusually two straightedges connected by pivoted arms areused to transfer lines of direction from one portion of achart to another. A transparent plotter is acombination protractor and straightedge employed in themeasurement of angles and distances and in drawing courselines on a chart.
In directing a course by dead reckoning, a device whichmeasures distance traveled is also essential to the navigator. These distance-measuringdevices include taffrail logs, or patent logs, andengine-revolution counters.  PILOTINGIn piloting, the navigator guides a shiplargely by the bearings of landmarks. A bearing is thehorizontal angle between an object and a reference pointfor example, true north is the reference point for truebearings. Bearings are usually measured clockwise from000° at the reference point through 360° and expressedin three digits, as 028°.
Bearings are used to determine, or fix, a ship's position. Drawn on a chart,a bearing forms a line of position a line on which somepoint must represent the ship's location. Therefore, whentwo or more bearings intersect (cross-bearings), theintersection must represent the ship's position.
Bearings of visible objects may be measured with suchinstruments as the alidade, pelorus, or azimuth circle.These devices usually have sighting vanes and referencecircles graduated in degrees. Bearings referred to a magnetic compass must be correctedfor compass errors deviation and magnetic variation. See also: 'How to Apply the CompassError'
When only one landmark is visible, a ship's position canbe fixed by determining the bearing of the object thenmeasuring the distance to it with a range finder or astadimeter. The range finder is an optical instrument formeasuring the distance to any clearly defined object. Ifthe height of the object is known, the stadimeter may beused. It operates on the principle that the closer anobject is, the bigger it appears to be.

In shallow water, soundings help fix a ship's position.Sonic, or echo, depth finders make use of the known speedof sound in water. Sound transmitted from the ship isreflected from the ocean floor to a receiver, whichmeasures elapsed time and calculates distance. Somedevices produce fathograms continuous profiles, orgraphs, of the ocean bottom. An older depth-findingdevice is the hand lead and line a marked cord with aweight on the end.

See also: 'Standard Marine NavigationalVocabulary'  Navigational Lights andBuoysLight stations and lightships are maintained alongcoastlines to warn approaching ships of potential dangerssuch as off-lying rocks. Most lights operate inon-and-off cycles. The length of time required for alight to complete a full cycle of changes is called theperiod of the light. Lights that are "off"longer than they are "on" are called flashinglights. Occulting lights are "on" as long as,or longer than, they are "off."
Floating navigational aids (other than lightships and weather ships) which are anchored ormoored are called buoys. UnitedStates waters are marked for safe navigation by the lateral system of buoyage. Simplearrangements of colors, shapes, numbers, and lights areemployed to indicate the side of a buoy on which a shipshould pass when moving in a given direction.

 See also:  'Buoyage'  DEAD RECKONINGIn dead reckoning, the navigator estimates a ship'sposition by keeping a careful recordof its movement. The initial point of departure for deadreckoning is usually the last fix the navigator obtainsfrom objects on land at thestart of a voyage. From this point, true courses steeredand distances traveled (as recorded bylog) are plotted on a chart.
Points along the dead-reckoning line, representingsuccessive positions of the ship, are labeled with theappropriate time and the notation "D.R." Deadreckoning commonly begins anew each time bearings,celestial observations, or electronic aids provide anaccurate fix. The dead-reckoning line on his chart isimportant to the navigator because it indicates at aglance the theoretical position of the ship, the trackthe ship should have followed, and the direction in whichthe ship is traveling.  ELECTRONIC NAVIGATIONModern electronic devices are important aids infinding position at sea. For example, the navigator whose ship is equippedwith a radio direction finder can determine the bearingsof radio transmitting stations on shore. Special radio beacons for navigation are established at lighthouses, lightships, andprominent points along coasts. Radio bearings may beplotted on a chart to obtain a fix.

Avariety of other electronic aids to navigation are in useor under development. Loran (long-range navigation) andshoran (short-range navigation) are among the most widelyknown. Radar is also of value,especially for a ship near the shore. Consol, bycontrast, is designed for operation over relatively longranges. A ship at sea can obtain a fix on its positionfrom Consol shore stations with the use of an ordinaryradio receiver.  CELESTIAL NAVIGATIONSeealso: 'Why Do You NeedCelestial In These Days Of GPS?'

For centuries sailors have guided their ships across theoceans by celestial navigation, or nautical astronomy.This is the art of finding position by observing the sun,moon, stars, and planets.
As they journey for some distance, travelers observe thatthe celestial bodies appear to change their paths acrossthe sky and to rise and set at new points along thehorizon. Since the apparent positions of celestial bodiesthus change with time and with changes in an observer'sposition on the nearly spherical Earth,the location of a ship or other craft may be determinedby careful observations of celestial bodies.  The Celestial SphereCelestial bodies,such as the stars, are so far from the Earth that they appear to be locatedon the inside surface of an imaginary hollow sphere. Thissphere, which has an infinite radius, is called thecelestial sphere. Its center coincides with the center ofthe Earth. All points on theEarth's surface are considered to be projected onto thecelestial sphere, as are the equator, the parallels oflatitude, and the meridians.
For the purpose of navigation, a system of coordinates isrequired on the celestial sphere in order that theposition of a celestial body at any time may beaccurately described. One such system is the celestialequator, or equinoctial, system.
In this system the celestial equator, or equinoctial, isthe base, or primary, circle. It corresponds to the Earth's equator. At right angles tothe celestial equator are the hour circles. An hourcircle is a great circle on the celestial sphere thatpasses through the poles and through a celestial body orpoint. Each meridian of the celestial sphere is identicalwith an hour circle.
The declination (dec.) of any point on the celestialsphere is its angular distance north or south from thecelestial equator, measured along the hour circle thatpasses through the point. Declination on the celestialsphere corresponds to latitude on the Earth's surface.
The Greenwich hour angle (GHA) of any point or body isthe angle, measured at the pole of the celestial sphere,between the celestial meridian of Greenwich and the hourcircle of the point. The angle is measured along thecelestial equator westward from the Greenwich celestialmeridian, from 000° through 360° . The GHA differs fromlongitude on the Earth's surfacein that longitude is measured east or west, from 000°through 180° , and remains constant. The GHA of a body,however, increases through each day as the Earth rotates.  The Theory of CelestialNavigationAt any instant of time every celestial body isdirectly above or in the zenith of some point on the Earth's surface. This point lies ona line connecting the body and the center of the Earth.It is called the geographical position, or GP, of thebody. Sometimes the GP of the sun is called the subsolarpoint; that of the moon, the sublunar point; and that ofa star, its substellar point.
A line from the center of the Earth through the GP of anobserver would extend to a point on the celestial sphere.This point is called the zenith of the observer; the lineis his local vertical.
The altitude of a celestial body is the angle, measuredby an observer on Earth, between the body and thehorizon. Were a celestial body say, a star directlyabove, or in the zenith of, an observer, its altitudewould be 90° . The observer would be at the GP of thestar.
Were the observer a distance away from the GP of a star,however, the altitude of the star would be less than 90°by an amount proportional to the distance. On thecelestial sphere, the observer's zenith would be apartfrom the star by a distance called the zenith distance,or ZD.
All points a given ZD from a star would form around thestar a circle of radius equal to the ZD. Were lines fromall points on the circle extended to the center of the Earth, a similar circle would beformed on the Earth's surface. From any point on thiscircle, the observed altitude of the star would be thesame; hence, it is called a circle of equal altitude. Itscenter is the GP of the star. A second circle of equalaltitude would exist around the GP of a second star.Ordinarily, the circles would intersect in two widelyseparated points. One of these points, of course, wouldbe the position of the observer on the surface of theEarth.  Celestial Navigation atSeaTo put this theory into practice, a navigator measures with a sextant the altitudes of two ormore celestial bodies. He carefully notes to the secondthe time at which he made his observations. He obtainsthe time from radio signals or from accurate clockscalled chronometers. These are kept set to Greenwich meantime, or GMT, for this is the time the navigator mustknow as he turns next to the Nautical Almanac.

The Nautical Almanac is a book of astronomicaltables from which may be found, for every second of everyday, the positions on the celestial sphere of the sun,the stars, the moon, and the planets used in navigation.The positions are given in declination and GHA. Fromthem, of course, the latitude and longitude of thebodies' GP's may be found.
Knowing the altitudes of the bodies he observed and theirGP's at the time, the navigator has the informationnecessary to construct the circles of equal altitudewhich define his position. Actually, the navigator doesnot plot on his chart the full circles. From deadreckoning or other means, he knows his approximatelatitude and longitude. All he needs, then, are segmentsof the circles so short that, without practical loss ofaccuracy, they may be drawn as straight lines. Like thelines obtained from bearings in piloting, they are calledlines of position.  The Continental ShelfAround each continent is an area, of varying distancefrom shore, that lies in water of relatively shallowdepth. It is called the continental shelf. In some ofthese areas, submerged river channels can be traced wellout to sea. Mariners and ocean navigators use thosesubmerged channels that have been charted as aids innavigation.
The ocean currents of the world have also been charted.The ocean floor is being charted in greater detail and atgreater depths than ever before. This is being donepartly to meet the requirements of antisubmarinedefenses. Electronic depth-finding instruments haveproduced much data for charts of the ocean floor. Inrecent years major advances have been made in underwaterarchaeology, in oceanography, and in meteorology.

See also: 'Terms Used In MarineMeteorology'