Navigation, science of determining the position of a ship, aircraft, or guided missile, and charting a course for guiding the craft safely and expeditiously from one point to another. The practice of navigation requires not only thorough knowledge of the science of navigation, but also considerable experience and judgement.
The science of nautical navigation is divided into four principal techniques: (1) so-called
dead reckoning, which is derived from the phrase deduced reckoning, and estimates the approximate position of a craft solely from its course and speed; (2) piloting, which involves guiding the craft by frequent reference to geographical landmarks and navigational aids and by use of sounding; (3) celestial navigation, which uses the observation of celestial bodies to determine position on the surface of the earth; and (4) electronic navigation, the most important and advanced system of navigation today, using radio and inertial electronic equipment. Aeronautical navigation is based on similar principles.Position and Direction on the Earth's Surface
The basic problems of navigating any craft involve the determination of its position and direction and the measurement of speed, distance, and time in proceeding from one point to another. Position on the earth's surface can be defined in terms of an accepted set of coordinates, such as
latitude and longitude. The direction of one place relative to another is usually indicated as the angular distance, measured in degrees of arc, from the direction of true north. Speed is often expressed in knots, or nautical miles per hour (1 knot = 1.853 km/hr or 1.15 mph). Altitude, or height above a reference level, is also important in aircraft navigation.
Map and Chart Projections
The initial planning and the end results of navigation are plotted on
maps and charts. The nearly-spherical surface of the earth is represented, or projected, on a plane surface upon which are superimposed the coordinates of latitude and longitude and also the desired features of surface and underwater topography. Charts are maps that accentuate the determination of position, direction, and distance, and stress points of interest to a navigator. Because no part of a sphere can be spread out flat without distortion, several projections have been developed to accommodate the navigator. Each projection has its own advantages and limitations and fulfils a specific need of the navigator.The most widely used projection in navigation charts is the Mercator, named after the 16th-century Flemish mathematician and geographer
Gerardus Mercator, who devised it. These charts portray the earth's surface projected on a cylinder tangent to the surface of the earth at the equator. When this cylinder is flattened out, the meridians, or lines of longitude, appear as equally spaced vertical lines and the parallels of latitude appear as parallel horizontal lines, spaced farther apart towards the poles. The Mercator projection, despite its great distortion, is popular because a course that follows a constant bearing—that is, makes a constant angle with the direction of north—is represented by a straight line on the chart.A navigator usually attempts to find the shortest route between two points; this can be accomplished by following a great-circle course. The great circle between two points on the surface of the earth represents the arc of a plane intersecting the sphere at its centre and is the shortest path on the spherical surface. Great-circle courses can be determined directly from great-circle charts, but because it is impracticable for a ship to travel on an ever-changing bearing, the usual practice is to follow a series of chords that approximate the great circle. These chords are normally plotted on a Mercator chart.
Most of the navigable waters of the world have been surveyed accurately by the hydrographic services of the principal maritime nations so that reliable charts of the waters are usually available to the navigator.
The hydrographic services of various countries also publish almanacs and sailing directions to assist the navigator. Sailing directions are descriptive books containing detailed information on coastal waters, harbour facilities, navigation aids, winds, tides, currents, dangers to navigation, directions for approaching and entering restricted waters, and other data that cannot be shown on the chart of the area. Similar information is provided for aircraft, with indications of features, such as ground topology and airways, that are a function of altitude.
Navigation Instruments
Many instruments are employed today to facilitate navigation; some are relatively simple to use and others require extensive instruction. In the latter category are some of the modern electronic and mechanical devices.
Navigation instruments are designed to fix position, measure direction and distance, determine speed, measure the depth of water, assist in plotting on charts, and observe the weather elements. Sometimes various instruments are used simultaneously to yield the required information.
The magnetic
compass is one of the oldest instruments used aboard ships. Although it has been generally supplanted by the gyrocompass on large ships, the magnetic compass retains its original role as the basic navigational instrument because it is not subject to electromechanical defects, and hence, on most seagoing ships, it is a necessary standby instrument. The magnetic compass serves as a directional device by aligning itself in the direction of the earth's magnetic poles.Because of the location of the magnetic poles, the needle of a compass will point to the geographical North Pole only in a few localities. In other places, it will point east or west of north. The difference in degrees between the direction of the compass needle and that of true north is called variation, or declination. For the convenience of navigators, the declination in many parts of the world has been measured, and charts have been prepared that show by isogonic lines—curves connecting points of equal declination—the approximate east or west declination for any place. On such charts, the line of zero declination, along which the compass points true north, is called an agonic line.
The gyrocompass, which uses a
gyroscope as its directive element, indicates true north. The gyroscope in this compass is a rapidly rotating body, free to move about one or two axes, perpendicular to the axis of rotation and to each other. Control elements are added to the gyroscope to convert it to a true direction indicator. The indications of the master gyrocompass may be repeated in various parts of the craft—for example, in bearing repeaters, steering repeaters, and radar repeaters.The azimuth circle is an important auxiliary device used for indicating the azimuth, or bearing, of an object, its direction measured from the north point. It is a graduated ring with sight vanes that is designed to fit snugly over a compass or a compass repeater; it provides a means of taking bearings of both terrestrial objects and celestial bodies.
An instrument known as the log is used to determine either the speed of a ship or the distance travelled through the water, or both simultaneously. Various types of logs are used, some operating on a simple mechanical principle and others based on ingenious electromechanical techniques. The airspeed indicator fulfils the same function in an aircraft.
To determine water depth a navigator uses either the lead or the echo sounder. The lead, which consists essentially of a lead weight at the end of a suitably marked line, is used in coastal or shallow waters under conditions of low visibility. The echo sounder, which is found on almost all seagoing ships, indicates the depth of water by measuring the time interval between the emission of a sonic or
ultrasonic signal and the return of its echo from the bottom. Aircraft height is determined by a barometric altimeter, a radio altimeter, or an inertial system.The plotting equipment used by the navigator resembles to a certain extent the tools used in
drafting. Dividers for measuring distances, compasses for drawing circles, plotters, protractors, and universal draughting machines are the rudimentary tools commonly found on the chart table of a ship.For celestial navigation the navigator uses a
sextant and a chronometer. The sextant is a double-reflecting instrument that measures the angle between two objects by bringing into coincidence rays of light received directly from one object and by reflection from the other. Its principal use is to determine the altitude (in degrees of arc) of celestial bodies above the horizon. The chronometer is a very accurate timepiece with a nearly constant rate of daily gain or loss. It is set to the time of a standard meridian, usually that of the former Greenwich Observatory, in London, and makes possible the determination of longitude at sea. Its daily rate of gain or loss is checked by radio time signals broadcast from various countries. (see Clocks and Watches). In addition to these instruments, most modern ships use several electronic navigation devices (see Electronic Navigation below).Navigation in Pilotage Waters
Piloting is the most exacting form of navigation because it entails the movement of ships under many potentially dangerous conditions. The greatest care and exactness are necessary for success in piloting, especially in poorly charted coastal waters or under unfavourable weather and visibility conditions. One of the chief concerns of the navigator in pilotage waters, where traffic is heavier than at sea, is to avoid collision with other ships.
Line of Position
A basic concept in piloting is known as the line of position, a line indicating a series of possible positions of a craft and determined usually by observation. The line may be straight, curved, or irregular, as when the line produced by plotting a series of soundings taken over a period. One line of position is not sufficient to determine the exact position of a ship. The point of intersection of two or more lines of position, taken simultaneously or adjusted for time lapse, is a positive position known as a fix. The navigator in pilotage waters strives constantly to plot such intersections of lines. Fixes then serve as reliable guideposts for future movements or decisions.
Visual piloting is accomplished generally by employing an azimuth circle on a gyrocompass repeater to take bearings of identifiable and chartered objects. These bearings are then plotted on a chart of the area to indicate graphically the position of the ship. A single navigational object may define a fix if both a bearing and a range can be taken simultaneously by the use of a rangefinder in addition to the azimuth circle, or by radar. In cases where only one line of position is available without an accompanying range, the navigator must resort to the use of the so-called estimated position, which is not as reliable as a fix but is more reliable than a dead-reckoning position. An estimated position requires continued extra caution in the navigation process until a fix is determined.
A line of position may be obtained by any one of the following methods: a range within which two known fixed objects appear in line, and the ship is placed somewhere on this line; a compass bearing of an object observed visually or by radar; a range obtained by rangefinder or by radar; a single sounding or a series of soundings of the bottom (usually referred to as a chain of soundings); a horizontal angle, measured by a sextant, between two known objects; a vertical angle, measured by a sextant, of an object of known height; an echo of the ship's whistle or siren; a radio direction-finder bearing; lines of position derived from one of several electronic systems; and astronomical lines of position.
Fixing the Position
Any combination of these methods of determining a line of position permits a fix. Fixes may be arrived at by cross bearings, by finding the bearing and distance of the same object, by a bearing and a sounding taken simultaneously, by horizontal sextant angles, and by two bearings of a single object taken at different times but adjusted for time lapse when plotted. The last-mentioned technique is known as a running fix.
In addition to these graphic methods, a ship's position can be deduced by the use of horizontal angles in conjunction with a three-arm protractor. Such a protractor consists of a circle, graduated in degrees, to which is attached one fixed arm and two arms pivoted at the centre. If horizontal angles taken on three identifiable fixed objects shown on a chart are set on the protractor and the latter is positioned on the chart with the objects lined up on the three arms, the position of the ship is fixed at the centre. Aids to navigation may consist of various types of
beacons, buoys, lighthouses, and light vessels; their characteristic shapes and colours provide at least partial daytime identification, and characteristic phases and colours of lights provide identification at night. Where these aids are absent, the navigator must resort to taking bearings of mountain peaks and of chartered structures such as water tanks or church spires, and taking tangent bearings of islands or points of land.Tides, Tidal Streams, and Ocean Currents
The practice of navigation is complicated by the presence of tidal effects and ocean currents. These effects, which may be favourable or unfavourable, tend to deflect the ship from its charted course and to reduce or increase its speed. A comparison of dead-reckoning positions and fixes reveals the extent of such effects and often helps the navigator to predict and adjust for future influences. See
Ocean and Oceanography; Tide; Wind.Celestial Navigation
In this classic method, used most commonly in the open sea, the navigator measures the positions of celestial bodies. Stars have been identified and grouped into
constellations, partly for this purpose, since ancient times (see Astronomy). Celestial navigation makes possible voyages across thousands of kilometres of unmarked water, but its one great limitation is that poor visibility, caused by clouds, fog, rain, snow, mist, or haze, may prevent the essential sightings of celestial bodies.A coordinate system similar to the earth's coordinates of latitude and longitude has been adopted to describe the positions of heavenly bodies. This system consists of declination, which corresponds to terrestrial latitude, and hour angle, which corresponds to terrestrial longitude. For practical purposes of navigation, the positions of the stars relative to one another are regarded as fixed on the
celestial sphere; the motions of the sun, the moon, and the planets are indicated in this system as a mean rate of progression across the sphere.The principal maritime nations publish yearly nautical
almanacs that tabulate the coordinates at any particular time of the celestial bodies used in navigation. The tables also provide other pertinent astronomical information.To use the nautical almanac, the navigator must establish the time of an observation accurately by means of the chronometer. The measurement of time is based on the rotation of the earth and the consequent apparent rotation of celestial bodies around the earth. In navigation, the primary system of time is based on the apparent movement of the sun westward at 15° of longitude per hour. Thus, a time difference is established between two places on the surface of the earth based on their difference of longitude. The longitude of New York, for example, is roughly 75° west and that of Greenwich is 0°. New York is therefore 5 hours to the west of Greenwich.
The navigational triangle, or astronomical triangle, which constitutes the most important part of celestial navigation, is a spherical
triangle, the three points of which represent the position of the observer, the geographical position of the celestial body, and the geographical pole that is nearer to the observer. The solution of such a triangle provides the basis for the derivation of an astronomical line of position. Spherical trigonometry was formerly required to solve such a problem, but this triangle can today be solved simply by using the nautical almanac in conjunction with one of several short tabular methods. The tabular methods include precomputed solutions of the astronomical triangle to accommodate any position of the observer and any celestial body observed.In the most modern approach to celestial navigation, the circle of equal altitude and the astronomical position line are used in conjunction with the solution of the navigational triangle. The circle of equal altitude is a circle on the surface of the earth, and at every point on this circle the altitude of a given celestial body is the same at a given instant.
Electronic Navigation
In this method of navigation,
electronic devices use the information provided by radio or radar to chart the position and route of a craft. Electronic and precision aids have increased the safety of navigation by supplying important information rapidly during periods of poor visibility, particularly in dangerous and congested waters. The modern navigator makes wide use of these devices, both in pilotage waters and in the open sea. Radio provides the navigator with auxiliary information, including time signals, regular weather reports, storm warnings, and general navigational warnings concerning such hazards as derelict ships, extinguished navigational lights, and buoys adrift.Radio was first used as an aid to navigation in the early 1900s, and in the 1930s aircraft were fitted with communications equipment to enable them to receive navigational direction from the ground; direction-finding loops enabled them to take bearings on ground transmitters. One basis for modern navigational aids is radio direction-finding, used in one of two ways: an aircraft or ship takes bearings on fixed transmitters on the ground and fixes its position relative to two or more of them; or bearings taken by ground stations on a transmission from an aircraft or a ship are centrally correlated and a position is passed on to the craft. The principal electronic devices and systems are described below.
Direction-Finder (D/F)
The radio direction-finder was the first navigational aid to come into general use. If the bearings of two transmitters with known locations can be measured, the position of the receiver may be determined. In its simplest form, a modern D/F consists of a conventional radio receiver with an
antenna in the form of a coil of wire called a loop. Such a loop antenna has strongly directional properties; if it is mounted so that its axis points directly to a radio station, it will receive no signal whatsoever from that station; if it is mounted so that the plane of the loop passes through the radio station, it receives a strong signal. At intermediate positions the signal is intermediate in strength. In practice, a known station is tuned in, and then the loop is rotated until no signal is heard; this position is called the aural null. The axis of the loop must then point directly towards (and away from) the station; this direction is plotted by the navigator as a line of position.An automatic direction-finder (ADF) has a motor that rotates the loop antenna, keeping the loop always in the null position. The motor also actuates a needle, similar in appearance to a compass needle, that indicates the position of the loop. This so-called radio compass points not towards north but towards whichever station is tuned in on the loop antenna. Such direction-finders can operate on any radio station broadcasting a continuous carrier on a frequency that the radio set can receive. Virtually all aircraft and ships are equipped with D/F equipment. Ground D/F stations have also been installed to aid lost aircraft. Radio D/F equipment is also used in police work and counter-espionage to locate hidden radio stations.
Radio Ranges
Radio ranges and D/F were the principal radio navigation aids in general use before World War II. They operate on low frequencies (200 to 415 kilohertz) and so are subject to bending, night effect, and other anomalies.
A radio range consists of two pairs of antennas broadcasting in
Morse code, one broadcasting the letter A (dot, dash), and the other broadcasting the letter N (dash, dot). The timing of the two letters is such that the space between letters just equals the time of a dash, while the space between the two parts of a letter just equals the time of a dot. The patterns thus interlock so that if both are heard at once, the sound is continuous. The transmission pattern from each pair of antennas is directional, and is projected into two opposite "quadrants", each covering about 90°. An aircraft in one of the quadrants will hear only a single letter, either A or N; however, if it is on the borderline between the two quadrants, the navigator will hear the continuous tone, which is called the on-course signal. This borderline is called the beam, and is generally about 3° wide. Directly above the range is an area in which no signal is heard. This area is called the cone of silence and is small at low altitudes, but increases in size at higher altitudes.Radio Beacons
A beacon is a radio station that is equipped with a nondirectional antenna; it is used principally for homing. Low-powered beacons are called locators and are used in conjunction with radio compasses.
Omnirange or Omnidirectional Range (MOR or VOR)
Omnirange is, in effect, a radio range with an infinite number of beams (or, in practice, 360 beams). Omnirange stations are operated on both VHF (very high frequency) and LF (low frequency): VHF omnirange is called VOR; the designation of low-frequency omnirange, originally LOR, was changed to MOR to avoid confusion with loran (see below). VOR is useful at ranges up to 160 km (100 mi).
The omnirange station has four antennas similar to the antennas of a range station, plus one central antenna. The central antenna broadcasts a continuous reference signal; the other antennas broadcast a variable signal in a beam that is rotated at 1,800 rpm. At the instant when the rotating signal points due north, it is in phase with the reference signal; at all other times it is out of phase with the reference signal by an amount that depends on its direction. The receiver, by measuring this phase difference, can determine its bearing from the station. In practice the omnirange receiver has three dials, one of which can be set manually to any desired course, the second of which tells whether the plane is to the left or right of the course, and the third of which resolves 180° ambiguity by indicating either "from" or "to". Omnirange can be used for homing and for determining a line of position.
Radio Altimeter
Radio altimeters measure the actual height of the craft above the ground or buildings, whereas ordinary
altimeters merely measure air pressure, which can be converted into altitude above ground only if the navigator knows the altitude above sea level of the nearest ground and the barometric reading there at that instant.Gee
This radar-like apparatus is a pulsed, three-station, hyperbolic system operating in the 20- to 85-megahertz band and providing navigational fixes slightly beyond optical range. Originally designed in 1937, Gee was not developed until 1940, during World War II, when stations built in Great Britain provided reliable navigational aid to aircraft operating over western Europe. A Gee chain comprises a master transmitter and two slave transmitters, located at distances of 80 to 160 km (50 to 100 mi) from the master. Pulses radiated from the master transmitter trigger pulse responses from the slave transmitters at precisely determined recurrence rates. The times at which the three pulses originate bear a known relationship to one another, and each master-slave time difference measured at the receiver by a
cathode-ray tube determines a hyperbolic position line. Two position lines derived from the two master-slave combinations provide a fix.Long-Range Navigation, or Loran
Loran
is the pulsed hyperbolic system developed by the United States during World War II to provide long-range navigation over sea for ships and aircraft. The radio frequency used in loran is approximately 2 megahertz, which permits long-distance reception over oceans but is not effective at long distances over land except at night. Its method of operation is similar to that of Gee, and a single airborne system, using both Gee and loran, was evolved in a cooperative effort between the United States and Great Britain.Rebecca-Eureka
This is probably the best-known responder system combination. Rebecca is the airborne interrogator, and Eureka the responder. The system is based on conventional secondary-radar techniques. The interrogating pulses are radiated from a central aerial near the nose of the aircraft, and the reply pulses from the responder are received by two aerials on either side of the nose. The pickup is indicated on a cathode-ray tube showing a vertical linear base. The responder pulses show as a horizontal "blip" across the line base; the range is indicated by the vertical position of the blip.
Consolan
This system provides coded signals from which the direction of a station can be determined, thus ensuring accurate bearings that are independent of all navigation equipment aboard. Consolan signals are usable up to about 1,300 km (800 mi) or more.
Global Positioning Systems (GPS)
The Transit system of six polar-orbit satellites provides a worldwide positioning service to military and research ships, but the system will probably be abandoned in the 1990s. The United States Navstar GPS and the former Soviet Union's Glonass system of military satellites are also available for civilian use. The European Space Agency is planning a Navsat system of 18 spacecraft. The International Maritime Satellite Organization (Inmarsat) is also developing a worldwide system of navigational aids. GPS are becoming accepted as the most convenient form of navigation aid for many applications.
Ground-Controlled Approach (GCA)
GCA is an instrument-approach system consisting of extremely high-precision microwave radar equipment that gives the position of an aircraft in range, azimuth, and elevation. It is primarily designed to bring the pilot through conditions of low visibility to make a normal landing by visual contact. Skilled operation of this system in the aircraft and on the ground permits emergency landing under conditions of nearly zero visibility. GCA uses two sets of radarscopes. One locates planes at a considerable distance, such as 15 to 25 km (about 9 to 15 mi). The controller using this set of scopes maintains communications with planes waiting to land, "stacks" them (that is, assigns each one to a separate altitude at which it can circle without danger of collision), and brings them in one at a time along a standard approach pattern until they are on the final leg of the approach. On the final approach leg the final-approach controller, using precision scopes, takes control. This controller also broadcasts verbal instructions, principally concerning altitude and lateral deviation from the desired glide path, and guides the pilot virtually to the end of the runway.
Instrument Landing System (ILS)
This system is primarily designed for instrument approach, but in emergency can be used for landing. It consists essentially of two beams, similar to radio range beams, one horizontal and the other vertical. The horizontal beam (called the localizer) is identical with the visual-aural range (VAR) beam, an ordinary radio range with only two beams instead of four. The vertical beam (called the glide-path) is extremely narrow and is inclined to the ground at an angle of 2.5°. The pilot follows the two beams by means of two pointers, one horizontal and one vertical, on a single dial.
Both ILS and GCA receive valuable supplementary aid from a standardized high-intensity lighting system along the runway and approach so that the pilot can make visual contact with the ground even in extremely bad weather and identify the position of the aircraft in relation to the runway.
Proposals have been made to replace ILS and GCA by a single microwave landing system (MLS). However, there is now some doubt as to whether this system will be implemented or whether it will be overtaken by the use of GPS.
Most radio navigational systems that are currently in use are operated in conjunction with high-speed computers.