Signaling is one way of passing information from one person or place to another. Signals are used when direct and unaided voice or direct written communication is impossible or undesirable.
There are three main forms of signals: (1) electrical, (2) visual, and (3) sound. Various electrical, mechanical, and hand devices are used to transmit signals. International codes—such as the International Morse Code and the international flag code—have been developed for some kinds of signals. Such signals are the same throughout the world and can be understood anywhere. For more information on these, see Flag (Flags that talk); Morse code.
Methods of signaling
Visual and sound signals of one kind or another have been used since ancient times. Early methods included trumpet sounding, drum beating, flag and smoke signals, and beacon fires. Electrical signals were first used in the early 1800’s.
Visual and sound signals
still have many uses today. People create sound signals with a variety of devices. These devices include bells, guns, horns, sirens, and whistles.
Visual signals are often made with flags. Semaphore signaling is done with two hand flags. The sender holds the flags in various positions to represent the letters of the alphabet and to give several other meanings. Flag hoist signals are made by hanging colored flags from crosspieces on the masts of ships.
Visual signaling can be done with colored lights, with each light having a different meaning. Lights are often flashed or blinked according to some code. Signals also may be passed by reflecting sunlight with mirrors. Fireworks used for signals include rockets, flares, smoke, and cartridges or shells fired from guns.
Electrical signals
can be transmitted in various ways. These include facsimile, radio, radar, television, telegraph, and telephone. Communications satellites are used to carry electrical communication to any part of the world. Coaxial cables are also used to carry several types of electrical communication. Fiber-optic systems translate electrical signals into flashes of light. These flashes of light are transmitted through transparent fibers of glass or plastic.
Important uses of signals
Throughout history, signals have been used in a wide range of activities. This article discusses only a few important uses. For information on other uses of signals, read the separate articles listed in the Related articles at the end of this article.
Military signaling.
Armies use signals in such modern electrical communications as radio, radar, telephone, and television. In addition, armies use messengers and visual and sound signals. Visual signals include arm and hand signals, hand flags, panels, lights, and fireworks. Sound signals include horns, gunshots, and whistles.
Aircraft signaling.
Radio signals are passed between ground stations and aircraft providing instructions for take-off and landing, the position of other planes, and weather information. In addition, some airplanes are equipped with an automatic direction finder, or radio compass, and use omnidirectional radio range equipment. This equipment enables pilots to locate their position at all times. Radar is used to guide aircraft to safe landings in any kind of weather and to guide military aircraft to their targets. Visual signals used to aid pilots include beacons, running lights, wind socks, and approach lights.
Marine signaling.
Various means of electrical communication are used by both commercial and naval vessels. Radio is used extensively in ship-to-ship communication, and for controlling naval aircraft. Radio also connects ships at sea with stations ashore. Radar helps reveal the position of ships. Enemy submarines and explosive mines can also be detected by an echo-sounding device called sonar.
Lights, semaphore flags, and flag hoists are the most common means of visual signaling at sea. Sound signals include electrical sound-producing devices and bells, gongs, and whistles on buoys (floating markers) that are operated by the motion of the sea. Lights and fog signals on lighthouses, lightships, and buoys warn that ships are approaching land or dangerous hidden objects.
Distress signals.
A ship or aircraft in need of help can send several internationally agreed upon distress signals. Two of the best known are the radio signal SOS in code, which is used at sea, and the aircraft call of Mayday sent over voice radio. Other well-known distress signals include a gun fired at short intervals; a continuous sounding of fog signals; colored flares thrown from shells or rockets; the international flag signal NC; and flying the national flag upside down.
