Radar
Radar - It makes you visible
Bats have the ability to navigate and catch insects even in total darkness. However, bats do not have a sensitive auditory sense nor an outstanding smell nor a sharp vision. What are the reasons for these? In fact, bats have the complicated structure of ears which can emit pulses of high-frequency sounds that are reflected back as echos to a bat's ears from surrounding surfaces, indicating the position, relative distance, and even the character of objects in its environment.
1. Bat sends out pulses of high-frequency sound.
2. Sound bounces off insect in bat’s path and returns to bat.
3. Bat may alter course as it repeats pulses.
4. Bat uses pattern of echoes to determine how close insects is.
5. With continual feedback, bat homes it on prey and captures a meal.
Human beings also have one thing, which is more powerful than bats’ ears. That is radar. In 1935, Sir Robert Watson-Watt, a British physicist, discovered a practical radar system, which helped England withstand the German Nazis’ attack during the World War II.
RADAR
Introduction
Radar, an electronic system, is used to locate objects beyond the range of vision, and to determine their distance by projecting radio waves against them.
The term radar is derived from the phrase "radio detection and ranging," and this name was used by the U.S. and its allies during World War II for a variety of devices concerned with radio detection and position finding. Such devices not only indicate the presence and range of a distant object, called the target, but also determine its position in space, its size and shape, and its velocity and direction of motion. Although originally developed as an instrument of war, radar today is used extensively in many peacetime pursuits, which include controlling air traffic, detecting weather patterns, and tracking spacecraft.
Principle
All radar systems employ a high-frequency radio transmitter to send out a beam of electromagnetic waves, ranging in wavelength from a few centimeters to about 1 m (about 3 ft). Distant objects in the path of the beam reflect these waves back to the transmitter.
Development
The principles by which radars function were known from the very early days of research into radio phenomena. Several attempts were made in the 1920s and early 1930s to devise useful radars, primarily to assist in ship collision avoidance. It was not, however, until just prior to the World War II that it became practically useful and attained the prominence it now holds for early warning, air traffic control, weapon control, ship and aircraft collision avoidance, and weather sensing. A summary of the milestones in the development of modern radars follows.
I. Continuous Wave Radars
The basic concepts of radar are based on the laws of radio-wave reflection, which are inherent in the equations governing the behavior of electromagnetic waves developed by the British physicist James Clerk Maxwell in 1864.
Between 1886 and 1920s, the principles by which radar operates were formulated and preliminary techniques derived.
In 1886-1888, Heinrich Hertz demonstrated the generation, reception, and scattering of electromagnetic waves in the radio bands.
In 1903-04, Christian Hulsmeyer developed and patented a primitive form of collision avoidance radar for ships. (The earliest proposed application of radiolocation)
In 1922, Guglielmo Marconi proposed a scheme for navigating ships by radio waves. He conducted an experiment in which a ship entered a harbour without visual assistance. The experiment inspired Marconi to make a high profile speech on 20 June 1922 in New York prophesying radar, a logical development of direction finding. Most of these early radars were Continuous Wave.
II. Pulsed Radars
The mid-1920s saw the introduction of pulsed radars.
The first successful radio range-finding experiment occurred in 1924, when the British physicist Sir Edward Victor Appleton used radio echoes to determine the height of the ionosphere, an ionized layer of the upper atmosphere that reflects longer radio waves.
In 1925 the first short-pulse echo from the ionosphere was observed on a cathode ray tube by G. Breit and M. Tuve of Johns Hopkins University.
Development of the latter was impossible until electronic techniques and equipment were improved in the 1930s.
During 1934, the first photo of a short-pulse echo from an aircraft was made by R.M. Page of the U.S. Naval Research Laboratory (NRL). NRL was destined to play an important role in the development of radar, a role which continues today. The first demonstrations of short-pulse range measurement of aircraft targets were made in 1935 by British and German scientists.
Operational radar systems came about in the mid-1930s. The first, the Chain Home system, was installed in Britain in 1937. It was designed by Sir Robert Watson-Watt and played a critical role in the Battle of Britain, pinpointing the location of German raids and allowing the Royal Air Force (RAF) to concentrate its forces in repelling these raids rather than having to search for enemy aircraft by patrolling.
Churchill said that three things played decisive roles in winning the Battle of Britain: the RAF pilots, the Spitfire fighter, and Radar.
The United States installed its first operational shipboard radar, the XAF, on the battleships USS New York. It had a surface search (for ships) range of 12 miles and an air search range of 85 miles.
Chain Home Radar Chain Home Low Radar
III. Microwave Radars
The single most important step in bringing about microwave radars was the development of resonant-cavity magnetron - the most important advance made in the technology of radar during World War II. It was invented by the physicist Henry Boot and biophysicist John T. Randall in 1939.
The magnetron is a tube capable of generating high-frequency radio pulses with a large amount of power, thus permitting the development of microwave radar, which operates in the very short wavelength band of less than 1 cm, using lasers. Microwave radar, also called LIDAR (light detection and ranging), is used in the present day for communications and to measure atmospheric pollution.
The cavity magnetron gave the Allies the advantage over German radar equipment, which was more like a scientific instrument in stability and precision of performance, but less innovative and functional in a battle situation.
The advanced radar systems developed in the 1930s played an important role in the Battle of Britain, an air battle from August through October 1940, in which Adolf Hitler's Luftwaffe failed to win control of the skies over England. Although the Germans had their own radar systems, throughout the rest of the war the British and the Americans were able to maintain technical superiority.
World War II changed the world in many ways, including the establishment of radar as an indispensable tool for remote sensing of the enemy and the directing of weapons toward that enemy. One area involved the development of tracking radars to more accurately locate enemy aircraft.
IV. Airborne Radars
Airborne radars also played a large role in the war. They proved particularly useful in finding Axis submarines and in guiding bombing raids over Europe in bad weather and at night. The development by the Allies of the magnetron allowed American and British radars to operate in the microwave bands and to produce small, light-weight airborne radars. Much of the world War II effort was directed to this end.
Operation
Radio waves travel at the speed of light (about 300,000 km/sec). Radar equipment consists of a transmitter, an antenna, a receiver, and an indicator.
The transmitter broadcasts a beam of electromagnetic waves by means of an antenna, which concentrates the waves into a shaped beam pointing in the desired direction. When these waves strike an object in the path of the beam, some are reflected from the object, forming an echo signal.
The antenna collects the energy contained in the echo signal and delivers it to the receiver. Through an amplification process and computer processing, the radar receiver produces a visual signal on the screen of the indicator, essentially a computer display monitor.
A typical radar scene
Transmitters
To operate radar successfully, the transmitter must emit a large burst of energy and receive, detect, and measure a tiny fraction (about a billionth of a billionth) of the total radio energy, returned in the form of an echo. One way to solve the problem of detecting the tiny echo in the presence of the enormously strong searching signal is by using the pulse system.
A pulse of energy is transmitted for 0.1 to 5 microseconds; thereafter the transmitter is silent for a period of hundreds or thousands of microseconds. During the pulse, or broadcast, phase the receiver is isolated from the antenna by means of a TR (transmit-receive) switch; during the period between pulses the transmitter is disconnected from the antenna by means of an ATR (anti-TR) switch.
Continuous-wave radar broadcasts a continuous signal rather than pulses. Doppler radar, which is often used to measure the speed of an object, such as an automobile or a baseball, transmits at a constant frequency. Signals reflected from objects that are moving relative to the antenna will be of different frequencies because of the Doppler effect. The difference in frequency bears the same ratio to the transmitted frequency as the target velocity bears to the speed of light. If a radar receiver is so arranged that it rejects echoes that have the same frequency as the transmitter and amplifies only those echoes that have different frequencies, it shows only moving targets. Such a receiver can pick out vehicles moving over terrain in darkness. In this way, police measure the speed of cars.
Antennas
Radar antennas must be highly directional; that is, they must produce a comparatively narrow beam. Because the width of the beam produced by the antenna is directly proportional to the wavelength of the radiation and inversely proportional to the width of the antenna, and because large antennas in mobile radar units are impracticable, it became necessary to develop microwave radar. Other advantages of microwave radar are its lower susceptibility to enemy countermeasures, such as jamming, and the greater resolution of targets. The necessary movement of the radar beam is obtained by moving the antenna; this movement is called scanning. The simplest form of scanning involves the continuous, slow rotation of the antenna.
Ground radars used for detecting aircraft often have two radar sets, one of which is scanned horizontally to detect the aircraft and determine its azimuth, or horizontal angular distance, and the other is scanned vertically after an aircraft has been reported, and determines the elevation of the aircraft. Many new radar antennas employ arrays with electronic steering.
Receivers
An ideal receiver must amplify and measure an extremely weak signal at extremely high frequency. The very high frequency of the radar signal requires an oscillator and a mixer with a precision far beyond that required in ordinary radio receivers, but suitable circuits have been developed, employing as oscillators high-power microwave tubes called klystrons. The intermediate frequency is amplified in conventional fashion. The signal is then fed to a computer.
Computer Processing
Most modern radars convert received analog signals to a string of numbers by means of an analog-to-digital converter. The numbers are processed by a high-speed computer to extract information about the target. First, the signal returns from the ground, where unwanted objects are removed by a moving target indicator (MTI) filter. Next, the signal is resolved into separate frequency components by means of a fast frequency transformer (FFT). Finally, after signals from multiple pulses are combined, target detection is determined by the constant false alarm rate (CFAR) processor.
The primary function of radar systems is to detect targets must indicate either the presence or the absence of a target. If the target is actually present, the radar will either correctly detect it or incorrectly miss it. If the target is not actually present, the radar may correctly indicate that no target is present, or it may set off a false alarm. The CFAR computer must balance detections against false alarms in an optimal manner.
Radar Displays
Modern radar displays resemble fancy video-game terminals. Target detection, speed, and position may be overlaid onto maps showing roads or other prominent features. Some airborne and space-based radar process the ground returns and display a high-resolution map of the terrain. Objects as small as a truck can often be seen at a distance of many miles, even at night during rainy conditions. Most of the recent advances in radar displays and processing are the result of advances in computer and high-speed electronics.
Pulse Modulator
A conventional radar set has another important component: the pulse modulator. This device draws current continuously from a power source such as a generator and delivers pulses of the necessary voltage, power, duration, and spacing to the magnetron in the transmitter. The pulse must start and end suddenly, but the power and voltage should not vary appreciably during the pulse.
Recent developments
Before and during World War II, several laboratories were established to develop radar, the most famous being the Radiation Laboratory (Rad Lab) at the Massachusetts Institute of Technology (MIT). Virtually all of the techniques used in modern radars were identified in this massive effort, although many of these discoveries were not implemented until recently because the technological advances required lagged far behind the theoretical.