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.
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Bat sends out pulses of high-frequency sound.
Sound bounces off insect in bat’s path and returns to bat.
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Bat may alter course as it repeats pulses.
Bat uses pattern of echoes to determine how close insects is.
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.