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Radar

Radar

:This article is about the device. For the fictional character from M
- A
- S
- H , see Corporal Walter (Radar) O'Reilly. Corporal Walter (Radar) O'Reilly) rotates on a track to observe activities near the horizon.]] RADAR is a system used to detect, range (determine the distance of), and map objects such as aircraft, ships, and rain, that was first suggested as a "ship finder" by Dr. Allen B. DuMont in 1932. Coined in 1941 as an acronym for Radio Detection and Ranging, it has since entered the English language as a standard word, losing the capitalization in the process.

Principles

Overview

Powerful radio waves are transmitted, and a receiver listens for any echoes. By analysing the reflected signal, the reflector can be located, and sometimes identified. Although the amount of signal returned is tiny, radio signals can easily be detected and amplified. Radar radio waves can be easily generated at any desired strength, detected at even tiny powers, and then amplified many times. Thus radar is suited to detecting objects at very large ranges where other reflections, like sound or visible light, would be too weak to detect. Radio waves can propagate with less attenuation than light in many conditions such as through clouds, fog, and smoke, enabling detection and tracking in such conditions.

Reflection

The extent to which an object reflects or scatters radio waves is called its radar cross section. radar cross section Electromagnetic waves reflect (scatter) from any large change in the dielectric or diamagnetic constants. This means that a solid object in air or vacuum, or other significant change in atomic density between object and what's surrounding it, will usually scatter radar (radio) waves. This is particularly true for electrically conductive materials such as metal and carbon fibre, making radar particularly well suited to the detection of aircraft and ships. Radar absorbing material, containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection. This is the radio equivalent of painting something a dark colour. Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light bounces from a mirror. If the wavelength is much longer than the size of the target, the target is polarized, like a dipole antenna. This is described by Rayleigh Scattering (like the blue sky). When the two length scales are comparable, there may be resonances. Early radars used very long wavelengths that were larger than the targets and received a vague signal, whereas some modern systems use shorter wavelengths (a few centimetres or shorter) that can image objects as small as a loaf of bread or smaller. Radio waves reflect from curves and corners, in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between the reflective surfaces. A structure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will always reflect waves entering its opening directly back at the source. These so-called corner reflectors are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect, and are often found on boats in order to improve their detection in a rescue situation and reduce collisions. For generally the same reasons objects attempting to avoid detection will angle their surfaces in a way to eliminate inside corners and avoid surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking stealth aircraft. These precautions do not completely eliminate reflection because of diffraction, especially at longer wavelengths. Half wavelength long wires or strips of conducting material such as chaff are very reflective but do not direct the scattered energy back toward the source. Electromagnetic waves do not travel well underwater; thus for underwater applications, sonar, based on sound waves, has to be used instead of radar.

Polarization

In the transmitted radar signal, the electric field is perpendicular to the direction of propagation, and this direction of the electric field is the Polarization of the wave. Radars use horizontal, vertical, and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces, and help a search radar ignore rain. Random polarization returns usually indicate a fractal surface like rock or dirt, and are used by navigational radars.

Interference

Radar systems must overcome several different sources of unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR) - the higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals.

Noise

Signal noise is an internal source of random variations in the signal, which is inherently generated to some degree by all electronic components (for a list of noise sources refer to the Signal noise article). Noise typically appears as random variations superimposed on the desired echo signal received in the radar receiver. The lower the power of the desired signal, the more difficult it is to discern it from the noise (analogous to trying to hear a whisper while standing near a loudly leaking air hose). Therefore, the most important noise sources appear in the receiver and much effort is made to minimize these factors. Noise figure is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized. Noise is also generated by external sources, most importantly the natural thermal radiation of the background scene surrounding the target of interest. In modern radar systems, due to the high performance of their receivers, the internal noise is typically about equal to or lower than the external scene noise. An exception is if the radar is aimed upwards at clear sky, where the scene is so cold that it generates very little thermal noise.

Clutter

Clutter refers to actual radio frequency (RF) echos returned from targets which are by definition uninteresting to the radar operators in general. Such targets mostly include natural objects such as ground, sea, rain/snow/hail and other precipitation forms, sand storms, animals (esp. birds), atmospheric turbulences, and other atmospheric effects (ionosphere reflections, meteor trails etc.). Clutter may also be returned from man-made objects such as buildings and chaff (this latter cause being intentional). It should be noted that while some clutter sources may be undesirable for some radar applications (e.g., storm clouds for air-defence radars), they may be desirable for others (meteorological radars in this example). Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar. There are several methods of detecting and neutralizing clutter. Many of these methods rely on the fact that clutter tends to appear static between radar scans. Therefore, when comparing subsequent scans echos, desirable targets will appear to move and all stationary echos can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with circular polarization (note that meteorological radars wish for the opposite effect, therefore using linear polarization the better to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio. CFAR (Constant False-Alarm Rate, sometimes called Automatic Gain Control, or AGC) is a method relying on the fact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected the gain with greater granularity, in specific detection cells. Automatic Gain Control Clutter may also originate from multipath echos from valid targets due to ground reflection, atmospheric ducting or ionospheric reflection/refraction. This specific clutter type is especially bothersome, since it appears to move and behave like nother normal (point) targets of interest, thereby creating a ghost. In a typical scenario, an aircraft echo is multipath-reflected from the ground below, appearing to the receiver as an identical target below the correct one. The radar may try to unify the targets, reporting the target at an incorrect height, or worse - eliminating it on the basis of jitter or a physical impossibility. These problems can be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below ground or above a certain height.

Jamming

Radar jamming refers to RF signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming may be intentional (as an anti-radar electronic warfare (EW) tactic) or unintentional (e.g., by friendly forces operating equipment that transmits using the same frequency range). Jamming is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals. Jamming is problematic to radar since the jamming signal only needs to travel one-way (from the jammer to the radar receiver) whereas the radar echos travel two-ways (radar-target-radar) and are therefore significantly reduced in power by the time they return to the radar receiver. Jammers therefore need be much less powerful than their jammed radars in order to effectively mask targets along the line of sight from the jammer to the radar (Mainlobe Jamming). Jammers have an added effect of affecting radars along other line-of-sights, due to the radar receiver's sidelobes (Sidelobe Jamming). While mainlobe jamming cannot generally be overcome, sidelobe jamming can be overcome by reducing receiving sidelobes in the radar antenna design and by using a highly directional antenna to narrow the angle of the mainlobe cone. Other anti-jamming techniques are frequency hopping and polarization. See Electronic counter-counter-measures for details.

Distance measurement

Transit time

Electronic counter-counter-measures The easiest way to measure the range of an object is to broadcast a short pulse of radio signal, and then evaluate the time it takes for the reflection to return. The distance is one-half the product of round trip time (because the signal has to travel to the target and then back to the receiver) and the speed of the signal.

Range = \frac

where c is the speed of light in a vacuum, and

\tau

is the round trip time. For radar the speed of signal is the speed of light, making the round trip times very short for terrestrial ranging. For this reason accurate distance measurement was difficult until the introduction of high performance electronics, with older systems being accurate to perhaps a few percent. The receiver cannot detect the return while the signal is being sent out – there is no way to tell if the signal it hears is the original or the return. This means that a radar has a distinct minimum range, which is the length of the pulse multiplied by the speed of light, divided by two. In order to detect closer targets one must use a shorter pulse length. A similar effect imposes a specific maximum range as well. If the return from the target comes in when the next pulse is being sent out, once again the receiver cannot tell the difference. In order to maximize range, one wants to use longer times between pulses, the inter-pulse time. These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. This is because the short pulses needed for a good minimum range broadcast have less total energy, making the returns much smaller and the target harder to detect. This could be offset by using more pulses, but this would shorten the maximum range again. So each radar uses a particular type of signal. Long range radars tend to use long pulses with long delays between them, and short range radars use smaller pulses with less time between them. This pattern of pulses and pauses is known as the Pulse Repetition Frequency (or PRF), and is one of the main ways to characterize a radar. As electronics have improved many radars now can change their PRF.

Frequency modulation

Another form of distance measuring radar is based on frequency modulation. Frequency comparison between two signals is considerably more accurate, even with older electronics, than timing the signal. By changing the frequency of the returned signal and comparing that with the original, the difference can be easily measured. This technique can be used in radar systems, and is often found in aircraft radar altimeters. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a sine wave or sawtooth pattern at audio frequencies. The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared. Since the signal frequency is changing, by the time the signal returns to the aircraft the broadcast has shifted to some other frequency. The amount of that shift is greater over longer times, so greater frequency differences mean a longer distance, the exact amount being the "ramp speed" selected by the electronics. The amount of shift is therefore directly related to the distance travelled, and can be displayed on an instrument. This signal processing is similar to that used in speed detecting doppler radar. See the article on continuous wave radar for more information.

Speed measurement

Speed is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a little memory to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making grease-pencil marks on the radar screen, and then calculating the speed using a slide rule. However there is another effect that can be used to make much more accurate speed measurements, and do so almost instantly (no memory required), known as the Doppler effect. Practically every modern radar uses this principle in the pulse-doppler radar system. It is also possible to make a radar without any pulsing, known as a continuous-wave radar (CW radar), by sending out a very pure signal of a known frequency. Return signals from targets are shifted away from this base frequency via the Doppler effect enabling the calculation of the speed of the object relative to the radar.

Position measurement

Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.

Early systems

Early systems tended to use omni-directional broadcast antennas, with directional receiver antennas which were pointed in various directions. For instance the first system to be deployed, Chain Home, used two straight antennas at right angles for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by rotating the antenna so one display showed a maximum while the other shows a minimum. One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is a small part of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional. More modern systems used a steerable parabolic "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combined two radar frequencies in the same antenna in order to allow automatic steering, or radar lock. parabolic

Phased array

Another method of steering is used in a phased array radar. This uses an array of similar aerials suitably spaced, the phase of the signal to each individual aerial being controlled so that the signal is reinforced in the desired direction and cancels in other directions. If the individual aerials are in one plane and the signal is fed to each aerial in phase with all others then the signal will reinforce in a direction perpendicular to that plane. By altering the relative phase of the signal fed to each aerial the direction of the beam can be moved because the direction of constructive interference will move. Because phased array radars require no physical movement the beam can scan at thousands of degrees per second, fast enough to irradiate many individual targets, and still run a wide-ranging search periodically. By simply turning some of the antennas on or off, the beam can be spread for searching, narrowed for tracking, or even split into two or more virtual radars. However, the beam cannot be effectively steered at small angles to the plane of the array, so for full coverage multiple arrays are required, typically disposed on the faces of a triangular pyramid (see picture). Phased array radars have been in use since the earliest years of radar use in World War II, but limitations of the electronics led to fairly poor accuracy. Phased array radars were originally used for missile defence. They are the heart of the ship-bourne Aegis combat system, and the Patriot Missile System, and are increasingly used in other areas because the lack of moving parts makes them more reliable, and sometimes permits a much larger effective antenna. As the price of electronics has fallen, phased array radars have become more and more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is far offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are still widely used in roles where cost is a significant factor such as air traffic surveillance, weather radars and similar systems. Phased array radars are also valued for use in aircraft, since they can track multiple targets. The first aircraft to use phased array radar was the Mikoyan MiG-31. The MiG-31M's SBI-16 Zaslon phased array radar is considered to be the world's most powerful fighter radar.

Radar equation

The amount of power P_r returning to the receiving antenna is given by the radar equation: :

P_r =

where
- P_t = transmitter power,
- G_t = gain of transmitting antenna,
- A_r = effective aperture (area) of receiving antenna,
- σ = Radar Cross Section, or scattering coefficient of target,
- R_t = distance from transmitter to target,
- R_r = distance from target to receiver. In the common case where the transmitter and receiver are at the same location, R_t = R_r and the term R_t² R_r² can be replaced by R⁴, where R is the range. This yields: :

P_r =

This shows that the received power declines as the fourth power of the range, which means that the reflected power from distant targets is very, very small. Note that the equation above is a simplification for vacuum without interference. In a real-world situation, pathloss effects should be considered, as well as other factors of the transmission medium. Other mathematical developments in radar signal processing include time-frequency analysis (Weyl Heisenberg or wavelet), as well as the chirplet transform which makes use of the fact that radar returns from moving targets typically "chirp" (change their frequency as a function of time, as does the sound of a bird or bat).

Frequency bands

The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world in the 21st century. They have been adopted in the United States by the IEEE, and internationally by the ITU. Most countries have additional regulations to control which parts of each band are available for civilian or military use. Other users of the radio spectrum, such as the broadcasting and electronic countermeasures (ECM) industries, have replaced the traditional military designations with their own systems.

Specific radar systems

- Active Electronically Scanned Array (AESA)
- Continuous-wave radar
- Doppler radar as weather radar
- Millimetre cloud radar
- NEXRAD
- Over-the-horizon radar
- Passive radar
- Pulse-doppler radar
- Radar gun traffic and sports radars
- Secondary surveillance radar (SSR)
- Synthetic aperture radar
- X-band radar
- Terrain-following radar

Radar modulators

Modulators are sometimes called pulsers and act to provide the short pulses of power to the magnetron In this way, the transmitted pulse of RF radiation is kept to a defined, and usually very short, duration. Modulators consist of a high voltage pulse generator formed from a HV supply, a pulse forming line or network (PFN) and a high voltage switch such as a thyratron.

Radar Signal Processing

External links

[http://www.radar-france.net The first operational radar in France 1934]

References

- Barrett, Dick, "[http://www.radarpages.co.uk/index.htm All you ever wanted to know about British air defence radar]". The Radar Pages. (History and details of various British radar systems)
- Buderi, "[http://www.privateline.com/TelephoneHistory3/radarhistorybuderi.html Telephone History: Radar History]". Privateline.com. (Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.)
- Ekco Radar [http://www.ekco-radar.co.uk/ WW2 Shadow Factory] The secret development of British Radar.
- ES310 "[http://www.fas.org/man/dod-101/navy/docs/es310/syllabus.htm Introduction to Naval Weapons Engineering.". (Radar fundamentals section)]
- Hollmann, Martin, "[http://www.radarworld.org/index.html Radar Family Tree]". [http://www.radarworld.org/ Radar World].
- Penley, Bill, and Jonathan Penley, "[http://www.penleyradararchives.org.uk/history/introduction.htm Early Radar History] - an Introduction". 2002.
- Sinnott, D.H., "[http://www.dsto.defence.gov.au/corporate/history/othr/ The Development of Over-the-Horizon Radar in Australia]"
- USAF Long Range Radar, "[http://www.rades.hill.af.mil/ 84th Radar Evaluation Squadron]". US Air Force Squadron responsible for long-range Radar Sensor (both civilian and military) operational availability, counterdrug, search and rescue, and flight safety information assurance to the operations community.. Category:Acronyms Category:Radar Category:Measuring instruments Category:Navigational equipment Category:Neologisms ms:Radar ja:レーダー

Horizon

:For other uses of horizon, see Horizon (disambiguation) Horizon (disambiguation) The horizon is the line that separates earth from sky. More precisely, it is the line that divides all of the directions you can possibly look into, into two categories: those which intersect the Earth, and those which do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc. The resulting intersection of earth and sky is instead known as the visible horizon. However, if you are on a ship at sea, the true horizon is strikingly apparent. Historically, the distance to the visible horizon has been extremely important as it represented the maximum range of communication and vision before the development of the radio and the telegraph. Even today, when flying an aircraft under Visual Flight Rules, a technique called attitude flying is used to control the aircraft, where the pilot uses the relationship between the aircraft's nose and the horizon to control the aircraft. He also retains his spatial orientation by referring to the horizon. spatial orientationIn astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points which have an altitude of zero degrees. The regular horizon is a little below that. The distance of the horizon on earth, in a plain (standing on the ground or on a tower, or from a plane) or on a hill or mountain surrounded by plains, is approximately

\sqrt

kilometers, where h is the height in meters of the eyes. Examples:
- standing on the ground with h = 1.70 m, the horizon is at a distance of 4.7 km
- standing on a hill or tower of 100 m height, the horizon is at a distance of 36 km These figures indicate theoretical visibility (what can be seen depends also on how clear the air is, of course) of objects at ground level. To compute to what distance the tip of a tower, the mast of a ship or a hill is above the horizon, add the horizon distance for that height. For example, standing on the ground with h = 1.70 m, one can see, weather permitting, the tip of a tower of 100 m height at a distance of 41 km. This formula is reasonable when h is much smaller than the radius of the Earth (6371 km). The exact formula for distance from the viewpoint to the horizon, applicable even for satellites, is :

\sqrt

where R is the radius of the Earth (note: both R and h in this equation are in kilometers). A different formula is given by :

\cos\frac=\frac.

This formula gives the arc length distance s along the curved surface of the Earth to to bottom of object, whereas the above formula is for the straight line of sight distance to the top of the object of view. Both formulas agree when the height of the object is negligible compared to the radius.

External links

- [http://newton.ex.ac.uk/people/sque/physics/horizon/ Derivation of the distance to the horizon]
- [http://www.robertbdance.com/PaintingAtmosphericEffects.html An artistic treatment of the horizon]

Acknowledgements

The first version of this article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml Category:Horizontal coordinate system Category:Spherical astronomy

Ships

vessel Amerigo Vespucci in New York Harbor, 1976]] A ship is a large, sea-going watercraft, sometimes with multiple decks. A ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it doesn't always apply) goes: "a boat can fit on a ship, but a ship can't fit on a boat". Often local law and regulation will define the exact size (or the number of masts) which a boat requires to become a ship. (Note that one refers to submarines as "boats"). Compare vessel. During the age of sail, ship signified a ship-rigged vessel, that is, one with three or more masts, usually three, all square-rigged. Such a vessel would normally have one fore and aft sail on her aftermost mast which was usually the mizzen. Almost invariably she would also have a bowsprit but this was not part of the definition. The same economic pressures which increased sizes to the point of carrying four or five masts, also introduced the fore and aft rig to larger vessels, so few ship-rigged vessels were built with more than three masts. The five-masted Preussen was the outstanding example but the big German ships and barques were built partly for prestige reasons. Nautical means related to sailors, particularly customs and practices at sea. Naval is the adjective pertaining to ships though in common usage, it has come to be more particularly associated with the noun 'navy'.

Measuring ships

One can measure ships in terms of overall length, length of the waterline, beam (breadth), depth (distance between the crown of the weather deck and the top of the keelson), draft (distance between the highest waterline and the bottom of the ship) and tonnage. A number of different tonnage definitions exist; most measure volume rather than weight and are used when describing merchant ships.
- Gross tonnage is a measure of the total internal volume of the ship.
- Net tonnage is expresses a merchant vessel's earning capacity and gives the internal capacity of that part of the ship available for cargo or passengers.
- Thames measurement tonnage was used for smaller vessels and worked to a formula: (length - beam) x beam x ½beam / 94
- Displacement tonnage is normally applied to warships and equals the actual weight of a ship complete with crew, fuel, stores and water.
- Light ship tonnage measures the actual weight of the ship with no fuel, no persons, no cargo, no water on board is not usually quoted.
- Deadweight tonnage is the weight of cargo, stores, passengers etc. which when added to the weight of the ship's structure and equipment, will bring the vessel down to her designed waterline. The word "displacement" arises from the basic physical law, discovered by Archimedes, that the weight of a floating object equates exactly to that of the water which would otherwise occupy the "hole in the water" displaced by the ship. In Britain until the Merchant Shipping Act of 1876, ship-owners could load their vessels until their decks were almost awash, resulting in a dangerously unstable condition. Additionally, anyone who signed onto such a ship for a voyage and, upon realizing the danger, chose to leave the ship, could end up in jail. Samuel Plimsoll, a member of Parliament, realised the problem and engaged some engineers to derive a fairly simple formula to determine the position of a line on the side of any specific ship's hull which, when it reached the surface of the water during loading of cargo, meant the ship had reached its maximum safe loading level. To this day, that mark, called the "Plimsoll Mark", exists on ships' sides, and consists of a circle with a horizontal line through the center. Because different types of water, (summer, fresh, tropical fresh, winter north Atlantic) have different densities, subsequent regulations required painting a group of lines forward of the Plimsoll mark to indicate the safe depth (or freeboard above the surface) to which a specific ship could load in water of various densities. Hence the "ladder" of lines seen forward of the Plimsoll mark to this day.

Propulsion

Until the application of the steam engine to ships in the early 19th century, oars propelled galleys or the wind propelled sailing ships. Before mechanisation, merchant ships always used sail, but as long as naval warfare depended on ships closing to ram or to fight hand-to-hand, galleys dominated in marine conflicts because of their maneuverability and speed. The Greek navies that fought in the Peloponnesian War used triremes, as did the Romans contesting the Battle of Actium. The use of large numbers of cannon from the 16th century meant that maneuverability took second place to broadside weight; this led to the dominance of the sail-powered warship. The development of the steamship became a complex process, the first commercial success accruing to Robert Fulton's North River Steamboat (often called Clermont) in the USA in 1807, followed in Europe by the 45-foot PS Comet of 1812. Steam propulsion progressed considerably over the rest of the 19th century. Notable developments included the condenser, which reduced the requirement for fresh water, and the multiple expansion engine, which improved efficiency. As the means of transmitting the engine's power, the paddle wheel gave way to the more efficient screw propeller. The marine steam turbine developed by Sir Charles Algernon Parsons, brought the power to weight ratio down. He had achieved publicity by demonstrating it unofficially in the 100-foot Turbinia at the Spithead Naval Review in 1897. This facilitated a generation of high-speed liners in the first half of the 20th century and rendered the reciprocating steam engine out of date, in warships. The marine diesel engine first came into use around 1912: either the Vulcanus or the Selandia (depending upon who you talk to) first deployed it. It soon offered even greater efficiency than the steam turbine but for many years had an inferior power-to-space ratio. About this period too, heavy fuel oil came into more general use and began to replace coal as the fuel of choice in steamships. Its great advantages were the convenience and the reduction in manning owing to the removal of the need for trimmers and of stokers in the old-fashioned numbers. Most ships built since around 1960 have used diesel power or motors; one exception, Queen Elizabeth 2 of 1968, started with steam turbines but subsequently converted to diesel as a cost-saving measure. A few ships have used nuclear reactors, but this is not a separate form of propulsion. It merely makes steam to drive the turbines. Nonetheless, it has caused concerns about safety and waste disposal. It has become usual only in large aircraft carriers and in submarines, where the ability to run submerged for long periods holds obvious advantage. In such long-endurance vessels, the saving in bunkerage too, is an important consideration.

General terminology

Ships may occur collectively as fleets, flotillas or squadrons. Convoys of ships commonly occur. A collection of ships for military purposes may compose a navy or a task force. In the past, people counting or grouping disparate types of ship may refer to the individual vessels as bottoms. Groups of sailing ships could constitute, say, a fleet of 40 sail. Groups of submarines (particularly German U-boats in the 1940s) may hunt in packs (often erroneously called wolf packs).

Shipboard terminology

See also: Glossary of nautical terms. The complexity of ships, particularly of sailing ships, led to the development of a rich and various vocabulary. Many of the following terms link to more detailed discussions of nautical terminology.
- Amidships - toward the middle of the vessel.
- Bow - strictly, one of the two curved structures where the hull broadens out from the stem (the pointed end). The bows is a term for the head of the vessel or front of the ship. Compare prow, a more poetical term for the ship's head.
- Stern - the after end of the ship.
- Aft - towards the stern when the relationship is within the ship.
- Astern beyond the stern where the relationship is outside the vessel.
- Starboard - the side of the ship which lies to the right when an observer within the ship faces forward.
- Port - the side of the ship which lies to the left when an observer within the ship faces forward. (A mnemonic to distinguish port and starboard notes that left and port both have four letters. Another incorporates the navigation light: Is there any red port left?)
- (Navigation) Bridge - A structure above the weather deck, extending the full width of the vessel, which houses a command centre, itself called by association, the bridge. A bridge usually extends a little beyond the ship's side to enable observation of boats alongside, or the proximity of a dock or lock gate; these projections are called bridge wings. In big vessels, a docking bridge used to be found aft. (See Lord, Walter. A Night to Remember (1976) p.96). It enabled an officer to observe docking manoeuvres before giving orders. RMS Titanic had one but they have been superseded by closed circuit television cameras.
- Bulkheads - internal "walls" in a ship. Bulkheads are the vertical equivalent of decks. They have a structural function as well as dividing spaces. They serve to prevent collapse of the hull under stress, to maintain stability, in the event of flooding, and to contain fire. Many bulkheads feature watertight doors which, in the case of certain types of ships, the crew may close remotely. An internal "wall" that is not load-bearing is usually referred to as a "partition". It is to a bulkhead as a flat is to a deck.
- Cabin - an enclosed room on a deck or flat.
- Capstan - a winch with a vertical axis.
- Coaming - Raised edges of hatches in decks for keeping water and articles free on the deck from falling into the hold.
- Decks - the structures forming the approximately horizontal surfaces in the ship's general structure. Unlike flats, they are a structural part of the ship.
- Deck Head - The under-side of the deck above. Sometimes panelled over to hide the pipe work. This panelling, like that lining the bottom and sides of the holds, is the ceiling.
- Draft - The vertical distance from the current waterline to the lowest point of the ship or in the part of the ship under consideration.
- Figurehead - symbolic image at the head of a traditional sailing ship or early steamer.
- Forecastle - a partial deck, above the upper deck and at the head of the vessel; traditionally the sailors' living quarters.
- Freeboard - The vertical distance from the current waterline to the highest continuous watertight deck. This usually varies from one part to another.
- Galley - the kitchen of the ship
- Gunwale - Formerly a fabricated band placed for strengthening around the ship at the main or upper deck level to accommodate the stresses imposed by the use of artillery. In later use it is the angle between the ship’s side and upper deck. It remained as a structural member, in wooden boats where it was mounted inboard of the sheer strake regardless of the need for gunnery.
- Bulwark - the extension of the ship's side above the level of the weather deck.
- Hold - In earlier use, below the orlop deck, the lower part of the interior of a ship's hull, especially when considered as storage space, as for cargo. In later merchant vessels it extended up through the decks to the underside of the weather deck.
- Hull - the shell and framework of the basic flotation-oriented part of a ship
- Keel - the central structural basis of the hull
- Kelson - the timber immediately above the keel of a wooden ship.
- Mast - a spar (in a ship, a very heavy one stepped in the keelson) formerly designed for the support of one or more sails. In modern ships, it is a steel or aluminium fabrication which carries navigation lights, radar antennae etc.
- Prow - a poetical alternative term for bows.
- Scupper - a drainage waterway at the edge of a deck, is drained by a pipe or, on the weather deck, a small opening in the bulwarks, leading overboard. It is called a scupper which is distinct from larger openings with hinged covers on the bulwarks, designed for relieving the ship of large quantities of water in a seaway. These are called freeing ports or wash ports..
- Windlass - A winch mechanism, usually with a horizontal axis. used where mechanical advantage greater than that obtainable by block and tackle was needed.
- Weather deck - whichever deck is that exposed to the weather – usually either the main deck or, in larger vessels, the upper deck.

Some types of ships and boats

- Aircraft carrier
- Auto carrier
- Bulk carrier
- Cable Layer
- Capital ship
- Cargo ship
- Catamaran
- Coaster
- Commerce raider
- Container ship
- Corvette
- Cruise ship
- Cruiser
- Cutter
- Destroyer
- Diving support vessel
- Ferry
- Frigate
- Guided missile cruiser
- Icebreaker
- Junk
- Laker
- Lugger
- Minesweeper
- Minehunter
- Ocean liner
- Panamax
- Reefer (refrigerated ship)
- Research vessel
- RO-RO ship (roll on, roll off)
- Sailing ship
- Sloop
- Submarine
- Supertanker
- Tanker
- Tender
- Train ferry
- Tugboat
- Shipyard
- Yacht

Some historical types of ships and boats

Yacht
- Barque A sailing vessel with three or more masts, fore-and-aft rigged on only the aftermost.
- Barquentine A sailing vessel with three or more masts, square-rigged only on the foremast.
- Battle cruiser A light battleship.
- Battleship a large, heavily-armoured and heavily-gunned warship. A term which generally post-dates sailing warships.
- Bilander
- Bireme An ancient vessel, propelled by two banks of oars.
- Birlinn
- Blockade runner A ship whose current business is to slip past a blockade.
- Brig A two-masted, square-rigged vessel.
- Brigantine A two-masted vessel, square-rigged on the foremast and fore-and-aft rigged on the main.
- Caravel
- Carrack
- Clipper
- Cog
- Collier A vessel designed for the coal trade.
- Dreadnought An early twentieth century class of battleship.
- Dromons are the precursors to galleys.
- East Indiaman An armed merchantman belonging to one of the East India companies (Dutch, British etc.)
- Fire ship A vessel of any sort, set on fire and sent into an anchorage with the aim of causing consternation and destruction. The idea is generally that of forcing an enemy fleet to put to sea in a confused, therefore vulnerable state.
- Galleass A sailing and rowing warship, equally well suited to sailing and rowing.
- Galleon A sixteenth century sailing warship.
- Galley A warship propelled by oars with a sail for use in a favourable wind.
- Galliot
- Ironclad A wooden warship with external iron plating.
- Knarr A type of Viking trade ship
- Liberty ship An American merchant ship of the late Second World War period, designed for rapid building in large numbers. (The earliest class of welded ships.)
- Longship A Viking raiding ship
- Man of war A sailing warship.
- Monitor A small, very heavily gunned warship with shallow draft. Designed for land bombardment.
- Paddle steamer A steam-propelled, paddle-driven vessel, a name commonly applied to nineteenth century excursion steamers.
- Pantserschip A Dutch ironclad. By the end of the nineteenth century, the name was applied to a heavy gunboat designed for colonial service.
- Penteconter An ancient warship propelled by 50 oars, 25 on each side.
- Pram A small dinghy, originally of a clinker construction and called in English, as in Danish, a praam. The Danish orthography has changed so that it would now be a pråm in its original language. It has a transom at both ends, the forward one usually small and steeply raked in the traditional design.
- Q-ship A commerce raider camouflaged as a merchant vessel.
- Quinquereme An ancient warship propelled by three banks of oars. On the upper row three rowers hold one oar, on the middle row - two rowers, and on the lower row - one man to an oar.
- Schooner A fore and aft-rigged vessel with two or more masts of which the foremast is shorter than the main.
- Shallop A large, heavily built, sixteenth century boat. Fore and aft rigged. More recently it has been a poetically frail open boat.
- Small Waterplane Area Twin Hull (SWATH) A modern ship design used for Research Vessels and other purposes needing a steady ship in rough seas.
- Steamship A ship propelled by a steam engine.
- Ship of the line A sailing warship of first, second or third rate. That is, with 64 or more guns. Before the late eighteenth century, fourth rates (50-60 guns) also served in the line of battle.
- Torpedo boat A small, fast surface vessel designed for launching torpedoes.
- Tramp steamer A steamer which takes on cargo when and where it can find it.
- Trireme An ancient warship propelled by three banks of oars.
- Xebec
- Victory ship

Quotations

:I must go down to the sea again, to the lonely sea and the sky, :And all I ask is a tall ship, and a star to steer her by... :-John Masefield

External links

- [http://www.shipsystems.net.tf - Reference page]
- [http://www.shipspotting.com/ ShipSpotting.com - shipping image archive]
- Category:Water transport Category:Transportation ja:船舶 ms:Kapal

Rain

: For other uses see Rain (disambiguation). Rain is a form of precipitation, other forms of which include snow, sleet, hail, and dew. Rain forms when separate drops of water fall to the Earth's surface from clouds. Not all rain reaches the surface, however; some evaporates while falling through dry air. When none of it reaches the ground, it is a precipitation called virga.

Rain in nature

Rain plays a major role in the hydrologic cycle in which [http://wiktionary.org/wiki/moisture moisture] from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again. There is also a small amount of water vapor that respires from plants and evaporates to join other water molecules in condensing into clouds. The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and can be measured to the nearest 0.25 mm or 0.01 in. It is sometimes expressed in litres per square metre (1 L/m² = 1 mm). Falling raindrops are often depicted in cartoons or anime as "tear-shaped", round at the bottom and narrowing towards the top, but this is incorrect (only drops of water dripping from some sources are tear-shaped at the moment of formation). Small raindrops are nearly spherical. Larger ones become increasingly flattened, like hamburger buns; very large ones are shaped like parachutes. [http://www.ems.psu.edu/~fraser/Bad/BadRain.html] On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 - some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water. Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain. Rain is said to be heavier immediately after a bolt of lightning. The cause of this phenomenon is traceable to the bipolar aspect of the water molecule. The intense electric and magnetic field generated by a lightning bolt forces many of the water molecules in the air surrounding the stroke to line up. These molecules then spontaneously create localized chains of water (similar to nylon or other 'poly' molecules). These chains then form water droplets when the electric/magnetic field is removed. These drops then fall as intensified rain.

Culture

lightning Cultural attitudes towards rain differ across the world. In the largely temperate Western world, rain traditionally has a sad and negative connotation — reflected in children's rhymes like Rain Rain Go Away — in contrast to the bright and happy sun. In dry places such as India and the Middle East, the rain is greeted with euphoria. Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats, and diversion devices such as gutters and storm drains that lead rains to sewers. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or rain is extremely heavy (monsoon). Rain may be collected for drinking water since rainwater is pure, or used as greywater. Excessive rain, particularly after a dry period has hardened the soil so that it cannot absorb water, can cause floods. Many people find the scent smelt during and immediately after rain especially pleasant or distinctive. The source of this smell is petrichor, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall.

Allen B. DuMont

Dr. Allen Balcom DuMont (January 29, 1901 - November 14, 1965) was an American scientist and inventor best known for improvements to the cathode ray tube in 1931 for use in television receivers. Seven years later he manufactured and sold the first commercially practical television set to the public. In 1938, his Model 180 television receiver was the first all-electronic television set ever sold to the public, a few months prior to RCA's first set. DuMont later went on to found in 1946 the first television network to be licensed, the DuMont Television Network, initially by linking station WABD (named for DuMont) in New York City to station WTTG in Washington, DC. (WTTG was named for Dr. Thomas T. Goldsmith, DuMont's Vice President of Research, and his best friend; Goldsmith turned 94 in January, 2004.) DuMont was born in Brooklyn, New York. At the age of 10, he was stricken with polio and was quarantined at his family's Eastern Parkway apartment for nearly a year. During his quarantine, his father brought home books and magazines for the young DuMont to read while bedridden. At this time, DuMont developed an interest in science, specifically wireless radio communication, and taught himself Morse code. His father bought him a crystal radio receiver which he assembled, took apart, reassembled and rebuilt several times. He improved his set each time he rebuilt it and later built a transmitter, while his father obtained the landlord's permission to erect a 30-foot high transceiving antenna on the roof. While recuperating from polio, DuMont was advised to swim to regain the use of his legs. In 1914, the family moved to Montclair, New Jersey, where there was an indoor year-round pool available at the local YMCA. He graduated from Montclair High School in 1919, and went to Rensselaer Polytechnic Institute in Troy, NY. While there he filed his first patent application for a sound activated switch, used to turn on and off lights and appliances with the clap of the hands. In 1915, DuMont became the youngest American to obtain a first class commercial radio operator's license at age 14. The following summer, he worked as a radio operator aboard a coastal steamer making runs from New York to Providence, Rhode Island. As the summers went by, he made his way to the Caribbean, South America and, after World War I, to Europe, where, during the summer of 1922, he was stuck in Copenhagen for months because of a dock workers strike. After graduating from Rensselaer in 1924, DuMont worked at the Westinghouse Lamp Company in Bloomfield, New Jersey, in charge of radio tube production. While there, he increased production from 500 tubes per day to an astounding 50,000 tubes per day. Management decided to give him a $500 bonus, a small raise, and the "Westinghouse Award", an award devised to recognize his accomplishments. The "Westinghouse Award" was later presented as a scholarship award to high school seniors showing promise in a field of science. By 1928, DuMont was searching for new opportunities and was wooed by Dr. Lee De Forest, a radio pioneer who developed the audion tube, the original voice amplifier for radio reception. De Forest had a checkered career as an inventor and had several failed business ventures. DuMont was hired as vice president and production manager for radio tubes. Here he came in contact with a mechanical television, one that De Forest had purchased from C. Francis Jenkins, another radio pioneer. DuMont worked to improve television transmission and reception and went to De Forest asking for funds to build a long lasting cathode ray tube for television reception. De Forest denied DuMont's request as De Forest's investors were demanding better returns. Subsequently, DuMont resigned at the same time that De Forest sold his radio manufacturing business to David Sarnoff at RCA. DuMont then started his own company, DuMont Laboratories, in the basement of his Cedar Grove, New Jersey home, building long-lasting cathode ray tubes. In 1931, he sold two tubes to two college science laboratories for $35 each. In 1932, DuMont proposed a "ship finder" device to the U.S. Signal Corps at Fort Monmouth, New Jersey, that used radio wave distortions to locate objects on a cathode ray tube screen—he had essentially invented radar. The military asked him, however, not to take out a patent for developing what they wanted to maintain as a secret, and so he is not often mentioned among those responsible for radar. He did, however, go on to develop long-range precision radar to aid the Allies during WWII. As a consequence the French Government knighted him in 1952. During the early years of World War II, DuMont received special government contracts to provide large 36" wide cathode ray tubes. These special tubes allowed scientists working on the Manhattan Project to study the action of accelerated electrons. DuMont produced black and white televisions in the 1940s and 1950s that were generally regarded as offering highest quality and durability. Many of these premium sets included a built in AM/FM radio and record player. The DuMont Television Network was not an unqualified success, being faced with the major problem of how to make a profit without the benefit of an already established radio network as a base. After ten years, DuMont shuttered the network and sold what remained of his television operations to John Kluge in 1956, which Kluge renamed Metromedia. DuMont sold his manufacturing operations in 1960. The television manufacturing division was sold to Emerson Radio. His research laboratory became part of Fairchild Camera and later developed semiconductor microchips. Robert Noyce, founder of Intel, originally worked for DuMont as an engineer. In the early 1960s, the Dumont laboratory, now owned by Fairchild, developed the original Sony Trinitron color picture tube, under a subcontract. DuMont was the first to provide funding for educational television broadcasting. He was the recipient of numerous honorary degrees and awards, among them the Cross of Knight awarded by the French Government, the Horatio Alger Award, the Westinghouse Award, and the DeForest Medal. DuMont died in 1965 and is buried in Mount Hebron Cemetery in Upper Montclair. The television center at Montclair State University bears his name. DuMont, Allen B. DuMont, Allen B. DuMont, Allen B. DuMont, Allen B.

1941

:For the movie, see 1941 (film) 1941 (MCMXLI) was a common year starting on Wednesday (link will take you to calendar).

Events

January-February

- January 6 - Franklin Delano Roosevelt delivers his Four Freedoms Speech in the State of the Union Address.
- January 10 - Lend-Lease is introduced into the U.S. Congress.
- January 19 - British troops attack Italian-held Eritrea.
- January 21 - World War II: Australian and British forces attack Tobruk, Libya.
- January 22 - World War II: British troops capture Tobruk from the Italians.
- January 23 - Charles Lindbergh testifies before the U.S. Congress and recommends that the United States negotiate a neutrality pact with Adolf Hitler.
- February 3 - World War II: The Nazis forcibly restore Pierre Laval to office in occupied Vichy, France.
- February 4 - World War II: The United Service Organization (USO) is created to entertain American troops.
- February 11 - World War II: Lieutenant-General Erwin Rommel arrives in Tripoli.
- February 19 - The start of the "three nights' Blitz" over Swansea, South Wales. Over these three nights of intensive bombing, which lasted a total of 13 hours and 48 minutes, Swansea town centre was almost completely obliterated by the 896 High Explosive bombs employed by the Luftwaffe. A total of 397 casualties and 230 deaths were reported. The Three nights Blitz ended in the early hours of February 22.

March

- March 1 - World War II: Bulgaria signs the Tripartite Pact thus joining the Axis powers.
- March 1 - W47NV begins operations in Nashville, Tennessee becoming the first FM radio station.
- March 1 - Arthur L. Bristol becomes Rear Admiral for the U.S. Navy's Support Force, Atlantic Fleet
- March 11 - World War II: President Franklin Delano Roosevelt signs the Lend-Lease Act into law, allowing American-built war supplies to be shipped to the Allies on loan.
- March 17 - In Washington, DC, the National Gallery of Art is officially opened by President Franklin D. Roosevelt.
- March 17 - British Minister of Labour, Ernest Bevin, calls for women to fill vital jobs
- March 22 - Washington's Grand Coulee Dam begins to generate electricity.
- March 25 - World War II: Kingdom of Yugoslavia in Vienna joins the Axis powers
- March 27 - World War II: Attack on Pearl Harbor - Japanese spy Takeo Yoshikawa arrives in Honolulu, Hawaii and begins to study the United States fleet at Pearl Harbor.
- March 29 - World War II: Battle of Cape Matapan - Off the Peloponnesus coast in the Mediterranean, British naval forces defeat those of Italy sinking five warships. Battle started on March 27.

April

- April 6 - World War II: Germany invades Yugoslavia and Greece.
- April 17 - World War II: Yugoslav Royal Army capitulates.
- April 21 - World War II: Greece capitulates. British troops withdraw to Crete.
- April 27 - World War II: German troops enter Athens.
- April - Russia and Japan sign a neutrality pact.

May

neutrality pact
- May 1 - Breakfast cereal Cheerios is introduced as CheeriOats by General Mills
- May 1 - Orson Welles' film, Citizen Kane, premieres in New York City
- May 5 - Emperor Haile Selassie enters Addis Ababa, which had been liberated from Italian forces; this date has been since commemorated as Liberation Day in Ethiopia.
- May 6 - At California's March Field, Bob Hope performs his first USO Show.
- May 9 - World War II: The German submarine U-110 is captured by the British Royal Navy. On board is the latest Enigma cryptography machine which Allied cryptographers later use to break coded German messages.
- May 10 - World War II: The United Kingdom's House of Commons is damaged by the Luftwaffe in an air raid.
- May 10 - World War II: Rudolf Hess parachutes into Scotland claiming to be on a peace mission.
- May 20 - World War II: Battle of Crete - Germany launches airborne invasion of Crete.
- May 21 - World War II: 950 miles off the coast of Brazil, the freighter SS Robin Moor becomes the first United States ship sunk by a German U-boat.
- May 24 - World War II: In the North Atlantic, the German battleship Bismarck sinks the HMS Hood killing all but three crewman on what was the pride of the Royal Navy.
- May 26 - World War II: In the North Atlantic, Fairey Swordfish aircraft from the carrier HMS Ark Royal fatally cripple the German battleship Bismarck in torpedo attack.
- May 27 - World War II: President Roosevelt proclaims an "unlimited national emergency."
- May 27 - World War II: German battleship Bismarck is sunk in North Atlantic killing 2,300.

June

- June 1 - World War II: Allies evacuate Crete.
- June 8 - World War II: Allies invade Syria and Lebanon.
- June 9 - World War II: Finland initiate mobilization and put some units under German command.
- June 14 - Mass deportations by Soviet Union authorities take place in Estonia, Latvia and Lithuania.
- June 22 - World War II: Germany attacks the Soviet Union in Operation Barbarossa
- June 25 - World War II: Finland attacked by the Soviet Union seeks the opportunity of revenge in the Continuation War.

July-August

- July 4 - Mass murder of Polish scientists and writers, committed by German troops in captured Polish city of Lwów.
- July 5 - World War II: German troops reach the Dnipro River.
- July 5-19 - War between Peru and Ecuador
- July 7 - World War II: American forces land in Iceland to forestall an invasion by the Nazis.
- July 13 - World War II - Montenegro starts the first popular uprising in Europe against the Axis Powers.
- July 26 - World War II: In response to the Japanese occupation of French Indo-China, US President Franklin D. Roosevelt orders the seizure of all Japanese assets in the United States.
- July 31 - Holocaust: Under instructions from Adolf Hitler, Nazi official Hermann Göring, orders SS general Reinhard Heydrich to "submit to me as soon as possible a general plan of the administrative material and financial measures necessary for carrying out the desired final solution of the Jewish question."
- August - Formation of the Political Warfare Executive in the United Kingdom
- August 1 - The first jeep is produced
- August 6 - 6-year-old Elaine Esposito goes to an appendix operation in Florida and lapses into a coma. She dies 1978, still in coma.
- August 18 - Adolf Hitler orders a temporary halt to Nazi Germany's systematic euthanasia of mentally ill and handicapped due to protests. However, graduates of the T-4 Euthanasia Program were then transferred to concentration camps, where they continued in their trade.

September-October

- September 6 - Holocaust: The requirement to wear the Star of David with the word "Jew" inscribed, is extended to all Jews over the age of 6 in German-occupied areas.
- September 8 - World War II: Siege of Leningrad begins - German forces begin a siege against the Soviet Union's second-largest city, Leningrad. Stalin orders the Volga Deutsche deported to Siberia.
- September 16 - Reza Pahlavi, Shah of Iran is forced to resign in favor of his son Mohammad Reza Pahlavi of Iran under pressure from the United Kingdom and the Soviet Union.
- October 2 - World War II: Operation Typhoon - Germany begins an all-out offensive against Moscow.
- October 8 - World War II: In their invasion of the Soviet Union, Germany reaches the Sea of Azov with the capture of Mariupol.
- October 21 - World War II: Germans rampage in Yugoslavia, killing thousands of civilians
- October 24 - Franz von Werra disappears during a flight over North Sea
- October 30 - World War II: Franklin Delano Roosevelt approves US$1 billion in Lend-Lease aid to the Soviet Union.
- October 31 - After 14 years of work, drilling is completed on Mount Rushmore.
- October 31 - World War II: The destroyer USS Reuben James is torpedoed by a German U-boat near Iceland, killing more than 100 United States Navy sailors.

November

United States Navy
- November 6 - World War II: Soviet leader Joseph Stalin addresses the Soviet Union for only the second time during his three-decade rule (the first time was earlier that year on July 2). He states that even though 350,000 troops were killed in German attacks so far, that the Germans have lost 4.5 million soldiers (a gross exaggeration) and that Soviet victory was near.
- November 12 - World War II: Battle of Moscow: Temperatures around Moscow drop to −12 °C and the Soviet Union launches ski troops for the first time against the freezing German forces near the city.
- November 13 - World War II: The aircraft carrier HMS Ark Royal is hit by German U-boat U-81
- November 14 - World War II: HMS Ark Royal capsizes and sinks, having been torpedoed by U 81.
- November 17 - World War II: Attack on Pearl Harbor - Joseph Grew, the United States ambassador to Japan, cables the State Department that Japan had plans to launch an attack against Pearl Harbor, Hawaii (his cable was ignored).
- November 19 - World War II: The Australian war cruiser HMAS Sydney sinks off the coast of Western Australia, killing 645 sailors.
- November 21 - The radio program King Biscuit Time is broadcast for the first time (it would later become the longest running daily radio broadcast in history and the most famous live blues radio program).
- November 24 - World War II: The United States grants Lend-Lease to the Free French.
- November 26 - US President Franklin Delano Roosevelt signs a bill establishing the fourth Thursday in November as Thanksgiving Day in the United States (this partly reversed a 1939 action by Roosevelt that changed the celebration of Thanksgiving to the third Thursday of November).
- November 26 - World War II: The Hull note ultimatum is delivered to Japan by the United States.
- November 26 - World War II: Attack on Pearl Harbor - A fleet of six aircraft carriers commanded by Japanese Vice Admiral Chuichi Nagumo leaves Hitokapu Bay for Pearl Harbor under strict radio silence.
- November 27 - A group of young men stop traffic on highway US 99 south of Yreka, California, handing out fliers proclaiming the establishment of the State of Jefferson.
- November 27 - World War II: Battle of Moscow - Germans reach their closest approach to Moscow. They are subsequently frozen by cold weather and attacks by the Soviets.

December

- December 1 - World War II: Former mayor of New York City, Fiorello LaGuardia, and the director of the Office of Civilian Defense, sign an order creating the Civil Air Patrol (CAP) as the civilian auxiliary of the United States Air Force (in April 1943 the CAP was placed under the jurisdiction of the United States Army Air Force).
- December 4 - State of Jefferson declared in Yreka, California, with judge John Childs as a governor
- December 7, December 6 (in Japan standard time) - Japanese Navy launches a surprise attack consisting of two full regiments on the United States fleet at Pearl Harbor, thus drawing the United States into World War II.
- December 8 - World War II: The United States officially declares war on Japan.
- December 11 - World War II: Germany declares war on the United States.
- December 12 - Hungary and Romania declare war on the United States. India declares war on Japan. United States seizes French ship Normandie.
- December 13 - Sweden's low temperature record with -53° C was set in a village within Vilhelmina Municipality.
- December 25 - World War II: British and Canadians are defeated by the Japanese at Hong Kong.
- December 27 - World War II: British Commandos raid the Norwegian port of Vaagso, causing Hitler to reinforce the garrison and defenses
- December 28 - World War II: starts the Operation Anthropoid (the assassination of Heydrich in Prague).

Unknown dates

- John Vincent Atanasoff and Clifford E. Berry developed the Atanasoff Berry Computer.
- Ives and Stilwell prove that ions radiate at frequencies affected by their motion.
- In Sweden, Victor Hasselblad forms the Hasselblad camera company.
- The Pinnacle Commune, a Rastafarian community, is destroyed by Jamaican authorities
- Indochina Communist party, led by Ho Chi Minh, combines with Nationalist party to form the Vietminh.
- Meet John Doe is brought out

Ongoing events

- Sino-Japanese War (1937-1945) (which may or may not be a part of World War II, depending on who's telling the tale)
- World War II (1939-1945)

Births

January

- January 3 - Van Dyke Parks, American composer, producer, and musician
- January 5 - Miyazaki Hayao, Japanese film maker
- January 7 - Iona Brown, British violinist and conductor (d. 2004)
- January 7 - John E. Walker, English chemist, Nobel Prize laureate
- January 8 - Graham Chapman, British comedian (d. 1989)
- January 9 - Joan Baez, American singer and activitist
- January 14 - Faye Dunaway, American actress
- January 14 - Milan Kučan, Slovenian politician and statesman
- January 15 - Captain Beefheart, American singer
- January 18 - David Ruffin, American singer (d. 1991)
- January 21 - Plácido Domingo, Spanish-born tenor
- January 21 - Richie Havens, American musician
- January 26 - Scott Glenn, American actor
- January 26 - Henry Jaglom, English director
- January 30 - Dick Cheney, Vice President of the United States
- January 31 - Dick Gephardt, American politician

February

- February 5 - Kaspar Villiger, Swiss Federal Councilor
- February 6 - Howard Phillips, founding member of the United States Constitution Party
- February 8 - Nick Nolte, American actor
- February 10 - Michael Apted, English director
- February 13 - Sigmar Polke, German painter
- February 16 - Kim Jong-il, leader of North

Radar

Principles

Overview

Reflection

Polarization

Interference

Noise

Clutter

Jamming

Distance measurement

Transit time

Frequency modulation

Speed measurement

Position measurement

Early systems

Phased array

Radar equation

Frequency bands

Specific radar systems

Radar modulators

Radar Signal Processing

See also

Further reading

External links

References

Horizon

See also

External links

Acknowledgements

Ships

Measuring ships

Propulsion

General terminology

Shipboard terminology

Some types of ships and boats

Some historical types of ships and boats

See also

Quotations

External links

Rain

Rain in nature

Culture

See also

Allen B. DuMont

1941

Events

January-February

March

April

May

June

July-August

September-October

November

December

Unknown dates

Ongoing events

Births

January

February