The Reeves AN/MSQ-35 Bomb Scoring Central was a United States Air Force dual radar system with computerized plotting board. It was used by the 1st Combat Evaluation Group to evaluate the accuracy of Strategic Air Command bomber crews.
117-405: The central had a 20 rpm Acquisition Radar System with a variable "fan-shaped beam" in elevation and an Interrogator Set AN/TPX-27 for identification friend or foe . The central's trailer van for operations had the separate AN/MSQ-54 Bomb Scoring Set with an automatic tracking radar group (OA-450/FSA-4 Receiver-Transmitter Control Group), a computer group with analog vacuum tube circuitry and on
234-637: 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
351-564: 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
468-494: A motorized switch, while an automatic gain control solved the problem of it sending out too much signal. Mark II was technically complete as the war began, but a lack of sets meant it was not available in quantity and only a small number of RAF aircraft carried it by the time of the Battle of Britain . Pip-squeak was kept in operation during this period, but as the Battle ended, IFF Mark II was quickly put into full operation. Pip-squeak
585-514: 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
702-765: 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
819-668: A program for a compatible system known as successor IFF (SIFF). Modes 4 and 5 are designated for use by NATO forces. In World War I , eight submarines were sunk by friendly fire and in World War II nearly twenty were sunk this way. Still, IFF has not been regarded a high concern before the 1990s by the US military as not many other countries possess submarines . IFF methods that are analogous to aircraft IFF have been deemed unfeasible for submarines because they would make submarines easier to detect. Thus, having friendly submarines broadcast
936-469: 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
1053-422: A set of tracking stations using HF/DF radio direction finders . Their aircraft radios were modified to send out a 1 kHz tone for 14 seconds every minute, allowing the stations ample time to measure the aircraft's bearing. Several such stations were assigned to each "sector" of the air defence system, and sent their measurements to a plotting station at sector headquarters, who used triangulation to determine
1170-508: A signal that the IFF responds to. There are two key differences, however. One is that the interrogation pulse is followed by a 12-bit code similar to the ones sent back by the Mark 3 transponders. The encoded number changes day-to-day. When the number is received and decoded in the aircraft transponder, a further cryptographic encoding is applied. If the result of that operation matches the value dialled into
1287-462: A signal, or somehow increase the submarine's signature (based on acoustics, magnetic fluctuations etc.), are not considered viable. Instead, submarine IFF is done based on carefully defining areas of operation. Each friendly submarine is assigned a patrol area, where the presence of any other submarine is deemed hostile and open to attack. Further, within these assigned areas, surface ships and aircraft refrain from any anti-submarine warfare (ASW); only
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#17330860201321404-743: 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,
1521-519: 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
1638-433: A way for ground controllers to determine whether an aircraft had the right code or not but it did not include a way for the transponder to reject signals from other sources. British military scientists found a way of exploiting this by building their own IFF transmitter called Perfectos , which were designed to trigger a response from any FuG 25a system in the vicinity. When an FuG 25a responded on its 168 MHz frequency,
1755-527: Is a combat identification system designed for command and control . It uses a transponder that listens for an interrogation signal and then sends a response that identifies the broadcaster. IFF systems usually use radar frequencies, but other electromagnetic frequencies, radio or infrared, may be used. It enables military and civilian air traffic control interrogation systems to identify aircraft, vehicles or forces as friendly, as opposed to neutral or hostile, and to determine their bearing and range from
1872-407: Is a more complex problem and, especially in the 1950s, added significantly to the cost of the radar system. By placing this function on the IFF, the same information could be returned for little additional cost, essentially that of adding a digitizer to the aircraft's altimeter . Modern interrogators generally send out a series of challenges on Mode 3/A and then Mode C, allowing the system to combine
1989-588: 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
2106-662: 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
2223-566: 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
2340-814: 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
2457-567: 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
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#17330860201322574-537: 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 ,
2691-682: 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
2808-693: 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
2925-677: 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
3042-603: 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
3159-581: 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
3276-580: 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
3393-689: 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
3510-682: 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
3627-654: The Telecommunications Research Establishment as the IFF Mark III . This was to become the standard for the Western Allies for most of the war. Mark III transponders were designed to respond to specific 'interrogators', rather than replying directly to received radar signals. These interrogators worked on a limited selection of frequencies, no matter what radar they were paired with. The system also allowed limited communication to be made, including
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3744-568: 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
3861-554: 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
3978-723: 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
4095-683: 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
4212-612: 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
4329-499: The air attack on the fortress of Koepenick over Germany. Already before the deployment of their Chain Home radar system (CH), the RAF had considered the problem of IFF. Robert Watson-Watt had filed patents on such systems in 1935 and 1936. By 1938, researchers at Bawdsey Manor began experiments with "reflectors" consisting of dipole antennas tuned to resonate to the primary frequency of
4446-430: The microwave -frequency cavity magnetron rendered this obsolete; there was simply no way to make a responder operating in this band using contemporary electronics. In 1940, English engineer Freddie Williams had suggested using a single separate frequency for all IFF signals, but at the time there seemed no pressing need to change the existing system. With the introduction of the magnetron, work on this concept began at
4563-479: 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
4680-584: The 40-week Keesler Air Force Base technical training school for Automatic Tracking Radar Specialists (AUTOTRACK). The Final Engineering Report for the AN/MSQ-35 was published in December 1965 after it was used as the basis for developing the 1965 Reeves AN/MSQ-77 Bomb Directing Central with integrating computer for Combat Skyspot ground-directed bombing in the then-ongoing Vietnam War . Identification friend or foe Identification, friend or foe ( IFF )
4797-415: The AN/MSQ-35 production program." In 1962 the Reeves Instrument Corporation production facility hosted the first class for AN/MSQ-35 operators, and the central's field testing was in early 1963 at White Sands Missile Range . The AN/MSQ-35 military school was initially at the Aberdeen Proving Ground (where the M-33 school had been located) and the central eventually became the instructional radar used at
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4914-409: 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
5031-400: 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
5148-421: 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,
5265-473: The British designs. This technique is now known as a cross-band transponder . When the Mark II was revealed in 1941 during the Tizard Mission , it was decided to use it and take the time to further improve their experimental system. The result was what became IFF Mark IV. The main difference between this and earlier models is that it worked on higher frequencies, around 600 MHz, which allowed much smaller antennas. However, this also turned out to be close to
5382-422: The CH radars. When a pulse from the CH transmitter hit the aircraft, the antennas would resonate for a short time, increasing the amount of energy returned to the CH receiver. The antenna was connected to a motorized switch that periodically shorted it out, preventing it from producing a signal. This caused the return on the CH set to periodically lengthen and shorten as the antenna was turned on and off. In practice,
5499-405: 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
5616-410: 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
5733-426: 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
5850-412: The IFF in the aircraft, the transponder replies with a Mode 3 response as before. If the values do not match, it does not respond. This solves the problem of the aircraft transponder replying to false interrogations, but does not completely solve the problem of locating the aircraft through triangulation. To solve this problem, a delay is added to the response signal that varies based on the code sent from
5967-412: The Mode S data. The IFF of World War II and Soviet military systems (1946 to 1991) used coded radar signals (called cross-band interrogation, or CBI) to automatically trigger the aircraft's transponder in an aircraft illuminated by the radar. Radar-based aircraft identification is also called secondary surveillance radar in both military and civil usage, with primary radar bouncing an RF pulse off of
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#17330860201326084-450: 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
6201-452: 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
6318-423: 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
6435-421: 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
6552-418: 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
6669-575: 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
6786-402: 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
6903-424: 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
7020-489: 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
7137-401: 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
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#17330860201327254-473: 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
7371-434: 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
7488-411: The ability to transmit a coded ' Mayday ' response. The IFF sets were designed and built by Ferranti in Manchester to Williams' specifications. Equivalent sets were manufactured in the US, initially as copies of British sets, so that allied aircraft would be identified upon interrogation by each other's radar. IFF sets were obviously highly classified. Thus, many of them were wired with explosives in
7605-605: The aircraft to determine position. George Charrier, working for RCA , filed for a patent for such an IFF device in 1941. It required the operator to perform several adjustments to the radar receiver to suppress the image of the natural echo on the radar receiver, so that visual examination of the IFF signal would be possible. By 1943, Donald Barchok filed a patent for a radar system using the abbreviation IFF in his text with only parenthetic explanation, indicating that this acronym had become an accepted term. In 1945, Emile Labin and Edwin Turner filed patents for radar IFF systems where
7722-415: The aircraft's location. Known as " pip-squeak ", the system worked, but was labour-intensive and did not display its information directly to the radar operators. A system that worked directly with the radar was clearly desirable. The first active IFF transponder (transmitter/responder) was the IFF Mark I which was used experimentally in 1939. This used a regenerative receiver , which fed a small amount of
7839-413: The amplified output back into the input, strongly amplifying even small signals as long as they were of a single frequency (like Morse code, but unlike voice transmissions). They were tuned to the signal from the CH radar (20–30 MHz), amplifying it so strongly that it was broadcast back out the aircraft's antenna. Since the signal was received at the same time as the original reflection of the CH signal,
7956-492: The area not involved in the combat, such as civilian airliners, will not be equipped with IFF. IFF is a tool within the broader military action of combat identification (CID), the characterization of objects detected in the field of combat sufficiently accurately to support operational decisions. The broadest characterization is that of friend, enemy, neutral, or unknown. CID not only can reduce friendly fire incidents, but also contributes to overall tactical decision-making. With
8073-405: 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
8190-400: The civilian air transport system, and it was decided to use slightly modified Mark X sets for these aircraft as well. These sets included a new military Mode 3 which was essentially identical to Mode 2, returning a four-digit code, but used a different interrogation pulse, allowing the aircraft to identify if the query was from a military or civilian radar. For civilian aircraft, this same system
8307-403: 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
8424-515: 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
8541-739: 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
8658-637: 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
8775-508: 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
8892-415: The event the aircrew bailed out or crash landed. Jerry Proc reports: Alongside the switch to turn on the unit was the IFF destruct switch to prevent its capture by the enemy. Many a pilot chose the wrong switch and blew up his IFF unit. The thud of a contained explosion and the acrid smell of burning insulation in the cockpit did not deter many pilots from destroying IFF units time and time again. Eventually,
9009-617: 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
9126-843: 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
9243-580: The frequencies used by the German Würzburg radar and there were concerns that it would be triggered by that radar and the transponder responses would be picked on its radar display. This would immediately reveal the IFF's operational frequencies. This led to a US–British effort to make a further improved model, the Mark V, also known as the United Nations Beacon or UNB. This moved to still higher frequencies around 1 GHz but operational testing
9360-501: The identity of the aircraft with its altitude and location from the radar. The current IFF system is the Mark XII. This works on the same frequencies as Mark X, and supports all of its military and civilian modes. It had long been considered a problem that the IFF responses could be triggered by any properly formed interrogation, and those signals were simply two short pulses of a single frequency. This allowed enemy transmitters to trigger
9477-479: The interrogator. IFF is used by both military and civilian aircraft. IFF was first developed during World War II , with the arrival of radar, and several friendly fire incidents. IFF can only positively identify friendly aircraft or other forces. If an IFF interrogation receives no reply or an invalid reply, the object is not positively identified as foe; friendly forces may not properly reply to IFF for various reasons such as equipment malfunction, and parties in
9594-427: The interrogator. When received by an enemy that does not see the interrogation pulse, which is generally the case as they are often below the radar horizon , this causes a random displacement of the return signal with every pulse. Locating the aircraft within the set of returns is a difficult process. During the 1980s, a new civilian mode, Mode S, was added that allowed greatly increased amounts of data to be encoded in
9711-512: 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
9828-677: 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
9945-454: The outgoing radar signal and the transponder's reply signal could each be independently programmed with a binary codes by setting arrays of toggle switches; this allowed the IFF code to be varied from day to day or even hour to hour. The United States and other NATO countries started using a system called Mark XII in the late twentieth century; Britain had not until then implemented an IFF system compatible with that standard, but then developed
10062-497: 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
10179-571: 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
10296-583: 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
10413-564: The release point to the vacuum impact point and then to an estimate impact point for drag and crosswind. By automatically using "ballistic data" such as time-of-fall to determine vacuum impact point, the AN/MSQ-35 computed bomb "score data [that was] printed out on tape" from a paper roll. The X-band Western Electric M-33 Fire Control System "was utilized as a basic building block" for the X-band AN/MSQ-35, and "techniques established during [the] AN/USQ-9 development program were utilized in
10530-693: 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 ,
10647-540: The resident submarine may target other submarines in its own area. Ships and aircraft may still engage in ASW in areas that have not been assigned to any friendly submarines. Navies also use database of acoustic signatures to attempt to identify the submarine, but acoustic data can be ambiguous and several countries deploy similar classes of submarines. World War II Asia-Pacific Mediterranean and Middle East Other campaigns Coups World War II or
10764-512: The response, and using triangulation , an enemy could determine the location of the transponder. The British had already used this technique against the Germans during WWII, and it was used by the USAF against VPAF aircraft during the Vietnam War . Mark XII differs from Mark X through the addition of the new military Mode 4. This works in a fashion similar to Mode 3/A, with the interrogator sending out
10881-521: The result was a lengthened "blip" on the CH display which was easily identifiable. In testing, it was found that the unit would often overpower the radar or produce too little signal to be seen, and at the same time, new radars were being introduced using new frequencies. Instead of putting Mark I into production, a new IFF Mark II was introduced in early 1940. Mark II had a series of separate tuners inside tuned to different radar bands that it stepped through using
10998-401: The return signal to contain up to 12 pulses, representing four octal digits of 3 bits each. Depending on the timing of the interrogation signal, SIF would respond in several ways. Mode 1 indicated the type of aircraft or its mission (cargo or bomber, for instance) while Mode 2 returned a tail code. Mark X began to be introduced in the early 1950s. This was during a period of great expansion of
11115-435: The returned signal. This was used to encode the location of the aircraft from the navigation system. This is a basic part of the traffic collision avoidance system (TCAS), which allows commercial aircraft to know the location of other aircraft in the area and avoid them without the need for ground operators. The basic concepts from Mode S were then militarized as Mode 5, which is simply a cryptographically encoded version of
11232-484: The roof, the antenna group. A communications group provided a link for receiving the aircraft's signal at simulated bomb release, and additional vehicles included a V-280 maintenance van and a V-287 flatbed trailer for transporting the radar/IFF antennas. In addition to providing a conventional plot of the aircraft bomb run, the computer group automated the previously manual "bomb plot" in which technicians used time-of-flight charts for air drag and drew lengths of line from
11349-469: The self destruct switch was secured by a thin wire to prevent its accidental use." FuG 25a Erstling (English: Firstborn, Debut) was developed in Germany in 1940. It was tuned to the low- VHF band at 125 MHz used by the Freya radar , and an adaptor was used with the low- UHF -banded 550–580 MHz used by Würzburg . Before a flight, the transceiver was set up with a selected day code of ten bits which
11466-458: The signal was received by the antenna system from an AI Mk. IV radar , which originally operated at 212 MHz. By comparing the strength of the signal on different antennas the direction to the target could be determined. Mounted on Mosquitos , the "Perfectos" severely limited German use of the FuG 25a. The United States Naval Research Laboratory had been working on their own IFF system since before
11583-410: 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
11700-494: 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
11817-575: 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,
11934-450: The successful deployment of radar systems for air defence during World War II , combatants were immediately confronted with the difficulty of distinguishing friendly aircraft from hostile ones; by that time, aircraft were flown at high speed and altitude, making visual identification impossible, and the targets showed up as featureless blips on the radar screen. This led to incidents such as the Battle of Barking Creek , over Britain, and
12051-451: The system was found to be too unreliable to use; the return was highly dependent on the direction the aircraft was moving relative to the CH station, and often returned little or no additional signal. It had been suspected this system would be of little use in practice. When that turned out to be the case, the RAF turned to an entirely different system that was also being planned. This consisted of
12168-510: 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
12285-504: 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
12402-475: 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
12519-489: The war. It used a single interrogation frequency, like the Mark III, but differed in that it used a separate responder frequency. Responding on a different frequency has several practical advantages, most notably that the response from one IFF cannot trigger another IFF on another aircraft. But it requires a complete transmitter for the responder side of the circuitry, in contrast to the greatly simplified regenerative system used in
12636-503: 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
12753-687: 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
12870-614: 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
12987-417: Was dialed into the unit. To start the identification procedure, the ground operator switched the pulse frequency of his radar from 3,750 Hz to 5,000 Hz. The airborne receiver decoded that and started to transmit the day code. The radar operator would then see the blip lengthen and shorten in the given code. The IFF transmitter worked on 168 MHz with a power of 400 watts (PEP). The system included
13104-427: Was known as Mode A, and because they were identical, they are generally known as Mode 3/A. Several new modes were also introduced during this process. Civilian modes B and D were defined, but never used. Mode C responded with a 12-bit number encoded using Gillham code , which represented the altitude as (that number) x 100 feet - 1200. Radar systems can easily locate an aircraft in two dimensions, but measuring altitude
13221-440: Was not complete when the war ended. By the time testing was finished in 1948, the much improved Mark X was beginning its testing and Mark V was abandoned. Mark X started as a purely experimental device operating at frequencies above 1 GHz; the name refers to "experimental", not "number 10". As development continued it was decided to introduce an encoding system known as the "Selective Identification Feature", or SIF. SIF allowed
13338-452: 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
13455-761: 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
13572-651: 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
13689-413: Was still used for areas over land where CH did not cover, as well as an emergency guidance system. Even by 1940 the complex system of Mark II was reaching its limits while new radars were being constantly introduced. By 1941, a number of sub-models were introduced that covered different combinations of radars, common naval ones for instance, or those used by the RAF. But the introduction of radars based on
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