Radar

• Type of physical science: Classical physics
• Field of study: Electromagnetism

Radar is a technology that transmits and receives electromagnetic waves in order to detect, locate, and gauge the speed of distant objects. Reflected waves are analyzed using mathematically derived equations, designed to overcome natural phenomena that affect the propagation of those waves.

Overview

The term "radar" is derived from the function it performs: radio detection and ranging.

It is a technology with many applications stemming from its ability to provide detailed images of objects at a distance regardless of location, visibility, weather, or other environmental conditions that might preclude observation. It can locate objects, track them, and establish the speed at which they are moving. Radar transmits and receives radio waves that bounce off distant objects.

It should not be confused with sonar (sound navigation ranging), a process whereby sound waves, not electromagnetic waves, are propagated through water and reflected to acquire information about objects under water. Despite the difference in approach, however, the goal for each is much the same: to detect, locate, and estimate the speed of objects.

Radio waves move through the atmosphere at a known speed in a way that can be projected mathematically. An echo indicates that an object is out there somewhere, and analysis of the returning signal provides information about the direction of that object from the transmitter. By calculating the amount of time it takes for the radio signal to move from transmitter to target and back, the operator arrives at an accurate measure of the distance. In existence since shortly before World War II, radar's development has been rapid, its deployment widespread.

There are two kinds of radar: monostatic and bistatic. Monostatic radar sends and receives signals from the same location, while bistatic radar transmits from one location and receives at a location or several locations at some distance from the transmitter.

Techniques have been established for determining the speed of objects using the Doppler frequency-shift relationship between transmitted and received signals. The nature of the object being observed—size, shape, and reflectivity—can also be deduced by comparing the waveform of the transmitted signal to the echo signal. Radar can also be programmed to lock onto targets for purposes of tracking them on the fly.

Radar information is displayed most often on a screen designed to be an electronic window whose field of vision is representative of the geographic area covered by the radar system. As signals are transmitted, the screen characterizes the echoes in ways that are immediately meaningful to the operator. While the image of an airplane on a radar screen does not necessarily look like a real airplane to the air traffic control officer, that image is distinct and instantly recognizable. There can be no mistake as to what the screen is showing.

It is important to note, however, that radar is affected by a number of factors that interfere with its accuracy. Many of these natural phenomena are both predictable and unpredictable. For example, radar signals behave differently in free space from the way they behave in the atmosphere. In the atmosphere, a number of conditions exist that affect the way radar waves behave. In the troposphere and ionosphere, for example, conditions are such that special mathematical techniques are required to calculate how radar signals will perform. Even close to the ground, radar waves appear to be affected by gravity and terrestrial electromagnetism, displaying a tendency to drop toward Earth as they travel. As a result, radar echoes could be misinterpreted except for techniques designed to compensate for this phenomenon.

Weather conditions also play a role in determining how radio waves behave as they pass through the atmosphere. It is known, for example, that these waves are sensitive to relative humidity in the atmosphere. Over the years, radar technicians have developed techniques for circumventing obstacles such as fog, rain, and snow. Conversely, when weather patterns are ideal, radar systems with specified ranges of 30 to 50 kilometers have picked up objects hundreds of kilometers away. Despite the unpredictable nature of radar during operation, current radar technology makes that unpredictability insignificant.

Applications

Radar has found many applications. It is used to locate and track aircraft as they fly across the horizon, to monitor the speed of automobile traffic on the highway, and to guard against unexpected enemy missile launches. It provides in-flight control for guided missiles and serves as an aid to navigation in the maritime industry. It is also an important tool for forecasting weather and for studying natural phenomena in the atmosphere. As the number of artificial Earth satellites continues to climb, radar is used to keep track of their locations and, where they occupy non-geostationary orbits, to follow them across the sky. It is also being employed in the study of the planets in the solar system and their satellites. In almost all cases, radar provides information that is impossible to gather any other way.

Over the centuries, a key function of all organized societies has been surveillance—keeping track of the events and issues that impact the daily lives of individual members of those societies. The survival of societies and cultures has often hinged on their ability to know about and prepare for what is happening around them, especially when such events call for defensive measures. In the last half of the twentieth century, the speed at which objects could be propelled through the atmosphere increased exponentially, making it possible for missiles and other craft to hit targets anywhere in the world with little warning. Radar is best known for its role as the major tool employed at the front lines of the early warning missile defense system—the main link in a vast defense network that stretches for thousands of kilometers along the northern frontier of North America.

The number of aircraft aloft at any given time has also increased significantly, creating a demand for information that can be used to help control air traffic. Radar systems are located at airports to monitor air traffic within specific geographic areas. Controllers use this information to place aircraft in proper takeoff and landing patterns to minimize the risk of mid-air collision and to keep traffic moving according to specified timetables. They are also able to track aircraft in flight, providing detailed and extremely accurate information about air speed, altitude, and location.

Airport-based radar systems are also used to monitor weather patterns that may affect aviation. These systems are usually operated by the National Oceanic and Atmospheric Administration (NOAA) to provide weather information to pilots and the general public. In the early twenty-first century, some broadcast weather services have begun employing special radar systems that provide extremely accurate and detailed imagery of weather formations located within the broadcast area of client stations. This information is useful to farmers, commuters, public service agencies, boaters, and many others. Weather radar is useful, especially in tracking large storm systems that cause widespread damage and disruption of essential services. With advance warning, residents can make preparations to ensure personal safety and minimize property damage.

Artificial Earth satellites are used increasingly to transmit or relay electronic signals over widespread areas of the Earth’s surface. Since the 1960s, more and more satellites have been placed in orbit to perform a multitude of tasks; some—called geostationary satellites—are placed directly over the equator and caused to move in the same direction at the same relative speed as Earth turns on its axis. This technique makes it appear that the satellites are stationary in space. It is extremely useful in deploying communications satellites, whose signals are transmitted and received by stationary uplinks and downlinks. Many other satellites are not stationary, however, and require tracking by land-based radar to control their operation and maintain fixes on their positions. Examples would be Landsats, Seasats, and other satellites designed to take high-resolution photographs of the Earth's surface from relatively low orbits. These satellites are designed to fly over the Earth's entire surface at prearranged speeds and trajectories. Occasionally, they are moved from one orbit to another by firing on-board maneuvering rockets. Some geostationary satellites are also moved from one position to another along the equatorial plane to accommodate the growing demand for satellite communications among continents.

Satellites are also used for military reconnaissance to support land-based radar and other surveillance technologies. While the advanced imaging systems aboard such satellites are not true radio detection and ranging, they evolved from radar, a successful solution to the problem of detecting, locating, and visualizing extremely distant objects through atmospheric conditions that make traditional photography impossible.

Context

Since World War II, advances in technology have led to a wealth of knowledge; much of it is concerned with an understanding of Earth and the universe beyond. Radar is one of the key technologies that have provided means of exploring the unexplorable; those regions that lie beyond the reach of human observation. In so doing, radar has allowed humans to go where they could not go previously and to prepare for unseen events that can be predicted now on the basis of radar information.

Some applications meet the needs of a changing world today, while others are helping lay the groundwork for future discoveries that will lead to greater understanding. Traffic law enforcement on busy commuter highways is made easier by radar that clocks the speed of oncoming vehicles instantaneously and which, by its very existence, encourages caution by drivers who fear the consequences if caught exceeding the speed limit. In much the same way, warring nations have found that the military advantage of surprise is severely compromised by the presence of radar on all sides, and the threat of retaliation based upon radar information may preclude military action. Radar is also the key to controlling missiles in-flight and makes it possible for them to lock on to moving targets, providing yet another preemptive military capability to discourage armed conflict.

Radar has been employed successfully in many areas of daily life, expanding the ability to control the environment. Airplanes can now fly at night and land at fog-shrouded airports. The ability to track weather systems with radar has improved agricultural production. Astronomers use radar to plot the location of distant planets and other extraterrestrial objects and, in some cases, describe the three-dimensional characteristics of their surfaces. Radar applications are useful and far-reaching.

As happens whenever a new technology appears, radar spawned many more sophisticated systems for locating, tracking, and gauging the speed of land-based and airborne objects. Satellites and other highly sophisticated surveillance equipment can now perform many tasks initially accomplished exclusively by land-based radar systems. Scientists continue learning how to achieve extremely high-quality imagery from afar using infrared scanning and heat-sensing techniques that detect and locate objects, but they can also provide other useful information. This technology provides accurate analytical data about the substances that encompass those objects. It can also determine whether moisture is present on some distant, unseen surface and, if so, in what quantities. In effect, the capabilities of radar have been enhanced to a great extent in the modern age.

Still, radar continues to perform many functions for which it remains the most efficient and economical means. With the assistance of modern-day computers that analyze radar data instantly, the information it provides becomes more detailed.

Radar was the first technology that gave scientists the ability to look beyond the horizon, beyond the normal field of vision, in real time. It provided a window into the unseen world and made it possible to expand the geographical area under humankind's visual control.

The unseen world could now be entered with confidence of the ability to detect and analyze environmental conditions in time to deal with them. In short, it was one of the first technologies that allowed scientists to break the shackles imposed by nature—a sixth sense that could be exploited in the public interest.

Principal terms

DOPPLER SHIFT: the change of frequency of a sound wave as its velocity changes in relation to the velocity of a receiver

EFFECTIVE ECHOING AREA OF TARGET: the area of a target that reflects the strongest signal strength value equally in all directions and returns to the receiver a signal equal in strength to that produced by the target

ELECTROMAGNETIC WAVE: a wave of electricity radiated into space from an antenna

IONOSPHERE: the outer layer of Earth's atmosphere that contains heavily ionized molecules

SONAR: a device that transmits sound waves through water and detects them when they reflect off objects

SOUND WAVE: a wave by which sound is transmitted through matter or through the atmosphere

Bibliography

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Leinwoll, Stanley. From Spark to Satellite: A History of Radio Communication. Charles Scribner's Sons, 1979.

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Reintjes, J. F., and G. T. Coat. Principles of Radar. Mcgraw-Hill, 1952.

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"What Is Radar?" SWISS International Air Lines, kids.swiss.com/en/take-part-and-win/kids-question/what-is-a-radar. Accessed 9 Feb. 2025.