Navigation is the process of finding an object’s position and directing its movement. The word navigate comes from two Latin words meaning ship and to drive.
Navigation has long been used to guide ships. Today, navigation techniques can help guide almost anything—from cars, trains, airplanes, and spacecraft to packages and people. Modern navigation systems make it easy for almost everyone to find their way. Such systems have been developed using advanced mathematics, physics, engineering, and computer science.
Today, radio transmitters, computers, and satellites automatically perform much of the work of navigation. But early navigators had to rely on more basic tools and techniques. Some traditional techniques are still in use.
Traditional navigation
Among the most basic navigation aids are charts, maps, and compasses. Charts and maps show the physical characteristics of an area. They may give such information as the location of ports, the height of mountains, or the depth of water. Compasses determine direction. The simplest is a magnetic compass with a magnetized needle that points to the North Pole. Magnetic compasses, however, become wildly inaccurate near the North and South poles. In these regions, navigators may use electronic compasses called gyrocompasses that do not depend on Earth’s magnetism.
There are three chief methods of traditional navigation: (1) dead reckoning, (2) piloting, and (3) celestial navigation. Navigators may use one or more of these methods, depending on the means of travel, the weather, and other factors.
Dead reckoning
(DR), sometimes called deduced reckoning, involves estimating how far and in what direction an object has moved. This method is not particularly accurate. It does not consider such factors as water currents, wind, or steering errors. Any of these factors can make it difficult for navigators to accurately keep track of a vehicle’s motion and direction, particularly in the case of ships and aircraft. Thus, navigators typically use other methods along with dead reckoning.
Piloting
involves finding a vehicle’s position in relation to one or more landmarks. Landmarks include such natural objects as mountains and islands. They also include artificial structures, such as buildings, buoys, and lighthouses. Successful piloting requires an up-to-date chart showing the positions of such landmarks.
In piloting, the navigator finds the bearing (direction) from the vehicle to a landmark in view. A magnetic compass can tell a landmark’s bearing relative to the North Pole. Alternatively, a device called a pelorus can measure a landmark’s bearing relative to a second landmark.
The navigator matches the landmark in view to its position on the chart. Then the navigator draws a line from the landmark on the chart in the same direction as the bearing. This line is called the line of position. The navigator can then figure out the vehicle’s rough position on the chart, because it is on the line of position. However, to pinpoint a vehicle’s position, two landmarks must be used. The two landmarks create two lines of position. The vehicle’s position is at the point where the two lines cross each other.
Navigators may also pilot with the aid of a depth finder, a device that measures the depth of the water. A navigator compares the depths indicated by this device with the depths on the chart.
Piloting can be used to navigate most types of vehicles. Ships use this method when entering or leaving ports, or when sailing close to land. Sailors pilot many small boats with only a chart and compass.
Celestial navigation
involves determining a vehicle’s location by observing the sun, moon, planets, and stars. The navigator measures the angle between the horizon and such astronomical objects, also called celestial (heavenly) bodies. This angle is then used in complex mathematical calculations to determine the vehicle’s position. Because the horizon is typically easy to view at sea, ships can make good use of celestial navigation.
Measuring angles.
A device called a sextant enables navigators to easily measure the angle between a celestial body and the horizon. The sextant has a sighting tube, a fixed mirror called a horizon glass in which the horizon appears, and an adjustable mirror called an index glass that moves to give the navigator a view of the sun or a star. The sextant enables a navigator to take accurate measurements, even if the device is unsteady or jerks around, as on a moving ship.
On land, the navigator may not be able to see the horizon. The horizon also may not be perfectly flat, as it is at sea. To use celestial navigation on land, a navigator must thus use special techniques to approximate a flat horizon, from which to measure angles. One such technique involves using a carpenter’s level, a device that forms a perfectly horizontal surface.
Keeping time.
The moon, the stars, and other objects in the sky are in constant motion. Measuring their angles against the horizon thus yields different results at different times. For this reason, a navigator must know the time to use celestial navigation effectively. An English clockmaker named John Harrison invented the first accurate timekeeping device in the mid-1700’s. The device, called a chronometer, made accurate celestial navigation possible.
Publications called almanacs list the positions of celestial bodies at all times during the year. Navigators could use almanacs along with accurate angle measurements and exact time measurements to determine position reliably.
One star never changes its position in the sky—the North Star, also called Polaris. In the Northern Hemisphere, stars seem to revolve around a point in the heavens called the celestial north pole, which lies above the North Pole. The North Star is so close to the celestial north pole that it appears stationary. Navigators in the Northern Hemisphere can derive latitude (position north or south on Earth’s surface relative to the equator) from the North Star’s position, without regard to time. However, people in the Southern Hemisphere cannot see the North Star, so this technique does not work there.
Modern use.
From Earth’s surface, celestial navigation is possible only when the sky is clear. But airplanes and spacecraft fly over clouds and so can always use it. On spacecraft, celestial navigation is used to control the craft’s orientation (direction in which it is pointed), rather than to find its position. A camera system called a star tracker does the work of a sextant, making celestial observations and measurements automatically.
Electronic navigation
The development of radio technology, computers, and artificial satellites has revolutionized navigation. Such technology can automatically make measurements and calculations that navigators once had to do by hand.
Radio navigation.
Most modern navigation systems use radio waves. Radio waves are invisible and can travel through air or space. Radio signals are useful for navigation because they can be used to measure distance and direction.
All radio navigation systems involve the use of one or more devices called transmitters. The transmitter sends out radio waves, which are eventually picked up by one or more devices called receivers.
In some radio navigation technologies, the transmitter sends radio waves to bounce off a moving object. Electronic equipment then measures how long the waves take to return to a receiver at their source. The returning waves reveal the distance and direction of the moving object, giving its position.
In other technologies, a receiver on a vehicle picks up radio waves from a transmitter. These waves may carry encoded information, such as the transmitter’s position or the time the signal was sent. This information can be used to calculate the receiver’s position and movement. Similarly, other systems use receivers on the ground to locate a moving transmitter.
Radio waves have various frequencies—that is, rates at which the wave oscillates (vibrates). Very high frequency (VHF) radio waves, such as those used in FM broadcasts, can more accurately pinpoint an object’s position. But they can only travel in a straight line from the transmitter, called a line of sight. Thus, they cannot be transmitted over the curve of Earth’s horizon. Airplanes and spacecraft avoid this problem because they fly high enough to have direct lines of sight to many places on the ground.
Low-frequency waves, such as those used in AM broadcasts, are less accurate. But they can travel beyond Earth’s horizon by bouncing off an electrically charged part of the atmosphere called the ionosphere.
Radar
uses radio waves with extremely high frequencies. It bounces the waves off moving objects to determine their direction and distance. Radar stands for radio detection and ranging. Air traffic controllers use radar to pinpoint the positions of aircraft in the sky. The controllers can share this information with the pilots through voice radio or text messaging. See Radar.
Radio direction finding
involves determining the bearing of a transmitter called a radio beacon. A device called a radio direction finder (RDF) receives the beacon’s signal. The navigator turns the RDF’s antenna, and the device shows when it is pointed toward the beacon.
Omnirange
is a short-range navigation system for aircraft flying over land. It makes use of ground stations shown on an aeronautical chart. They are called VOR’s, for Very High Frequency Omnidirectional Range. The stations and the aircraft transmit radio waves to each other. These radio waves help determine the aircraft’s range and bearing to each station.
The military uses a similar navigation system called TACAN, standing for Tactical Air Navigation. When VOR and TACAN equipment are on the same ground station, the station is called a VORTAC station. Almost all commercial airports have such stations.
Satellite navigation
makes use of radio signals broadcast from artificial satellites orbiting Earth. There are several satellite-based navigation systems. Perhaps the best known is the Global Positioning System (GPS). The GPS includes at least 24 Navstar satellites, which are controlled by the United States Air Force. The word Navstar stands for Navigation Satellite Tracking and Ranging.
The satellites circle Earth in six different orbits, with four or more satellites spread along each orbit. Each satellite broadcasts its exact position and time. A receiver picks up signals from at least three different satellites, analyzing how long each signal takes to reach it. The receiver then determines its location by calculating its distance from these satellites.
GPS makes accurate navigation possible in any weather condition. Many drivers, boaters, and hikers use a portable GPS receiver that displays their position on an electronic map. Many cell phones are also equipped with GPS receivers.
In addition, GPS signals serve as accurate time-keepers. Each satellite holds an extremely accurate atomic clock. The satellites broadcast their times to receivers. These signals are used around the world to synchronize communication systems—that is, to make them agree on what time it is.
Another satellite navigation system, GLONASS, is controlled by Russia. The European Commission and the European Space Agency built a satellite navigation system called Galileo. China also built a global navigation system as part of its BeiDou satellite system. The name BeiDou comes from the Chinese name for the Big Dipper constellation.
Other radio navigation systems
include loran and omega. They have been replaced by satellite navigation systems.
Loran stands for long range navigation. It was used to guide ships and airplanes as they approached coastal waters from the sea. Multiple loran stations sent out low- or medium-frequency radio waves, which vehicles could detect over long distances. The vehicle’s position could be determined accurately by comparing the signals from different stations.
Omega broadcast even lower frequency radio waves than loran did. It used just eight transmitters throughout the world and required international cooperation to operate.
Inertial guidance
involves the use of a computer and a type of gyroscope called an inertial measurement unit (IMU). The IMU constantly monitors changes in the vehicle’s motion and sends this information to the computer. Specifically, IMU’s measure the force produced when a vehicle accelerates. The computer uses these measurements to calculate the distance and direction traveled, and to continuously update the vehicle’s velocity and acceleration. Inertial navigation is used to guide satellites, aircraft, ships, and submarines. See Inertial guidance.
History
In ancient times, sailors navigated by observing heavenly bodies and constellations. They also studied the seasonal patterns in wind direction. During the Middle Ages (about the A.D. 400’s through the 1400’s), navigators drew simple charts. These charts included wind directions for different seasons, in addition to compass directions.
Early instruments.
Many of today’s navigation instruments developed from crude equipment used hundreds of years ago. For example, the first compass consisted of a magnetized piece of metal on a straw. The straw floated in a container of water.
For many centuries, the primary navigation tool was the sextant. Navigators used it to calculate a ship’s latitude by measuring the angle of the sun or of a star above the horizon. The sextant developed from the astrolabe and the quadrant, two early instruments for measuring the angle between a celestial body and the horizon. An astrolabe consisted of a disk, along with a sight that turned on a pivot. A quadrant had the same arrangement of two mirrors, one fixed and one adjustable, as a sextant had. A quadrant could measure larger angles than could the sextant. It could measure up to 90 degrees, or one-fourth of a circle, compared with the sextant’s 60 degrees, or one-sixth of a circle. But navigators eventually preferred the sextant because it was more portable, and modern sextants can move up to 120 degrees.
Explorers.
In the 1700’s, the British navigator James Cook became the first person to use highly developed celestial navigation methods. Cook used these techniques in his three voyages to the Pacific Ocean.
The Lewis and Clark expedition, led by U.S. Army officers Meriwether Lewis and William Clark, used a quadrant and a sextant on their expedition up the Missouri River in 1803. The mountainous terrain often made it impossible to see the horizon. Instead, Lewis and Clark used a mirror and a level to create an “artificial horizon” from which to measure the relative angles to the sun and moon.
Electronic navigation.
The development of radio, followed by its use on ships and airplanes in the early 1900’s, marked the start of electronic navigation. During World War II (1939-1945), German scientists developed navigation systems to guide V-1 and V-2 rockets. Among the Allies, British and American scientists made important advances in radar technology that aided navigation.
Satellite navigation began with the launch of Sputnik in 1957. Scientists realized that a “constellation” of such satellites could be used for worldwide navigation. In 1960, the U.S. Navy launched a navigation satellite called TRANSIT 1B. The TRANSIT satellite navigation system operated from 1964 to 1996. It consisted of six operating satellites in widely spaced orbits, in addition to several spares. Several TRANSIT satellites are still in orbit and are used in scientific research.
In 1979, Canada, France, the United States, and the Soviet Union developed the COSPAS-SARSAT search and rescue satellite system. In this system, a transmitter on the ground sends a signal to a satellite. When an aircraft or ship is in distress, its SARSAT transmitter is activated automatically. This transmitter is known as an Emergency Position Indicating Beacon, or EPIRB. One or more satellites receive the signal and locate the EPIRB. The system broadcasts this information to ground stations throughout the world, beginning search and rescue operations. The COSPAS-SARSAT system has helped save thousands of lives. Many countries share the responsibilities for its operation.
In the 1970’s, the United States began to launch the first several satellites of the Navstar GPS. The GPS system became fully operational in 1995.
