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''.]] Navigation is the process of planning, recording, and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The word ''navigate'' is derived from the Latin roots ''navis'' meaning "ship" and ''agere'' meaning "to move" or "to direct." Different navigational techniques have evolved over the ages in different cultures, but all involve locating one's position compared to known locations or patterns. BASIC CONCEPTS Latitude The latitude of a place on the earth's surface is the angular distance north or south of the (marked with °) ranging from 0° at the Equator to 90° at the North and South poles. The latitude of the North Pole is 90° N, and the latitude of the South Pole is 90° S. Longitude Similar to latitude, the longitude of a place on the earth's surface is the angular distance east or west of the Prime Meridian or Greenwich Meridian . Longitude is usually expressed in Degrees (marked with °) ranging from 0° at the Greenwich meridian to 180° east and west. Sydney, Australia , for example, has a longitude of about 151° east. New York City has a longitude of about 74° west. MODERN TECHNIQUE Most modern navigation relies primarily on positions determined electronically by receivers collecting information from satellites. Most other modern techniques rely on crossing 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.Maloney, 2003:618. Lines (or circles) of position can be derived from a variety of sources:
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.Bowditch, 2002:1. 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.Bowditch, 2002:99. 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:
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 Marine 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.Bowditch, 2002:105. 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 See Also: Celestial navigation Celestial navigation systems are based on observation of the positions of the Sun , Moon and Star s. By knowing which point on the rotating earth a celestial object is currently above and measuring it's height above the observer's horizon, the navigator can determine his distance from that subpoint. A Nautical Almanac and a Chronometer are used to compute the subpoint on earth a celestial body is over, and a Sextant is used to measure the body's angular height above the horizon. That height can then be used to compute ones distance from the subpoint to create a circular line of position. A navigator shoots a number of stars in succession to give a series of overlapping lines of position. Where they intersect is his celestial fix. The moon and sun may also be used. The sun can also be used by itself to shoot a succession of lines of position (best done around local noon) to determine a position as well. Marine chronometer .]] In order to accurately measure longitude, one must record the precise time of a sextant sighting (down to the second, if possible). Various types of chronometers are widely used. The spring-driven marine chronometer is a precision timepiece used aboard ship to provide accurate time for celestial observations.Bowditch, 2002:269. A chronometer differs from a springdriven watch principally in that it contains a variable lever device to maintain even pressure on the mainspring, and a special balance designed to compensate for temperature variations. A spring-driven chronometer is set approximately to Greenwich mean time (GMT) and is not reset until the instrument is overhauled and cleaned, usually at three-year intervals. The difference between GMT and chronometer time is carefully determined and applied as a correction to all chronometer readings. Spring-driven chronometers must be wound at about the same time each day. At maximum intervals of about three years, a spring-driven chronometer should be sent to a chronometer repair shop for cleaning and overhaul. Quartz crystal marine chronometers have replaced spring-driven chronometers aboard many ships because of their greater accuracy. They are maintained on GMT directly from radio time signals. This eliminates chronometer error and watch error corrections. Should the second hand be in error by a readable amount, it can be reset electrically. The basic element for time generation is a quartz crystal oscillator. The quartz crystal is temperature compensated and is hermetically sealed in an evacuated envelope. A calibrated adjustment capability is provided to adjust for the aging of the crystal. The chronometer is designed to operate for a minimum of 1 year on a single set of batteries. A good marine chronometer has a built-in push button battery test meter. The meter face is marked to indicate when the battery should be replaced. The chronometer continues to operate and keep the correct time for at least 5 minutes while the batteries are changed. The chronometer is designed to accommodate the gradual voltage drop during the life of the batteries while maintaining accuracy requirements. A chronometer should not be removed from its case to time sights. Observations may be timed and ship’s clocks set with a comparing watch, which is set to chronometer time and taken to the bridge wing for recording sight times. In practice, a wrist watch coordinated to the nearest second with the chronometer will be adequate. A stop watch, either spring wound or digital, may also be used for celestial observations. In this case, the watch is started at a known GMT by chronometer, and the elapsed time of each sight added to this to obtain GMT of the sight. All chronometers and watches should be checked regularly with a radio time signal. Times and frequencies of radio time signals are listed in publications such as Radio Navigational Aids . The marine sextant is used to measure the elevation of celestial bodies above the horizon.]] 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. Electronic navigation 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 Platform s. 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 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. Radar navigation Marine radar systems can provide very useful navigation information in a variety of situations. When the vessel is within radar range of land or special radar aids to navigation, the navigator can take distances and angular bearings to charted objects and use these to establish arcs of position and lines of position on a chart.Maloney, 2003:744. A fix consisting of only radar information is called a radar fix.Bowditch, 2002:816. Some types of radar fixes include the relatively self-explanatory methods of "range and bearing to a single object,"National Imagery and Mapping Agency, 2001:163. "two or more bearings," "tangent bearings," and "two or more ranges." Parallel indexing is a technique defined by William Burger in the 1957 book ''The Radar Observer's Handbook''.National Imagery and Mapping Agency, 2001:169. This technique involves creating a line on the screen that is parallel to the ship's course, but offset to the left or right by some distance. This parallel line allows the navigator to maintain a given distance away from hazards. Some techniques have been developed for special situations. One, known as the "contour method," involves marking a transparent plastic template on the radar screen and moving it to the chart to fix a position.National Imagery and Mapping Agency, 2001:164. Another special technique, known as the Franklin Continuous Radar Plot Technique, involves drawing the path a radar object should follow on the radar display if the ship stays on its planned course.National Imagery and Mapping Agency, 2001:182. During the transit, the navigator can check that the ship is on track by checking that the pip lies on the drawn line. 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 Metre s using Time Signal s transmitted along a Line Of Sight by Radio from Satellite s. 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 Russia n 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 Earthquake s, and Synchronization of telecommunications networks. Developed by the , 2006 . including the replacement of aging satellites, and research and development. Despite this fact, GPS is free for civilian use as a Public Good . NAVIGATION PROCESSES Day's Work in Navigation The Day's Work in Navigation is an attempt to summarize a minimal set of tasks consistent with prudent navigation. The definition will vary on military and civilian vessels, and from ship to ship, but takes a form resemblingTurpin and McEwen, 1980:6-18.: # Maintain continuous dead reckoning plot. #Take two or more star observations at morning twilight for a celestial fix. #Morning sun observation. Can be taken on or near prime vertical for longitude, or at any time for a line of position. #Determine compass error by azimuth observation of the sun. #Computation of the interval to noon, watch time of local apparent noon, and constants for meridian or ex-meridian sights. #Noontime meridian or ex-meridian observation of the sun for noon latitude line. Running fix or cross with Venus line for noon fix. #Noontime determination the day's run and day's set and drift. #At least one afternoon sun line, in case the stars are not visible at twilight. #Determine compass error by azimuth observation of the sun. #Take two or more star observations at evening twilight for a celestial fix. Passage planning See Also: Passage planning |
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