Overview
Primary radar
The rapid wartime development of radar had obvious applications forSecondary radar
The need to be able to identify aircraft more easily and reliably led to another wartime radar development, theStandards and specifications
TheOperation
The purpose of SSR is to improve the ability to detect and identify aircraft while automatically providing theInterrogation modes
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 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 inDeficiencies
A number of problems are described in an ICAO publication of 1983 entitled ''Secondary Surveillance Radar Mode S Advisory Circular''.ICAO Circular 174-AN/110 ''Secondary Surveillance Radar Mode S Advisory Circular''Mode A
Although 4,096 different identity codes available in 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 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.Mode C
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 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.FRUIT
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 the airspace, the amount of FRUIT generated will also increase.Garble
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.Synchronous garble
If two aircraft paths cross within about two miles slant range from 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.Capture
While an aircraft is replying to one ground interrogation it is unable to respond to another interrogation, reducing detection efficiency. For a Mode A or C interrogation the transponder reply may take up to 120 µs before it can reply to a further interrogation.Antenna
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 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 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.Stevens, M.C. "Multipath and interference effects in secondary surveillance radar systems", Proc. Inst.Electr. Eng., Part F, 128(1), 43–53, 1981Developments to address the deficiencies
The deficiencies in modes A and C were recognised quite early in the use of SSR and in 1967 Ullyatt published a paper and in 1969 an expanded paper,Ullyatt, C. ''Sensors for the ATC environment with special reference to SSR'', Electron. Civil Aviat., 3, C1–C3, 1969 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 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 were also considering similar problems but were assuming that 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 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.Stevens, M.C. ''Precision secondary radar'', Proc. Inst. Electr. Eng., 118(12), 1729–1735, 1971 The FAA engaged Lincoln Laboratory of MIT to further design the system and it produced a series of ATC Reports defining all aspects of the new joint development. Notable additions to the concept proposed by Ullyatt 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. Further the proposed aircraft identity code also comprised 24 bits with 16 million permutations. This allowed each aircraft to be wired with its own unique address. Blocks of addresses are allocated to different countries and further allocated to particular airlines so that knowledge of the address could identify a particular aircraft. 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''.Bowes R.C., Drouilhet P.R., Weiss H.G. and Stevens M.C., ''ADSEL/DABS – A Selective Address Secondary Surveillance Radar'',AGARD Conference Proceedings No. 188. 20th Symposium of the Guidance and Control Panel held in Cambridge, Massachusetts, USA, 20–23 May 1975 This was followed at a conference at ICAO Headquarters in Montreal, at which a low-power interrogation constructed 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 airbound transponders, again with modification. The best way of showing that this 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, which described the new system.Improved antenna
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 was to shape the vertical beam. This necessitated a vertical array of dipoles suitably fed to produce the desired shape. A five-foot vertical dimension was found to be optimum and this has become the international standard.Monopulse secondary surveillance radar
The new 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 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 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 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 today. 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 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.Stevens, M.C. S''urveillance in the Mode S Era'', CAA/IEE Symposium on ATC, London. March 1990 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 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 the existing SSRs by the 1990s and its accuracy provided for a reduction of separation minima in en-route ATC from to 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 the delay of Mode S.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 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 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 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 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 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 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 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 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 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 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 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 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 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.''Carriage of SSR Mode S Transponders for IFR Flights Operating as General Traffic'', www.caa.co.uk/docs/810/ Other registers are intended for use with TCAS and ADS-B. The Comm-B Data Selector (BDS) numbers are in hexadecimal notation.Extended squitter
Starting in 2009, the ICAO defined an "extended squitter" mode of operation; it supplements the requirements contained in ICAO Annex 10, Volumes III and IV. The first edition specified earlier versions of extended squitter messages: ;Version 0: Extends Mode S to deal with basic ADS-B exchanges, to add traffic information broadcast (TIS-B) format information, as well as uplink and downlink broadcast protocol information. ;Version 1: Better describes surveillance accuracy and integrity information (navigation accuracy category, navigation integrity category, surveillance integrity level), and additional parameters for TIS-B and ADS-B rebroadcast (ADS-R). ;Version 2: The second edition introduced yet a new version of extended squitter formats and protocols to:See also
*References
Further reading
;Industry specificationsExternal links