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 submarines, as well as in various adaptations for secondary purposes like search and rescue . 500 units were also supplied to the Soviet Union during the war.
130-412: Mark III replaced the earlier Mark II which had been in service since 1940. Mark II had an antenna that received signals from radar systems, amplified them, and returned them. This caused the blip on the radar display to become larger, indicating a friendly aircraft. As the number of radar systems on different frequencies proliferated through the mid-war period, the number of models of Mark II had to do
260-579: A Stirling bomber came back from a raid on the Ruhr. It got lost, and it was assumed to be hostile. Two Beaufighters went to intercept it. One of them shot it down, and then it was itself shot down by the other Beaufighter. Two aircraft and a dozen lives lost! What are you going to do about it? They responded by working night and day until the system was completed, which was "introduced quickly" and went into production at Ferranti in Manchester. A big trial
390-401: A "difference" beam. To produce the sum beam the signal is distributed horizontally across the antenna aperture. This feed system is divided into two equal halves and the two parts summed again to produce the original sum beam. However the two halves are also subtracted to produce a difference output. A signal arriving exactly normal, or boresight, to the antenna will produce a maximum output in
520-507: A 16.125 μs data block. This can include an indication of the interrogator transmitting the All-Call with the request that if the aircraft has already replied to this interrogator then do not reply again as aircraft is already known and a reply unnecessary. The Mode S interrogation can take three forms: The first five bits, known as the uplink field (UF) in the data block indicate the type of interrogation. The final 24 bits in each case
650-417: A Mode A or C interrogation the transponder reply may take up to 120 μs before it can reply to a further interrogation. The ground antenna has a typical horizontal 3 dB beamwidth of 2.5° which limits the accuracy in determining the bearing of the aircraft. Accuracy can be improved by making many interrogations as the antenna beam scans an aircraft and a better estimate can be obtained by noting where
780-565: 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
910-631: A certain pattern of pulses from the interrogator, and respond with a similarly custom set of pulses. This made it very difficult for an enemy to trigger the IFF without knowing the proper code. The fact that the Soviet Union had been supplied with 500 Mark III units was a serious concern for US Navy planners. It was assumed that the Soviets would use these units during the Korean War , and this caused
1040-412: A chance overlapping pulse from another ground interrogator. The interrogation may be short with P6 = 16.125 μs, mainly used to obtain a position update, or long, P6 = 30.25 μs, if an additional 56 data bits are included. The final 24 bits contain both the parity and address of the aircraft. On receiving an interrogation, an aircraft will decode the data and calculate the parity. If the remainder
1170-593: A directed turn by the aircraft. Primary radar is still used by ATC as a backup/complementary system to secondary radar, although its coverage and information is more limited. The need to be able to identify aircraft more easily and reliably led to another wartime radar development, the Identification Friend or Foe (IFF) system, which had been created as a means of positively identifying friendly aircraft from unknowns. This system, which became known in civil use as secondary surveillance radar (SSR), or in
1300-628: A few feet could cross a threshold and be indicated as the next increment up and a change of 100 feet. Smaller increments were desirable. Since all aircraft reply on the same frequency of 1090 MHz, a ground station will also receive aircraft replies originating from responses to other ground stations. These unwanted replies are known as FRUIT (False Replies Unsynchronized with Interrogator Transmissions or alternatively False Replies Unsynchronized In Time). Several successive FRUIT replies could combine and appear to indicate an aircraft which does not exist. As air transport expands and more aircraft occupy
1430-404: 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
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#17327901077191560-405: A long phase-modulated pulse. The ground antenna is highly directional but cannot be designed without sidelobes. Aircraft could also detect interrogations coming from these sidelobes and reply appropriately. However these replies can not be differentiated from the intended replies from the main beam and can give rise to a false aircraft indication at an erroneous bearing. To overcome this problem
1690-431: A low-power interrogation test by Lincoln Laboratory successfully communicated with an upgraded commercial SSR transponder of UK manufacture. The only thing needed was an international name. Much had been made of the proposed new features but the existing ground SSR interrogators would still be used, albeit with modification, and the existing aircraft transponders, again with modification. The best way of showing that this
1820-536: A means of providing continuous surveillance of air traffic disposition. Precise knowledge of the positions of aircraft would permit a reduction in the normal procedural separation standards, which in turn promised considerable increases in the efficiency of the airways system. This type of radar (called a primary radar ) can detect and report the position of anything that reflects its transmitted radio signals including, depending on its design, aircraft, birds, weather and land features. For air traffic control purposes this
1950-421: A mode A & C transponder as coming from an antenna sidelobe and therefore a reply is not required. The following long P6 pulse is phase modulated with the first phase reversal, after 1.25 μs, synchronising the transponder's phase detector. Subsequent phase reversals indicate a data bit of 1, with no phase reversal indicating a bit of value 0. This form of modulation provides some resistance to corruption by
2080-405: A mode A reply may seem enough, once particular codes have been reserved for emergency and other purposes, the number is significantly reduced. Ideally an aircraft would keep the same code from take-off until landing even when crossing international boundaries, as it is used at the air traffic control centre to display the aircraft's callsign using a process known as code/callsign conversion. Clearly
2210-405: A new frequency band, between 157 and 187 MHz, just below most VHF radars, was selected for this role. The only downside to this design is that the radar itself no longer provided the trigger signal for the transponder, so a separate transmitter and receiver was needed at the radar stations. The Mark III began to replace the Mark II in 1942 and 1943, in a somewhat lengthy switchover period. It
2340-527: A new pair of frequencies would be required. Ullyatt showed that the existing 1030 MHz and 1090 MHz frequencies could be retained and the existing ground interrogators and airbornes transponders, with suitable modifications, could be used. The result was a Memorandum of Understanding between the US and the UK to develop a common system. In the US the programme was called DABS (Discrete Address Beacon System), and in
2470-409: A primary radar) by transmitting a coded reply signal containing the requested information. Both the civilian SSR and the military IFF have become much more complex than their war-time ancestors, but remain compatible with each other, not least to allow military aircraft to operate in civil airspace. SSR can provide much more detailed information, for example, the aircraft altitude, as well as enabling
2600-416: A second pulse inserted in the other half of the bit period. Much more likely is that both halves are confused and the decoded bit is flagged as "low confidence". The reply also has parity and address in the final 24 bits. The ground station tracks the aircraft and uses the predicted position to indicate the range and bearing of the aircraft so it can interrogate again and get an update of its position. If it
2730-598: A single frequency, they were more like the original Mark I in a technical sense, but used the Mark III internals to gain all the advantages of the newer electronics and production capability. When the Blind Approach Beacon System (BABS) was introduced on 173.5 MHz, the ASV beacons had to move to 177 MHz. A similar system for RAF airfields was quickly adopted by the night fighters , operating on
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#17327901077192860-424: 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
2990-485: A single pulse, hence monopulse, but accuracy can be improved by averaging measurements made on several or all of the pulses received in a reply from an aircraft. A monopulse receiver was developed early in the UK Adsel programme and this design is still used widely. Mode S reply pulses are deliberately designed to be similar to mode A and C replies so the same receiver can be used to provide improved bearing measurement for
3120-467: A specific channel so that they could be sure they were receiving the signals from their interrogator and not an enemy broadcaster. The system also included many more variations on the return signal, which allowed ground operators to set a day code and then ignore signals that didn't respond with the proper code. At that time, Controller of Research and Development of the Navy was Admiral Ernest King , who put
3250-630: 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
3380-415: 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
3510-422: Is a radar system used in air traffic control (ATC), that unlike primary radar systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder , that reply to each interrogation signal by transmitting encoded data such as an identity code, the aircraft's altitude and further information depending on its chosen mode. SSR
3640-408: Is based on the military identification friend or foe (IFF) technology originally developed during World War II ; therefore, the two systems are still compatible. Monopulse secondary surveillance radar ( MSSR ), Mode S , TCAS and ADS-B are similar modern methods of secondary surveillance. The rapid wartime development of radar had obvious applications for air traffic control (ATC) as
3770-411: Is both an advantage and a disadvantage. Its targets do not have to co-operate, they only have to be within its coverage and be able to reflect radio waves, but it only indicates the position of the targets, it does not identify them. When primary radar was the only type of radar available, the correlation of individual radar returns with specific aircraft typically was achieved by the controller observing
3900-488: Is combined aircraft address and parity. Not all permutations have yet been allocated but those that have are shown: Similarly the Mode S reply can take three forms: The first five bits, known as the downlink field (DF) in the data block indicate the type of reply. The final 24 bits in each case is combined aircraft address and parity. Eleven permutations have been allocated. A transponder equipped to transmit Comm-B replies
4030-422: Is divided into two parts. If a 0.5 μs pulse occupies the first half and there is no pulse in the second half then a binary 1 is indicated. If it is the other way round then it represents a binary 0. In effect the data is transmitted twice, the second time in inverted form. This format is very resistant to error due to a garbling reply from another aircraft. To cause a hard error one pulse has to be cancelled and
IFF Mark III - Misplaced Pages Continue
4160-428: Is expecting a reply and if it receives one then it checks the remainder from the parity check against the address of the expected aircraft. If it is not the same then either it is the wrong aircraft and a re-interrogation is necessary, or the reply has been corrupted by interference by being garbled by another reply. The parity system has the power to correct errors as long as they do not exceed 24 μs, which embraces
4290-540: Is fitted with 256 data registers each of 56 bits. The contents of these registers are filled and maintained from on-board data sources. If the ground system requires this data then it requests it by a Surveillance or Comm-A interrogation. ICAO Annex 10 Volume III, Chapter 5 lists the contents of all those currently allocated. A reduced number are required for current operational use. Other registers are intended for use with TCAS and ADS-B. The Comm-B Data Selector (BDS) numbers are in hexadecimal notation. Starting in 2009,
4420-493: Is not the address of the aircraft then either the interrogation was not intended for it or it was corrupted. In either case it will not reply. If the ground station was expecting a reply and did not receive one then it will re-interrogate. The aircraft reply consists of a preamble of four pulses spaced so that they cannot be erroneously formed from overlapping mode A or C replies. The remaining pulses contain data using pulse position amplitude modulation . Each 1 μs interval
4550-475: Is not used), indicating aircraft altitude as indicated by its altimeter in 100-foot increments. Mode B gave a similar response to mode A and was at one time used in Australia. Mode D has never been used operationally. The new mode, Mode S, has different interrogation characteristics. It comprises pulses P1 and P2 from the antenna main beam to ensure that Mode-A and Mode-C transponders do not reply, followed by
4680-490: Is separately labelled with direction this information can be used to unscramble two overlapping mode A or C replies. The process is presented in ATC-65 "The ATCRBS Mode of DABS". The approach can be taken further by also measuring the strength of each reply pulse and using that as a discriminate as well. The following table compares the performance of conventional SSR, monopulse SSR (MSSR) and Mode S. The MSSR replaced most of
4810-425: 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
4940-419: 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
5070-606: 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
5200-728: The Pacific Theatre in WWII but it was never used in Europe. Bowden stayed on in the US, joining the NRL group in 1942 to begin development of the further improved Mark V, later known as the United Nations Beacon or UNB. This moved to even higher frequencies between 950 and 1150 MHz, dividing up this band into twelve discrete "channels". This allowed the ground operators to instruct the aircraft to change their transponder to
5330-460: The anti-aircraft cruiser HMS Delhi reported that over a period of a month they interrogated Mark I, Mark II, Mark IIG, Mark IIN and Mark III, as well as many friendly aircraft that displayed no IFF at all. Mark III was still considered a qualified success during this era. One of the few modifications to the basic Mark III was the Mark IIIG, also known as ARI.5131 in the UK or SCR-695 in
IFF Mark III - Misplaced Pages Continue
5460-423: The cavity magnetron operating in the 3 GHz range this process could not be continued. These frequencies required entirely different electronics to detect and amplify. It was at this point that Williams's suggestion was first taken seriously. During the development of the new Mark III in 1941, Vivian Bowden was in charge. Converting the Mark II to this new concept was straightforward; they simply removed all of
5590-487: 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
5720-497: The 212 MHz of the AI Mark IV they carried. To use the system, the aircraft would first fly in the rough direction of the airfield so their radar signals would hit the transponder. The transponder would then reply to the pulses of the fighter's radar, providing a powerful signal that could be received at ranges as great as 100 miles (160 km). The signal was received by two antennas that were aimed slightly left or right of
5850-636: The Air Traffic Control systems in all countries that may be visited. Volume III, Part 1 is concerned with digital data communication systems including the data link functions of Mode S while volume IV defines its operation and signals in space. The American Radio Technical Commission for Aeronautics (RTCA) and the European Organization for Civil Aviation Equipment (Eurocae) produce Minimum Operational Performance Standards for both ground and airborne equipment in accordance with
5980-559: The IFF Mark II began in October 1939 and the first units were available in early 1940. This used a complex mechanical system to select among several separate radio tuners and sweep through each one's band of frequencies, ensuring it would hear the radar signal from any of the systems in service at some point in the 10 second cycle. Mark II was the first system to be operationally deployed, and was widespread by late 1940. Even as Mark II
6110-628: 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
6240-469: 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
6370-451: The SSR interrogation signal and transmits a reply on 1090 MHz that provides aircraft information. The reply sent depends on the interrogation mode. The aircraft is displayed as a tagged icon on the controller's radar screen at the measured bearing and range. An aircraft without an operating transponder still may be observed by primary radar, but would be displayed to the controller without
6500-418: The SSR mode A and C system with the advantage that the interrogation rate can be substantially reduced thereby reducing the interference caused to other users of the system. Lincoln Laboratory exploited the availability of a separate bearing measurement on each reply pulse to overcome some of the problems of garble whereby two replies overlap making associating the pulses with the two replies. Since each pulse
6630-429: The UK Adsel (Address selective). Monopulse, which means single pulse, had been used in military track-and-follow systems whereby the antenna was steered to follow a particular target by keeping the target in the centre of the beam. Ullyatt proposed the use of a continuously rotating beam with bearing measurement made wherever the pulse may arrive in the beam. The FAA engaged MIT Lincoln Laboratory to further develop
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#17327901077196760-402: The US as the air traffic control radar beacon system (ATCRBS), relies on a piece of equipment aboard the aircraft known as a " transponder ." The transponder is a radio receiver and transmitter pair which receives on 1030 MHz and transmits on 1090 MHz. The target aircraft transponder replies to signals from an interrogator (usually, but not necessarily, a ground station co-located with
6890-469: The US. This combined the normal Mark III transponder with a second one tuned to the frequency of the newer ground control radars, notably the AMES Type 7 at 209 MHz. A motorized switch was used to turn on the second frequency for 1 ⁄ 5 of a second, once every second. This produced a signal similar to the one from the original Mark I but because Type 7 used a plan-position indicator display,
7020-459: The address could readily identify them also. The Lincoln Laboratory report ATC 42 entitled Mode S Beacon System: Functional Description gave details on the proposed new system. The two countries reported the results of their development in a joint paper, ADSEL/DABS – A Selective Address Secondary Surveillance Radar . This was followed at a conference at ICAO Headquarters in Montreal, at which
7150-455: The aircraft switched on its radar, and then only for a few minutes during the drop, they were very secure as German radio operators did not have much time to use a radio direction finder on the signals. A similar system was introduced in 1943 as "Walter". This was a small version of the beacon system that was carried aboard aircraft life rafts and activated if they were forced down on water. This allowed search and rescue aircraft to home in on
7280-509: 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
7410-428: The aircraft was not upside down. The other advantage was that the return pulse no longer had to be short or singular. With Mark II the IFF signals were displayed on the same display as the radar signals, so if the IFF returned too many of these signals or ones that were too long, they could hide the blips from other aircraft on the display. With Mark III, the signal was separately received and did not have to be sent to
7540-463: The aircraft. A third pulse, P2, is for side lobe suppression and is described later. Not included are additional military (or IFF) modes, which are described in Identification Friend or Foe . A mode-A interrogation elicits a 12-pulse reply, indicating an identity number associated with that aircraft. The 12 data pulses are bracketed by two framing pulses, F1 and F2. The X pulse is not used. A mode-C interrogation produces an 11-pulse response (pulse D1
7670-515: The airspace, the amount of FRUIT generated will also increase. FRUIT replies can overlap with wanted replies at a ground receiver, thus causing errors in extracting the included data. A solution is to increase the interrogation rate so as to receive more replies, in the hope that some would be clear of interference. The process is self-defeating as increasing the reply rate only increases the interference to other users and vice versa. If two aircraft paths cross within about two miles slant range from
7800-558: The attention of a night fighter that just happened to be flying in the area and saw an odd return on their display. When the Co-Operation Command observers complained that it was a setup, their Blenheim repeated the trick a second time after the transponder was moved. Further development of this basic concept led to the Rebecca/Eureka transponding radar system. The only major change to the original beacon concept
7930-416: The axis. This made the signals both easier to see as well as allowing them to be modified in order to identify individual aircraft or provide security. Another problem that had been seen in Mark II as the number of radar sets in use increased was that the number of interrogation signals being received began to swamp the transponder's ability to reply. A related issue made tracking distant targets difficult; in
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#17327901077198060-434: The bearing of the aircraft thereby reducing to one the number of interrogations/replies per aircraft on each scan of the antenna. Further each interrogation would be preceded by main beam pulses P1 and P2 separated by 2 μs so that transponders operating on modes A and C would take it as coming from the antenna sidelobe and not reply and not cause unnecessary FRUIT. The FAA was considering similar problems but assumed that
8190-399: The benefit of SSR derived data. It is typically a requirement to have a working transponder in order to fly in controlled air space and many aircraft have a back-up transponder to ensure that condition is met. There are several modes of interrogation, each indicated by the difference in spacing between two transmitter pulses, known as P1 and P3. Each mode produces a different response from
8320-453: The case where two aircraft were being interrogated by a single radar, their responses would not overlap because the more distant aircraft was not triggered until the signal reached it at a later time. However, if the nearer aircraft was being interrogated by more than one radar, its responses to those other radars might occur at the same time as the other aircraft's response to the first, masking it. Mark III fixed both of these problems. The first
8450-537: 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
8580-401: The complex mechanical switch and multiple tuners. At the time it was not considered a serious enough problem to warrant a change, instead the Mark II's would be given different tuners depending on which radars they were expected to encounter. It was not long before there was a profusion of different versions of the Mark II covering different combinations of radars. After the 1941 introduction of
8710-544: 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. Secondary surveillance radar Secondary surveillance radar ( SSR )
8840-532: The concern that an aircraft carrier might find itself being attacked by a group of planes displaying proper IFF responses. In May 1951, the US Far East Air Force ordered its units to not assume an aircraft displaying Mark III was friendly. By this time the US had already begun switching over to Mark X although this caused just as much confusion as the switch to Mark III. The British and Commonwealth ships had not yet begun this conversion. The result
8970-522: The crews to adjust the strength of the return signal while the aircraft was on the ground (or on the deck of an aircraft carrier ) and no adjustments were needed in flight. This greatly improved the reliability of the system. Shortly after Bowden took over the development of the Mark III, he was summoned by the Commander in Chief, Fighter Command, Hugh Dowding . Dowding stated Well, last Saturday night,
9100-685: The delay of Mode S. A more detailed description of Mode S is given in the Eurocontrol publication Principles of Mode S and Interrogator Codes and the ICAO circular 174-AN/110 Secondary Surveillance Radar Mode S Advisory Circular . The 16 million permutations of the 24-bit aircraft address codes have been allocated in blocks to individual states and the assignment is given in ICAO Annex 10, Volume III, Chapter 9. A mode S interrogation comprises two 0.8 μs wide pulses, which are interpreted by
9230-594: The direct exchange of data between aircraft for collision avoidance. Most SSR systems rely on Mode C transponders, which report the aircraft pressure altitude . The pressure altitude is independent of the pilot's altimeter setting , thus preventing false altitude transmissions if altimeter is adjusted incorrectly. Air traffic control systems recalculate reported pressure altitudes to true altitudes based on their own pressure references, if necessary. Given its primary military role of reliably identifying friends, IFF has more secure (encrypted) messages to prevent "spoofing" by
9360-672: The direction of travel, and by comparing the length of the resulting blips on the radar display , the operator could tell the pilot which direction to turn to point the nose directly at it. In June 1941, a battery-powered version of the same equipment was used by Robert Hanbury Brown in a demonstration for the RAF Army Co-Operation Command . He told them to hide the transponder anywhere within 15 miles (24 km) of their HQ in Bracknell . Not only did their RAF Bristol Blenheim easily find it, but it also attracted
9490-583: The downed aircraft from very long range. In practice these proved useful but variable; the system had to be small and lightweight which made its performance less than ideal. Around the same time, the ground-based homing beacons were placed in Coastal Command aircraft operating in the Mediterranean area. These installations were known as "Rooster". Aircraft flying patrols would not attack targets directly, but instead turn on its Rooster and follow
9620-402: The duration of a mode A or C reply, the most expected source of interference in the early days of Mode S. The pulses in the reply have individual monopulse angle measurements available, and in some implementations also signal strength measurements, which can indicate bits that are inconsistent with the majority of the other bits, thereby indicating possible corruption. A test is made by inverting
9750-655: The enemy, and is used on many types of military platforms including air, sea and land vehicles. The International Civil Aviation Organization (ICAO) is a specialized agency of the United Nations headquartered in Montreal, Quebec , Canada. It publishes annexes to the Convention and Annex 10 addresses Standards and Recommended Practices for Aeronautical Telecommunications. The objective is to ensure that aircraft crossing international boundaries are compatible with
9880-536: The energy is directed at the ground where it is reflected back up, and interferes with, the upward energy causing deep nulls at certain elevation angles and loss of contact with aircraft. Second, if the surrounding ground is sloping, then the reflected energy is partly offset horizontally, distorting the beam shape and the indicated bearing of the aircraft. This was particularly important in a monopulse system with its much improved bearing measurement accuracy. The deficiencies in modes A and C were recognised quite early in
10010-621: The equipment bay of the aircraft. The purpose of SSR is to improve the ability to detect and identify aircraft while automatically providing the Flight Level (pressure altitude) of an aircraft. An SSR ground station transmits interrogation pulses on 1030 MHz (continuously in Modes A, C and selectively, in Mode S) as its antenna rotates, or is electronically scanned, in space. An aircraft transponder within line-of-sight range 'listens' for
10140-405: The existing SSRs by the 1990s and its accuracy provided for a reduction of separation minima in en-route ATC from 10 nautical miles (19 km; 12 mi) to 5 nautical miles (9.3 km; 5.8 mi) MSSR resolved many of the system problems of SSR, as changes to the ground system only, were required. The existing transponders installed in aircraft were unaffected. It undoubtedly resulted in
10270-481: The existing tuner equipment and replaced it with a much simpler one tuned to a single band. The chosen band was 157 to 187 MHz, which the motorized tuner swept through every two seconds. Things were not so simple on the radar station's side. Since the radar signal itself was no longer the trigger for the IFF transceiver, a new transmitter had to be added, known in British terminology as an interrogator . To ensure
10400-523: The frequencies used by the German Würzburg radar . There were concerns that a Würzburg might trigger the Mark IV and cause a reply on their display, immediately revealing the presence of the system and its working frequencies. For this reason, the Mark IV was held in reserve in case Mark III was compromised. This did occur very late in the war but too late to be a concern. Some Mk. IVs were used in
10530-548: The ground antenna is provided with a second, mainly omni-directional, beam with a gain which exceeds that of the sidelobes but not that of the main beam. A third pulse, P2, is transmitted from this second beam 2 μs after P1. An aircraft detecting P2 stronger than P1 (therefore in the sidelobe and at the incorrect main lobe bearing), does not reply. A number of problems are described in an ICAO publication of 1983 entitled Secondary Surveillance Radar Mode S Advisory Circular . Although 4,096 different identity codes available in
10660-402: The ground interrogator, their replies will overlap and the interference caused will make their detection difficult. Typically the controller will lose the longer range aircraft, just when the controller may be most interested in monitoring them closely. While an aircraft is replying to one ground interrogation it is unable to respond to another interrogation, reducing detection efficiency. For
10790-448: The ground station and 493.5 MHz for the reply from the aircraft. This separation of frequencies meant that separate transmitters and receivers had to be used, making the sets more complex but had the significant advantage that a response from one aircraft could not trigger IFF units in nearby aircraft. As the Mark II and Mark III went into service, the NRL design was given the name Mark IV. The selected frequency happened to be close to
10920-486: The highest possible national priority on the development of UNB. To house the development team, a new 60,000 square feet (5,600 m) building was constructed by a huge work gang working 24 hours. In contrast to the development of Mark III, which had a team of a few dozen, UNB's team was over ten times that. The first systems were available in August 1944 but the end of the war in 1945 ended major effort. Testing continued and
11050-523: 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
11180-473: 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
11310-404: The name " secondary radar ", which remains in use to this day. This change also realized two additional advantages. Radar signals were typically horizontally polarized which improved the interaction with the ground or sea surface. This also meant the antenna on the aircraft should ideally be horizontal as well. This was not easy to arrange, on the Supermarine Spitfire , for instance, the antenna
11440-459: 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
11570-418: 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
11700-498: The position of any aircraft carrying a Mark III transponder. A less important problem was that as electronics improved it became possible to move to higher frequencies in the UHF region, which allowed for smaller antennas and thus less drag on the aircraft. The US Naval Research Laboratory (NRL) had already been working on IFF-like devices before being introduced to the Mark II. Their system used separate frequencies of 470 MHz from
11830-473: 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
11960-418: 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
12090-406: 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
12220-468: The reflection of the station's own broadcast signal, and was more powerful. The result was that the aircraft's blip on the radar display grew larger and stretched out. The same blip would be produced if the radar was tracking a group of targets in formation, so the transponder also had a motorized switch that turned the signal on and off, causing the blip to oscillate on the Chain Home display. Mark I
12350-449: The replies started and where they stopped, and taking the centre of the replies as the direction of the aircraft. This is known as a sliding window process. The early system used an antenna known as a hogtrough . This has a large horizontal dimension to produce a narrow horizontal beam and a small vertical dimension to provide coverage from near the horizon to nearly overhead. There were two problems with this antenna. First, nearly half
12480-442: The reply process on receipt of pulse P3. However a Mode S transponder will abort this procedure upon the detection of pulse P4, and instead respond with a short Mode S reply containing its 24 bit address. This form of All-Call interrogation is now not much used as it will continue to obtain replies from aircraft already known and give rise to unnecessary interference. The alternative form of All-Call uses short Mode S interrogation with
12610-515: The result was a series of small blips on either side of the target return. This was known as the "crown of thorns". A further version, Mark IIIQ or ARI.5640, does not appear to have been deployed. James Rennie Whitehead used the Mark III electronics to produce beacons that responded on the 176 MHz frequency of ASV Mk. II radar . These were placed at naval bases and Fleet Air Arm airfields, allowing aircraft to use their anti-shipping radars as long-range navigation systems. As they only responded to
12740-410: The same display. Generally, the signal was sent through an inverter and then sent to a second channel on the radar's cathode ray tube . The result was a normal radar display on top (or bottom) half of the screen, and a second similar display below it (or above) with the IFF signals only. This allowed the Mark III to send back longer pulses as they no longer overlapped aircraft reflections which were above
12870-503: The same mode A code should not be given to two aircraft at the same time as the controller on the ground could be given the wrong callsign with which to communicate with the aircraft. The mode C reply provides height increments of 100 feet, which was initially adequate for monitoring aircraft separated by at least 1000 feet. However, as airspace became increasingly congested, it became important to monitor whether aircraft were not moving out of their assigned flight level. A slight change of
13000-443: The same. Aircraft could never be sure their IFF would respond to the radars they flew over. Freddie Williams had suggested using a single separate frequency for IFF as early as 1940, but at that time the problem had not become acute. The introduction of microwave radars based on the cavity magnetron was the main impetus for adopting this solution, as the Mark II could not easily be adapted to respond on these frequencies. In 1942,
13130-443: The signals remained in synchronicity with the radar, the interrogator had a trigger input that was fed a small amount of the radar signal so that the ground station sent out its interrogation pulse at the same time as the main radar signal. The aircraft's transponder received and rebroadcast the interrogation pulse. This signal was received by the respondor at the radar station. The second transmitter and receiver quickly gave rise to
13260-435: The standards specified in ICAO Annex 10. Both organisations frequently work together and produce common documents. ARINC (Aeronautical Radio, Incorporated) is an airline run organisation concerned with the form, fit and function of equipment carried in aircraft. Its main purpose is to ensure competition between manufacturers by specifying the size, power requirements, interfaces and performance of equipment to be located in
13390-411: The state of some or all of these bits (a 0 changed to a 1 or vice versa) and if the parity check now succeeds the changes are made permanent and the reply accepted. If it fails then a re-interrogation is required. Mode S operates on the principle that interrogations are directed to a specific aircraft using that aircraft's unique address. This results in a single reply with aircraft range determined by
13520-471: The sum beam but a zero signal in the difference beam. Away from boresight the signal in the sum beam will be less but there will be a non-zero signal in the difference beam. The angle of arrival of the signal can be determined by measuring the ratio of the signals between the sum and difference beams. The ambiguity about boresight can be resolved as there is a 180° phase change in the difference signal either side of boresight. Bearing measurements can be made on
13650-643: The system and it produced a series of ATC Reports defining all aspects of the new joint development. Added to Ullyatt's concept was the use of a more powerful 24-bit parity system using a cyclic redundancy code , which not only ensured the accuracy of the received data without the need for repetition but also allowed errors caused by an overlapping FRUIT reply to be corrected. A proposed aircraft identity code comprised 24 bits with 16 million permutations. This allowed each aircraft to be assigned its own unique address. Blocks of addresses are allocated to different countries and further allocated to particular airlines so that
13780-459: 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
13910-502: The target. This made a blip appear on all of the other aircraft's ASV Mark II radar displays which they would then use to find the indicated area. These IIF Mark IIIG(R) (for Rooster) allowed the aircraft to converge en masse . Although very successful, Mark III had problems of its own. Primary among them was that it would respond to any signal across a wide variety of frequencies around 180 MHz. An enemy who knew this could send out random signals on this band and receive signals about
14040-503: 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 ;
14170-472: The time taken to receive the reply and monopulse providing an accurate bearing measurement. In order to interrogate an aircraft its address must be known. To meet this requirement the ground interrogator also broadcasts All-Call interrogations, which are in two forms. In one form, the Mode A/C/S All-Call looks like a conventional Mode A or C interrogation at first and a transponder will start
14300-409: 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
14430-480: 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
14560-400: 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
14690-500: The use of SSR and in 1967 Ullyatt published a paper and in 1969 an expanded paper, which proposed improvements to SSR to address the problems. The essence of the proposals was new interrogation and reply formats. Aircraft identity and altitude were to be included in the one reply so collation of the two data items would not be needed. To protect against errors a simple parity system was proposed – see Secondary Surveillance Radar – Today and Tomorrow . Monopulse would be used to determine
14820-398: Was a friendly fire incident on 23 June 1950 when HMS Hart opened fire on two P-51 Mustangs when bombs were dropped nearby. In July 1951, Scott-Moncrieff stated that "identification has been one of the more unsatisfactory features of this war" and in August the decision was made to treat all aircraft as friendly to avoid friendly-fire incidents. IFF Mark II IFF Mark II
14950-453: 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
15080-406: Was addressed by adding a delay so the transponder responded only after receiving 4, 5 or 6 pulses. The second was somewhat more complex; as the interrogation rate increased, the Mark III began to lower its output signal, so that more distant aircraft signals were not masked. The new design also included a number of detail improvements, most notably a new power supply for the transponder. This allowed
15210-415: Was also used as the basis for several other transponder systems such as Walter and Rebecca/Eureka , which allowed suitably equipped aircraft to home in on locations on the ground. These found use for dropping paratroopers and supplies in Europe, locating downed aircraft, and other roles. Several newer IFF designs were trialled, but none of them offered enough of an advantage to warrant a switchover. Mark III
15340-417: Was an evolution not a revolution was to still call it SSR but with a new mode letter. Mode S was the obvious choice, with the S standing for select. In 1983 ICAO issued an advisory circular describing the new system. The problem with the existing standard "hogtrough" antenna was caused by the energy radiated toward the ground, which was reflected up and interfered with the upwards directed energy. The answer
15470-402: Was being deployed, it was clear that the number of radars being introduced would shortly present a problem even for that system. In 1940 Freddie Williams had suggested that the IFF systems should work on their own frequency band instead of trying to listen for every possible radar that might come along. This would also have the advantage that the radio electronics would be much simpler, eliminating
15600-486: Was building more IFF units than all other radars in the US combined. IFF only works if the aircraft being queried is carrying it; this makes the switchover from one IFF to another a difficult affair as it has to be carried out all-or-nothing in any given area of operations. This was almost impossible to arrange and led to great confusion. For instance, during the Operation Avalanche period in September 1943,
15730-542: Was carried out in Pembrokeshire with transponders installed in all sorts of aircraft. This successful demonstration was one of the reasons the US Army Air Force selected Mark III for their own aircraft, instead of their own designs that were somewhat more sophisticated. This led to a massive production effort in the US, where Bowden was sent to help get things started. At one point, Hazeltine Corporation
15860-472: Was completed in 1948. Mark III was finally replaced in the early 1950s by the IFF Mark X . This moved to even higher frequencies, 1030 MHz for interrogation and 1090 MHz for replies. Using separate frequencies helped reduce crosstalk between the electronics. Later versions included the "Selective Identification Feature" (or "Facility"), or SIF for short. This introduced the ability to respond only to
15990-423: Was replaced by IFF Mark X over an extended time starting in 1952. IFF Mark I was the first IFF system to see experimental use, with a small number of units installed in 1939. Mark I was a simple system that listened for signals on the 5 meter band used by Chain Home radars and responded by sending out a short pulse on the same frequency. At the Chain Home station, this signal would be received slightly after
16120-585: 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
16250-423: Was stretched along the fuselage toward the tail, and only operated properly if the aircraft was flying roughly perpendicular to the radar so the antenna was visible. With the move to a separate transmitter, the signal could be vertically polarized instead. Mark III antennas were a simple quarter-wave unipole projecting down from the bottom of the aircraft, which provided excellent omnidirectional reception as long as
16380-412: 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
16510-430: Was to respond on a second frequency, to avoid the noise created by the original radar signal reflecting off the ground. This required a similar change in the radar to receive this second frequency. The transponders, known as Eureka, were dropped to resistance groups in occupied Europe, allowing them to accurately guide Rebecca-equipped aircraft dropping supplies and agents. Since the system did not broadcast any signals until
16640-442: Was to shape the vertical beam. This necessitated a vertical dipole array suitably fed to produce the desired shape. A five-foot vertical dimension was found to be optimum and it has become the international standard. The Mode S system was intended to operate with just a single reply from an aircraft, a system known as monopulse. The accompanying diagram shows a conventional main or "sum" beam of an SSR antenna to which has been added
16770-471: 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
16900-524: Was used only experimentally, with about 50 sets completed in total. The problem with Mark I was that it operated only on the 23 MHz Chain Home frequency. By 1939 there were already several other radars being introduced that operated on different frequencies, notably the 75 MHz used by the GL Mk. I radar and the 43 MHz used by the Royal Navy 's Type 79 radar . To address this, development of
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