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Navigation

"Navigation" in the engineering sense has two meanings:
  • travel by sea and
  • determination of position and direction on the surface of the Earth.

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

Traditional maritime navigation uses multiple redundant sources of position information to locate the ship's position. A navigator starts with dead reckoning based on the ship's logged course and speed. Using this estimated position, the navigator will select several other objects at known locations and measure their bearing. The lines of position can be plotted on a map, with the point where they cross being the ship's current location. Addition lines of position can be measured in order to validate the results taken against other objects. This is known as a fix.

Early navigation required visual fixes with land, forcing all ships to stay close to shore. The development of accurate systems for taking lines of position based on the measurement of stars and planets with the sextant allowed ships to navigate the open ocean. Later developments included the addition of lighthouses and buoys close to shore to add more accurate information when approaching land after a long sea voyage. Eventually the addition of radio beacons and radio direction finders allowed for accurate land-based fixes even hundreds of miles from shore.

Traditional navigation systems were based on observation of the relative position of the Sun, Moon and stars. Navigators could determine their latitude by measuring the sun's angle over the southern horizon at noon, and comparing that to the known angle at the same date at their home port. Conceptually they could determine their longitude by measuring the angle over the eastern or western horizon at noon, but to do so would require a much more accurate determination of "noon" – the sun moves north and south only a degree or so per hour near noon, but contines to move to the west at 15 degrees per hour, making it considerably more difficult to determine when it reaches "the top". The determination of longitude thus became a technological issue, requiring the development of an accurate shipborne chronometer that could tell them exactly when noon was. The need for accurate navigation led to the development of progressively more accurate clocks.

In modern celestial navigation, a navigational almanac and trigonometric sight reduction tables permit navigators to measure the Sun, Moon, visible planets or any of 57 navigational stars at any time of day or night. From a single sight, a time within a second and an estimated position, a position can be determined within a third of a mile. The math required for navigation is simple addition and subtraction, if sight reduction tables are available. The numerous celestial objects permit navigators to shoot through holes in clouds. Most navigation is performed with the sun and moon.

Time is measured with a chronometer, a quartz watch or a short wave radio broadcast from an atomic clock.

A quartz wristwatch normally keeps time within a half-second per day. If it is worn constantly, keeping it near body heat, its 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 are 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 no chronometers are ever risked to the elements. Winding the chronometers was a crucial duty of the navigator.

The angle is measured with a special optical instrument called a "sextant." Sextants use two mirrors to cancel the relative motion of the sextant. During a sight, the user's view of the star and horizon remains steady as the boat rocks. An arm moves a split image of the star relative to the split image of the horizon, When the image of the star touches the horizon, the angle can be read from the sextant's scale. Some sextants create an artificial horizon by reflecting a bubble. Inexpensive plastic sextants are available, though they have less accuracy than the more expensive metal models.

Automated navigation systems are almost all based on measuring the time-of-flight of radio waves using the well-known speed of light to measure distance from a number of points. This is possible because of the widespread availability of clocks with high precision and stability.

History

There are two great traditions of navigation, the Western tradition, and the Polynesian tradition.

Polynesian Navigation

The polynesian navigators routinely crossed thousands of miles of open ocean, to tiny inhabited islands, without using any instruments besides themselves.

In Eastern Polynesia, navigators memorized extensive facts about stars and weather to locate directions according to the times of day and year. They also memorized facts about times of travel, wildlife (which congregate at particular positions), directions of swells and colors of the sea. They also memorized stars and angles to approach harbors. These, and canoe construction methods were all kept as guild secrets. These secrets were almost lost, except that one of the last living navigators trained a professional small boat captain so that he could write a book.

Generally each island maintained a guild of navigators, and the navigators were nearly worshipped, because in times of famine or difficulty, only the navigators could trade for aid or evacuate people.

Western Navigation

In the West, navigation was at first performed exclusively by dead-reckoning, the process of estimating one's present position based on the navigators' experience with wind, tide and currents.

Most sailors have always been able find absolute north from the stars, which rotate around Polaris, or by using a dual sundial called a diptych.

When combined with a plumb bob, some diptychs 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 1/4 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 lucrative ports.

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 successfuly sailed from the eastern mediterranean to the south coast of the British Isles.

Some time later, around 300 A.D., the magnetic compass[?] was invented in china. This let masters continue sailing a course when the weather limited visibility of the sky.

Around 400 A.D., 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 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.

At first, the best avilable 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 Nikolai Tesla invented radio 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. An analogous system for aircraft, VOR[?] 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:

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.

See also:


For fundamentals about navigation (not only the engineering part) see Navigation research.



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