Navigation

V

Electronic Navigation

Modern navigators rarely rely exclusively on their own measurements and calculations. They often use position calculations derived by high-tech electronic navigational instruments. These instruments usually can determine positions faster and more accurately than humans. They function in nearly all weather conditions, day or night, and have a range far beyond that of the human senses alone.

A

Global Position System

Global positioning system (GPS) is a navigation system using 24 Earth-orbiting artificial satellites. Satellite navigation, like celestial navigation, works on the principle that if the navigator knows the exact location of a celestial body and can measure a relationship between the craft and the body, the navigator can calculate the craft’s position. In celestial navigation, this relationship takes the form of a celestial body’s altitude above the horizon. In GPS, it is the distance and direction from the navigator’s receiver to two or more satellites orbiting Earth.

GPS consists of three types of components: satellites in space, control devices on land, and receivers on a craft or carried by hand. Twenty-four artificial satellites orbit 20,100 km (12,500 mi) above the Earth’s surface. On land, electronic control devices track the movement of the satellites and send them signals coded with their precise locations. The satellites continuously broadcast signals telling their exact location and the time each signal was sent. A craft’s receiver picks up these signals. When the receiver picks up signals from two satellites, its internal computer calculates the position of the craft and displays it for the navigator to read and use.

GPS receivers, some as small as a cellular phone, are used on boats, ships, airplanes, cars, and ambulances as well as by hikers and mountain climbers. A GPS receiver keeps steady track of its geographical position in latitude and longitude. It can calculate the speed and compass heading of the person or craft carrying it. It also can be programmed with the positions (in latitude and longitude coordinates) of selected destinations, such as campgrounds, harbors, and fishing areas, so that the receiver displays the bearing and distance to a destination. These positions are called waypoints. GPS is accurate to about 9 m (about 30 ft).

---

---

GPS can be combined with other electronic instruments called course plotters to display the craft’s changing position and motion on a monitor, like a television screen. Course plotters show computerized charts and radar data for the area surrounding a vessel or aircraft. Some displays are almost as realistic as video games, depicting tiny boats and the waves in the wakes behind them as they move through the water.

B

Loran-C

Loran (abbreviated from the words long range navigation) works by measuring the time difference in reception of radio signals sent by remote transmitters. The type of loran in use today is Loran-C. In Loran-C, pairs of land-based transmitters simultaneously send radio signals toward each other. Special onboard receiver-computers intercept these signals, then calculate position by measuring the difference in the time of reception. If the craft lies exactly halfway between the two stations, on what is called the centerline, there is no time difference between reception of the two signals. But anywhere else, the craft receives one signal before the other signal. The receiver-computer converts the time difference between signals into a line of position. With two or more pairs of transmitters, the craft’s position can be fixed and displayed in latitude and longitude.

Like GPS, Loran-C can also calculate speed and be programmed with waypoints. Loran is called a hyperbolic navigation system because the lines of time differences are curved like hyperbolas. Loran-C was the leading electronic navigation system from the 1970s until GPS was perfected in the 1990s. It may be phased out early in the 21st century.

C

Radar

Many vessels and aircraft use radar to detect potentially dangerous objects that are out of visual range and also in navigation near landmarks and land. A radar instrument sends out a radio pulse through a rotating antenna, called a scanner. When the pulse hits a target, it bounces back to the scanner. The instrument calculates the time difference between transmission and reception. It converts this information into a visual display on a monitor called a scope, which shows the object as a point of light called a pip. The scope displays the bearing to the pip and the distance to it. This information can be used to quickly produce accurate estimated positions and fixes. Radar is especially effective for ships and boats navigating narrow channels or in areas that have many obstructions.

D

Radio Direction Finder

An electronic navigation instrument called a radio direction finder (RDF) takes bearings on radio beacons, many of which are in lighthouses and towers. Each signal in an area has its own identity consisting of its radio frequency or of Morse code sequences of dashes and dots. Using the same basic techniques used in sight bearings, navigators plot lines of position on a chart extending from the radio beacons back to the vessel. Beginning in the first decade of the 20th century, radio beacons were placed at entrances to harbors, and later at airports, to enable ships or airplanes to home in on and follow the signals to their destination. However, radio beacons have been largely supplanted by more advanced electronic navigation technologies, and most are expected to be dismantled in the 21st century.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain