Navigation



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.

History
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.

In the West, before the invention of the magnetic compass, the primary means of navigation was to stay in sight of land. Travel away from land was possible for short distances, using the sun and stars. This early celestial navigation was problemmatic and often resulted in the navigator becoming lost.

In the China Sea and Indian Ocean, a navigator could take advantage of the fairly constant monsoon winds to judge direction. This made long one-way voyages possible twice a year.

The voyage of Pytheas of Massalia, between 350BC and 300 BC is one of the best records of an early voyage.

The Middle Ages
In China between 1040 and 1117, the magnetic compass was being developed and applied to navigation. This let masters continue sailing a course when the weather limited visibility of the sky. The true mariner's compass using a pivoting needle in a dry box was invented in Europe no later than 1300.

Nautical charts began to appear in Italy at the end of the 13th century. However, their use did not seem to spread quickly: there are no reports of the use of a nautical chart on an English vessel until 1489.

The Age of Discovery
The commercial activities of Portugal in the early 15th century marked an epoch of distinct progress in practical navigation. These trade expositions sent out by Henry the Navigator led to the discovery of the Azores in 1419, the Cape Verde Islands in 1447 and Sierra Leone in 1460. Henry worked to systemize the practice of navigation. In order to develop more accurate tables on the sun's declination, he established an observatory at Sagres.

Henry's successor, John II continued this research, forming a committee on navigation. This group computed tables of the sun's declination and improved the astrolabe, believing it a good replacement for the cross-staff. These resources improved the ability of a navigator at sea to judge his latitude.

The compass, a cross-staff or astrolabe, a method to correct for the altitude of Polaris and rudimentary nautical charts were all the tools available to a navigator at the time of Christopher Columbus. In his notes on Ptolemy's geography, John Werner of Nurenberg wrote in 1514 that the cross-staff was a very ancient instrument, but was only beginning to be used on ships.

Prior to 1577, no method of judging the ship's speed was mentioned that was more advanced than the "Dutchman's log". This method involved throwing a floating object overboard at the ship's bow and timing it's passage. In 1577, a slightly more advanced technique was mentioned: attaching a log of wood to a line, throwing the log over the stern, and observing how quickly the line paid out. In 1578, a patent was registered for a device that would judge the ship's speed by counting the revolutions of a wheel mounted below the ship's waterline.

Accurate time-keeping is necessary for the determination of longitude. As early as 1530, precursors to modern techniques were being explored. However, the most accurate clocks available to these early navigators were water clocks and sand clocks, such as hourglass. Hourglasses were still in use by the Royal Navy of Britain until 1839.

In 1545, Pedro de Medina published the Arte de navigar, which appears to be the first book ever published professionally on navigation. The book was translated into French and Italian, and many years later into English.

In the late 16th century, Gerardus Mercator made vast improvements to nautical charts.

In 1594, John Davis pubished an 80-page pamphlet called The Seaman's Secrets which, among other things describes great circle sailing. It's said that the explorer Sebastian Cabot had used great circle methods in a crossing of the North Atlantic in 1495.

In 1631, Pierre Vernier described his newly invented quadrant that was accurate to one minute of arc. In theory, this level of accuracy could give a line of position within a nautical mile of the navigator's actual position.

In 1635, Henry Gellibrand published an account of yearly change in magnetic variation.

In 1637, using a specially built sextant with a 5-foot radius, Richard Norwood measured the length of a nautical mile with chains. His definition of 2040 yards is fairly close to the modern International System of Units (SI) definition of 2025.372 yards. Norwood is also credited with the discovery of magnetic dip 59 years earlier, in 1576.

Modern Times
In 1714, the British "Commissioners for the discovery of longitude at sea" came into prominence. This group, which existed until 1828, offered grants and rewards for the solution of various navigational problems. Between 1737 and 1828, the commissioners disbursed some £101,000. The government of the United Kingdom also offered significant rewards for navigational accomplishments in this era, such as £20,000 for the discovery of the Northwest passage and £5,000 for the navigator that could sail within a degree of latitude of the North pole.

In 1731 the sextant was invented, replacing earlier cross-staffs and astrolabes. This had the immediate effect of making latitude calculations much more accurate. But it was only four years later that the marine chronometer was invented. The combination of chronometer and sextant allowed for accurate determination of longitude.

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.In 1912 Nils Gustaf Dalén was awarded the Nobel Prize in Physics for his invention of automatic valves designed to be used in combination with gas accumulators in lighthouses

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.

Modern Navigation
Most modern techniques of navigation rely on crossing lines of position or LOP. A line of position can refer to two different things: a line on a chart and a line between the observer and an object in real life. A bearing is a measure of the direction to an object. If the navigator measures the direction in real life, he can then draw the angle on a nautical chart and presume he lies on that line on the chart.

In addition to bearings, navigators also often measure distances to objects. On the chart, a distance produces a circle or arc of position. Circles, arcs, and hyperbolae of positions are often referred to as lines of position.

If the navigator draws two lines of position, and they intersect he must be at that position. A fix is the intersection of two or more LOPs.

If only one line of position is available, this may be evaluated against the dead reckoning position to establish an estimated position.

Lines (or circles) of position can be derived from a variety of sources:


 * celestial observation (actually, a short segment of the circle of equal altitude, but generally represented as a line),
 * terrestrial range (natural or man made) when two charted points are observed to be in line with each other,
 * compass bearing to a charted object,
 * radar range to a charted object,
 * on certain coastlines, a depth sounding from echo sounder or hand leadline.

There are some older methods seldom used today such as "dipping a light" to calculate the geographic range from observer to lighthouse

Methods of navigation have changed through history. Each new method has enhanced the mariner’s ability to complete his voyage safely and expeditiously. One of the most important judgments the navigator must make involves choosing the best method to use. Some commonly recognized types of navigation are depicted in the table.

The practice of navigation usually involves a combination of these different methodologies.

Dead reckoning
Dead reckoning is the process of estimating one’s present position by projecting course and speed from a known past position. It is also used to predict a future position by projecting course and speed from a known present position. The DR position is only an approximate position because it does not allow for the effect of leeway, current, helmsman error, compass error, or any other external influences.

The navigator uses dead reckoning in many ways, such as:
 * to determine sunrise and sunset,
 * to predict landfall, sighting lights and arrival times,
 * to evaluate the accuracy of electronic positioning information,
 * to predict which celestial bodies will be available for future observation.

The most important use of dead reckoning is to project the position of the ship into the immediate future and avoid hazards to navigation.

A prudent navigator carefully tends the DR plot, updating it when required, and uses it to evaluate external forces acting on the ship. The navigator also consults the DR plot to avoid potential navigation hazards. A fix taken at each DR position will reveal the effects of current, wind, and steering error, and allow the navigator to stay on track by correcting for them.

The use of DR when an Electronic Charts Display and Information System (ECDIS) is the primary plotting method will vary with the type of system. An ECDIS allows the display of the ship’s heading projected out to some future position as a function of time, the display of waypoint information, and progress toward each waypoint in turn.

Until ECDIS is proven to provide the level of safety and accuracy required, the use of a traditional DR plot on paper charts is a prudent backup, especially in restricted waters.

Before the development of the chronometer, dead reckoning was the primary method of determining longitude available to mariners such as Christopher Columbus and John Cabot on their trans-Atlantic voyages.

Piloting
Piloting (also called pilotage) involves navigating a vessel in restricted waters and fixing its position as precisely as possible at frequent intervals. More so than in other phases of navigation, proper preparation and attention to detail are important. Procedures vary from vessel to vessel, and between military, commercial, and private vessels. It is the responsibility of the navigator to choose the procedures applicable to his own situation, to train the piloting team in their execution, and to ensure that duties are carried out properly.

A military navigation team will nearly always consist of several people. A military navigator might have bearing takers stationed at the gyro repeaters on the bridge wings for taking simultaneous bearings, while the civilian navigator must often take and plot them himself. While the military navigator will have a bearing book and someone to record entries for each fix, the civilian navigator will simply plot the bearings on the chart as they are taken and not record them at all.

If the ship is equipped with an ECDIS, it is reasonable for the navigator to simply monitor the progress of the ship along the chosen track, visually ensuring that the ship is proceeding as desired, checking the compass, sounder and other indicators only occasionally. If a pilot is aboard, as is often the case in the most restricted of waters, his judgement can generally be relied upon explicitly, further easing the workload. But should the ECDIS fail, the navigator will have to rely on his skill in the manual and time-tested procedures discussed in this chapter.

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.

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 marine sextant
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.

Radio navigation
A radio direction finder or RDF is a device for finding the direction to a radio source. Due to radio's ability to travel very long distances "over the horizon", it makes a particularly good navigation system for ships and aircraft that might be flying at a distance from land.

RDF's work by pointing a directional antenna in "various directions" and then listening for the direction in which the signal from a known station comes through most strongly. This sort of system was widely used in the 1930s and 1940s. RDF antennas are particularly very easy to spot on German World War II aircraft, as loops under the rear section of the fuselage, whereas most US aircraft enclosed the antenna in a small teardrop-shaped fairing.

In navigational applications, RDF signals are provided in the form of radio beacons, the radio version of a lighthouse. The signal is typically a simple AM broadcast of a morse code series of letters, which the RDF can tune in to see if the beacon is "on the air". Most modern detectors can also tune in any commercial radio stations, which is particularly useful due to their high power and location near major cities.

Decca, OMEGA, and LORAN-C are three similar hyperbolic navigation systems. Decca was a hyperbolic low frequency radio navigation system (also known as multilateration) that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings. As was the case with Loran C, its primary use was for ship navigation in coastal waters. Fishing vessels were major post-war users, but it was also used on aircraft, including a very early (1949) application of moving-map displays. The system was deployed extensively in the North Sea and was used by helicopters operating to oil platforms. After being shut down in the spring of 2000, it has been superseded by systems such as the American GPS and the planned European Galileo positioning system.

The OMEGA Navigation System was the first truly global radio navigation system for aircraft, operated by the United States in cooperation with six partner nations. OMEGA was originally developed by the United States Navy for military aviation users. It was approved for development in 1968 and promised a true worldwide oceanic coverage capability with only eight transmitters and the ability to achieve a four mile accuracy when fixing a position. Initially, the system was to be used for navigating nuclear bombers across the North Pole to Russia. Later, it was found useful for submarines.  Due to the success of the Global Positioning System the use of Omega declined during the 1990s, to a point where the cost of operating Omega could no longer be justified. Omega was permanently terminated on September 30, 1997 and all stations ceased operation.

LORAN is a terrestrial navigation system using low frequency radio transmitters that use the time interval between radio signals received from three or more stations to determine the position of a ship or aircraft. The current version of LORAN in common use is LORAN-C, which operates in the low frequency portion of the EM spectrum from 90 to 110 kHz. Many nations are users of the system, including the United States, Japan, and several European countries. Russia uses a nearly exact system in the same frequency range, called CHAYKA. LORAN use is in steep decline, with GPS being the primary replacement. However, there are current attempts to enhance and re-popularize LORAN.

Satellite navigation
Global Navigation Satellite System or GNSS is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. A GNSS allow small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line of sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.

As of 2007, the United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS. The Russian GLONASS is a GNSS in the process of being restored to full operation. The European Union's Galileo positioning system is a next generation GNSS in the initial deployment phase, scheduled to be operational in 2010. China has indicated it may expand its regional Beidou navigation system into a global system.

More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the receiver's location, speed and direction.

Since the first experimental satellite was launched in 1978, GPS has become an indispensable aid to navigation around the world, and an important tool for map-making and land surveying. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Developed by the United States Department of Defense, GPS is officially named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year, including the replacement of aging satellites, and research and development. Despite this fact, GPS is free for civilian use as a public good.

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, however on larger vessels, the task will be delegated to the ship's navigator.

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. The practice of voyage planning has evolved from penciling lines on nautical charts to a process of risk management.

Passage planning consists of four stages: appraisal, planning, execution, and monitoring, which are specified in International Maritime Organization Resolution A.893(21), Guidelines For Voyage Planning, 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 vessel's progress in relation to the plan and responding to deviations and unforseen circumstances.

Integrated bridge systems
Electronic integrated bridge concepts are driving future navigation system planning. Integrated systems take inputs from various ship sensors, electronically display positioning information, and provide control signals required to maintain a vessel on a preset course. The navigator becomes a system manager, choosing system presets, interpreting system output, and monitoring vessel response.