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Radar and Sonar
William C. Vergara 

Sometimes, when conditions are right, you can hear your own echo. If you shout "hello," the sound may bounce back at you from a large object. You then hear your own voice coming back. Your returning voice is called an echo. Radar and sonar are electronic devices that use the principle of an echo to detect and locate objects.

Both radar and sonar locate objects from the echo of a signal that is bounced off the object. Radar uses radio waves, which are a type of electromagnetic energy. Sonar uses the echo principle by sending out sound waves underwater or through the human body to locate objects. Sound waves are a type of acoustic energy. Because of the different type of energy used in radar and sonar, each has its own applications.

The word "radar" was formed from the first letters of the term "radio detection and ranging." A radio wave is a type of electromagnetic radiation. (Microwaves, X-rays, and light waves are other types.) It is the fundamental part of this form of technology. "Detection," as used here, means finding an object or target by sending out a radio signal that will bounce back off the target as a radio echo. "Ranging" means measuring the distance to the target from the radar set (the device that sends out the radio signal and picks up the returning echo).

A true radar system uses radio waves. Another system, called optical radar or lidar (from the first letters of the term "light detection and ranging"), is based on the same principle as radar but uses light waves.

How Radar Works
Radar sets, also called radar systems, come in many different sizes, depending on the job they are expected to do. But all have four main parts -- a transmitter, an antenna, a receiver, and an indicator (display screen). The transmitter produces the radio waves. When a radio wave strikes an object such as an airplane, part of the wave is reflected back to the radar set. The signal is detected by the antenna as a radio echo. The returning echo is sent to the receiver, where its strength is increased, or amplified. The echo is usually displayed as an image that can be seen on the indicator.

The usual type of indicator is the plan position indicator, or PPI. On the face of its large tube, the operator sees a map-like picture of the surrounding region. This picture looks as if it were made looking down at the area from high above the radar set. On the indicator, the echoes appear as bright spots, called blips. The blips show where land areas are located. Blips also show the position of targets, such as planes and ships. The radar operator can pick out these targets because they are moving, while the land areas are not.

A common type of radar is called pulse radar. This type of radar sends out radio waves in short bursts, or pulses. The distance to a target is determined by the time it takes the signal to reach the target and the echo to return. Radio signals travel at a known speed -- about 186,000 miles (300,000 kilometers) per second, the speed of light. If the radio signal comes back in 1/1,000 second, then the round trip is 186 miles (300 kilometers). The target must be half that far, or 93 miles (150 kilometers) away.
Pulsed transmission helps determine the distance more accurately. Why is this so? Imagine that you are about to shout across a canyon to make an echo. If you shout a long sentence, the first words will come back before you can finish. It would be impossible to hear the entire echo clearly because it would be mixed with your own speech. But if you shout a short word the echo comes back crisp and clear with no interference from the transmitter (you).

The location of the target in relation to the radar set is found in a different way. The radar antenna sends out radio pulses in a narrow beam, much like the beam of a flashlight. The antenna and its beam are rotated slowly through all possible directions, searching the entire horizon for targets. An echo is reflected from a ship or other object only if the narrow beam happens to strike it. The returning echoes are amplified by the receiver, then go to the indicator, which displays the range and direction of the target.

Uses of Radar
Radar has both military and civilian uses. The most common civilian use is to help navigate ships and planes. Radar sets carried on a ship or located at an airport pick up echoes from other ships and planes and help prevent collisions. On ships, they also pick up echoes from buoys in channels when the ships enter or leave port. Radar sets help commercial airplanes land when visibility is bad or in the event of mechanical failure.

Radar is also used in meteorology, including weather prediction. Weather forecasters use it, normally combined with lidar (optical radar), to study storms and locate hurricanes and blizzards. Doppler radar is based on the principle of the Doppler effect -- that is, the frequency of a wave changes as the source of the wave moves toward or away from the receiver. By analyzing changes in the frequency of reflected radio waves, Doppler radar can track the movement of storms and the development of tornadoes. An improved Doppler radar system called Next-Generation Radar (NEXRAD) can predict weather more accurately and farther into the future.

Scientists use radar to track the migrations of birds and insects and to map distant planets. Because it can tell how fast and in which direction a target is moving, radar is used by police to locate speeding automobiles and control street traffic. Similar systems are used in tennis to measure the speed of serves and to call faults. Balloon-borne radar supports officials fighting drug trafficking. Surface-wave radar detects surface waves of the ocean to warn ships of icebergs and nearby vessels.

Historically, there have been two main military uses of radar: search radar and fire-control radar. Search radar is the kind already discussed. It continually searches the horizon to find targets. Fire-control radar helps aim a gun or missile so that it will hit the target when fired and must be more accurate than search radar. The U.S. military has also developed specialized types of radar. For example, Miniature Synthetic Aperture Radar (MSAR) is used on aircraft to provide high-quality images in all kinds of weather.

History of Radar
Radar technology began with experiments using radio waves in the laboratory of German physicist Heinrich Hertz in 1887. He discovered that these waves could be sent through many different materials but were reflected by others. In 1900, a radio pioneer, Nikola Tesla, noticed that large objects could produce reflected radio waves that are strong enough to be detected. He knew that reflected radio waves were really radio echoes. So he predicted that such echoes could be used to find the position and course of ships at sea.

Pulse radar was introduced in the United States in 1925. In 1935, radar was patented under British patent law partly as a result of the research led by Scottish physicist Sir Robert Alexander Watson-Watt. This patented radar later developed into the radar system that proved effective against German air raids on Britain during World War II (1939-45). The term "radar" was first used by U.S. Navy scientists during that war.

Advances in both military and civilian applications of radar continued throughout the 1900's. By the early 2000's, researchers were targeting their efforts at improving radar's range, quality, imaging, and size and reducing its cost.

The word "sonar" comes from the first letters of "sound navigation ranging." Sonar can detect and locate objects under the sea by echoes, much as porpoises and other marine animals navigate using their natural sonar systems.

How Sonar Works
There are two types of sonar sets: active and passive. An active sonar set sends out sound pulses called pings, then receives the returning sound echo. Passive sonar sets receive sound echoes without transmitting their own sound signals.

In active sonar sets, the sound signals are very powerful compared with ordinary sounds. Most sonar sets send out sounds that are millions of times more powerful than a shout. Each ping lasts a fraction of a second.

Some sonar sets emit sounds you can hear. Other sonar signals are pitched so high that the human ear cannot hear them. These signals are called ultrasonic waves. ("Ultra" means "beyond," and "sonic" means "sound.") The sonar set has a special receiver that can pick up the returning echoes. The location of underwater objects can then be determined by the length of time that elapses between sending the signal and hearing the returning echo.

Uses of Sonar
Sonar has many uses. Submarines use sonar to detect other vessels. Sonar is also used to measure the depth of water, by means of a device called a Fathometer. (One fathom equals 6 feet, or about 1.8 meters.) The Fathometer measures the time it takes for a sound pulse to reach the bottom of the sea and return to the ship. Fishing boats use Fathometers to locate schools of fish.

Oceanographers use sonar to map the contours of the ocean floor. Sound signals can also be sent into the mud or sand on the ocean floor and strike a layer of rock underneath. An echo then comes back, giving the distance to the rock layer.

The same principle is used in searching for oil on land. A sonar pulse is sent into the ground. Echoes come back from the different layers of soil and rock and tell geologists what kinds of soils and rocks are present. This helps them identify areas for drilling that are most likely to contain oil or gas. This subterranean mapping is called seismic exploration.

A special kind of sonar used in medicine is called ultrasonography or echoscopy. High-frequency sound waves produce different echoes when reflected by different body organs. Doctors can use these echoes to detect disease and to monitor the growth of an unborn child.

Extremely high-frequency sound waves are used in medicine and industry to clean many kinds of materials by shaking loose tiny particles of dirt or other matter.

History of Sonar
It was nature itself that invented "sonic radar," or sonar, well before humans did. For example, bats fly in the dark with poor sight without hitting obstacles and locate prey by means of sound pulses humans cannot hear.

In 1906, American naval architect Lewis Nixon invented the first sonar-like listening device to detect icebergs. During World War I (1914-18), a need to detect submarines increased interest in sonar. French physicist Paul Langévin constructed the first sonar set to detect submarines in 1915. At first, these sonar sets could only "listen" to returning signals. By 1918, Britain and the United States had built sonar sets that could send out, as well as receive, sound signals. The U.S. military began using the term "sonar" during World War II. As with radar, new military applications for sonar are constantly being developed. For example, in the early 2000's, the U.S. Navy introduced a sonar system to help clear military mines.
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