Navigation

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Navigation is the art and science of determining one's position so as to safely travel to a desired destination. Different techniques have evolved over the ages in different cultures, but all involve locating one's position compared to known locations or patterns.

Contents 1 Modern Navigation 2 Branches 3 History 4 Passage planning 5 Celestial navigation 5.1 Timekeeping requirement 6 Navigation in other cultures 6.1 Austronesian Navigation 6.2 Polynesian navigation 7 "Point system" measure of direction 8 See also 9 Sources 10 External links

[edit] Modern Navigation

Simple navigation begins with pilotage which is knowing your position in familiar territory by orienting oneself to visual landmarks. One can also use a map, nautical chart or aeronautical chart to perform pilotage in unfamiliar locales. Maps are normally used on land where they depict surface features such as roads that might be followed; charts are used on sea and in the air where they depict obstacles that must be avoided. A compass can assist in orienting the map or chart, obtaining bearings to landmarks and for maintaining a steady direction of travel. A sextant can be used to measure the angle of a star above the horizon or the angle between two terrestrial landmarks.

Where landmarks are not visible, other techniques must be used. Dead reckoning allows the navigator to deduce their position by advancing their position on a chart using their known course, speed and time of travel. After compensating for the effect of winds or currents, an estimated position is derived. Celestial navigation is most often used out of sight of land using the Sun, Moon, stars and planets as reference points. In addition to the sextant mentioned above, celestial navigation requires an accurate time source such as a chronometer and a nautical almanac. A calculator or sight reduction tables ease the required spherical geometry calculations. Some facility for plotting the resultant lines of position is also required.

The basic technique of the navigator is to derive a line or circle of position. This can be accomplished by taking the bearing to or distance from a landmark and plotting the resultant line of position or cicle of position on his chart. Where two lines of position intersect establishes a fix. If only one line of position is available, this may be evaluated against the dead reckoning position to establish an estimated position. A third line of position is always sought in order to verify the first two lines of position. The three lines of position rarely meet exactly but normally form a triangle (called a "cocked hat") on a chart. The size of the triangle is affected by many factors, the vessel may be within or occasionally outside the triangle, but as a general rule the larger the triangle, the less certain the position.

Lines (or circles) of position can be derived from a variety of sources: a) celestial observation (actually, a short segment of the circle of equal altitude, but generally represented as a line, b) terrestrial range (natural or man made) when two charted points are observed to be in line with each other, c) compass bearing to a charted object, d) radar range to a charted object, e) on certain coastlines, a depth sounding from echo sounder of hand leadline. There are some older methods seldom used today such as "dipping a light" to calculate the geographic range from observer to lighthouse

Radio aids to navigation provide additional electronic reference points to the navigator, and in some cases, handle the plotting work as well. Satellite systems such as GPS measure the distance to artificial satellites to determine position. Increasingly, Electronic Chart Display and Information Systems take these electronic inputs to provide a moving chart showing the travelers location on an electronic chart display. However, it is prudent for the traveler to always know, practice, and have the materials at hand for manual navigation in the event that electronic aids fail or electrical power is lost.

[edit] Branches

There are several different branches of navigation, including but not limited to:

• celestial navigation - navigation by observation of the sun, moon, stars, and planets
• pilotage - using visible natural and man made features such as sea marks and beacons
• dead reckoning - using course and speed to determine position
• Off-course navigation - allows for variables in heading by deliberately aiming to the one side of the destination.
• electronic navigation - using electronic equipment such as radio navigation and satellite navigation system to follow a course to a waypoint Also Electronic Chart Display and Information System
• position fixing - determining current position by visual and electronic means
• collision avoidance using radar

[edit] History

The earliest form of navigation was "land navigation". Marine navigation began when pre-historic man attempted to guide his craft, perhaps a log, across the water using a form of piloting which uses familiar landmarks as guides. Dead reckoning was probably next, used to navigate when landmarks were out of sight. While celestial bodies were used to steer by, celestial navigation, as known today, was not used until the motion of the sun and stars was understood. The voyage of Pythease of Massalia, between 350BC and 300 BC is one of the best records of an early voyage. Use of a magnetic compass could allow a course to be maintained. The log and a sand glass could be used to determine distance run. This allowed a dead reckoning estimate of the ship's position to be calculated. When approaching land the lead line was used to assist with landfall. Nautical charts were developed to record new navigational and pilotage information for use by other navigators. The development of accurate celestial navigation for taking lines of position based on the measurement of stars and planets with the sextant allowed ships to more accurately determine position. Most sailors have always been able find absolute north from the stars, which currently rotate around Polaris, or by using a dual sundial called a diptych.

When combined with a plumb bob, some diptyches could also determine latitude. Basically, when the diptych's two sundials indicated the same time, the diptych was aligned to the current latitude and true north.

Another early invention was the compass rose, a cross or painted panel of wood oriented with the pole star or diptych. This was placed in front of the helmsman.

Latitude was determined with a "cross staff" an instrument vaguely similar to a carpenter's angle with graduated marks on it. Most sailors could use this instrument to take sun sights, but master navigators knew that sightings of Polaris were far more accurate, because they were not subject to time-keeping errors involved in finding noon.

Time-keeping was by precision hourglasses, filled and tested to ¼ of an hour, turned by the helmsman, or a young boy brought for that purpose.

The most important instrument was a navigators' diary, later called a rutter. These were often crucial trade secrets, because they enabled travel to ports, whose location may have only been known by the pilots..

The above instruments were a powerful technology, and appear to have been the technique used by ancient Cretan bronze-age trading empire. Using these techniques, masters successfully sailed from the eastern Mediterranean to the south coast of the British Isles.

Some time later, around 1300, the magnetic compass was invented in China. This let masters continue sailing a course when the weather limited visibility of the sky.

Around 1400, metallurgy allowed construction of astrolabes graduated in degrees, which replaced the wooden latitude instruments for night use. Diptychs remained in use during the day, until shadowing astrolabes were constructed.

After Isaac Newton published the Principia, navigation was transformed. Starting in 1670, the entire world was measured using essentially modern latitude instruments and the best available clocks.

In 1730 the sextant was invented and navigators rapidly replaced their astrolabes. A sextant uses mirrors to measure the altitude of celestial objects with regard to the horizon. Thus, its "pointer" is as long as the horizon is far away. This eliminates the "cosine" error of an astrolabe's short pointer. Modern sextants measure to 0.2 minutes of arc, an error that translates to a distance of about 0.2 nautical miles (400 m).

At first, the best available "clocks" were the moons of Jupiter, and the calculated transits of selected stars by the moon. These methods were too complex to be used by any but skilled astronomers, but they sufficed to map most of the world. A number of scientific journals during this period were started especially to chronicle geography.

Later, mechanical chronometers enabled navigation at sea and in the air using relatively unskilled procedures.

In the late 19th century radio technology was invented and direction-finding was quickly adapted to navigation. Up until 1960 it was commonplace for ships and aircraft to use radio direction-finding on commercial stations in order to locate islands and cities within the last several miles of error.

Around 1960, LORAN was developed. This used time-of-flight of radio waves from antennas at known locations. It revolutionized navigation by permitting semiautomated equipment to locate geographic positions to less than a half mile (800 m). An analogous system for aircraft, VHF omnidirectional range and DME, was developed around the same time.

At about the same, TRANSIT, the first satellite-based navigation system was developed. It was the first electronic navigation system to provide global coverage.

Other radionavigation systems include:

• Decca
• Omega, a longwave system developed by the United States Navy
• Alpha, a longwave system developed by the Soviet Union

In 1974, the first GPS satellite was launched. The GPS system now permits accurate geographic location with an error of only a few metres, and precision timing to less than a microsecond. GLONASS is a positioning system launched by the Soviet Union. It relies on a slightly different geodesic model of the Earth. Galileo is a competing system, that will be placed into service by the European Union.

Later developments included the placing of lighthouses and buoys close to shore to act as marine signposts identifying ambiguous features, highlighting hazards and pointing to safe channels for ships approaching some part of a coast after a long sea voyage. The invention of the radio lead to radio beacons and radio direction finders providing accurate land-based fixes even hundreds of miles from shore. These were made obsolete by satellite navigation systems.

In the pre-modern history of human migration and discovery of new lands by navigating the oceans, a few peoples have excelled as sea-faring explorers. Prominent examples are the Phoenicians, the Ancient Greeks, the Persians, Arabians, the Norse and the Austronesian peoples including the Malays and especially the Polynesians and the Micronesians of the Pacific Ocean. With the advent of the airplane, the art of aerial navigation, an offshoot of sea navigation, was developed.

[edit] Passage planning

Main article: Passage planning

Passage planning or voyage planning is a procedure to develop a complete description of vessel's voyage from start to finish. The plan includes leaving the dock and harbor area, the enroute portion of a voyage, approaching the destination, and mooring. According to international law, a vessel's captain is legally responsible for passage planning,^[1] however on larger vessels, the task will be delegated to the ship's navigator.^[2]

Studies show that human error is a factor in 80 percent of naviational accidents and that in many cases the human making the error had access to information that could have prevented the accident.^[2] The practice of voyage planning has evolved from penciling lines on nautical charts to a process of risk management.^[2]

Passage planning consists of four stages: appraisal, planning, execution, and monitoring,^[2] which are specified in International Maritime Organization Resolution A.893(21), Guidelines For Voyage Planning,^[3] and these guidelines are reflected in the local laws of IMO signatory countries (for example, Title 33 of the U.S. Code of Federal Regulations), and a number of professional books and publications. There are some fifty elements of a comprehensive passage plan depending on the size and type of vessel, each applicable according to the individual situation.

The appraisal stage deals with the collection of information relevant to the proposed voyage asa well as ascertaining risks and assessing the key features of the voyage. In the next stage, the written plan is created. The third stage is the execution of the finalised voyage plan, taking into account any special circumstances which may arise such as changes in the weather, which may require the plan to be reviewed or altered. The final stage of passage planning consists of monitoring the the vessel's progress in relation to the plan and responding to deviations and unforseen circumstances.

[edit] Celestial navigation

Main article: Celestial navigation

Celestial navigation systems are based on observation of the positions of the Sun, Moon and stars relative to the observer and a known location. In ancient times, the vessel's home port or home capital was used as the known location. With the rise of the British Navy and merchant marine, the Greenwich Meridian or Prime Meridian at Greenwich, England eventually became the starting location for most celestial almanacs.

Early navigators on the northern hemisphere could determine their latitude by measuring the angular altitude of the North Star. The earliest sailors simply used measurements of hand or finger widths to determine latitude; later, cross-staffs and astrolabes were developed to increase the precision of the sighting. Eventually quadrants, octants, and sextants were invented, along with the introduction of printed tables of the positions of the sun, moon, and stars for various times and days of the year. Determining latitude by the sun is more complicated, since one has to measure the sun's altitude at noon (or: the sun's highest point in the sky for a given day) which changes during the year for a given location.

[edit] Timekeeping requirement

In order to accurately measure longitude, one must record the precise time of a sextant sighting (down to the second, if possible). Time is measured with a chronometer, a quartz watch or read from a shortwave radio broadcast by a distant atomic clock.

A quartz wristwatch normally keeps time within a half-second per day. If it is worn constantly, keeping it near body heat, rate of drift can be measured with the radio, and by compensating for this drift, a navigator can keep time to better than a second per month.

Traditionally, three chronometers were kept in gimbals in a dry room near the center of the ship, and used to set a watch for the actual sight, so that the chronometers themselves did not risk exposure to the elements. Winding the chronometers at nearly exact 24 hour intervals, and comparing the rate with a radio time signal daily, was a crucial duty of the navigator. Mechanical chronometers required shop overhaul at regular intervals. In modern practice, quartz movement chronometers have replaced mechanical timepieces.

The second critical component of modern celestial navigation is to measure the angle formed at the observer's eye between the celestial body and the sensible horizon. The sextant, a clever optical instrument, is used to perform this function. The sextant consists of two primary assemblies. The frame is a rigid triangular structure with a pivot at the top and a graduated segment of a circle, referred to as the "arc", at the bottom. The second component is the index arm, which is attached to the pivot at the top of the frame. At the bottom is an endless vernier which clamps into teeth on the bottom of the "arc". The optical system consists of two mirrors and, generally, a low power telescope. One mirror, referred to as the "index mirror" is fixed to the top of the index arm, over the pivot. As the index arm is moved, this mirror rotates, and the graduated scale on the arc indicates the measured angle ("altitude"). The second mirror, referred to as the "horizon glass", is fixed to the front of the frame. One half of the horizon glass is silvered and the other half is clear. Light from the celestial body strikes the index mirror and is reflected to the silvered portion of the horizon glass, then back to the observer's eye through the telescope. The observer manipulates the index arm so the reflected image of the body in the horizon glass is just resting on the visual horizon, seen through the clear side of the horizon glass.

Adjustment of the sextant consists of checking and aligning all the optical elements to eliminate "index correction". Index correction should be checked, using the horizon or more preferably a star, each time the sextant is used. The practice of taking celestial observations from the deck of a rolling ship, often through cloud cover and with a hazy horizon, is by far the most challenging part of celestial navigation. The mechanics of celestial navigation can be mastered in the classroom, but proficiency with a sextant at sea is a matter for expert instruction and extensive practice.

The LORAN system is based on measuring the phase shift of radio waves sent simultaneously from a master and slave station. Signals from these two points establish a hyperbolic curve for possible positions. A third source along with dead-reckoning will generally resolve to a single position.

Today, the Global Positioning System has largely replaced both celestial and LORAN position-finding systems. GPS fixes one's position in 3D trilateration based on the timing signals sent by four or more satellites.

[edit] Navigation in other cultures

[edit] Austronesian Navigation

The Austronesians were some of the early people that crossed vast open seas and settled farflung islands in search of new land to settle. The Austronesian expansion around 2500 BC and onwards is widely considered by some contemporary scholars to be one of the great movements of population in history.^{[citation needed]}

[edit] Polynesian navigation

The Polynesian navigators routinely crossed thousands of miles of open ocean, to tiny inhabited islands, using only their own senses and knowledge, passed by oral tradition, from navigator to apprentice.

In Eastern Polynesia, navigators, in order to locate directions at various times of day and year, memorized extensive facts concerning:

• the motion of specific stars, and where they would rise and set on the horizon of the ocean
• weather
• times of travel
• wildlife species (which congregate at particular positions)
• directions of swells on the ocean, and how the crew would feel their motion
• colors of the sea and sky, especially how clouds would cluster at the locations of some islands
• angles for approaching harbors

These, and outrigger canoe construction methods, were kept as guild secrets. Generally each island maintained a guild of navigators who had very high status, since in times of famine or difficulty, only they could trade for aid or evacuate people. The guild secrets might have been lost, had not one of the last living navigators trained a professional small boat captain so that he could write a book. But only in a Polynesian Outlier, Taumako Island of Solomon Islands, there still be the original traditional method of Polynesian Navigation.

The first settlers of the Hawaiian Islands were said to have used these navigation methods to sail to the Hawaiian Islands from the Marquesas Islands. In 1973, the Polynesian Voyaging Society was established in Hawaii to research Polynesian navigation methods. They built a replica of an ancient double-hulled canoe called the Hokule'a, whose crew, in 1976, successfully navigated the Pacific Ocean from Hawaii to Tahiti using no instruments. But this voyage was navigated by the Micronesian navigator Pius Piailug. Therefore this is not the voyage by Polynesian method.

Later in 1980, a Hawai'ian young guy Nainoa Thompson had invented a whole new method of non instrument navigation (called "modern Hawai'ian wayfinding system") and completed the voyage from Hawai'i to Tahiti and then back.

And in 1987, a Maori young man Matahi Whakataka a.k.a. Greg Brightwell and his mentor Francis Cowan sailed from Tahiti to Aotearoa by non instrument navigation.

Thus there are now three non instrument navigation systems in Polynesia. Taumakoan, Modern Hawai'ian and Modern Maori.

• Wayfinding Summary
• Wayfinding Main Page

[edit] "Point system" measure of direction

A "point" is defined as one eighth of a right angle, and therefore equals exactly 11.25 degrees. The full circle of 360 degrees contains 32 points. For example, a bearing of northwest by north differs by one point from a northwest bearing, and by a point from a north-northwest one. Naming the points of the compass from memory is called "boxing the compass"

[edit] See also

• Air navigation
• Great-circle distance explains how to find that quantity if one knows the two latitudes and longitudes.
• Localization
• Satellite navigation system
• Global Positioning System
• GLONASS
• Galileo positioning system
• Beidou navigation system
• Decca Navigation System
• Loran
• Chronometer
• Sextant
• Nautical chart
• Franz Xaver, Baron Von Zach, a scientific editor and astronomer, first located many places geographically.
• Geodetic system
• Astrogation
• Marshall Islands stick chart

[edit] Sources

• Admiralty Manual of Seamanship ISBN 0-11-772696-6
• American Practical Navigator (Bowditch)
• Dutton's Navigation & Pilotage
• USAF Manual 51-40 Air Navigation

[edit] External links

• Navigation - U.S. Army Manual.
• Celestial Navigation
• Bowditch Online - complete online edition of Nathaniel Bowditch's American Practical Navigator
• Navigational Algorithms
• (Danish) traditional compass navigation
• How to navigate with less than a compass or GPS
• LOCUS research project about mobile navigation using a digital compass and a GPS.