Math Is Fun Forum

2459) Compass

Gist

A compass is a tool for finding direction. A simple compass is a magnetic needle mounted on a pivot, or short pin. The needle, which can spin freely, always points north. The pivot is attached to a compass card. The compass card is marked with the directions. To use a compass, a person lines up the needle with the marking for north. Then the person can figure out all the other directions.

A compass works because Earth is a huge magnet. A magnet has two main centers of force, called poles—one at each end. Lines of magnetic force connect these poles. Bits of metal near a magnet always arrange themselves along these lines. A compass needle acts like these bits of metal. It points north because it lines up with Earth’s lines of magnetic force.

Earth’s magnetic poles are not the same as the geographic North and South poles. The geographic poles are located at the very top and bottom of a globe. The magnetic poles are nearby but not at exactly the same places. A compass points to the magnetic North Pole, not the geographic North Pole. Therefore, a compass user has to make adjustments to find true north.

A special kind of compass called a gyrocompass does point to true north. The gyrocompass uses a device called a gyroscope, which always points in the same direction. Today large ships carry both magnetic compasses and gyrocompasses.

People in China and Europe first learned how to make magnetic compasses during the 1100s. They discovered that when a magnetized bit of iron floated in water, it pointed north. Sailors soon began to use compasses to navigate, or find their way, at sea. 

Summary

A compass, in navigation or surveying, is the primary device for direction-finding on the surface of the Earth. Compasses may operate on magnetic or gyroscopic principles or by determining the direction of the Sun or a star.

The oldest and most familiar type of compass is the magnetic compass, which is used in different forms in aircraft, ships, and land vehicles and by surveyors. Sometime in the 12th century, mariners in China and Europe made the discovery, apparently independently, that a piece of lodestone, a naturally occurring magnetic ore, when floated on a stick in water, tends to align itself so as to point in the direction of the polestar. This discovery was presumably quickly followed by a second, that an iron or steel needle touched by a lodestone for long enough also tends to align itself in a north-south direction. From the knowledge of which way is north, of course, any other direction can be found.

The reason magnetic compasses work as they do is that the Earth itself acts as an enormous bar magnet with a north-south field that causes freely moving magnets to take on the same orientation. The direction of the Earth’s magnetic field is not quite parallel to the north-south axis of the globe, but it is close enough to make an uncorrected compass a reasonably good guide. The inaccuracy, known as variation (or declination), varies in magnitude from point to point upon the Earth. The deflection of a compass needle due to local magnetic influences is called deviation.

Over the centuries a number of technical improvements have been made in the magnetic compass. Many of these were pioneered by the English, whose large empire was kept together by naval power and who hence relied heavily upon navigational devices. By the 13th century the compass needle had been mounted upon a pin standing on the bottom of the compass bowl. At first only north and south were marked on the bowl, but then the other 30 principal points of direction were filled in. A card with the points painted on it was mounted directly under the needle, permitting navigators to read their direction from the top of the card. The bowl itself was subsequently hung on gimbals (rings on the side that let it swing freely), ensuring that the card would always be level. In the 17th century the needle itself took the shape of a parallelogram, which was easier to mount than a thin needle.

During the 15th century navigators began to understand that compass needles do not point directly to the North Pole but rather to some nearby point; in Europe, compass needles pointed slightly east of true north. To counteract this difficulty, British navigators adopted conventional meridional compasses, in which the north on the compass card and the “needle north” were the same when the ship passed a point in Cornwall, England. (The magnetic poles, however, wander in a predictable manner—in more recent centuries Europeans have found magnetic north to be west of true north—and this must be considered for navigation.)

In 1745 Gowin Knight, an English inventor, developed a method of magnetizing steel in such a way that it would retain its magnetization for long periods of time; his improved compass needle was bar-shaped and large enough to bear a cap by which it could be mounted on its pivot. The Knight compass was widely used.

Some early compasses did not have water in the bowl and were known as dry-card compasses; their readings were easily disturbed by shocks and vibration. Although they were less affected by shock, liquid-filled compasses were plagued by leaks and were difficult to repair when the pivot became worn. Neither the liquid nor the dry-card type was decisively advantageous until 1862, when the first liquid compass was made with a float on the card that took most of the weight off the pivot. A system of bellows was invented to expand and contract with the liquid, preventing most leaks. With these improvements liquid compasses made dry-card compasses obsolete by the end of the 19th century.

Modern mariners’ compasses are usually mounted in binnacles, cylindrical pedestals with provision for illuminating the compass face from below. Each binnacle contains specially placed magnets and pieces of steel that cancel the magnetic effects of the metal of the ship. Much the same kind of device is used aboard aircraft, except that, in addition, it contains a corrective mechanism for the errors induced in magnetic compasses when airplanes suddenly change course. The corrective mechanism is a gyroscope, which has the property of resisting efforts to change its axis of spin. This system is called a gyromagnetic compass.

Gyroscopes are also employed in a type of nonmagnetic compass called the gyrocompass. The gyroscope is mounted in three sets of concentric rings connected by gimbals, each ring spinning freely. When the initial axis of spin of the central gyroscope is set to point to true north, it will continue to do so and will resist efforts to realign it in any other direction; the gyroscope itself thus functions as a compass. If it begins to precess (wobble), a pendulum weight pulls it back into line. Gyrocompasses are generally used in navigation systems because they can be set to point to true north rather than to magnetic north.

Details

A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It typically consists of a magnetized needle or another element, such as a compass card or compass rose, that pivots to align itself with magnetic north. Other methods may be used, including gyroscopes, magnetometers, and GPS receivers.

Compasses often show angles in degrees: north corresponds to 0°, and the angles increase clockwise, so east is 90°, south is 180°, and west is 270°. These numbers allow the compass to show azimuths or bearings which are commonly stated in degrees. If local variation between magnetic north and true north is known, then direction of magnetic north also gives direction of true north.

Among the Four Great Inventions, the magnetic compass was first invented as a device for divination as early as the Chinese Han dynasty (since c. 206 BC), and later adopted for navigation by the Song dynasty Chinese during the 11th century. The first usage of a compass recorded in Western Europe and the Islamic world occurred around 1190.

The magnetic compass is the most familiar compass type. It functions as a pointer to "magnetic north", the local magnetic meridian, because the magnetized needle at its heart aligns itself with the horizontal component of the Earth's magnetic field. The magnetic field exerts a torque on the needle, pulling the North end or pole of the needle approximately toward the Earth's North magnetic pole, and pulling the other toward the Earth's South magnetic pole. The needle is mounted on a low-friction pivot point, in better compasses a jewel bearing, so it can turn easily. When the compass is held level, the needle turns until, after a few seconds to allow oscillations to die out, it settles into its equilibrium orientation.

In navigation, directions on maps are usually expressed with reference to geographical or true north, the direction toward the Geographical North Pole, the rotation axis of the Earth. Depending on where the compass is located on the surface of the Earth the angle between true north and magnetic north, called magnetic declination can vary widely with geographic location. The local magnetic declination is given on most maps, to allow the map to be oriented with a compass parallel to true north. The locations of the Earth's magnetic poles slowly change with time, which is referred to as geomagnetic secular variation. The effect of this means a map with the latest declination information should be used. Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions.

Additional Information

A compass is a device that indicates direction. It is one of the most important instruments for navigation.

A compass is a device that indicates direction. It is one of the most important instruments used for navigation. Magnetic compasses are the best-known type of compass. While the design and construction of the magnetic compass have changed significantly over the centuries, the concept of how it works remains the same. A magnetic compass consists of a magnetized needle that rotates to line up with Earth's magnetic field. The ends point to what are known as the north magnetic pole and the south magnetic pole.

History of Compasses

The principle of magnetism has been observed by humans for thousands of years. Ancient Greeks observed the principle of magnetism, but they did not understand its relationship to the Earth or that a magnetized metal would point north. People in Ancient China also recognized magnetism. They learned that a magnetized bar of lodestone tied to a string would always point in the same direction. This observation became part of their spiritual beliefs as religious leaders used a magnetic spoon balanced on a plate to predict the future. Some consider this to be the earliest form of a compass, although compasses are more accurately defined as instruments devised for navigational purposes. A unique aspect of these early compasses in China is that they were oriented to the south and were referred to as “south pointing spoons” or “south pointers.” Later Chinese compasses were similarly oriented to point south rather than north like today’s compasses.

There is evidence that explorers from China and Europe were using compasses to navigate the seas as far back as the 1100s. In fact, many historians believe that people in China were using compasses to navigate long before that time. Miners in search of jade, for instance, appear to have used “south pointing spoons.” Some historians also believe that compasses originated in China and traveled to Europe through trade routes, but others think that Europeans developed the technology independently.

Early Compasses

Very early compasses were made of a magnetized needle attached to a piece of wood or cork that floated freely in a dish of water. As the needle settled, the marked end would point toward magnetic north.

As engineers and scientists learned more about magnetism, the compass needle was mounted and placed in the middle of a card that showed the cardinal directions: north, south, east, and west. In time, 32 points of direction were added to the compass card.

In their earliest use, compasses were likely used as a backup navigational tool for when the sun, stars, or other landmarks could not be seen. As compasses became more reliable and more explorers understood how to use them, compasses became an essential tool for travelers.

Adjustments and Adaptations

By the 15th century, explorers realized that the “north” indicated by a compass needle was not the same as Earth’s true geographic north. This discrepancy between magnetic north and true north is called variation (by mariners or pilots) or magnetic declination (by land navigators), and it varies depending on location. Because of this variation, a compass could lead a novice user many kilometers off-course. Navigators learn to adjust their compass readings to account for variation.

Other adaptations have been made to magnetic compasses over time, especially for their use in marine navigation. When ships evolved from being made of wood to being made of iron and steel, the magnetism of the ship affected compass readings. This difference is called deviation. Adjustments, such as placing soft iron balls (called Kelvin spheres) and bar magnets (called Flinders bars) near the compass, help increase the accuracy of the readings. Deviation must also be taken into account on aircraft using compasses, due to the metal in the construction of an airplane.

Magnetic compasses come in many forms. The most basic are portable compasses for use on casual hikes. Magnetic compasses can have additional features, such as magnifiers for use with maps, a prism or mirror that allows the user to see the landscape and compass reading at the same time, or markings in Braille for people with a visual impairment. The most complicated compasses are complex devices on ships or planes that can calculate and adjust for motion, variation, and deviation.

Other Types of Compasses

Some compasses do not use Earth’s magnetism to indicate direction. The gyrocompass, invented in the early 20th century, uses a spinning gyroscope to follow Earth’s axis of rotation to point to true north. Since magnetic north is not measured, variation is not an issue. Once the gyroscope begins spinning, motion will not disturb it. This type of compass is often used on ships and aircraft.

A solar compass uses the sun as a navigational tool. It was used in the 19thand 20thcenturies to survey land. Because a solar compass is not affected by iron metal deposits or location relative to the poles, a solar compass can be more accurate than a magnetic compass, particularly near the poles. The most common method is to use a compass card and the angle of the shadow of the sun to indicate direction.

Another type of solar compass is an old-fashioned analog (not digital) watch. Using the watch’s hands and the position of the sun, it is possible to determine north or south. Simply hold the watch parallel to the ground (in your hand) and point the 12 o'clock mark in the direction of the sun. Find the angle between the hour hand and the 12 o’clock mark. This is the north-south line. In the Southern Hemisphere, north will be the direction closer to the sun. In the Northern Hemisphere, north will be the direction further from the sun.

Receivers from the global positioning system (GPS) have begun to take the place of compasses. A GPS receiver coordinates with satellites orbiting Earth and monitoring stations on Earth to pinpoint the receiver's location. GPS receivers can plot latitude, longitude, and altitude on a map. In open areas and optimal conditions, standard GPS is accurate to about 6 meters (20 feet), and researchers have developed a so-called “SuperGPS” that is accurate within 10 centimeters (3.9 inches).

Many people throughout history have used knowledge of the stars and constellations as a kind of compass. For example, Polynesians have been using patterns in the sky to navigate the ocean for at least 2,000 years. A native Hawaiian historian, Charles Nainoa Thompson, developed a Hawaiian star compass in the mid-2000s to illustrate how Polynesians use the constellations for navigation. Much like a magnetic compass, there are 32 stars situated around a center point on a star compass.

Impact of the Compass

The compass had a major impact on the world, particularly for people who were navigating the sea. Because compasses were more accurate than other naval navigational tools, explorers used them to explore parts of the world that were unknown to them during the so-called Age of Exploration that began in the early 15th century, a development with both positive and negative impacts. European exploration contributed to trade and the circulation of knowledge, but it would also lead to the spread of disease, the colonization of new lands, and the enslavement of Africans and other indigenous populations. Because today’s global relationships have emerged from these relationships, the legacy of colonization—and the invention of the compass—continues to have a profound impact on the world today.

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#5887. What does the adjective compliant mean?

#5888. What does the noun complicity mean?

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#9819.

Hi,

2668.

2458) RADAR

Gist

RADAR stands for Radio Detection And Ranging, a system that uses radio waves to detect, locate, and track objects by sending out signals and analyzing the returning echoes to determine distance, speed, and direction, effective even in poor visibility for things like aircraft, ships, weather, and vehicles. It's crucial in aviation, shipping, meteorology, and traffic control.

The principle of Radar (Radio Detection And Ranging) is to detect objects by sending out electromagnetic waves (radio waves) and analyzing the returning echoes, much like SONAR uses sound. A transmitter sends pulses, an antenna radiates them, and the signal reflects off targets, returning as weak echoes. By measuring the time delay and direction of these returning signals, the radar system calculates an object's distance, bearing (direction), and velocity (using Doppler shift), displaying the information visually. 

Summary

Radar is a system that uses radio waves to determine the distance (ranging), direction (azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations and terrain. The term RADAR was coined in 1940 by the United States Navy as an acronym for "radio detection and ranging". The term radar has since entered English and other languages as an anacronym, a common noun, losing all capitalization.

A radar system consists of a transmitter producing electromagnetic waves in the radio or microwave domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds. This device was developed secretly for military use by several countries in the period before and during World War II. A key development was the cavity magnetron in the United Kingdom, which allowed the creation of relatively small systems with sub-meter resolution.

The modern uses of radar are highly diverse, including air and terrestrial traffic control, radar astronomy, air-defense systems, anti-missile systems, marine radars to locate landmarks and other ships, aircraft anti-collision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring, radar remote sensing, altimetry and flight control systems, guided missile target locating systems, self-driving cars, and ground-penetrating radar for geological observations.  Modern high tech radar systems use digital signal processing and machine learning and are capable of extracting useful information from very high noise levels.

Other systems which are similar to radar make use of other regions of the electromagnetic spectrum. One example is lidar, which uses predominantly infrared light from lasers rather than radio waves. With the emergence of driverless vehicles, radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents.

Details

A radar is an electromagnetic sensor used for detecting, locating, tracking, and recognizing objects of various kinds at considerable distances. It operates by transmitting electromagnetic energy toward objects, commonly referred to as targets, and observing the echoes returned from them. The targets may be aircraft, ships, spacecraft, automotive vehicles, and astronomical bodies, or even birds, insects, and rain. Besides determining the presence, location, and velocity of such objects, radar can sometimes obtain their size and shape as well. What distinguishes radar from optical and infrared sensing devices is its ability to detect faraway objects under adverse weather conditions and to determine their range, or distance, with precision.

Radar is an “active” sensing device in that it has its own source of illumination (a transmitter) for locating targets. It typically operates in the microwave region of the electromagnetic spectrum—measured in hertz (cycles per second), at frequencies extending from about 400 megahertz (MHz) to 40 gigahertz (GHz). It has, however, been used at lower frequencies for long-range applications (frequencies as low as several megahertz, which is the HF [high-frequency], or shortwave, band) and at optical and infrared frequencies (those of laser radar, or lidar). The circuit components and other hardware of radar systems vary with the frequency used, and systems range in size from those small enough to fit in the palm of the hand to those so enormous that they would fill several football fields.

Radar underwent rapid development during the 1930s and ’40s to meet the needs of the military. It is still widely employed by the armed forces, where many technological advances have originated. At the same time, radar has found an increasing number of important civilian applications, notably air traffic control, weather observation, remote sensing of the environment, aircraft and ship navigation, speed measurement for industrial applications and for law enforcement, space surveillance, and planetary observation.

Fundamentals of radar

Radar typically involves the radiating of a narrow beam of electromagnetic energy into space from an antenna. The narrow antenna beam scans a region where targets are expected. When a target is illuminated by the beam, it intercepts some of the radiated energy and reflects a portion back toward the radar system. Since most radar systems do not transmit and receive at the same time, a single antenna is often used on a time-shared basis for both transmitting and receiving.

A receiver attached to the output element of the antenna extracts the desired reflected signals and (ideally) rejects those that are of no interest. For example, a signal of interest might be the echo from an aircraft. Signals that are not of interest might be echoes from the ground or rain, which can mask and interfere with the detection of the desired echo from the aircraft. The radar measures the location of the target in range and angular direction. Range, or distance, is determined by measuring the total time it takes for the radar signal to make the round trip to the target and back. The angular direction of a target is found from the direction in which the antenna points at the time the echo signal is received. Through measurement of the location of a target at successive instants of time, the target’s recent track can be determined. Once this information has been established, the target’s future path can be predicted. In many surveillance radar applications, the target is not considered to be “detected” until its track has been established.

Pulse radar

The most common type of radar signal consists of a repetitive train of short-duration pulses. The figure shows a simple representation of a sine-wave pulse that might be generated by the transmitter of a medium-range radar designed for aircraft detection. The sine wave represents the variation with time of the output voltage of the transmitter. The numbers given in parentheses in the figure are meant only to be illustrative and are not necessarily those of any particular radar. They are, however, similar to what might be expected for a ground-based radar system with a range of about 50 to 60 nautical miles (90 to 110 km), such as the kind used for air traffic control at airports. The pulse width is given in the figure as 1 microsecond ({10}^{-6} second). It should be noted that the pulse is shown as containing only a few cycles of the sine wave; however, in a radar system having the values indicated, there would be 1,000 cycles within the pulse. In the figure the time between successive pulses is given as 1 millisecond ({10}^{-3} second), which corresponds to a pulse repetition frequency of 1 kilohertz (kHz). The power of the pulse, called the peak power, is taken here to be 1 megawatt. Since a pulse radar does not radiate continually, the average power is much less than the peak power. In this example, the average power is 1 kilowatt. The average power, rather than the peak power, is the measure of the capability of a radar system. Radars have average powers from a few milliwatts to as much as one or more megawatts, depending on the application.

A weak echo signal from a target might be as low as 1 picowatt ({10}^{-12} watt). In short, the power levels in a radar system can be very large (at the transmitter) and very small (at the receiver).

Another example of the extremes encountered in a radar system is the timing. An air-surveillance radar (one that is used to search for aircraft) might scan its antenna 360 degrees in azimuth in a few seconds, but the pulse width might be about one microsecond in duration. Some radar pulse widths are even of nanosecond ({10}^{-9} second) duration.

Radar waves travel through the atmosphere at roughly 300,000 km per second (the speed of light). The range to a target is determined by measuring the time that a radar signal takes to travel out to the target and back. The range to the target is equal to cT/2, where c = velocity of propagation of radar energy, and T = round-trip time as measured by the radar. From this expression, the round-trip travel of the radar signal through air is at a rate of 150,000 km per second. For example, if the time that it takes the signal to travel out to the target and back was measured by the radar to be 0.0006 second (600 microseconds), then the range of the target would be 90 km. The ability to measure the range to a target accurately at long distances and under adverse weather conditions is radar’s most distinctive attribute. There are no other devices that can compete with radar in the measurement of range.

The range accuracy of a simple pulse radar depends on the width of the pulse: the shorter the pulse, the better the accuracy. Short pulses, however, require wide bandwidths in the receiver and transmitter (since bandwidth is equal to the reciprocal of the pulse width). A radar with a pulse width of one microsecond can measure the range to an accuracy of a few tens of metres or better. Some special radars can measure to an accuracy of a few centimetres. The ultimate range accuracy of the best radars is limited by the known accuracy of the velocity at which electromagnetic waves travel.

Directive antennas and target direction

Almost all radars use a directive antenna—i.e., one that directs its energy in a narrow beam. (The beamwidth of an antenna of fixed size is inversely proportional to the radar frequency.) The direction of a target can be found from the direction in which the antenna is pointing when the received echo is at a maximum. A precise means for determining the direction of a target is the monopulse method—in which information about the angle of a target is obtained by comparing the amplitudes of signals received from two or more simultaneous receiving beams, each slightly offset (squinted) from the antenna’s central axis. A dedicated tracking radar—one that follows automatically a single target so as to determine its trajectory—generally has a narrow, symmetrical “pencil” beam. (A typical beamwidth might be about 1 degree.) Such a radar system can determine the location of the target in both azimuth angle and elevation angle. An aircraft-surveillance radar generally employs an antenna that radiates a “fan” beam, one that is narrow in azimuth (about 1 or 2 degrees) and broad in elevation (elevation beamwidths of from 20 to 40 degrees or more). A fan beam allows only the measurement of the azimuth angle.

Doppler frequency and target velocity

Radar can extract the Doppler frequency shift of the echo produced by a moving target by noting how much the frequency of the received signal differs from the frequency of the signal that was transmitted. (The Doppler effect in radar is similar to the change in audible pitch experienced when a train whistle or the siren of an emergency vehicle moves past the listener.) A moving target will cause the frequency of the echo signal to increase if it is approaching the radar or to decrease if it is receding from the radar. For example, if a radar system operates at a frequency of 3,000 MHz and an aircraft is moving toward it at a speed of 400 knots (740 km per hour), the frequency of the received echo signal will be greater than that of the transmitted signal by about 4.1 kHz. The Doppler frequency shift in hertz is equal to 3.4 f0vr, where f0 is the radar frequency in gigahertz and vr is the radial velocity (the rate of change of range) in knots.

Since the Doppler frequency shift is proportional to radial velocity, a radar system that measures such a shift in frequency can provide the radial velocity of a target. The Doppler frequency shift can also be used to separate moving targets from stationary targets even when the echo signal from undesired clutter is much more powerful than the echo from the desired moving targets. A form of pulse radar that uses the Doppler frequency shift to eliminate stationary clutter is called either a moving-target indication (MTI) radar or a pulse Doppler radar, depending on the particular parameters of the signal waveform.

The above measurements of range, angle, and radial velocity assume that the target is a “point-scatterer.” Actual targets, however, are of finite size and can have distinctive shapes. The range profile of a finite-sized target can be determined if the range resolution of the radar is small compared with the target’s size in the range dimension. (The range resolution of a radar, given in units of distance, is a measure of the ability of a radar to separate two closely spaced echoes.) Some radars can have resolutions much smaller than one metre, which is quite suitable for determining the radial size and profile of many targets of interest.

The resolution in angle, or cross range, that can be obtained with conventional antennas is poor compared with that which can be obtained in range. It is possible, however, to achieve good resolution in angle by resolving in Doppler frequency (i.e., separating one Doppler frequency from another). If the radar is moving relative to the target (as when the radar is on an aircraft and the target is the ground), the Doppler frequency shift will be different for different parts of the target. Thus, the Doppler frequency shift can allow the various parts of the target to be resolved. The resolution in cross range derived from the Doppler frequency shift is far better than that achieved with a narrow-beam antenna. It is not unusual for the cross-range resolution obtained from Doppler frequency to be comparable to that obtained in the range dimension.

Radar imaging

Radar can distinguish one kind of target from another (such as a bird from an aircraft), and some systems are able to recognize specific classes of targets (for example, a commercial airliner as opposed to a military jet fighter). Target recognition is accomplished by measuring the size and speed of the target and by observing the target with high resolution in one or more dimensions. Propellers and jet engines modify the radar echo from aircraft and can assist in target recognition. The flapping of the wings of a bird in flight produces a characteristic modulation that can be used to recognize that a bird is present or even to distinguish one type of bird from another.

Cross-range resolution obtained from Doppler frequency, along with range resolution, is the basis for synthetic aperture radar (SAR). SAR produces an image of a scene that is similar, but not identical, to an optical photograph. One should not expect the image seen by radar “eyes” to be the same as that observed by optical eyes. Each provides different information. Radar and optical images differ because of the large difference in the frequencies involved; optical frequencies are approximately 100,000 times higher than radar frequencies.

SAR can operate from long range and through clouds or other atmospheric effects that limit optical and infrared imaging sensors. The resolution of a SAR image can be made independent of range, an advantage over passive optical imaging where the resolution worsens with increasing range. Synthetic aperture radars that map areas of the Earth’s surface with resolutions of a few metres can provide information about the nature of the terrain and what is on the surface.

A SAR operates on a moving vehicle, such as an aircraft or spacecraft, to image stationary objects or planetary surfaces. Since relative motion is the basis for the Doppler resolution, high resolution (in cross range) also can be accomplished if the radar is stationary and the target is moving. This is called inverse synthetic aperture radar (ISAR). Both the target and the radar can be in motion with ISAR.

Additional Information

Radars are critical for understanding the weather; they allow us to “see” inside clouds and help us to observe what is really happening. Working together, engineers, technicians, and scientists collectively design, develop and operate the advanced technology of radars that are used to study the atmosphere.

What are Weather Radars?

Doppler weather radars are remote sensing instruments and are capable of detecting particle type (rain, snow, hail, insects, etc), intensity, and motion. Radar data can be used to determine the structure of storms and to help with predicting severity of storms.

The Electromagnetic Spectrum

Energy is emitted in various frequencies and wavelengths from large wavelength radio waves to shorter wavelength gamma rays. Radars emit microwave energy, a longer wavelength, highlighted in yellow.

How Do Radars Work?

The radar transmits a focused pulse of microwave energy (yup, just like a microwave oven or a cell phone, but stronger) at an object, most likely a cloud. Part of this beam of energy bounces back and is measured by the radar, providing information about the object. Radar can measure precipitation size, quantity, speed and direction of movement, within about 100 mile radius of its location.

Doppler radar is a specific type of radar that uses the Doppler effect to gather velocity data from the particles that are being measured. For example, a Doppler radar transmits a signal that gets reflected off raindrops within a storm. The reflected radar signal is measured by the radar's receiver with a change in frequency. That frequency shift is directly related to the motion of the raindrops.

Why does NCAR use radars for research?
Atmospheric scientists use different types of ground-based and aircraft-mounted radar to study weather and climate. Radar can be used to help study severe weather events such tornadoes and hurricanes, or long-term climate processes in the atmosphere.

Ground-based Research Radar

The NCAR S-Band Dual-Polarization Doppler Radar (S-PolKa) is a 10-cm wavelength weather radar initially designed and fielded by NCAR in the 1990s. Continuously modified and improved, this state-of-the-art radar system now includes dual-wavelength capability. When the Ka-band is added, a 0.8-cm wavelength radar, it is known as S-PolKa. S-PolKa’s mission is to promote a better understanding of weather and its causes and thereby ultimately provide improved forecasting of severe storms, tornadoes, floods, hail, damaging winds, aircraft icing conditions, and heavy snow.

Airborne Research Radar

In the air, research aircraft can be outfitted with an array of radars. The NCAR HIAPER Cloud Radar (HCR) can be mounted to the underside of the wing of the NSF/NCAR HIAPER research aircraft (a modified Gulfstream V jet) and delivers high quality observations of winds, precipitation and other particles. It was designed and manufactured by a collaborative team of mechanical, electrical, aerospace, and software engineers; research scientists; and instrument makers from EOL.

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#5885. What does the verb (used with object) qualify mean?

#5886. What does the adjective qualitative mean?

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#2538. What does the medical term Colectomy mean?

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#6312.

2404) John Eccles (neurophysiologist)

Gist:

Work

The nervous system in people and animals consists of many different cells. In cells, signals are conveyed by small electrical currents and by chemical substances. By measuring small variations in electrical charges at contact surfaces between nerve cells, or synapses, in the early 1950s John Eccles showed how nerve impulses are conveyed from one cell to another. The synapses are of different types, which has either a stimulating or inhibiting effect. A nerve cell receives signals from many different synapses, and the effect is determined by which type prevails.

Summary

Sir John Carew Eccles (born Jan. 27, 1903, Melbourne, Australia—died May 2, 1997, Contra, Switz.) was an Australian research physiologist who received (with Alan Hodgkin and Andrew Huxley) the 1963 Nobel Prize for Physiology or Medicine for his discovery of the chemical means by which impulses are communicated or repressed by nerve cells (neurons).

After graduating from the University of Melbourne in 1925, Eccles studied at the University of Oxford under a Rhodes scholarship. He received a Ph.D. there in 1929 after having worked under the neurophysiologist Charles Scott Sherrington. He held a research post at Oxford before returning to Australia in 1937, teaching there and in New Zealand over the following decades.

Eccles conducted his prizewinning research while at the Australian National University, Canberra (1951–66). He demonstrated that one nerve cell communicates with a neighbouring cell by releasing chemicals into the synapse (the narrow cleft, or gap, between the two cells). He showed that the excitement of a nerve cell by an impulse causes one kind of synapse to release into the neighbouring cell a substance (probably acetylcholine) that expands the pores in nerve membranes. The expanded pores then allow free passage of sodium ions into the neighbouring nerve cell and reverse the polarity of electric charge. This wave of electric charge, which constitutes the nerve impulse, is conducted from one cell to another. In the same way, Eccles found, an excited nerve cell induces another type of synapse to release into the neighbouring cell a substance that promotes outward passage of positively charged potassium ions across the membrane, reinforcing the existing polarity and inhibiting the transmission of an impulse.

Eccles’s research, which was based largely on the findings of Hodgkin and Huxley, settled a long-standing controversy over whether nerve cells communicate with each other by chemical or by electric means. His work had a profound influence on the medical treatment of nervous diseases and research on kidney, heart, and brain function.

Among his scientific books are Reflex Activity of the Spinal Cord (1932), The Physiology of Nerve Cells (1957), The Inhibitory Pathways of the Central Nervous System (1969), and The Understanding of the Brain (1973). He also wrote a number of philosophical works, including Facing Reality: Philosophical Adventures by a Brain Scientist (1970) and The Human Mystery (1979).

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Sir John Carew Eccles (27 January 1903 – 2 May 1997) was an Australian neurophysiologist and philosopher who won the 1963 Nobel Prize in Physiology or Medicine for his work on the synapse. He shared the prize with Andrew Huxley and Alan Lloyd Hodgkin.

Life and work:

Early life

Eccles was born in Melbourne, Australia. He grew up there with his two sisters and his parents: William and Mary Carew Eccles (both teachers, who home schooled him until he was 12). He initially attended Warrnambool High School (now Warrnambool College) (where a science wing is named in his honour), then completed his final year of schooling at Melbourne High School. Aged 17, he was awarded a senior scholarship to study medicine at the University of Melbourne.[3] As a medical undergraduate, he was never able to find a satisfactory explanation for the interaction of mind and body; he started to think about becoming a neuroscientist. He graduated (with first class honours) in 1925, and was awarded a Rhodes Scholarship to study under Charles Scott Sherrington at Magdalen College, Oxford University, where he received his Doctor of Philosophy in 1929.

In 1937 Eccles returned to Australia, where he worked on military research during World War II. During this time, Eccles was the director of the Kanematsu Institute at Sydney Medical School, where he and Bernard Katz gave research lectures at the University of Sydney, strongly influencing the intellectual environment of the university. After the war, he became a professor at the University of Otago in New Zealand. From 1952 to 1962, he worked as a professor at the John Curtin School of Medical Research (JCSMR) of the Australian National University. From 1966 to 1968, Eccles worked at the Feinberg School of Medicine at Northwestern University in Chicago.

Career

In the early 1950s, Eccles and his colleagues performed the research that would lead to his receiving the Nobel Prize. To study synapses in the peripheral nervous system, Eccles and colleagues used the stretch reflex as a model, which is easily studied because it consists of only two neurones: a sensory neurone (the muscle spindle fibre) and the motor neurone. The sensory neurone synapses onto the motor neurone in the spinal cord. When a current is passed into the sensory neurone in the quadriceps, the motor neurone innervating the quadriceps produced a small excitatory postsynaptic potential (EPSP). When a similar current is passed through the hamstring, the opposing muscle to the quadriceps, an inhibitory postsynaptic potential (IPSP) is produced in the quadriceps motor neurone. Although a single EPSP was not enough to fire an action potential in the motor neurone, the sum of several EPSPs from multiple sensory neurones synapsing onto the motor neurone can cause the motor neurone to fire, thus contracting the quadriceps. On the other hand, IPSPs could subtract from this sum of EPSPs, preventing the motor neurone from firing.

Apart from these seminal experiments, Eccles was key to a number of important developments in neuroscience. Until around 1949, Eccles believed that synaptic transmission was primarily electrical rather than chemical. Although he was wrong in this hypothesis, his arguments led him and others to perform some of the experiments which proved chemical synaptic transmission. Bernard Katz and Eccles worked together on some of the experiments which elucidated the role of acetylcholine as a neurotransmitter in the brain.

Honours

He was appointed a Knight Bachelor in 1958 in recognition of services to physiological research.

He won the Australian of the Year Award in 1963, the same year he won the Nobel Prize.

In 1964, he became an honorary member to the American Philosophical Society, and in 1966 he moved to the United States to work as a professor at the Institute for Biomedical Research at the Feinberg School of Medicine in Chicago. Unhappy with the working conditions there, he left to become a professor at The State University of New York at Buffalo from 1968 until he retired in 1975. After retirement, he moved to Switzerland and wrote on the mind–body problem.

In 1981, Eccles became a founding member of the World Cultural Council.

In 1990 he was appointed a Companion of the Order of Australia (AC) in recognition of service to science, particularly in the field of neurophysiology. He died at the age of 94 in 1997 in Tenero-Contra, Locarno, Switzerland.

In March 2012, the Eccles Institute of Neuroscience was constructed in a new wing of the John Curtin School of Medical Research, with the assistance of a $63M grant from the Commonwealth Government. In 2021, a new $60M animal research building was opened at the University of Otago, Dunedin, New Zealand, and named the Eccles Building.