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69-596: IFF Mark II was the first operational identification friend or foe system. It was developed by the Royal Air Force just before the start of World War II . After a short run of prototype Mark I s, used experimentally in 1939, the Mark II began widespread deployment at the end of the Battle of Britain in late 1940. It remained in use until 1943, when it began to be replaced by the standardised IFF Mark III , which

138-510: 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

207-403: 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 (μs) pulses, the second setting sent 40 μs pulses and the final setting switched between

276-445: 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

345-677: 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

414-515: 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

483-462: 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

552-424: 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

621-433: 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,

690-629: A wide range of frequencies from the RAF's 200 MHz systems used on night fighters and Chain Home Low 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

759-412: 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

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828-407: 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

897-424: 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

966-466: 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 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

1035-418: Is to amplify the radio signal and send it into an LC circuit , 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 which greatly amplifies the signal. As long as the input signal is relatively constant, like Morse code signals, a single vacuum tube can provide significant amplification. One problem with regeneration

1104-554: The Battle of Barking Creek 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

1173-683: 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

1242-543: The cavity magnetron 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. Before Chain Home (CH) systems began deployment, Robert Watt had considered

1311-430: 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

1380-591: 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

1449-412: 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

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1518-678: 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 had matured and a new generation of radars operating in the microwave 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

1587-416: The Mark   I 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

1656-463: 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

1725-469: 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

1794-658: 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

1863-444: 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 (RAF) introduced a different system that consisted of a set of tracking stations using HF/DF radio direction finders . The standard aircraft radios were modified to send out a 1   kHz tone for 14 seconds every minute, allowing

1932-471: 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

2001-413: 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,

2070-411: The antenna was turned on and off. In practice, 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

2139-400: 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

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2208-536: The coast there was no way to track them. Even if one of the rare Mark   I 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 required two wire antennas on the tail that slowed the top speed by 2 miles per hour (3.2 km/h) and added 40 pounds (18 kg) of weight. Pip-squeak

2277-578: The complex multi-band system. The only disadvantage was that a second transmitter was needed at radar stations. Production of the IFF Mark III 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. Identification friend or foe IFF can only positively identify friendly aircraft or other forces. If an IFF interrogation receives no reply or an invalid reply,

2346-415: 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,

2415-596: 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

2484-506: 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

2553-521: 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 with an order for 18,000 sets as the SCR-535 in July 1942. The system was never entirely reliable. 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

2622-472: 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. Seeking a system that would be as simple as possible, the Bawdsey researchers began work with a regenerative receiver . The idea behind regeneration

2691-427: 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

2760-457: 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 and an order for 1,000 was placed with Ferranti in September 1939. 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 Mark   I was being tested, the RAF, Royal Navy and British Army were introducing new systems, spanning

2829-418: 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 cams 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

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2898-490: 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 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

2967-454: 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

3036-399: The primary frequency of 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

3105-470: The problem of identifying friendly aircraft on a radar display . He filed initial patents on such systems in 1935 and 1936. In 1938, researchers at the Bawdsey Manor radar research establishment began working with the first of Watt's concepts. This was a simple "reflector" system consisting of a set of dipole antennas that were tuned to resonate at the frequency of the CH radars. When a pulse from

3174-418: 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

3243-405: 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

3312-486: 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. Regenerative circuit Too Many Requests If you report this error to

3381-522: 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

3450-476: 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

3519-401: 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

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3588-435: 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

3657-528: 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

3726-461: 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

3795-459: 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

3864-576: 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 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

3933-502: The time of the Battle of Britain 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 . 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 ;

4002-409: 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 eliminated

4071-478: 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 to determine the aircraft's location. Known as " pip-squeak ", 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

4140-399: 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 " across

4209-438: 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

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4278-507: 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 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

4347-464: Was also being planned. This consisted of 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

4416-417: 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

4485-427: 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

4554-440: 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

4623-413: 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

4692-520: 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 in November 1940. US researchers were already working on their own IFF system of some complexity. They realised

4761-471: 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 radar systems, causing the aircraft's "blip" to extend on the radar display , identifying the aircraft as friendly. Mark   I had the problem that the gain had to be adjusted in flight to keep it working; in the field, it was correct only half

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