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IFF Mark II was the first operational
identification friend or foe Identification, friend or foe (IFF) 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 syst ...
system. It was developed by the
Royal Air Force The Royal Air Force (RAF) is the Air force, air and space force of the United Kingdom, British Overseas Territories and Crown Dependencies. It was formed towards the end of the World War I, First World War on 1 April 1918, on the merger of t ...
just before the start of
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. After a short run of prototype Mark Is, used experimentally in 1939, the Mark II began widespread deployment at the end of the
Battle of Britain The Battle of Britain () was a military campaign of the Second World War, in which the Royal Air Force (RAF) and the Fleet Air Arm (FAA) of the Royal Navy defended the United Kingdom (UK) against large-scale attacks by Nazi Germany's air force ...
in late 1940. It remained in use until 1943, when it began to be replaced by the standardised
IFF Mark III IFF Mark III, also known as ARI.5025 in the UK or SCR.595 in the US, was the Allied Forces standard identification friend or foe (IFF) system from 1943 until well after the end of World War II. It was widely used by aircraft, ships, and submarin ...
, which was used by all Allied aircraft until long after the war ended. The Mark I was a simple system that amplified the signals of the British
Chain Home Chain Home, or CH for short, was the codename for the ring of coastal early warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF, and given the off ...
radar systems, causing the aircraft's "blip" to extend on the
radar display A radar display is an electronic device that presents radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and re ...
, identifying the aircraft as friendly. MarkI had the problem that the gain had to be adjusted in flight to keep it working; in the field, it was correct only half the time. Another problem was that it was sensitive to only one frequency and had to be manually tuned to different radar stations. In 1939, Chain Home was the only radar of interest and operated on a limited set of frequencies but new radars were already entering service and the number of frequencies was beginning to multiply. Mark II addressed both these problems. An
automatic gain control Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the inpu ...
eliminated the need to adjust the gain, making the device much more likely to be working properly when interrogated. To work with many types of radar, a complex system of motorised gears and
cam Cam or CAM may refer to: Science and technology * Cam (mechanism), a mechanical linkage which translates motion * Camshaft, a shaft with a cam * Camera or webcam, a device that records images or video In computing * Computer-aided manufacturin ...
s constantly shifted the frequency through three wide bands, scanning each every few seconds. These changes automated the operation of the device and made it truly useful for the first time; previously, operators could not be sure if a blip was an enemy aircraft or a friendly one with a maladjusted IFF. Originally ordered in 1939, installation was delayed during the Battle of Britain and the system became widely used from the end of 1940. Although the Mark II's selection of frequencies covered the early war period, by 1942 so many radars were in use that a series of sub-versions had been introduced to cover particular combinations of radars. The introduction of new radars based on the
cavity magnetron The cavity magnetron is a high-power vacuum tube used in early radar systems and subsequently in microwave ovens and in linear particle accelerators. A cavity magnetron generates microwaves using the interaction of a stream of electrons wit ...
required different frequencies to which the system was not easily adapted. This led to the introduction of the Mark III, which operated on a single frequency that could be used with any radar; it also eliminated the need for the complex gear and cam system. Mark III began entering service in 1943 and quickly replaced the Mark II.


History


Early efforts

Before
Chain Home Chain Home, or CH for short, was the codename for the ring of coastal early warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF, and given the off ...
(CH) systems began deployment, Robert Watt had considered the problem of identifying friendly aircraft on a
radar display A radar display is an electronic device that presents radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and re ...
. He filed initial patents on such systems in 1935 and 1936. In 1938, researchers at the
Bawdsey Manor Bawdsey Manor stands at a prominent position at the mouth of the River Deben close to the village of Bawdsey in Suffolk, England, about north-east of London. Built in 1886, it was enlarged in 1895 as the principal residence of Sir Will ...
radar research establishment began working with the first of Watt's concepts. This was a simple "reflector" system consisting of a set of
dipole antenna In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...
s that were tuned to resonate at the frequency of the CH radars. When a pulse from the radar hit them, they would resonate for a short period and cause an additional signal to be received by the station. The antennas were connected to a motorised switch that periodically shorted the antenna out and cancelled the broadcast, causing the signal to turn on and off. On the CH display, this caused the "blip" to periodically lengthen and contract. The system proved highly unreliable; it worked only when the aircraft was at certain locations and flying in certain directions. It was always suspected that this system would be of little use in practice. When that turned out to be the case, the
Royal Air Force The Royal Air Force (RAF) is the Air force, air and space force of the United Kingdom, British Overseas Territories and Crown Dependencies. It was formed towards the end of the World War I, First World War on 1 April 1918, on the merger of t ...
(RAF) introduced a different system that consisted of a set of tracking stations using
HF/DF High-frequency direction finding, usually known by its abbreviation HF/DF or nickname huff-duff, is a type of radio direction finder (RDF) introduced in World War II. High frequency (HF) refers to a radio band that can effectively communicate ove ...
radio direction finder Direction finding (DF), radio direction finding (RDF), or radiogoniometry is the use of radio waves to determine the direction to a radio source. The source may be a cooperating radio transmitter or may be an inadvertent source, a natural ...
s. The standard aircraft radios were modified to send out a 1kHz tone for 14 seconds every minute, allowing the tracking 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. There they used
triangulation In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle m ...
to determine the aircraft's location. Known as "
pip-squeak Pip-squeak was a radio navigation system used by the British Royal Air Force during the early part of World War II. Pip-squeak used an aircraft's voice radio set to periodically send out a tone which was picked up by ground-based high-frequency ...
", the system worked but was very labour-intensive, requiring operators at several stations and at plotting boards in sector HQs. More operators were needed to merge the information from the pip-squeak system with that from the radar systems to provide one view of the airspace. It also meant the pilots were constantly interrupted when talking to their ground controllers. A system that worked directly with the radar was desired.


Mark I

Seeking a system that would be as simple as possible, the Bawdsey researchers began work with a
regenerative receiver A regenerative circuit is an amplifier circuit that employs positive feedback (also known as regeneration or reaction). Some of the output of the amplifying device is applied back to its input to add to the input signal, increasing the amplific ...
. The idea behind regeneration is to amplify the radio signal and send it into an
LC circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can act ...
, or "tank", that resonates at a selected frequency. A small part of the tank's output is sent back into the amplifier's input, causing
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause and effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handle ...
which greatly amplifies the signal. As long as the input signal's frequency is relatively constant, like
Morse code Morse code is a telecommunications method which Character encoding, encodes Written language, text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code i ...
signals, a single
vacuum tube A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
can provide significant amplification. One problem with regeneration is that if the feedback is too strong, the signal will grow to the point where it begins to broadcast back out of the antenna and cause interference on other receivers. In the case of the IFF system, this is precisely what was desired. When the radar signal was received, and the gain was properly adjusted, the signal grew until it turned the system from a receiver to a broadcaster. The signal levels were still small, but the receivers in the radar systems were extremely sensitive and the signal from the transceiver was larger than what would normally be received from the reflection of the original radar pulse alone. This extra signal would cause the aircraft's blip on the radar screen to suddenly grow to be much larger. Since it might be difficult to distinguish the resulting larger signal from IFF from the return of a larger aircraft or formation without IFF, the circuit was connected to a motorised switch that rapidly disconnected and reconnected the receiver, causing the blip to oscillate on the radar display. A switch on the cockpit control panel allowed the pattern to be controlled; one setting sent back 15
microsecond A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available. A microsecond is to one second, ...
(μs) pulses, the second setting sent 40 μs pulses and the final setting switched between the two with every received pulse. There were two major disadvantages of the design. One was that the pilot had to carefully set the feedback control; if it was too low the system would not create an output signal and nothing would be received by the radar station, and if it was too high, the circuit would amplify its own electronic noise and give off random signals known as "
squitter Squitter refers to random pulses, pulse-pairs and other non-solicited messages used in various aviation radio systems' signal maintenance. Squitter pulses were originally, and are still, used in the DME/TACAN air navigation systems. Squitter puls ...
" across a wide range of frequencies. This caused significant interference over a large area and was a major problem for radar operators. It was too easy to forget to adjust the gain during flight, especially in single-seat fighters, and it was estimated a usable signal was returned only about 50 per cent of the time. The other problem was that the CH stations operated on a small but distinct set of frequencies, and the system worked on only a single frequency at a time. An aircraft on a typical mission profile might be visible only to a single CH station, or perhaps two or three over their operational area. To address this, the cockpit panel had a card with the frequencies of local CH stations on it, which the pilot had to tune as they moved about. Pilots often forgot to do this, and if they were lost or off-course, they would not know which frequency to tune to, or the nearest station might not be on the card at all. The Mark I was used only experimentally. Thirty sets were hand-made at
AMES AMES, short Air Ministry Experimental Station, was the name given to the British Air Ministry's radar development team at Bawdsey Manor (afterwards RAF Bawdsey) in the immediate pre-World War II era. The team was forced to move on three occasion ...
and an order for 1,000 was placed with
Ferranti Ferranti International PLC or simply Ferranti was a UK-based electrical engineering and equipment firm that operated for over a century, from 1885 until its bankruptcy in 1993. At its peak, Ferranti was a significant player in power grid system ...
in September 1939.


Mark II

Beyond the operational problems with the Mark I, a more serious issue was the growing number of new radar systems being deployed. Even as the MarkI was being tested, the RAF,
Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...
and
British Army The British Army is the principal Army, land warfare force of the United Kingdom. the British Army comprises 73,847 regular full-time personnel, 4,127 Brigade of Gurkhas, Gurkhas, 25,742 Army Reserve (United Kingdom), volunteer reserve perso ...
were introducing new systems, spanning a wide range of frequencies from the RAF's 200 MHz systems used on
night fighter A night fighter (later known as all-weather fighter or all-weather interceptor post-Second World War) is a largely historical term for a fighter aircraft, fighter or interceptor aircraft adapted or designed for effective use at night, during pe ...
s and
Chain Home Low Chain Home Low (CHL) was the name of a British early warning radar system operated by the RAF during World War II. The name refers to CHL's ability to detect aircraft flying at altitudes below the capabilities of the original Chain Home (CH) r ...
to the Army's 75 MHz gun-laying radars and on to the CH at 20 to 30 MHz. Attempting to manually tune among these would be impractical and impossible if the aircraft were visible to more than one radar, which was increasingly the case. A solution was already under development in early 1939, similar to the MarkI but employing tuned circuits sensitive to many radar sets. It used a "complicated system of cams and cogs and Geneva mechanisms" to switch among the bands by connecting to oscillators covering a band and then used a motorised tuning capacitor to sweep through the frequency range within that band. To ensure the signal was the right strength and did not cause squitter, an
automatic gain control Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the inpu ...
was added. These changes eliminated the need for tuning or gain adjustments in flight, greatly improving the chance it would respond correctly to a radar. Only periodic adjustments on the ground were needed to keep it working properly. An order for 1,000 sets was sent to
Ferranti Ferranti International PLC or simply Ferranti was a UK-based electrical engineering and equipment firm that operated for over a century, from 1885 until its bankruptcy in 1993. At its peak, Ferranti was a significant player in power grid system ...
in October 1939 and they had completed the first 100 sets by November. The rapid expansion of the RAF precluded a significant proportion of its force being equipped by the time of the
Battle of Britain The Battle of Britain () was a military campaign of the Second World War, in which the Royal Air Force (RAF) and the Fleet Air Arm (FAA) of the Royal Navy defended the United Kingdom (UK) against large-scale attacks by Nazi Germany's air force ...
in mid-1940. In any case, the action took place mostly over southern England, where IFF would not be very useful as the CH stations were positioned along the coast and could see the fighters only if they were out over the
English Channel The English Channel, also known as the Channel, is an arm of the Atlantic Ocean that separates Southern England from northern France. It links to the southern part of the North Sea by the Strait of Dover at its northeastern end. It is the busi ...
. There was no pressing need to install the systems and pip-squeak continued in use during the battle. The lack of IFF led to problems including
friendly fire In military terminology, friendly fire or fratricide is an attack by belligerent or neutral forces on friendly troops while attempting to attack enemy or hostile targets. Examples include misidentifying the target as hostile, cross-fire while ...
; the
Battle of Barking Creek The Battle of Barking Creek was a friendly fire incident over the East Coast of England in the earliest days of the Second World War. On 6 September 1939, aircraft from several No. 11 Group RAF, 11 Group, RAF Fighter Command squadrons, were scra ...
in September 1939 would not have occurred if IFF had been installed. It also meant that enemy aircraft could not be identified if they were close to known RAF aircraft. In July 1940, the Germans began to take advantage of this by inserting their bombers into formations of RAF bombers returning from night missions over Europe. To the ground operators these appeared to be more RAF aircraft and once they crossed the coast there was no way to track them. Even if one of the rare MarkI sets was available, the unreliability of their signals made it difficult for controllers to trust it. As the Battle of Britain ended, Mark II was rapidly installed in RAF aircraft. Its installation on the
Supermarine Spitfire The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and other Allies of World War II, Allied countries before, during, and after World War II. It was the only British fighter produced conti ...
required two wire antennas on the tail that slowed the top speed by and added of weight. Pip-squeak was still used for areas over land where CH did not cover, as well as an emergency guidance system. Mark II also found a use on Royal Navy ships, where it was produced as the Type 252 so that ships could identify each other by radar. A Mark II set was taken to the US as part of the
Tizard Mission The Tizard Mission, officially the British Technical and Scientific Mission, was a delegation from the United Kingdom that visited the United States during World War II to share secret research and development (R&D) work that had military applicat ...
in November 1940. US researchers were already working on their own IFF system of some complexity. They realised the importance of using a common IFF system and in early 1941 they decided to install Mark II in their own aircraft. Production was taken up by
Philco Philco (an acronym for Philadelphia Battery Company) is an American electronics industry, electronics manufacturer headquartered in Philadelphia. Philco was a pioneer in battery, radio, and television production. In 1961, the company was purchase ...
with an order for 18,000 sets as the SCR-535 in July 1942. The system was never entirely reliable.


Mark III

The profusion of radars that led to the Mark II continued and by 1942 there were almost a dozen sub-types of the Mark II covering sets of frequencies. Some, like the IIIN, were tuned to the radars commonly used by the Navy, while others, like the IIIG, to those used by ground radars in the Army and Air Force. No one unit could respond to them all. To add to the problem, the
cavity magnetron The cavity magnetron is a high-power vacuum tube used in early radar systems and subsequently in microwave ovens and in linear particle accelerators. A cavity magnetron generates microwaves using the interaction of a stream of electrons wit ...
had matured and a new generation of radars operating in the
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
region was about to enter service, using frequencies on which the IFF receivers could not operate. In 1940, English engineer Freddie Williams had considered this problem and suggested that all IFF operations move to a single frequency. Instead of responding on the radar's frequency and thus mixing with their signal in the receiver, a separate unit would transmit "interrogation" pulses in synchronicity with the radar's pulses, and the received signals would be amplified independently and then mixed with the radar's signals on the display. This greatly simplified the airborne equipment because it operated on one frequency, eliminating the complex multi-band system. The only disadvantage was that a second transmitter was needed at radar stations. Production of the
IFF Mark III IFF Mark III, also known as ARI.5025 in the UK or SCR.595 in the US, was the Allied Forces standard identification friend or foe (IFF) system from 1943 until well after the end of World War II. It was widely used by aircraft, ships, and submarin ...
began at Ferranti and was quickly taken up in the US by Hazeltine. It remained the Allies' primary IFF system for the rest of the war; the 176 MHz common frequency was used for many years after.


Versions

:''From Shayler.'' * Mark I – prototype version that worked with CH radars * Mark II – automatic scanning of three bands covering CH, GL and Navy
Type 79 radar The Type 79 radar was a British naval early-warning radar developed before World War II. It was the first radar system deployed by the Royal Navy. The first version of this radar, Type 79X, was mounted on the RN Signal School's tender, the minesw ...
* Mark IIG – "G"round version with bands covering common ground-based radars like CH, CHL, GL, and
AMES Type 7 The Air Ministry Experimental Station, AMES Type 7, also known as the Final GCI, was a ground-based radar system introduced during World War II by the Royal Air Force (RAF). The Type 7 was the first truly modern radar used by the Allies, providing ...
* Mark IIN – "N"aval version with bands covering various
Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...
radars like Type 286 * ABE (SCR-535 and SCR-535/A) – US version covering
US Army The United States Army (USA) is the primary land service branch of the United States Department of Defense. It is designated as the Army of the United States in the United States Constitution.Article II, section 2, clause 1 of the United Stat ...
radars like
SCR-268 The SCR-268 (for ''Signal Corps Radio no. 268'') was the United States Army's first radar system. Introduced in 1940, it was developed to provide accurate aiming information for antiaircraft artillery and was also used for gun laying systems and ...
,
SCR-270 The SCR-270 was one of the first operational early-warning radars. It was the U.S. Army's primary long-distance radar throughout World War II and was deployed around the world. It is also known as the Pearl Harbor Radar, since it was an SCR-27 ...
, SCR-271 and SCR-516 * ABK – US version covering
US Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...
radars as well as common ground radars


Notes


References


Citations


Bibliography

* * * * *


Further reading

* {{cite web , url=http://www.qsl.net/vk2dym/radio/iff.htm , title=Radio Identification Systems – Identification, Friend or Foe or I.F.F. , website=VK2DYM's Radio and Radar Information contains a list of various Mark II submodels


External links


Mark II and Mark III IFF displays
shows the effect of IFF Mark II on the radar display. World War II British electronics Identification friend or foe Military equipment introduced in the 1930s