Misplaced Pages

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176-401: ILS uses two directional radio signals , the localizer (108 to 112 MHz frequency), which provides horizontal guidance, and the glideslope (329.15 to 335 MHz frequency) for vertical guidance. The relationship between the aircraft's position and these signals is displayed on an aircraft instrument , often additional pointers in the attitude indicator . The pilot attempts to manoeuvre

352-659: A Neutrality Pact in April 1941, and Japan adopted the doctrine of Nanshin-ron , promoted by the Navy, which took its focus southward and eventually led to war with the United States and the Western Allies. In Europe, Germany and Italy were becoming more aggressive. In March 1938, Germany annexed Austria , again provoking little response from other European powers. Encouraged, Hitler began pressing German claims on

528-586: A Pennsylvania Central Airlines Boeing 247 D flew from Washington, D.C., to Pittsburgh, Pennsylvania, and landed in a snowstorm using only the Instrument Landing System. The first fully automatic landing using ILS occurred in March 1964 at Bedford Airport in the UK. The instrument landing systems market revenue was US$ 1,215 million in 2019, and is expected to reach US$ 1,667 million in 2025, with

704-426: A carrier frequency of 75 MHz are provided. When the transmission from a marker beacon is received it activates an indicator on the pilot's instrument panel and the tone of the beacon is audible to the pilot. The distance from the runway at which this indication should be received is published in the documentation for that approach, together with the height at which the aircraft should be if correctly established on

880-578: A cautious French probe into the Saarland . The Western Allies also began a naval blockade of Germany , which aimed to damage the country's economy and war effort. Germany responded by ordering U-boat warfare against Allied merchant and warships, which would later escalate into the Battle of the Atlantic . On 8 September, German troops reached the suburbs of Warsaw . The Polish counter-offensive to

1056-434: A head-up display (HUD) guidance that provides the pilot with an image viewed through the windshield with eyes focused at infinity, of necessary electronic guidance to land the airplane with no true outside visual references. In the United States, airports with CAT III approaches have listings for CAT IIIa and IIIb or just CAT III on the instrument approach plate (U.S. Terminal Procedures). CAT IIIb RVR minimums are limited by

1232-412: A microphone , a video signal representing moving images from a video camera , or a digital signal representing data from a computer . In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency , called the carrier wave because it creates the radio waves that "carry" the information through the air. The information signal is used to modulate

1408-418: A resonator , similarly to a tuning fork . The tuned circuit has a natural resonant frequency at which it oscillates. The resonant frequency is set equal to the frequency of the desired radio station. The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by

1584-406: A runway , using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC) , such as low ceilings or reduced visibility due to fog, rain, or blowing snow. Previous blind landing radio aids typically took the form of beam systems of various types. These normally consisted of a radio transmitter that

1760-552: A space heater or wood fire. The oscillating electric field of the wave causes polar molecules to vibrate back and forth, increasing the temperature; this is how a microwave oven cooks food. Radio waves have been applied to the body for 100 years in the medical therapy of diathermy for deep heating of body tissue, to promote increased blood flow and healing. More recently they have been used to create higher temperatures in hyperthermia therapy and to kill cancer cells. However, unlike infrared waves, which are mainly absorbed at

1936-509: A transmitter , which is connected to an antenna , which radiates the waves. They are received by another antenna connected to a radio receiver , which processes the received signal. Radio waves are very widely used in modern technology for fixed and mobile radio communication , broadcasting , radar and radio navigation systems, communications satellites , wireless computer networks and many other applications. Different frequencies of radio waves have different propagation characteristics in

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2112-808: A CAGR of 5.41% during 2020–2025 even with the negative effects of the COVID-19 pandemic . The top 10 manufacturers in the instrument landing systems market are: Other manufacturers include: The advent of the Global Positioning System (GPS) provides an alternative source of approach guidance for aircraft. In the US, the Wide Area Augmentation System (WAAS) has been available in many regions to provide precision guidance to Category I standards since 2007. The equivalent European Geostationary Navigation Overlay Service (EGNOS)

2288-473: A CAT I ILS approach supported by a 1,400-to-3,000-foot-long (430 to 910 m) ALS, and 3 ⁄ 8 mile (600 m) visibility 1,800-foot (550 m) visual range is possible if the runway has high-intensity edge lights, touchdown zone and centerline lights, and an ALS that is at least 2,400 feet (730 m) long (see Table 3-3-1 "Minimum visibility values" in FAA Order 8260.3C). In effect, ALS extends

2464-418: A CAT I localizer must shut down within 10 seconds of detecting a fault, but a CAT III localizer must shut down in less than 2 seconds. In contrast to other operations, CAT III weather minima do not provide sufficient visual references to allow a manual landing to be made. CAT IIIb minima depend on roll-out control and redundancy of the autopilot, because they give only enough time for the pilot to decide whether

2640-468: A fail-operational system, along with a crew who are qualified and current, while CAT I does not. A HUD that allows the pilot to perform aircraft maneuvers rather than an automatic system is considered as fail-operational. A HUD allows the flight crew to fly the aircraft using the guidance cues from the ILS sensors such that if a safe landing is in doubt, the crew can respond in an appropriate and timely manner. HUD

2816-461: A human user. The radio waves from many transmitters pass through the air simultaneously without interfering with each other. They can be separated in the receiver because each transmitter's radio waves oscillate at a different rate, in other words each transmitter has a different frequency , measured in kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The bandpass filter in the receiver consists of one or more tuned circuits which act like

2992-403: A left-hand sense. Right circularly polarized radio waves consist of photons spinning in a right hand sense. Left circularly polarized radio waves consist of photons spinning in a left hand sense. Plane polarized radio waves consist of photons in a quantum superposition of right and left hand spin states. The electric field consists of a superposition of right and left rotating fields, resulting in

3168-429: A modulation index of 100%. The determination of angle within the beam is based on the comparison of the audible strength of the two signals. sa In ILS, a more complex system of signals and antennas varies the modulation of two signals across the entire width of the beam pattern. The system relies on the use of sidebands , secondary frequencies that are created when two different signals are mixed. For instance, if one takes

3344-462: A noisy aircraft, often while communicating with the tower. Accuracy of the system was normally on the order of 3 degrees in azimuth. While this was useful for bringing the aircraft onto the direction of the runway, it was not accurate enough to safely bring the aircraft to visual range in bad weather; the radio course beams were used only for lateral guidance, and the system was not enough on its own to perform landings in heavy rain or fog. Nevertheless,

3520-605: A plane oscillation. Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable propagation properties, stemming from their large wavelength . Radio waves have the ability to pass through the atmosphere in any weather, foliage, and through most building materials. By diffraction , longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength. The study of radio propagation , how radio waves move in free space and over

3696-402: A plane perpendicular to the direction of motion. In a horizontally polarized radio wave the electric field oscillates in a horizontal direction. In a vertically polarized wave the electric field oscillates in a vertical direction. In a circularly polarized wave the electric field at any point rotates about the direction of travel, once per cycle. A right circularly polarized wave rotates in

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3872-524: A pretext to initiate the invasion. The first German attack of the war came against the Polish defences at Westerplatte . The United Kingdom responded with an ultimatum for Germany to cease military operations, and on 3 September, after the ultimatum was ignored, Britain and France declared war on Germany. During the Phoney War period, the alliance provided no direct military support to Poland, outside of

4048-781: A pretext to worsen relations. On 29 August, Hitler demanded that a Polish plenipotentiary immediately travel to Berlin to negotiate the handover of Danzig , and to allow a plebiscite in the Polish Corridor in which the German minority would vote on secession. The Poles refused to comply with the German demands, and on the night of 30–31 August in a confrontational meeting with the British ambassador Nevile Henderson , Ribbentrop declared that Germany considered its claims rejected. On 1 September 1939, Germany invaded Poland after having staged several false flag border incidents as

4224-456: A radio frequency signal at 10 MHz and mixes that with an audible tone at 2500 Hz, four signals will be produced, at the original signals' frequencies of 2500 and 10000000 hertz, and sidebands 9997500 and 10002500 hertz. The original 2500 Hz signal's frequency is too low to travel far from an antenna, but the other three signals are all radio frequency and can be effectively transmitted. ILS starts by mixing two modulating signals to

4400-404: A right-hand sense about the direction of travel, while a left circularly polarized wave rotates in the opposite sense. The wave's magnetic field is perpendicular to the electric field, and the electric and magnetic field are oriented in a right-hand sense with respect to the direction of radiation. An antenna emits polarized radio waves, with the polarization determined by the direction of

4576-475: A secret protocol that defined German and Soviet "spheres of influence" (western Poland and Lithuania for Germany; eastern Poland , Finland, Estonia , Latvia and Bessarabia for the Soviet Union), and raised the question of continuing Polish independence. The pact neutralised the possibility of Soviet opposition to a campaign against Poland and assured that Germany would not have to face the prospect of

4752-722: A similar pact and rejected ceding part of its territory to the Soviet Union. The Soviet Union invaded Finland in November 1939, and was subsequently expelled from the League of Nations for this crime of aggression. Despite overwhelming numerical superiority, Soviet military success during the Winter War was modest, and the Finno-Soviet war ended in March 1940 with some Finnish concessions of territory . In June 1940,

4928-414: A single signal entirely in electronics, it provides angular resolution of less than a degree, and allows the construction of a precision approach . Although the encoding scheme is complex, and requires a considerable amount of ground equipment, the resulting signal is both far more accurate than the older beam-based systems and is far more resistant to common forms of interference. For instance, static in

5104-531: A two-front war, as it had in World War   I. Immediately afterwards, Hitler ordered the attack to proceed on 26 August, but upon hearing that the United Kingdom had concluded a formal mutual assistance pact with Poland and that Italy would maintain neutrality, he decided to delay it. In response to British requests for direct negotiations to avoid war, Germany made demands on Poland, which served as

5280-422: A view outside the cockpit. A basic system, fully operative, was introduced in 1932 at Berlin- Tempelhof Central Airport (Germany) named LFF or " Lorenz beam " after its inventor, the C. Lorenz AG company. The Civil Aeronautics Board (CAB) of the United States authorized installation of the system in 1941 at six locations. The first landing of a scheduled U.S. passenger airliner using ILS was on January 26, 1938, when

5456-414: A widespread standard to this day. The introduction of precision approaches using global navigation satellite systems (GNSSs) instead of requiring expensive airport infrastructure is leading to the replacement of ILS. Providing the required accuracy with GNSS normally requires only a low-power omnidirectional augmentation signal to be broadcast from the airport, which is dramatically less expensive than

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5632-404: Is absorbed within a few meters, so the atmosphere is effectively opaque. In radio communication systems, information is transported across space using radio waves. At the sending end, the information to be sent, in the form of a time-varying electrical signal, is applied to a radio transmitter . The information, called the modulation signal , can be an audio signal representing sound from

5808-463: Is also sent to the antenna array. For lateral guidance, known as the localizer , the antenna is normally placed centrally at the far end of the runway and consists of multiple antennas in an array normally about the width of the runway. Each individual antenna has a particular phase shift and power level applied only to the SBO signal such that the resulting signal is retarded 90 degrees on the left side of

5984-499: Is becoming increasingly popular with "feeder" airlines and most manufacturers of regional jets are now offering HUDs as either standard or optional equipment. A HUD can provide capability to take off in low visibility. Some commercial aircraft are equipped with automatic landing systems that allow the aircraft to land without transitioning from instruments to visual conditions for a normal landing. Such autoland operations require specialized equipment, procedures and training, and involve

6160-403: Is done in simulators with various degrees of fidelity. At a controlled airport, air traffic control will direct aircraft to the localizer course via assigned headings, making sure aircraft do not get too close to each other (maintain separation), but also avoiding delay as much as possible. Several aircraft can be on the ILS at the same time, several miles apart. An aircraft that has turned onto

6336-442: Is extremely small, from 10 to 10   joules . So the antenna of even a very low power transmitter emits an enormous number of photons every second. Therefore, except for certain molecular electron transition processes such as atoms in a maser emitting microwave photons, radio wave emission and absorption is usually regarded as a continuous classical process, governed by Maxwell's equations . Radio waves in vacuum travel at

6512-411: Is indicated to the flight crew by means of a display dial (a carryover from when an analog meter movement indicated deviation from the course line via voltages sent from the ILS receiver). The output from the ILS receiver goes to the display system (head-down display and head-up display if installed) and may go to a Flight Control Computer . An aircraft landing procedure can be either coupled where

6688-406: Is noted on the instrument approach procedure and the aircraft must have at least one operating DME unit, or an IFR-approved system using a GNSS (an RNAV system meeting TSO-C129/ -C145/-C146), to begin the approach. Some installations include medium- or high-intensity approach light systems (abbreviated ALS ). Most often, these are at larger airports but many small general aviation airports in

6864-436: Is published for each ILS approach to provide the information needed to fly an ILS approach during instrument flight rules (IFR) operations. A chart includes the radio frequencies used by the ILS components or navaids and the prescribed minimum visibility requirements. An aircraft approaching a runway is guided by the ILS receivers in the aircraft by performing modulation depth comparisons. Many aircraft can route signals into

7040-426: Is the wavelength of a 1  hertz radio signal. A 1  megahertz radio wave (mid- AM band ) has a wavelength of 299.79 meters (983.6 ft). Like other electromagnetic waves, a radio wave has a property called polarization , which is defined as the direction of the wave's oscillating electric field perpendicular to the direction of motion. A plane-polarized radio wave has an electric field that oscillates in

7216-400: Is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones. Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called a Faraday cage . A metal screen shields against radio waves as well as a solid sheet as long as the holes in the screen are smaller than about 1 ⁄ 20 of wavelength of

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7392-604: The Anglo-German Naval Agreement and the German–Polish declaration of non-aggression . The situation became a crisis in late August as German troops continued to mobilise against the Polish border. On 23 August the Soviet Union signed a non-aggression pact with Germany, after tripartite negotiations for a military alliance between France, the United Kingdom, and Soviet Union had stalled. This pact had

7568-689: The British Empire , with fighting in the Balkans , Mediterranean, and Middle East , the aerial Battle of Britain and the Blitz , and naval Battle of the Atlantic . Through a series of campaigns and treaties, Germany took control of much of continental Europe and formed the Axis alliance with Italy , Japan , and other countries. In June 1941, Germany led the European Axis in an invasion of

7744-581: The German Revolution of 1918–1919 , and a democratic government, later known as the Weimar Republic , was created. The interwar period saw strife between supporters of the new republic and hardline opponents on both the political right and left. Italy, as an Entente ally, had made some post-war territorial gains; however, Italian nationalists were angered that the promises made by the United Kingdom and France to secure Italian entrance into

7920-853: The German invasion of Poland and the United Kingdom and France 's declaration of war on Germany two days later on 3 September 1939. Dates for the beginning of the Pacific War include the start of the Second Sino-Japanese War on 7 July 1937, or the earlier Japanese invasion of Manchuria , on 19 September 1931. Others follow the British historian A. J. P. Taylor , who stated that the Sino-Japanese War and war in Europe and its colonies occurred simultaneously, and

8096-576: The Italian mainland , and Allied offensives in the Pacific—cost the Axis powers their initiative and forced them into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France at Normandy , while the Soviet Union regained its territorial losses and pushed Germany and its allies westward. At the same time, Japan suffered reversals in mainland Asia, while the Allies crippled

8272-553: The Japanese Navy and captured key islands . The war in Europe concluded with the liberation of German-occupied territories ; the invasion of Germany by the Western Allies and the Soviet Union, culminating in the fall of Berlin to Soviet troops; Hitler's suicide ; and the German unconditional surrender on 8 May 1945 . Following the refusal of Japan to surrender on the terms of the Potsdam Declaration ,

8448-695: The League of Nations as a force to preserve peace. Both Italy and Ethiopia were member nations, but the League did little when the former clearly violated Article X of the League's Covenant . The United Kingdom and France supported imposing sanctions on Italy for the invasion, but the sanctions were not fully enforced and failed to end the Italian invasion. Italy subsequently dropped its objections to Germany's goal of absorbing Austria . When civil war broke out in Spain, Hitler and Mussolini lent military support to

8624-731: The Nationalist rebels , led by General Francisco Franco . Italy supported the Nationalists to a greater extent than the Nazis: Mussolini sent more than 70,000 ground troops, 6,000 aviation personnel, and 720 aircraft to Spain. The Soviet Union supported the existing government of the Spanish Republic . More than 30,000 foreign volunteers, known as the International Brigades , also fought against

8800-690: The Rome–Berlin Axis . A month later, Germany and Japan signed the Anti-Comintern Pact , which Italy joined the following year. The Kuomintang (KMT) party in China launched a unification campaign against regional warlords and nominally unified China in the mid-1920s, but was soon embroiled in a civil war against its former Chinese Communist Party (CCP) allies and new regional warlords . In 1931, an increasingly militaristic Empire of Japan , which had long sought influence in China as

8976-619: The Second World War (1 September 1939 – 2 September 1945) was a global conflict between two coalitions: the Allies and the Axis powers . Nearly all the world's countries —including all the great powers —participated, with many investing all available economic, industrial, and scientific capabilities in pursuit of total war , blurring the distinction between military and civilian resources. Tanks and aircraft played major roles , with

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9152-590: The Soviet–Japanese Neutrality Pact in April 1941. By contrast, the Germans were steadily making preparations for an attack on the Soviet Union, massing forces on the Soviet border. Hitler believed that the United Kingdom's refusal to end the war was based on the hope that the United States and the Soviet Union would enter the war against Germany sooner or later. On 31 July 1940, Hitler decided that

9328-598: The Stresa Front in April 1935 in order to contain Germany, a key step towards military globalisation ; however, that June, the United Kingdom made an independent naval agreement with Germany, easing prior restrictions. The Soviet Union, concerned by Germany's goals of capturing vast areas of Eastern Europe , drafted a treaty of mutual assistance with France. Before taking effect, though, the Franco-Soviet pact

9504-709: The Sudetenland , an area of Czechoslovakia with a predominantly ethnic German population. Soon the United Kingdom and France followed the appeasement policy of British Prime Minister Neville Chamberlain and conceded this territory to Germany in the Munich Agreement , which was made against the wishes of the Czechoslovak government, in exchange for a promise of no further territorial demands. Soon afterwards, Germany and Italy forced Czechoslovakia to cede additional territory to Hungary, and Poland annexed

9680-707: The Tanggu Truce was signed in 1933. Thereafter, Chinese volunteer forces continued the resistance to Japanese aggression in Manchuria , and Chahar and Suiyuan . After the 1936 Xi'an Incident , the Kuomintang and CCP forces agreed on a ceasefire to present a united front to oppose Japan. The Second Italo-Ethiopian War was a brief colonial war that began in October 1935 and ended in May 1936. The war began with

9856-579: The Trans-Olza region of Czechoslovakia. Although all of Germany's stated demands had been satisfied by the agreement, privately Hitler was furious that British interference had prevented him from seizing all of Czechoslovakia in one operation. In subsequent speeches Hitler attacked British and Jewish "war-mongers" and in January 1939 secretly ordered a major build-up of the German navy to challenge British naval supremacy. In March 1939, Germany invaded

10032-558: The Treaty of Craiova . The loss of one-third of Romania's 1939 territory caused a coup against King Carol II, turning Romania into a fascist dictatorship under Marshal Ion Antonescu , with a course set towards the Axis in the hopes of a German guarantee. Meanwhile, German-Soviet political relations and economic co-operation gradually stalled, and both states began preparations for war. In April 1940, Germany invaded Denmark and Norway to protect shipments of iron ore from Sweden , which

10208-749: The United Kingdom and France to declare war on Germany. Poland was divided between Germany and the Soviet Union under the Molotov–Ribbentrop Pact , in which they had agreed on " spheres of influence " in Eastern Europe. In 1940, the Soviets annexed the Baltic states and parts of Finland and Romania . After the fall of France in June 1940, the war continued mainly between Germany and

10384-519: The United Kingdom during World War II , which led to it being selected as the international standard after the formation of the International Civil Aviation Organization (ICAO) in 1947. Several competing landing systems have been developed, including the radar -based ground-controlled approach (GCA) and the more recent microwave landing system (MLS), but few of these systems have been deployed. ILS remains

10560-709: The Wehrmacht rapidly advanced to the Channel and cut off the Allied forces in Belgium, trapping the bulk of the Allied armies in a cauldron on the Franco-Belgian border near Lille. The United Kingdom was able to evacuate a significant number of Allied troops from the continent by early June, although they had to abandon almost all their equipment. On 10 June, Italy invaded France , declaring war on both France and

10736-630: The aftermath of World War I and the rise of fascism in Europe and militarism in Japan . Key events leading up to the war included Japan's invasion of Manchuria , the Spanish Civil War , the outbreak of the Second Sino-Japanese War , and Germany's annexations of Austria and the Sudetenland . World War II is generally considered to have begun on 1 September 1939, when Nazi Germany , under Adolf Hitler , invaded Poland , prompting

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10912-410: The autopilot to fly the approach automatically. An ILS consists of two independent sub-systems. The localizer provides lateral guidance; the glide slope provides vertical guidance. A localizer (LOC, or LLZ until ICAO standardisation) is an antenna array normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. The localizer will allow

11088-414: The electromagnetic spectrum , typically with frequencies below 300 gigahertz (GHz) and wavelengths greater than 1 millimeter ( 3 ⁄ 64 inch), about the diameter of a grain of rice. Radio waves with frequencies above about 1 GHz and wavelengths shorter than 30 centimeters are called microwaves . Like all electromagnetic waves, radio waves in vacuum travel at the speed of light , and in

11264-479: The reunification of East and West Germany to take place and resolved most post–World War   II issues. No formal peace treaty between Japan and the Soviet Union was ever signed, although the state of war between the two countries was terminated by the Soviet–Japanese Joint Declaration of 1956 , which also restored full diplomatic relations between them. World War I had radically altered

11440-467: The speed of light c {\displaystyle c} . When passing through a material medium, they are slowed depending on the medium's permeability and permittivity . Air is tenuous enough that in the Earth's atmosphere radio waves travel at very nearly the speed of light. The wavelength λ {\displaystyle \lambda } is the distance from one peak (crest) of

11616-482: The "near field" zone, the physical relationships between the electric and magnetic components of the field can be complex, and it is best to use the field strength units discussed above. Power density is measured in terms of power per unit area, for example, with the unit milliwatt per square centimeter (mW/cm ). When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in

11792-467: The 1909 Nobel Prize in physics for his radio work. Radio communication began to be used commercially around 1900. The modern term " radio wave " replaced the original name " Hertzian wave " around 1912. Radio waves are radiated by charged particles when they are accelerated . Natural sources of radio waves include radio noise produced by lightning and other natural processes in the Earth's atmosphere, and astronomical radio sources in space such as

11968-474: The ALS counts as runway end environment. In the U.S., an ILS without approach lights may have CAT I ILS visibility minimums as low as 3 ⁄ 4 mile (1.2 km) (runway visual range of 4,000 feet (1,200 m)) if the required obstacle clearance surfaces are clear of obstructions. Visibility minimums of 1 ⁄ 2 mile (0.80 km) (runway visual range of 2,400 feet (730 m)) are possible with

12144-409: The Allies were attempting to cut off . Denmark capitulated after six hours , and despite Allied support , Norway was conquered within two months. British discontent over the Norwegian campaign led to the resignation of Prime Minister Neville Chamberlain , who was replaced by Winston Churchill on 10   May 1940. On the same day, Germany launched an offensive against France . To circumvent

12320-400: The Axis war against the Soviet Union, in Romania's case partially to recapture territory ceded to the Soviet Union . In early June 1940, the Italian Regia Aeronautica attacked and besieged Malta , a British possession. From late summer to early autumn, Italy conquered British Somaliland and made an incursion into British-held Egypt . In October, Italy attacked Greece , but the attack

12496-421: The Blitz , but largely ended in May 1941 after failing to significantly disrupt the British war effort. Using newly captured French ports, the German Navy enjoyed success against an over-extended Royal Navy , using U-boats against British shipping in the Atlantic . The British Home Fleet scored a significant victory on 27   May 1941 by sinking the German battleship Bismarck . In November 1939,

12672-399: The Earth's atmosphere at a slightly lower speed. Radio waves are generated by charged particles undergoing acceleration , such as time-varying electric currents . Naturally occurring radio waves are emitted by lightning and astronomical objects , and are part of the blackbody radiation emitted by all warm objects. Radio waves are generated artificially by an electronic device called

12848-447: The Earth's atmosphere; long waves can diffract around obstacles like mountains and follow the contour of the Earth ( ground waves ), shorter waves can reflect off the ionosphere and return to Earth beyond the horizon ( skywaves ), while much shorter wavelengths bend or diffract very little and travel on a line of sight , so their propagation distances are limited to the visual horizon. To prevent interference between different users,

13024-636: The European Axis declaring war on the US. Japan conquered much of coastal China and Southeast Asia , but its advances in the Pacific were halted in mid-1942 after its defeat in the naval Battle of Midway ; Germany and Italy were defeated in North Africa and at Stalingrad in the Soviet Union. Key setbacks in 1943—including German defeats on the Eastern Front, the Allied invasions of Sicily and

13200-702: The French possessions of Syria and Lebanon , assisted by the Free French . With the situation in Europe and Asia relatively stable, Germany, Japan, and the Soviet Union made preparations for war. With the Soviets wary of mounting tensions with Germany, and the Japanese planning to take advantage of the European War by seizing resource-rich European possessions in Southeast Asia , the two powers signed

13376-486: The Germans, and the last large operational unit of the Polish Army surrendered on 6   October . Despite the military defeat, Poland never surrendered; instead, it formed the Polish government-in-exile and a clandestine state apparatus remained in occupied Poland. A significant part of Polish military personnel evacuated to Romania and Latvia; many of them later fought against the Axis in other theatres of

13552-452: The ILS glide slope to the pilot, and does not require an installation outside the airport boundary. When used in conjunction with an ILS, the DME is often sited midway between the reciprocal runway thresholds with the internal delay modified so that one unit can provide distance information to either runway threshold. For approaches where a DME is specified in lieu of marker beacons, DME required

13728-451: The ILS is automatically switched off or the navigation and identification components are removed from the carrier. Either of these actions will activate an indication ('failure flag') on the instruments of an aircraft using the ILS. Modern localizer antennas are highly directional . However, usage of older, less directional antennas allows a runway to have a non-precision approach called a localizer back course . This lets aircraft land using

13904-603: The ILS signals are pointed in one direction by the positioning of the arrays, glide slope supports only straight-line approaches with a constant angle of descent. Installation of an ILS can be costly because of siting criteria and the complexity of the antenna system. ILS critical areas and ILS sensitive areas are established to avoid hazardous reflections that would affect the radiated signal. The location of these critical areas can prevent aircraft from using certain taxiways leading to delays in takeoffs, increased hold times, and increased separation between aircraft . In addition to

14080-514: The ILS. This provides a check on the correct function of the glide slope. In modern ILS installations, a DME is installed, co-located with the ILS, to augment or replace marker beacons. A DME continuously displays the aircraft's distance to the runway. Distance measuring equipment (DME) provides pilots with a slant range measurement of distance to the runway. DMEs are augmenting or replacing markers in many installations. The DME provides more accurate and continuous monitoring of correct progress on

14256-512: The Nationalists. Both Germany and the Soviet Union used this proxy war as an opportunity to test in combat their most advanced weapons and tactics. The Nationalists won the civil war in April 1939; Franco, now dictator, remained officially neutral during World War   II but generally favoured the Axis . His greatest collaboration with Germany was the sending of volunteers to fight on the Eastern Front . In July 1937, Japan captured

14432-511: The Royal Navy putting three Italian battleships out of commission after a carrier attack at Taranto , and neutralising several more warships at the Battle of Cape Matapan . Italian defeats prompted Germany to deploy an expeditionary force to North Africa; at the end of March 1941, Rommel 's Afrika Korps launched an offensive which drove back Commonwealth forces. In less than a month, Axis forces advanced to western Egypt and besieged

14608-602: The Soviet Union occupied the entire territories of Estonia, Latvia and Lithuania, as well as the Romanian regions of Bessarabia, Northern Bukovina, and the Hertsa region . In August 1940, Hitler imposed the Second Vienna Award on Romania which led to the transfer of Northern Transylvania to Hungary. In September 1940, Bulgaria demanded Southern Dobruja from Romania with German and Italian support, leading to

14784-719: The Soviet Union , opening the Eastern Front and initially making large territorial gains. Japan aimed to dominate East Asia and the Asia-Pacific , and by 1937 was at war with the Republic of China . In December 1941, Japan attacked American and British territories in Southeast Asia and the Central Pacific , including Pearl Harbor in Hawaii , which resulted in the US and the UK declaring war against Japan, and

14960-530: The Soviet Union and Mongolia . The Japanese doctrine of Hokushin-ron , which emphasised Japan's expansion northward, was favoured by the Imperial Army during this time. This policy would prove difficult to maintain in light of the Japanese defeat at Khalkin Gol in 1939, the ongoing Second Sino-Japanese War and ally Nazi Germany pursuing neutrality with the Soviets. Japan and the Soviet Union eventually signed

15136-756: The Soviet Union should be eliminated and aimed for the conquest of Ukraine , the Baltic states and Byelorussia . However, other senior German officials like Ribbentrop saw an opportunity to create a Euro-Asian bloc against the British Empire by inviting the Soviet Union into the Tripartite Pact. In November 1940, negotiations took place to determine if the Soviet Union would join the pact. The Soviets showed some interest but asked for concessions from Finland, Bulgaria, Turkey, and Japan that Germany considered unacceptable. On 18 December 1940, Hitler issued

15312-463: The Sun, galaxies and nebulas. All warm objects radiate high frequency radio waves ( microwaves ) as part of their black body radiation . Radio waves are produced artificially by time-varying electric currents , consisting of electrons flowing back and forth in a specially shaped metal conductor called an antenna . An electronic device called a radio transmitter applies oscillating electric current to

15488-501: The U.S. have approach lights to support their ILS installations and obtain low-visibility minimums. The ALS assists the pilot in transitioning from instrument to visual flight, and to align the aircraft visually with the runway centerline. Pilot observation of the approach lighting system at the Decision Altitude allows the pilot to continue descending towards the runway, even if the runway or runway lights cannot be seen, since

15664-402: The US dropped the first atomic bombs on Hiroshima and Nagasaki on 6 and 9 August. Faced with an imminent invasion of the Japanese archipelago , the possibility of further atomic bombings, and the Soviet declaration of war against Japan and its invasion of Manchuria , Japan announced its unconditional surrender on 15 August and signed a surrender document on 2 September 1945 , marking

15840-581: The United Kingdom and France guaranteed their support for Polish independence ; when Italy conquered Albania in April 1939, the same guarantee was extended to the Kingdoms of Romania and Greece . Shortly after the Franco - British pledge to Poland, Germany and Italy formalised their own alliance with the Pact of Steel . Hitler accused the United Kingdom and Poland of trying to "encircle" Germany and renounced

16016-542: The United Kingdom attacked on 3   July in an attempt to prevent its seizure by Germany. The air Battle of Britain began in early July with Luftwaffe attacks on shipping and harbours . The German campaign for air superiority started in August but its failure to defeat RAF Fighter Command forced the indefinite postponement of the proposed German invasion of Britain . The German strategic bombing offensive intensified with night attacks on London and other cities in

16192-461: The United Kingdom. The Germans turned south against the weakened French army, and Paris fell to them on 14   June. Eight days later France signed an armistice with Germany ; it was divided into German and Italian occupation zones , and an unoccupied rump state under the Vichy Regime , which, though officially neutral, was generally aligned with Germany. France kept its fleet, which

16368-660: The United States was assisting China and the Western Allies, and had amended the Neutrality Act to allow "cash and carry" purchases by the Allies. In 1940, following the German capture of Paris, the size of the United States Navy was significantly increased . In September the United States further agreed to a trade of American destroyers for British bases . Still, a large majority of the American public continued to oppose any direct military intervention in

16544-606: The Yellow River ; this manoeuvre bought time for the Chinese to prepare their defences at Wuhan , but the city was taken by October. Japanese military victories did not bring about the collapse of Chinese resistance that Japan had hoped to achieve; instead, the Chinese government relocated inland to Chongqing and continued the war. In the mid-to-late 1930s, Japanese forces in Manchukuo had sporadic border clashes with

16720-426: The aircraft to keep the indicators centered while they approach the runway to the decision height . Optional marker beacon(s) provide distance information as the approach proceeds, including the middle marker (MM), placed close to the position of the (CAT 1) decision height. Markers are largely being phased out and replaced by distance measuring equipment (DME). The ILS usually includes high-intensity lighting at

16896-450: The aircraft to turn and match the aircraft with the runway. After that, the pilots will activate approach phase (APP). The pilot controls the aircraft so that the glide slope indicator remains centered on the display to ensure the aircraft is following the glide path of approximately 3° above horizontal (ground level) to remain above obstructions and reach the runway at the proper touchdown point (i.e. it provides vertical guidance). Due to

17072-606: The aircraft will land in the touchdown zone (basically CAT IIIa) and to ensure safety during rollout (basically CAT IIIb). Therefore, an automatic landing system is mandatory to perform Category III operations. Its reliability must be sufficient to control the aircraft to touchdown in CAT IIIa operations and through rollout to a safe taxi speed in CAT IIIb (and CAT IIIc when authorized). However, special approval has been granted to some operators for hand-flown CAT III approaches using

17248-553: The aircraft, airport, and the crew. Autoland is the only way some major airports such as Charles de Gaulle Airport remain operational every day of the year. Some modern aircraft are equipped with enhanced flight vision systems based on infrared sensors, that provide a day-like visual environment and allow operations in conditions and at airports that would otherwise not be suitable for a landing. Commercial aircraft also frequently use such equipment for takeoffs when takeoff minima are not met. For both automatic and HUD landing systems,

17424-404: The airport they would tune in the signal and listen to it in their headphones. They would hear dots and dashes (Morse code "A" or "N"), if they were to the side of the runway, or if they were properly aligned, the two mixed together to produce a steady tone, the equisignal . The accuracy of this measurement was highly dependent on the skill of the operator, who listened to the signal on earphones in

17600-540: The antenna, and the antenna radiates the power as radio waves. Radio waves are received by another antenna attached to a radio receiver . When radio waves strike the receiving antenna they push the electrons in the metal back and forth, creating tiny oscillating currents which are detected by the receiver. From quantum mechanics , like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged elementary particles called photons . In an antenna transmitting radio waves,

17776-401: The array will receive both of these signals mixed together. Using simple electronic filters, the original carrier and two sidebands can be separated and demodulated to extract the original amplitude-modulated 90 and 150 Hz signals. These are then averaged to produce two direct current (DC) signals. Each of these signals represents not the strength of the original signal, but the strength of

17952-657: The artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunication Union (ITU), which defines radio waves as " electromagnetic waves of frequencies arbitrarily lower than 3000  GHz , propagated in space without artificial guide". The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses. Higher-frequency, shorter-wavelength radio waves are called microwaves . Radio waves were first predicted by

18128-410: The autopilot or Flight Control Computer directly flies the aircraft and the flight crew monitor the operation, or uncoupled where the flight crew flies the aircraft manually to keep the localizer and glideslope indicators centered. Tests of the ILS began in 1929 in the United States, with Jimmy Doolittle becoming the first pilot to take off, fly and land an airplane using instruments alone, without

18304-461: The capital Nanking in December 1937. After the fall of Nanking, tens or hundreds of thousands of Chinese civilians and disarmed combatants were murdered by the Japanese . In March 1938, Nationalist Chinese forces won their first major victory at Taierzhuang , but then the city of Xuzhou was taken by the Japanese in May. In June 1938, Chinese forces stalled the Japanese advance by flooding

18480-410: The carrier, altering some aspect of it, encoding the information on the carrier. The modulated carrier is amplified and applied to an antenna . The oscillating current pushes the electrons in the antenna back and forth, creating oscillating electric and magnetic fields , which radiate the energy away from the antenna as radio waves. The radio waves carry the information to the receiver location. At

18656-463: The carrier, one at 90 Hz and another at 150. This creates a signal with five radio frequencies in total, the carrier and four sidebands. This combined signal, known as the CSB for "carrier and sidebands", is sent out evenly from an antenna array. The CSB is also sent into a circuit that suppresses the original carrier, leaving only the four sideband signals. This signal, known as SBO for "sidebands only",

18832-422: The complexity of ILS localizer and glide slope systems, there are some limitations. Localizer systems are sensitive to obstructions in the signal broadcast area, such as large buildings or hangars. Glide slope systems are also limited by the terrain in front of the glide slope antennas. If terrain is sloping or uneven, reflections can create an uneven glidepath, causing unwanted needle deflections. Additionally, since

19008-403: The conflict well into 1941. In December 1940, Roosevelt accused Hitler of planning world conquest and ruled out any negotiations as useless, calling for the United States to become an " arsenal of democracy " and promoting Lend-Lease programmes of military and humanitarian aid to support the British war effort; Lend-Lease was later extended to the other Allies, including the Soviet Union after it

19184-409: The correct ILS. The glide slope station transmits no identification signal, so ILS equipment relies on the localizer for identification. It is essential that any failure of the ILS to provide safe guidance be detected immediately by the pilot. To achieve this, monitors continually assess the vital characteristics of the transmissions. If any significant deviation beyond strict limits is detected, either

19360-526: The creation of a " New Roman Empire ". Adolf Hitler , after an unsuccessful attempt to overthrow the German government in 1923, eventually became the Chancellor of Germany in 1933 when Paul von Hindenburg and the Reichstag appointed him. Following Hindenburg's death in 1934, Hitler proclaimed himself Führer of Germany and abolished democracy, espousing a radical, racially motivated revision of

19536-472: The descent to a landing; otherwise, the pilot must execute a missed approach procedure, then try the same approach again, try a different approach, or divert to another airport. Usually, the decision on whether or not the pilot continues with the approach relies on whether the runway is visible or not, or if the runway is clear or not. Smaller aircraft generally are equipped to fly only a CAT I ILS. On larger aircraft, these approaches typically are controlled by

19712-757: The dissolution of the Austro-Hungarian, Ottoman, and Russian Empires. To prevent a future world war, the League of Nations was established in 1920 by the Paris Peace Conference . The organisation's primary goals were to prevent armed conflict through collective security, military, and naval disarmament , as well as settling international disputes through peaceful negotiations and arbitration. Despite strong pacifist sentiment after World War   I , irredentist and revanchist nationalism had emerged in several European states. These sentiments were especially marked in Germany because of

19888-421: The electrons in the antenna emit the energy in discrete packets called radio photons, while in a receiving antenna the electrons absorb the energy as radio photons. An antenna is a coherent emitter of photons, like a laser , so the radio photons are all in phase . However, from Planck's relation E = h ν {\displaystyle E=h\nu } , the energy of individual radio photons

20064-662: The end of May completed the German conquest of the Balkans. Partisan warfare subsequently broke out against the Axis occupation of Yugoslavia , which continued until the end of the war. In the Middle East in May, Commonwealth forces quashed an uprising in Iraq which had been supported by German aircraft from bases within Vichy-controlled Syria . Between June and July, British-led forces invaded and occupied

20240-467: The end of the runways to help the pilot locate the runway and transition from the approach to a visual landing. A number of radio-based landing systems were developed between the 1920s and 1940s, notably the Lorenz beam which saw relatively wide use in Europe prior to World War II . The US-developed SCS-51 system was more accurate while also adding vertical guidance. Many sets were installed at airbases in

20416-463: The end of the war. World War II changed the political alignment and social structure of the world, and it set the foundation of international relations for the rest of the 20th century and into the 21st century. The United Nations was established to foster international cooperation and prevent conflicts, with the victorious great powers—China, France, the Soviet Union, the UK, and the US—becoming

20592-438: The energy is deposited. For example, the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm . Looking into a source of radio waves at close range, such as the waveguide of a working radio transmitter, can cause damage to the lens of the eye by heating. A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts . Since

20768-467: The equipment requires special approval for its design and also for each individual installation. The design takes into consideration additional safety requirements for operating an aircraft close to the ground and the ability of the flight crew to react to a system anomaly. The equipment also has additional maintenance requirements to ensure that it is capable of supporting reduced visibility operations. Nearly all of this pilot training and qualification work

20944-758: The far field zone. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm World War II Asia-Pacific Mediterranean and Middle East Other campaigns Coups World War II or

21120-512: The final decision to land was made at only 300 metres (980 ft) from the airport. The ILS, developed just prior to the start of World War II , used a more complex system of signals and an antenna array to achieve higher accuracy. This requires significantly more complexity in the ground station and transmitters, with the advantage that the signals can be accurately decoded in the aircraft using simple electronics and displayed directly on analog instruments. The instruments can be placed in front of

21296-575: The first step of what its government saw as the country's right to rule Asia , staged the Mukden incident as a pretext to invade Manchuria and establish the puppet state of Manchukuo . China appealed to the League of Nations to stop the Japanese invasion of Manchuria. Japan withdrew from the League of Nations after being condemned for its incursion into Manchuria. The two nations then fought several battles, in Shanghai , Rehe and Hebei , until

21472-425: The flight control system with the flight crew providing supervision. CAT I relies only on altimeter indications for decision height, whereas CAT II and CAT III approaches use radio altimeter (RA) to determine decision height. An ILS must shut down upon internal detection of a fault condition. Higher categories require shorter response times; therefore, ILS equipment is required to shut down more quickly. For example,

21648-864: The former Chinese imperial capital of Peking after instigating the Marco Polo Bridge incident , which culminated in the Japanese campaign to invade all of China. The Soviets quickly signed a non-aggression pact with China to lend materiel support, effectively ending China's prior cooperation with Germany . From September to November, the Japanese attacked Taiyuan , engaged the Kuomintang Army around Xinkou , and fought Communist forces in Pingxingguan . Generalissimo Chiang Kai-shek deployed his best army to defend Shanghai , but after three months of fighting, Shanghai fell. The Japanese continued to push Chinese forces back, capturing

21824-422: The glideslope has a corresponding set of 40 channels between 328.6 and 335.4 MHz. The higher frequencies generally result in the glideslope radiating antennas being smaller. The channel pairs are not linear; localizer channel 1 is at 108.10 and paired with glideslope at 334.70, whereas channel two is 108.15 and 334.55. There are gaps and jumps through both bands. Many illustrations of the ILS concept often show

22000-527: The heating effect is in principle no different from other sources of heat, most research into possible health hazards of exposure to radio waves has focused on "nonthermal" effects; whether radio waves have any effect on tissues besides that caused by heating. Radiofrequency electromagnetic fields have been classified by the International Agency for Research on Cancer (IARC) as having "limited evidence" for its effects on humans and animals. There

22176-405: The inbound heading and is within two and a half degrees of the localizer course (half scale deflection or less shown by the course deviation indicator) is said to be established on the approach. Typically, an aircraft is established by at least 2 nautical miles (3.7 km) prior to the final approach fix (glideslope intercept at the specified altitude). Aircraft deviation from the optimal path

22352-481: The industry in anticipation of the certification of the first GBAS ground stations in Memphis, TN; Sydney, Australia; Bremen, Germany; Spain; and Newark, NJ. All four countries have installed GBAS ground stations and are involved in technical and operational evaluation activities. Radio signal Radio waves are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in

22528-525: The invasion of the Ethiopian Empire (also known as Abyssinia ) by the armed forces of the Kingdom of Italy ( Regno d'Italia ), which was launched from Italian Somaliland and Eritrea . The war resulted in the military occupation of Ethiopia and its annexation into the newly created colony of Italian East Africa ( Africa Orientale Italiana , or AOI); in addition it exposed the weakness of

22704-413: The landings. FAA Order 8400.13D limits CAT III to 300 ft RVR or better. Order 8400.13D (2009) allows special authorization CAT II approaches to runways without ALSF-2 approach lights and/or touchdown zone/centerline lights, which has expanded the number of potential CAT II runways. In each case, a suitably equipped aircraft and appropriately qualified crew are required. For example, CAT IIIb requires

22880-682: The latter enabling the strategic bombing of population centres and delivery of the only two nuclear weapons ever used in war. World War II was the deadliest conflict in history, resulting in 70 to 85 million deaths , more than half being civilians. Millions died in genocides , including the Holocaust of European Jews, as well as from massacres, starvation, and disease. Following the Allied powers' victory, Germany , Austria , Japan , and Korea were occupied, and war crimes tribunals were conducted against German and Japanese leaders . The causes of World War II included unresolved tensions in

23056-467: The levels of electric and magnetic field strength at a measurement location. Another commonly used unit for characterizing an RF electromagnetic field is power density . Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the far field zone of the radiation pattern. In closer proximity to the transmitter, i.e., in

23232-424: The lights on the high intensity, five times to medium intensity or three times for low intensity. Once established on an approach, the pilot follows the ILS approach path indicated by the localizer and descends along the glide path to the decision height. This is the height at which the pilot must have adequate visual reference to the landing environment (e.g. approach or runway lighting) to decide whether to continue

23408-434: The metal antenna elements. For example, a dipole antenna consists of two collinear metal rods. If the rods are horizontal, it radiates horizontally polarized radio waves, while if the rods are vertical, it radiates vertically polarized waves. An antenna receiving the radio waves must have the same polarization as the transmitting antenna, or it will suffer a severe loss of reception. Many natural sources of radio waves, such as

23584-424: The modulation relative to the carrier, which varies across the beam pattern. This has the great advantage that the measurement of angle is independent of range. The two DC signals are then sent to a conventional voltmeter , with the 90 Hz output pulling the needle right and the other left. Along the centreline the two modulating tones of the sidebands will be cancelled out and both voltages will be zero, leaving

23760-464: The multiple, large and powerful transmitters required for a full ILS implementation. By 2015, the number of US airports supporting ILS-like LPV approaches exceeded the number of ILS installations, and this is expected to lead to the eventual removal of ILS at most airports. An instrument landing system operates as a ground-based instrument approach system that provides precision lateral and vertical guidance to an aircraft approaching and landing on

23936-423: The needle centered in the display. If the aircraft is far to the left, the 90 Hz signal will produce a strong DC voltage (predominates), and the 150 Hz signal is minimised, pulling the needle all the way to the right. This means the voltmeter directly displays both the direction and magnitude of the turn needed to bring the aircraft back to the runway centreline. As the measurement compares different parts of

24112-430: The other radio signals picked up by the antenna, then amplifies the signal so it is stronger, then finally extracts the information-bearing modulation signal in a demodulator . The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce a visible image, or other devices. A digital data signal is applied to a computer or microprocessor , which interacts with

24288-438: The pattern of the two directional signals, which demanded that they be relatively narrow. The ILS pattern can be much wider. ILS installations are normally required to be usable within 10 degrees on either side of the runway centerline at 25 nautical miles (46 km; 29 mi), and 35 degrees on either side at 17 nautical miles (31 km; 20 mi). This allows for a wide variety of approach paths. The glideslope works in

24464-502: The permanent members of its security council . The Soviet Union and the United States emerged as rival superpowers , setting the stage for the Cold War . In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia . Most countries whose industries had been damaged moved towards economic recovery and expansion . World War II began in Europe on 1 September 1939 with

24640-403: The pilot, eliminating the need for a radio operator to continually monitor the signals and relay the results to the pilot over the intercom . Key to its operation is a concept known as the amplitude modulation index , a measure of how strongly the amplitude modulation is applied to the carrier frequency . In the earlier beam systems, the signal was turned on and off entirely, corresponding to

24816-589: The political European map with the defeat of the Central Powers —including Austria-Hungary , Germany , Bulgaria , and the Ottoman Empire —and the 1917 Bolshevik seizure of power in Russia , which led to the founding of the Soviet Union. Meanwhile, the victorious Allies of World War I , such as France, Belgium, Italy, Romania, and Greece, gained territory, and new nation-states were created out of

24992-599: The port of Tobruk . By late March 1941, Bulgaria and Yugoslavia signed the Tripartite Pact ; however, the Yugoslav government was overthrown two days later by pro-British nationalists. Germany and Italy responded with simultaneous invasions of both Yugoslavia and Greece , commencing on 6 April 1941; both nations were forced to surrender within the month. The airborne invasion of the Greek island of Crete at

25168-412: The previously mentioned navigational signals, the localizer provides for ILS facility identification by periodically transmitting a 1,020 Hz Morse code identification signal. For example, the ILS for runway 4R at John F. Kennedy International Airport transmits IJFK to identify itself, while runway 4L is known as IHIQ. This lets users know the facility is operating normally and that they are tuned to

25344-401: The range of practical radio communication systems decreases with increasing frequency. Below about 20 GHz atmospheric attenuation is mainly due to water vapor. Above 20 GHz, in the millimeter wave band, other atmospheric gases begin to absorb the waves, limiting practical transmission distances to a kilometer or less. Above 300 GHz, in the terahertz band , virtually all the power

25520-421: The reality of Maxwell's electromagnetic waves by experimentally generating electromagnetic waves lower in frequency than light, radio waves, in his laboratory, showing that they exhibited the same wave properties as light: standing waves , refraction , diffraction , and polarization . Italian inventor Guglielmo Marconi developed the first practical radio transmitters and receivers around 1894–1895. He received

25696-446: The receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna. This voltage is applied to the radio receiver , which extracts the information signal. The receiver first uses a bandpass filter to separate the desired radio station's radio signal from all

25872-722: The remainder of Czechoslovakia and subsequently split it into the German Protectorate of Bohemia and Moravia and a pro-German client state , the Slovak Republic . Hitler also delivered an ultimatum to Lithuania on 20 March 1939, forcing the concession of the Klaipėda Region , formerly the German Memelland . Greatly alarmed and with Hitler making further demands on the Free City of Danzig ,

26048-506: The runway and advanced 90 degrees on the right. Additionally, the 150 Hz signal is inverted on one side of the pattern, another 180 degree shift. Due to the way the signals mix in space the SBO signals destructively interfere with and almost eliminate each other along the centerline, leaving the CSB signal predominating. At any other location, on either side of the centerline, the SBO and CSB signals combine in different ways so that one modulating signal predominates. A receiver in front of

26224-408: The runway environment out towards the landing aircraft and allows low-visibility operations. CAT II and III ILS approaches generally require complex high-intensity approach light systems, while medium-intensity systems are usually paired with CAT I ILS approaches. At some non-towered airports , the pilot controls the lighting system ; for example, the pilot can key the microphone seven times to turn on

26400-587: The runway/taxiway lighting and support facilities, and are consistent with the airport surface movement guidance control system (SMGCS) plan. Operations below 600 ft RVR require taxiway centerline lights and taxiway red stop bar lights. If the CAT IIIb RVR minimums on a runway end are 600 feet (180 m), which is a common figure in the U.S., ILS approaches to that runway end with RVR below 600 feet (180 m) qualify as CAT IIIc and require special taxi procedures, lighting, and approval conditions to permit

26576-407: The same general fashion as the localizer and uses the same encoding, but is normally transmitted to produce a centerline at an angle of 3 degrees above horizontal from an antenna beside the runway instead of the end. The only difference between the signals is that the localizer is transmitted using lower carrier frequencies, using 40 selected channels between 108.10 MHz and 111.95 MHz, whereas

26752-451: The signal transmitted from the back of the localizer array. Highly directional antennas do not provide a sufficient signal to support a back course. In the United States, back course approaches are typically associated with Category I systems at smaller airports that do not have an ILS on both ends of the primary runway. Pilots flying a back course should disregard any glide slope indication. On some installations, marker beacons operating at

26928-452: The signal will affect both sub-signals equally, so it will have no effect on the result. Similarly, changes in overall signal strength as the aircraft approaches the runway, or changes due to fading , will have little effect on the resulting measurement because they would normally affect both channels equally. The system is subject to multipath distortion effects due to the use of multiple frequencies, but because those effects are dependent on

27104-410: The significant territorial, colonial, and financial losses imposed by the Treaty of Versailles . Under the treaty, Germany lost around 13 percent of its home territory and all its overseas possessions , while German annexation of other states was prohibited, reparations were imposed, and limits were placed on the size and capability of the country's armed forces . The German Empire was dissolved in

27280-498: The start or prelude to World War II. The exact date of the war's end also is not universally agreed upon. It was generally accepted at the time that the war ended with the armistice of 15 August 1945 ( V-J Day ), rather than with the formal surrender of Japan on 2 September 1945, which officially ended the war in Asia . A peace treaty between Japan and the Allies was signed in 1951. A 1990 treaty regarding Germany's future allowed

27456-593: The strong Maginot Line fortifications on the Franco-German border, Germany directed its attack at the neutral nations of Belgium , the Netherlands , and Luxembourg . The Germans carried out a flanking manoeuvre through the Ardennes region, which was mistakenly perceived by the Allies as an impenetrable natural barrier against armoured vehicles. By successfully implementing new Blitzkrieg tactics,

27632-416: The sun, stars and blackbody radiation from warm objects, emit unpolarized waves, consisting of incoherent short wave trains in an equal mixture of polarization states. The polarization of radio waves is determined by a quantum mechanical property of the photons called their spin . A photon can have one of two possible values of spin; it can spin in a right-hand sense about its direction of motion, or in

27808-412: The surface of objects and cause surface heating, radio waves are able to penetrate the surface and deposit their energy inside materials and biological tissues. The depth to which radio waves penetrate decreases with their frequency, and also depends on the material's resistivity and permittivity ; it is given by a parameter called the skin depth of the material, which is the depth within which 63% of

27984-588: The surface of the Earth, is vitally important in the design of practical radio systems. Radio waves passing through different environments experience reflection , refraction , polarization , diffraction , and absorption . Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate: At microwave frequencies, atmospheric gases begin absorbing radio waves, so

28160-412: The system operating more similarly to beam systems with the 90 Hz signal on one side and the 150 on the other. These illustrations are inaccurate; both signals are radiated across the entire beam pattern, it is their relative difference in the depth of modulation (DDM) that changes dependent upon the position of the approaching aircraft. An instrument approach procedure chart (or ' approach plate ')

28336-407: The terrain, they are generally fixed in location and can be accounted for through adjustments in the antenna or phase shifters. Additionally, because it is the encoding of the signal within the beam that contains the angle information, not the strength of the beam, the signal does not have to be tightly focussed in space. In the older beam systems, the accuracy of the equisignal area was a function of

28512-437: The theory of electromagnetism that was proposed in 1867 by Scottish mathematical physicist James Clerk Maxwell . His mathematical theory, now called Maxwell's equations , predicted that a coupled electric and magnetic field could travel through space as an " electromagnetic wave ". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicist Heinrich Hertz demonstrated

28688-411: The tuned circuit and not passed on. Radio waves are non-ionizing radiation , which means they do not have enough energy to separate electrons from atoms or molecules , ionizing them, or break chemical bonds , causing chemical reactions or DNA damage . The main effect of absorption of radio waves by materials is to heat them, similarly to the infrared waves radiated by sources of heat such as

28864-469: The two wars became World War II in 1941. Other proposed starting dates for World War II include the Italian invasion of Abyssinia on 3 October 1935. The British historian Antony Beevor views the beginning of World War   II as the Battles of Khalkhin Gol fought between Japan and the forces of Mongolia and the Soviet Union from May to September 1939. Others view the Spanish Civil War as

29040-513: The war were not fulfilled in the peace settlement. From 1922 to 1925, the Fascist movement led by Benito Mussolini seized power in Italy with a nationalist, totalitarian , and class collaborationist agenda that abolished representative democracy, repressed socialist, left-wing, and liberal forces, and pursued an aggressive expansionist foreign policy aimed at making Italy a world power, promising

29216-475: The war. Germany annexed western Poland and occupied central Poland ; the Soviet Union annexed eastern Poland ; small shares of Polish territory were transferred to Lithuania and Slovakia . On 6 October, Hitler made a public peace overture to the United Kingdom and France but said that the future of Poland was to be determined exclusively by Germany and the Soviet Union. The proposal was rejected and Hitler ordered an immediate offensive against France, which

29392-420: The wave's electric field to the next, and is inversely proportional to the frequency f {\displaystyle f} of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is where Equivalently, c {\displaystyle c} , the distance that a radio wave travels in vacuum in one second, is 299,792,458 meters (983,571,056 ft), which

29568-442: The waves. Since radio frequency radiation has both an electric and a magnetic component, it is often convenient to express intensity of radiation field in terms of units specific to each component. The unit volt per meter (V/m) is used for the electric component, and the unit ampere per meter (A/m) is used for the magnetic component. One can speak of an electromagnetic field , and these units are used to provide information about

29744-512: The west halted the German advance for several days, but it was outflanked and encircled by the Wehrmacht . Remnants of the Polish army broke through to besieged Warsaw . On 17 September 1939, two days after signing a cease-fire with Japan , the Soviet Union invaded Poland under the supposed pretext that the Polish state had ceased to exist. On 27 September, the Warsaw garrison surrendered to

29920-707: The world order , and soon began a massive rearmament campaign . France, seeking to secure its alliance with Italy, allowed Italy a free hand in Ethiopia , which Italy desired as a colonial possession. The situation was aggravated in early 1935 when the Territory of the Saar Basin was legally reunited with Germany, and Hitler repudiated the Treaty of Versailles, accelerated his rearmament programme, and introduced conscription. The United Kingdom, France and Italy formed

30096-614: Was invaded by Germany. The United States started strategic planning to prepare for a full-scale offensive against Germany. At the end of September 1940, the Tripartite Pact formally united Japan, Italy, and Germany as the Axis powers . The Tripartite Pact stipulated that any country—with the exception of the Soviet Union—that attacked any Axis Power would be forced to go to war against all three. The Axis expanded in November 1940 when Hungary , Slovakia , and Romania joined. Romania and Hungary later made major contributions to

30272-411: Was certified for use in safety of life applications in March 2011. As such, the number of Cat I ILS installations may be reduced, however there are no plans in the United States to phase out any Cat II or Cat III systems. Local Area Augmentation System (LAAS) is under development to provide for Category III minimums or lower. The FAA Ground-Based Augmentation System (GBAS) office is currently working with

30448-441: Was connected to a motorized switch to produce a pattern of Morse code dots and dashes. The switch also controlled which of two directional antennae the signal was sent to. The resulting signal sent into the air consists of dots sent to one side of the runway and dashes to the other. The beams were wide enough so they overlapped in the center. To use the system an aircraft only needed a conventional radio receiver. As they approached

30624-456: Was postponed until the spring of 1940 due to bad weather. After the outbreak of war in Poland, Stalin threatened Estonia , Latvia , and Lithuania with military invasion, forcing the three Baltic countries to sign pacts allowing the creation of Soviet military bases in these countries; in October 1939, significant Soviet military contingents were moved there. Finland refused to sign

30800-772: Was repulsed with heavy Italian casualties; the campaign ended within months with minor territorial changes. To assist Italy and prevent Britain from gaining a foothold, Germany prepared to invade the Balkans, which would threaten Romanian oil fields and strike against British dominance of the Mediterranean. In December 1940, British Empire forces began counter-offensives against Italian forces in Egypt and Italian East Africa . The offensives were successful; by early February 1941, Italy had lost control of eastern Libya, and large numbers of Italian troops had been taken prisoner. The Italian Navy also suffered significant defeats, with

30976-557: Was required to go through the bureaucracy of the League of Nations, which rendered it essentially toothless. The United States, concerned with events in Europe and Asia, passed the Neutrality Act in August of the same year. Hitler defied the Versailles and Locarno Treaties by remilitarising the Rhineland in March 1936, encountering little opposition due to the policy of appeasement . In October 1936, Germany and Italy formed

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