The Very, Very Early Days
The oldest known precursor to modern radar systems evolved in bats millions of years ago, and is known to us today as sonar2. Bats emit a short 'cry' from their noses, receiving the echo with a set of two antennae which happen to be ears. True, a bat's radar doesn't use electromagnetic rays, but the working principle is the same as that of a modern radar, with a chirped signal, target-tracking by Doppler estimation, PRF agility, terrain avoidance function, and fine angle measurement based on the monopulse principle.
The oldest radar warning device also was developed millions of years ago. Tiger moths (which frequently appear on bats' menus) are equipped with ears that can detect and jam the ultrasonic signal of a bat, and they have also developed tactics to evade a bat's attack. Thus, Electronic Combat also came into being a long time ago.
Heinrich Hertz
Heinrich Hertz in Germany calculated that an electric current swinging very rapidly back and forth in a conducting wire would radiate electromagnetic waves into the surrounding space (today we would call such a wire an "antenna"). With such a wire he created (in 1886) and detected such oscillations in his lab, using an electric spark, in which the current oscillates rapidly (that is how lightning creates its characteristic crackling noise on the radio!). Today we call such waves "radio waves". At first however they were "Hertzian waves, " and even today we honor the memory of their discoverer by measuring frequencies in Hertz (Hz), oscillations per second--and at radio frequencies, in megahertz (MHz).
Hertz lived from 1857 to 1894 and was the first to demonstrate experimentally the production and detection of Maxwell's waves. This discovery of course lead directly to radio.
RADAR History
The history of radar will show that it began in 1904. German engineer Christian Hulsmeyer created an apparatus capable of detecting an object’s presence some distance away. However, no single scientist invented the modern radar; scientists from several nations worked on it, especially during the 1930s and 40s.
Pioneers in Radar Research
Hulsmeyer received a patent for his invention in 1904. However it was Nikola Tesla who discovered that frequency could be used to detect the presence of vehicles as well as their course.
The succeeding years saw American and European scientists develop various radar devices. Coming on the heels of World War I, nations began to realize how important it could be for warfare.
One of the pioneers in the history of radar development was the Frenchman Emile Girardeau in 1934. He got a patent for his work. In 1935 it was put in the Normandie liner. Also that year, America had its first monopulse radar courtesy of Dr. Robert Page.
The Russian engineer P.K. Oschepkov invented the RAPID. It could sense the presence of a vehicle within 3 km. A similar model was produced in Hungary a year later by Zoltan Ray.
In 1922, Guglielmo Marconi gave a speech which demonstrated that he had a clear idea that it was possible to detect remote objects by radio signals. But it was not before 1933 that he was able to show a first working device.
In 1925/26, the American physicians Breit and Tuve, as well as the British researchers Appleton and Barnett, performed measurements of the Earth's ionosphere, using a pulsed radio transmitter which could be described as a radar.
It was 1928 when HM Signal School of the UK received the first patent on Radio Location, credited to L. S. Alder.
In 1930, a team of engineers from the US Naval Research Lab performed measurements of a radio antenna, and more or less by serendipity they independently discovered radar. Their radio link happened to stretch across an aircraft landing strip, and the signal quality changed significantly when an aircraft crossed the beam.
In 1933 when Hitler took over power in Germany, the German Kriegsmarine (Navy) started research into what they called Funkmesstechnik, or remote radio measuring technology.
Research in Russia began in 1934, but was somewhat hindered by quarrels between different authorities. However, one of the earlier devices was a success, with a 70km detection range against aircraft.
Robert Watson’s Radar System
But it was Robert Watson’s invention that showed the radar’s full potential. In 1935, Watson showed his work to the British Air Ministry. During this time, the British were more concerned about the alleged German death ray.
Radar (for RAdio Detection And Ranging) was developed over the years with input from many sources, but it was Robert Watson-Watt, a Scottish physicist looking for a reliable method to help airmen locate and avoid approaching thunderstorms, who designed the first set put into practical use. Watson-Watt realized, as he perfected his device, that radio waves could be used to detect more than storms.
A Royal Air Force Heyforth bomber was used for the War Ministry demonstration at Daventry. Three times the plane passed overhead and three times the main beam of a BBC short-wave radio transmitter picked up reflected signals.
Impressed, the air ministers embraced the new technology and by September 1939, when war broke out in Europe, the British had a network of radar installations covering the English Channel and North Sea coasts.
It was radar, even more than the pluck of the dashing RAF pilots, that tipped the scales in England's favor in the Battle of Britain.
Christian Andreas Doppler
Doppler RADAR is named after Christian Andreas Doppler. Doppler was an Austrian physicist who first described in 1842, how the observed frequency of light and sound waves was affected by the relative motion of the source and the detector. This phenomenon became known as the Doppler effect.
This is most often demonstrated by the change in the sound wave of a passing train. The sound of the train whistle will become "higher" in pitch as it approaches and "lower" in pitch as it moves away. This is explained as follows: the number of sound waves reaching the ear in a given amount of time (this is called the frequency) determines the tone, or pitch, perceived. The tone remains the same as long as you are not moving. As the train moves closer to you the number of sound waves reaching your ear in a given amount of time increases. Thus, the pitch increases. As the train moves away from you the opposite happens.
This discovery of C. A. Doppler led to the invention of Doppler Radio detection system named Doppler Radar.
Hitler's strategic aerial onslaught, meant to clear the skies over the Channel and southeastern England preparatory to an invasion of the British Isles, might have succeeded if not for radar. The RAF was outnumbered by the Luftwaffe, and radar saved already-stretched Fighter Command from having to maintain constant air surveillance.
With radar providing an early-warning system, well-rested RAF pilots could be scrambled and rising to meet the incoming enemy formations in a matter of minutes. As the German fighters ran low on fuel and were forced to turn back, the Spitfires and Hurricanes could pick off the German bombers as they moved deeper into England.
World War II and the Cold War
In 1937, a prototype RDF station (called Chain Home or CH) was built at the Bawdsey Research Station and handed over to the Royal Air Force. The CM station operated at a frequency of 22 MHz (13.6 m wavelength) and aircraft at 3,000 m (10,000 ft) as far out as 150 km (80 miles) could be spotted in good weather. In rain, a range one half or more, depended on the target altitude. By September 1939 at the outbreak of war, 20 CH stations were operational. Because of the low operating frequency which is in the radio range, it has often been questioned if the CH was really a radar set or simply a RDF system.
Later, frequencies from 22 to 55 MHz were used to avoid interference between adjacent towers, noise or jamming. Although the CH had an elevation limit from 1.5 –16 degrees and could not detect low flying aircraft, the system worked and was used in the Battle of Britain.
Development work in 1937 led to "beamed radar" for airborne sets and for Coastal Defense (CD) radar that operated on 1.5 m wavelength. The CD system was also called the Chain Home Low (CHL). The CHL used a rotating antenna, which rotated at 1-2.3 rpm and had a range of 160 km with an azimuth accuracy of 1.5 degrees. A PPI display was used. Height could not be determined. By 1941, 11 CHL were operational. The use of higher frequency made the radar less dependent on weather.
The Navy used a similar set to the CHL. Called the type 281; it was tested on the HMS Dido in October of 1940 and the HMS Prince of Wales in January of 1941. Over 59 sets were produced during the war. This set could operated on a wavelength of 50 cm and it could locate ships up to a distance of 20 km.
World War II saw more rapid developments in radar technology. Both the British and the Germans were engaged in a race to produce larger and more sophisticated radars. However the Germans (as did the Japanese) were not able to fully harness it. It was the British that were able to utilize it more effectively.
The Cold War led to the development of more sophisticated radar systems. One of them was the Pinetree Line established by the US in the early 1950s. This was followed up by the DEW Line. This was followed by the ballistic Missile Early Warning System.
Radar Configurations
The history of radar has seen the appearance of various types of configurations and systems. These include continuous wave radar, Doppler radar, monopulse radar and Bistastic radar. Different types of radars are used for biological work and tackling the weather.
Different frequencies are also used. For coastal radar systems, the frequency range is 3 to 30 MHz. For ground breaking it is 30 to 330 MHz; the ballistic missile warning system uses 300 to 1000 MHz. For air traffic control it is 1 to 2 GHz. For missile guidance it is 8 to 12 GHz. The frequency for airport surveillance is 24 to 40 GHz.
The history of radar has been marked by rapid development and this continues to this day. Today, it is also being used in air traffic control, sensing of traffic violators and has meteorological uses as well.
Generic Block diagram of Radar
Operation
It is convenient to consider radars composed of four principal parts: the transmitter, antenna, receiver, and display (see illustration).
Block diagram of a pulse radar.
The transmitter provides the rf signal in sufficient strength (power) for the radar sensitivity desired and sends it to the antenna, which causes the signal to be radiated into space in a desired direction. The signal propagates (radiates) in space, and some of it is intercepted by reflecting bodies. These reflections, in part at least, are radiated back to the antenna. The antenna collects them and routes all such received signals to the receiver, where they are amplified and detected. The presence of an echo of the transmitted signal in the received signal reveals the presence of a target. The echo is indicated by a sudden rise in the output of the detector, which produces a voltage (video) proportional to the sum of the rf signals being received and the rf noise inherent in the receiver itself. The time between the transmission and the receipt of the echo discloses the range to the target. The direction or bearing of the target is disclosed by the direction the antenna is pointing when an echo is received.
A duplexer permits the same antenna to be used on both transmit and receive, and is equipped with protective devices to block the very strong transmit signal from going to the sensitive receiver and damaging it. The antenna forms a beam, usually quite directive, and, in the search example, rotates throughout the region to be searched. See also Antenna (electromagnetism).
The radar reflections are among the signals received by the antenna in the period between transmissions. Most search radars have a pulse repetition frequency (prf), antenna beam-width, and rotation rate such that several pulses are transmitted (perhaps 20 to 40) while the antenna scans past a target. This allows a buildup of the echo being received. Most radars are equipped with low-noise rf preamplifiers to improve sensitivity. The signal is then “mixed” with (multiplied by) a local oscillator signal to produce a convenient intermediate-frequency (i-f) signal, commonly at 30 or 60 MHz; the same principle is used in all heterodyne radio receivers. The local oscillator signal, kept offset from the transmit frequency by precisely this intermediate frequency, is supplied by the transmitter oscillators during reception. After other significant signal processing in the i-f circuitry (of a digital nature in many newer radars), a detector produces a video signal, a voltage proportional to the strength of the processed i-f signal. This video can be applied to a cathode-ray-tube (CRT) display so as to form a proportionately bright spot (a blip), which could be judged to originate from a target echo. However, increasingly radars use artificial computerlike displays based on computer analysis of the video. Automatic detection and automatic tracking (based on a sequence of dwells) are typical of such data processing, reports being displayed for radar operator management and also made instantly available to the user system. See also Cathode-ray tube; Electronic display; Heterodyne principle; Mixer; Preamplifier; Radio receiver.
Radar carrier frequencies are broadly identified by a nomenclature that originated in wartime secrecy and has since been found very convenient and widely accepted. The spectrum is divided into bands, the frequencies and wavelengths of which are given in the table. The charged layers of the ionosphere present a highly refractive shell at radio frequencies well below the microwave frequencies of most radars. Consequently, over-the-horizon radars have been built in the 10-MHz area to exploit this skip path. See also Continuous-wave radar; Monopulse radar; Radio spectrum allocations.
Source: Various)
0 comments:
Post a Comment