Navigation

From Academic Kids


There are several traditions of navigation.

Contents

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.

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.

Western navigation

Modern methods

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

Knowing the ship's current position is the main problem for all navigators. Early navigators used pilotage, relying on local knowledge of land marks and coastal features, forcing all ships to stay close to shore. The magnetic compass allowing a course to be maintained and estimates of the ship's location to be calculated. Nautical charts were developed to record new navigational and pilotage information for use by other navigators. 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 without needing to see land marks.

Later developments included the placing of lighthouses and buoys close to shore to act a 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.

Traditional maritime navigation with a compass uses multiple redundant sources of position information to locate the ship's position. A navigator uses the ship's last known position and dead reckoning, based on the ship's logged compass course and speed, to calculate the current position. If the set and drift, due to tide and wind, can be determined, an estimated position can also be calculated.

Periodically, the navigator needs confirm the accuracy of the dead reckoning or estimated position calculations using position fixing techniques. This is done by correctly identifying reference points and measuring their bearings from the ship. These lines of position can be plotted on a nautical chart, with the intersection being the ship's current location. Addition lines of position can be measured in order to validate the results taken against other reference points. This is known as a fix.

Celestial navigation systems are based on observation of the positions of the Sun, Moon and stars relative to the observer and a known location. Anciently the home port was used as the known location, currently the Greenwich Meridian or Prime Meridian is used as the known location for celestial charts.

Navigators could determine their latitude by measuring the angular altitude of Polaris any time that it was visible. Determining latitude by the sun was a little more difficult since the sun's altitude at noon during the year changes for a given location.

Calculating the anticipated altitude of the sun for a given day and known position is done easily using Calculus. But prior to its invention by Newton around 1700, tables of the sun's altitude during the year for a known port were used. The sun's angle over the horizon at noon was measured, and compared to the known angle at the same date as the known port. Local noon is easily determined by recording periodic readings of the altitude of the sun. Since periodic readings of the altitude will plot a sine wave, the maximum reading is the one used for local noon.

Longitude is calculated as a time difference between the same celestial event at different locations. Noon was an easy event to observe. Local noon is determined while shooting the azimuth as described above. The time of the maximum altitude is easily determined by interpolating between periodic readings. The time of noon at the known location is carried by the navigator on an accurate clock. Then the local time of local noon is observed by the navigator. The difference of longitude is determined knowing that the sun moves to the west at 15 degrees per hour.

The need for accurate navigation led to the development of progressively more accurate clocks. Once accurate clocks were available, detailed tables for celestial bodies were created so that navigational activities could take place anytime during the day or night, rather than at noon.

In modern celestial navigation, a nautical 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 (500 m).

Conceptually, the angle to the celestial object establishes a ring of possible positions on the surface of the Earth. A second sighting on a different object establishes an intersecting ring. Usually the navigator knows his position well enough to pick which of the two intersections is the current position. The math required for sight reduction 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.

Accurately knowing the time of an observation is important. 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.

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 point establish a hyperbolic curve for possible positions. A third source along with dead-reckoning will generally resolve to a single position.

GPS uses 3D trilateration based on measuring the time-of-flight of radio waves using the well-known speed of light to measure distance from at least three satelites. This can be accomplished using low cost quarts clocks because the satellites send time correction signals to the GPS receivers.

History

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 currently 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.

Compass with rose in center
Enlarge
Compass with rose in center

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 successfully sailed from the eastern Mediterranean to the south coast of the British Isles.

Some time later, around 300, the magnetic compass was invented in China. This let masters continue sailing a course when the weather limited visibility of the sky.

Missing image
Astrolab.JPG
Astrolabe

Around 400, 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 Nikola 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 (800 m). 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.

"Point" measure of direction

A "point" is defined as one eighth of a right angle, and therefore equals exactly 11.25 degrees. 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.

See also

External links

  • Navigation (http://www.wildernessmanuals.com/manual_6/chpt_2/index.html) - U.S Army Manual.
  • Bowditch Online (http://www.irbs.com/bowditch) - complete online edition of Nathaniel Bowditch's American Practical Navigator
Navigation

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