Paper 2000 Question: 1 (a) Al-Beruni
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| The military uses surface-to-air systems for defense against ballistic missiles as well as aircraft (see Defense Systems). During the Cold War both the United States and the Union of Soviet Socialist Republics (USSR) did a great deal of research into defense against intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs). The United States and the USSR signed the Anti-Ballistic Missile (ABM) treaty in 1972. This treaty limited each of the superpowers to a single, limited capability system. The U.S. system consisted of a low-frequency (UHF) phased-array radar around the perimeter of the country, another phased-array radar to track incoming missiles more accurately, and several very high speed missiles to intercept the incoming ballistic missiles. The second radar guided the interceptor missiles.
Airborne air defense systems incorporate the same functions as ground-based air defense, but special aircraft carry the large area search radar systems. This is necessary because it is difficult for high-performance fighter aircraft to carry both large radar systems and weapons. Modern warfare uses air-to-ground radar to detect targets on the ground and to monitor the movement of troops. Advanced Doppler techniques and synthetic aperture radar have greatly increased the accuracy and usefulness of air-to-ground radar since their introduction in the 1960s and 1970s. Military forces around the world use air-to-ground radar for weapon aiming and for battlefield surveillance. The United States used the Joint Surveillance and Tracking Radar System (JSTARS) in the Persian Gulf War (1991), demonstrating modern radar’s ability to provide information about enemy troop concentrations and movements during the day or night, regardless of weather conditions. C2 Countermeasures The military uses several techniques to attempt to avoid detection by enemy radar. One common technique is jamming—that is, sending deceptive signals to the enemy’s radar system. During World War II (1939-1945), flyers under attack jammed enemy radar by dropping large clouds of chaff—small pieces of aluminum foil or some other material that reflects radar well. “False” returns from the chaff hid the aircraft’s exact location from the enemy’s air defense radar. Modern jamming uses sophisticated electronic systems that analyze enemy radar, then send out false radar echoes that mask the actual target echoes or deceive the radar about a target’s location. Stealth technology is a collection of methods that reduce the radar echoes from aircraft and other radar targets (see Stealth Aircraft). Special paint can absorb radar signals and sharp angles in the aircraft design can reflect radar signals in deceiving directions. Improvements in jamming and stealth technology force the continual development of high-power transmitters, antennas good at detecting weak signals, and very sensitive receivers, as well as techniques for improved clutter rejection. D Traffic safety Since the 1950s, police have used radar to detect motorists who are exceeding the speed limit. Most older police radar “guns” use Doppler technology to determine the target vehicle’s speed. Such systems were simple, but they sometimes produced false results. The radar beam of such systems was relatively wide, which meant that stray radar signals could be detected by motorists with radar detectors. Newer police radar systems, developed in the 1980s and 1990s, use laser light to form a narrow, highly selective radar beam. The narrow beam helps insure that the radar returns signals from a single, selected car and reduces the chance of false results. Instead of relying on the Doppler effect to measure speed, these systems use pulse radar to measure the distance to the car many times, then calculate the speed by dividing the change in distance by the change in time. Laser radar is also more reliable than normal radar for the detection of speeding motorists because its narrow beam is more difficult to detect by motorists with radar detectors. E Meteorology Meteorologists use radar to learn about the weather. Networks of radar systems installed across many countries throughout the world detect and display areas of rain, snow, and other precipitation. Weather radar systems use Doppler radar to determine the speed of the wind within the storm. The radar signals bounce off of water droplets or ice crystals in the atmosphere. Gaseous water vapor does not reflect radar waves as well as the liquid droplets of water or solid ice crystals, so radar returns from rain or snow are stronger than that from clouds. Dust in the atmosphere also reflects radar, but the returns are only significant when the concentration of dust is much higher than usual. The Terminal Doppler Weather Radar can detect small, localized, but hazardous wind conditions, especially if precipitation or a large amount of dust accompanies the storm. Many airports use this advanced radar to make landing safer. F Scientific Applications Scientists use radar in several space-related applications. The Spacetrack system is a cooperative effort of the United States, Canada, and the United Kingdom. It uses data from several large surveillance and tracking radar systems (including the Ballistic Missile Early Warning System) to detect and track all objects in orbit around the earth. This helps scientists and engineers keep an eye on space junk—abandoned satellites, discarded pieces of rockets, and other unused fragments of spacecraft that could pose a threat to operating spacecraft. Other special-purpose radar systems track specific satellites that emit a beacon signal. One of the most important of these systems is the Global Positioning System (GPS), operated by the U.S. Department of Defense. GPS provides highly accurate navigational data for the U.S. military and for anyone who owns a GPS receiver. During space flights, radar gives precise measurements of the distances between the spacecraft and other objects. In the U.S. Surveyor missions to the moon in the 1960s, radar measured the altitude of the probe above the moon’s surface to help the probe control its descent. In the Apollo missions, which landed astronauts on the moon during the 1960s and 1970s, radar measured the altitude of the Lunar Module, the part of the Apollo spacecraft that carried two astronauts from orbit around the moon down to the moon’s surface, above the surface of the moon. Apollo also used radar to measure the distance between the Lunar Module and the Command and Service Module, the part of the spacecraft that remained in orbit around the moon. Astronomers have used ground-based radar to observe the moon, some of the larger asteroids in our solar system, and a few of the planets and their moons. Radar observations provide information about the orbit and surface features of the object. The U.S. Magellan space probe mapped the surface of the planet Venus with radar from 1990 to 1994. Magellan’s radar was able to penetrate the dense cloud layer of the Venusian atmosphere and provide images of much better quality than radar measurements from Earth. Many nations have used satellite-based radar to map portions of the earth’s surface. Radar can show conditions on the surface of the earth and can help determine the location of various resources such as oil, water for irrigation, and mineral deposits. In 1995 the Canadian Space Agency launched a satellite called RADARsat to provide radar imagery to commercial, government, and scientific users. V HISTORY Although British physicist James Clerk Maxwell predicted the existence of radio waves in the 1860s, it wasn’t until the 1890s that British-born American inventor Elihu Thomson and German physicist Heinrich Hertz independently confirmed their existence. Scientists soon realized that radio waves could bounce off of objects, and by 1904 Christian Hülsmeyer, a German inventor, had used radio waves in a collision avoidance device for ships. Hülsmeyer’s system was only effective for a range of about 1.5 km (about 1 mi). The first long-range radar systems were not developed until the 1920s. In 1922 Italian radio pioneer Guglielmo Marconi demonstrated a low-frequency (60 MHz) radar system. In 1924 English physicist Edward Appleton and his graduate student from New Zealand, Miles Barnett, proved the existence of the ionosphere, an electrically charged upper layer of the atmosphere, by reflecting radio waves off of it. Scientists at the U.S. Naval Research Laboratory in Washington, D.C., became the first to use radar to detect aircraft in 1930. A Radar in World War II None of the early demonstrations of radar generated much enthusiasm. The commercial and military value of radar did not become readily apparent until the mid-1930s. Before World War II, the United States, France, and the United Kingdom were all ca rying out radar research. Beginning in 1935, the British built a network of ground-based aircraft detection radar, called Chain Home, under the direction of Sir Robert Watson-Watt. Chain Home was fully operational from 1938 until the end of World War II in 1945 and was extremely instrumental in Britain’s defense against German bombers. The British recognized the value of radar with frequencies much higher than the radio waves used for most systems. A breakthrough in radar technology came in 1939 when two British scientists, physicist Henry Boot and biophysicist John Randall, developed the resonant-cavity magnetron. This device generates high-frequency radio pulses with a large amount of power, and it made the development of microwave radar possible. Also in 1939, the Massachusetts Institute of Technology (MIT) Radiation Laboratory was formed in Cambridge, Massachusetts, bringing together U.S. and British radar research. In March 1942 scientists demonstrated the detection of ships from the air. This technology became the basis of antiship and antisubmarine radar for the U.S. Navy. The U.S. Army operated air surveillance radar at the start of World War II. The army also used early forms of radar to direct antiaircraft guns. Initially the radar systems were used to aim searchlights so the soldier aiming the gun could see where to fire, but the systems evolved into fire-control radar that aimed the guns automatically. B Radar during the Cold War With the end of World War II, interest in radar development declined. Some experiments continued, however; for instance, in 1946 the U.S. Army Signal Corps bounced radar signals off of the moon, ushering in the field of radar astronomy. The growing hostility between the United States and the Union of Soviet Socialists Republics—the so-called Cold War—renewed military interest in radar improvements. After the Soviets detonated their first atomic bomb in 1949, interest in radar development, especially for air defense, surged. Major programs included the installation of the Distant Early Warning (DEW) network of long-range radar across the northern reaches of North America to warn against bomber attacks. As the potential threat of attack by ICBMs increased, the United Kingdom, Greenland, and Alaska installed the Ballistic Missile Early Warning System (BMEWS). C Modern Radar Radar found many applications in civilian and military life and became more sophisticated and specialized for each application. The use of radar in air traffic control grew quickly during the Cold War, especially with the jump in air traffic that occurred in the 1960s. Today almost all commercial and private aircraft have transponders. Transponders send out radar signals encoded with information about an aircraft and its flight that other aircraft and air traffic controllers can use. American traffic engineer John Barker discovered in 1947 that moving automobiles would reflect radar waves, which could be analyzed to determine the car’s speed. Police began using traffic radar in the 1950s, and the accuracy of traffic radar has increased markedly since the 1980s. Doppler radar came into use in the 1960s and was first dedicated to weather forecasting in the 1970s. In the 1990s the United States had a nationwide network of more than 130 Doppler radar stations to help meteorologists track weather patterns. Earth-observing satellites such as those in the SEASAT program began to use radar to measure the topography of the earth in the late 1970s. The Magellan spacecraft mapped most of the surface of the planet Venus in the 1990s. The Cassini spacecraft, scheduled to reach Saturn in 2004, carries radar instruments for studying the surface of Saturn’s moon Titan. As radar continues to improve, so does the technology for evading radar. Stealth aircraft feature radar-absorbing coatings and deceptive shapes to reduce the possibility of radar detection. The Lockheed F-117A, first flown in 1981, and the Northrop , first flown in 1989, are two of the latest additions to the U.S. stealth aircraft fleet. In the area of civilian radar avoidance, companies are introducing increasingly sophisticated radar detectors, designed to warn motorists of police using traffic radar. Contributed By: Robert E. Millett Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved. (v) 1. Tape Recording In analog tape recording, electrical signals from a microphone are transformed into magnetic signals. These signals are encoded onto a thin plastic ribbon of recording tape. Recording tape is coated with tiny magnetic particles. Chromium dioxide and ferric oxide are two magnetic materials commonly used. A chemical binder coats the particles to the tape, and a back coating prevents the magnetic charge from traveling from one layer of tape to the next. Tape is wound onto reels, which can vary in diameter and size. Professional reel-to-reel tape, which is 6.2 mm (0.25 in) wide, is wound on large metal or plastic reels. Reel-to-reel tapes must be loaded onto a reel-to-reel tape recorder by hand. Cassette tape is only 3.81 mm (0.15 in) wide and is completely self-enclosed for convenience. Regardless of size, all magnetic tape is drawn from a supply reel on the left side of the recorder to a take-up reel on the right. A drive shaft, called a capstan, rolls against a pinch roller and pulls the tape along. Various guides and rollers are used to mechanically regulate the speed and tension of the tape, since any variations in speed or tension will affect sound quality. As the tape is drawn from the supply reel to the take-up reel, it passes over a series of three magnetic coils called heads. The erase head is activated only while recording. It generates a current that places the tape's magnetic particles in a neutral position in order to remove any previous sounds. The record head transforms the electrical signal coming into the recorder into a magnetic flux and thus applies the original electrical signal onto the tape. The sound wave is now physically present on the analog tape. The playback head reads the magnetic field on the tape and converts this field back to electric energy. Unwanted noise, such as hiss, is a frequent problem with recording on tape. To combat this problem, sound engineers developed noise reduction systems that help reduce unwanted sounds. Many different systems exist, such as the Dolby System, which is used to reduce hiss on musical recordings and motion-picture soundtracks. Most noise occurs around the weakest sounds on a tape recording. Noise reduction systems work by boosting weak signals during recording. When the tape is played, the boosted signals are reduced to their normal levels. This reduction to normal levels also minimizes any noise that might have been present. Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved. Q7: (i) Deoxyribonucleic Acid I INTRODUCTION Deoxyribonucleic Acid (DNA), genetic material of all cellular organisms and most viruses. DNA carries the information needed to direct protein synthesis and replication. Protein synthesis is the production of the proteins needed by the cell or virus for its activities and development. Replication is the process by which DNA copies itself for each descendant cell or virus, passing on the information needed for protein synthesis. In most cellular organisms, DNA is organized on chromosomes located in the nucleus of the cell. II STRUCTURE A molecule of DNA consists of two chains, strands composed of a large number of chemical compounds, called nucleotides, linked together to form a chain. These chains are arranged like a ladder that has been twisted into the shape of a winding staircase, called a double helix. Each nucleotide consists of three units: a sugar molecule called deoxyribose, a phosphate group, and one of four different nitrogen-containing compounds called bases. The four bases are adenine (A), guanine (G), thymine (T), and cytosine (C). The deoxyribose molecule occupies the center position in the nucleotide, flanked by a phosphate group on one side and a base on the other. The phosphate group of each nucleotide is also linked to the deoxyribose of the adjacent nucleotide in the chain. These linked deoxyribose-phosphate subunits form the parallel side rails of the ladder. The bases face inward toward each other, forming the rungs of the ladder. The nucleotides in one DNA strand have a specific association with the corresponding nucleotides in the other DNA strand. Because of the chemical affinity of the bases, nucleotides containing adenine are always paired with nucleotides containing thymine, and nucleotides containing cytosine are always paired with nucleotides containing guanine. The complementary bases are joined to each other by weak chemical bonds called hydrogen bonds. In 1953 American biochemist James D. Watson and British biophysicist Francis Crick published the first description of the structure of DNA. Their model proved to be so important for the understanding of protein synthesis, DNA replication, and mutation that they were awarded the 1962 Nobel Prize for physiology or medicine for their work. III PROTEIN SYNTHESIS DNA carries the instructions for the production of proteins. A protein is composed of smaller molecules called amino acids, and the structure and function of the protein is determined by the sequence of its amino acids. The sequence of amino acids, in turn, is determined by the sequence of nucleotide bases in the DNA. A sequence of three nucleotide bases, called a triplet, is the genetic code word, or codon, that specifies a particular amino acid. For instance, the triplet GAC (guanine, adenine, and cytosine) is the codon for the amino acid leucine, and the triplet CAG (cytosine, adenine, and guanine) is the codon for the amino acid valine. A protein consisting of 100 amino acids is thus encoded by a DNA segment consisting of 300 nucleotides. Of the two polynucleotide chains that form a DNA molecule, only one strand contains the information needed for the production of a given amino acid sequence. The other strand aids in replication. Protein synthesis begins with the separation of a DNA molecule into two strands. In a process called transcription, a section of one strand acts as a template, or pattern, to produce a new strand called messenger RNA (mRNA). The mRNA leaves the cell nucleus and attaches to the ribosomes, specialized cellular structures that are the sites of protein synthesis. Amino acids are carried to the ribosomes by another type of RNA, called transfer RNA (tRNA). In a process called translation, the amino acids are linked together in a particular sequence, dictated by the mRNA, to form a protein. A gene is a sequence of DNA nucleotides that specify the order of amino acids in a protein via an intermediary mRNA molecule. Substituting one DNA nucleotide with another containing a different base causes all descendant cells or viruses to have the altered nucleotide base sequence. As a result of the substitution, the sequence of amino acids in the resulting protein may also be changed. Such a change in a DNA molecule is called a mutation. Most mutations are the result of errors in the replication process. Exposure of a cell or virus to radiation or to certain chemicals increases the likelihood of mutations. IV REPLICATION In most cellular organisms, replication of a DNA molecule takes place in the cell nucleus and occurs just before the cell divides. Replication begins with the separation of the two polynucleotide chains, each of which then acts as a template for the assembly of a new complementary chain. As the old chains separate, each nucleotide in the two chains attracts a complementary nucleotide that has been formed earlier by the cell. The nucleotides are joined to one another by hydrogen bonds to form the rungs of a new DNA molecule. As the complementary nucleotides are fitted into place, an enzyme called DNA polymerase links them together by bonding the phosphate group of one nucleotide to the sugar molecule of the adjacent nucleotide, forming the side rail of the new DNA molecule. This process continues until a new polynucleotide chain has been formed alongside the old one, forming a new double-helix molecule. V TOOLS AND PROCEDURES Several tools and procedures facilitate are used by scientists for the study and manipulation of DNA. Specialized enzymes, called restriction enzymes, found in bacteria act like molecular scissors to cut the phosphate backbones of DNA molecules at specific base sequences. Strands of DNA that have been cut with restriction enzymes are left with single-stranded tails that are called sticky ends, because they can easily realign with tails from certain other DNA fragments. Scientists take advantage of restriction enzymes and the sticky ends generated by these enzymes to carry out recombinant DNA technology, or genetic engineering. This technology involves removing a specific gene from one organism and inserting the gene into another organism. Another tool for working with DNA is a procedure called polymerase chain reaction (PCR). This procedure uses the enzyme DNA polymerase to make copies of DNA strands in a process that mimics the way in which DNA replicates naturally within cells. Scientists use PCR to obtain vast numbers of copies of a given segment of DNA. DNA fingerprinting, also called DNA typing, makes it possible to compare samples of DNA from various sources in a manner that is analogous to the comparison of fingerprints. In this procedure, scientists use restriction enzymes to cleave a sample of DNA into an assortment of fragments. Solutions containing these fragments are placed at the surface of a gel to which an electric current is applied. The electric current causes the DNA fragments to move through the gel. Because smaller fragments move more quickly than larger ones, this process, called electrophoresis, separates the fragments according to their size. The fragments are then marked with probes and exposed on X-ray film, where they form the DNA fingerprint—a pattern of characteristic black bars that is unique for each type of DNA. A procedure called DNA sequencing makes it possible to determine the precise order, or sequence, of nucleotide bases within a fragment of DNA. Most versions of DNA sequencing use a technique called primer extension, developed by British molecular biologist Frederick Sanger. In primer extension, specific pieces of DNA are replicated and modified, so that each DNA segment ends in a fluorescent form of one of the four nucleotide bases. Modern DNA sequencers, pioneered by American molecular biologist Leroy Hood, incorporate both lasers and computers. Scientists have completely sequenced the genetic material of several microorganisms, including the bacterium Escherichia coli. In 1998, scientists achieved the milestone of sequencing the complete genome of a multicellular organism—a roundworm identified as Caenorhabditis elegans. The Human Genome Project, an international research collaboration, has been established to determine the sequence of all of the three billion nucleotide base pairs that make up the human genetic material. An instrument called an atomic force microscope enables scientists to manipulate the three-dimensional structure of DNA molecules. This microscope involves laser beams that act like tweezers—attaching to the ends of a DNA molecule and pulling on them. By manipulating these laser beams, scientists can stretch, or uncoil, fragments of DNA. This work is helping reveal how DNA changes its three-dimensional shape as it interacts with enzymes. 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