Secondary surveillance radar

SSR-antenna of Deutsche Flugsicherung at Neubrandenburg, in Mecklenburg/Western Pommerania

Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), which not only detects and measures the position of aircraft but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems, which measure only the range and bearing of targets by detecting reflected radio signals, rather like seeing an object in a beam of light, SSR relies on its targets being equipped with a radar transponder, which replies to each interrogation signal by transmitting its own response containing encoded data. SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II, and the two systems are still compatible today. Monopulse secondary surveillance radar (MSSR), Mode S, TCAS and ADS-B are modern improved versions of SSR.

Overview

Primary radar

The rapid wartime development of radar had obvious applications for air traffic control (ATC) as 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 (now 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 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 a directed turn by the aircraft. Primary radar is still used by ATC today as a backup/complementary system to secondary radar, although its coverage and information is more limited.

Secondary radar

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 enemy. This system, which became known in civil use as secondary surveillance radar (SSR) or in the USA 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 which receives on one frequency (1030 MHz) and transmits on another (1090 MHz). The target aircraft's transponder replies to signals from an interrogator (usually, but not necessarily, a ground station co-located with 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 now provide much more detailed information, for example, the aircraft's altitude, and it also permits the exchange of data directly between aircraft for collision avoidance. Given its primary military role of reliably identifying friends, IFF has much more secure (encrypted) messages to prevent "spoofing" by the enemy, and also is used on all kinds of military platforms including air, sea and land vehicles.

Standards and specifications

The International Civil Aviation Organization (ICAO) is a branch of the United Nations and its headquarters are in Montreal. It publishes annexes to the Convention and Annex 10 addresses Standards and Recomended Practices for Aeronautical Telecommunications with volume IV being concerned with SSR. The objective is to ensure that aircraft crossing international boundaries are compatible with the Air Traffic Control systems in all countries that may be visited. It is mainly concerned with signals in space and the data contained in those signals.

The American Radio Technical Commission for Aeronautics (RTCA) and the European European Organization for Civil Aviation Equipment (Eurocae) produce Minimum Operational Performance Standards for both ground and airborne equipment in accordance with the standards specified in 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 the equipment bay of the aircraft.

Operation

The purpose of SSR is to improve the ability to detect and identify aircraft while it additionally provides automatically the Flight Level (pressure altitude) of a flight. An SSR continuously transmits interrogation pulses (continuously in Modes A and C and selectively in Mode S) as its antenna rotates, or is electronically scanned in space. A transponder on an aircraft that is within line-of-sight range 'listens' for the SSR interrogation signal and sends back a reply that provides aircraft information. The reply sent depends on the mode that was interrogated (see below). The aircraft is then 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 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 this condition is met.

A cross-band beacon is used, which simply means that the interrogation pulses are at one frequency (1030 MHz) and the reply pulses are at a different frequency (1090 MHz).

Interrogation Modes

Main article: Aviation transponder interrogation modes

There are several modes of interrogation each requiring a different response from the aircraft. The mode is indicated by the difference in spacing between two transmitter pulses, known as P1 and P3 according to the table below. Not included are additional military modes.

Mode A and C interrogation format

Mode	Pulse spacing	Purpose
A	8µS	identity
B	17µS	identity
C	21µS	Altitude
D	25µS	undefined
S	2µS	multipurpose

Sum and Control antenna beams

A mode A interrogation elicits a 12 pulse reply indicating an identity number associated with that aircraft - see diagram below. 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 is not used) indicating aircraft altitude as indicated by its altimiter 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, see below. It comprises a P1 and P2 pulses from the antenna main beam to ensure that Mode A and C transponders do not reply followed by a long, phase modulated, pulse as described below.

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 the ground antenna is provided with a second, mainly onmidirectional, 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 2µS after P1 and an aircraft detecting P2 stronger than P1 does not reply.

Deficiencies

Mode A

Mode A and C reply format

Although 4096 different identity codes available in a mode A reply may seem enough, but once particular codes have been reserved for emergency and other special purposes, the number is significantly reduced. Ideally an aircraft would keep the same code from take-off until landing 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 aircrafts at the same time as the controller on the ground could be given the wrong callsign with which to communicate with the aircraft. There have also been cases where an aircraft was delayed from takeoff as no mode A code for that route was available until another aircraft had landed.

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 aircrafts were not moving out of their assigned flight level. A slight change of a few feet could cross a treshold and be indicated as the next increment up and a change of 100 feet. Smaller increments were desireable.

Fruit

Since all aircraft reply on the same frequency of 1090 MHz, a ground station will also receive 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 exists. 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.

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 former 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.

Antenna

Original SSR antenna providing a narrow horizontal beam and a wide vertical beam

Regions of weak signal due to ground reflection

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 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 cover from close to the horizon to nearly overhead. There were two problems. Firstly nearly half the energy is directed into the ground where it is then reflected back up and to interfer with the upward energy causes deep nulls at certain elevation angles and loss of contact with aircraft. Secondly 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.

Developments to address the above deficiences

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 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 collelation of the two data items would not be needed. Monopulse would be used to determine the bearing of the aircraft thereby reducing to one the number of interrogations/replies per aircarft on each scan of the antenna. Further each interrogation would be preceeded by main beam pulses P1 and P2 separated by 2µS 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 a 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 measured made wherever the pulse may arrive in the beam.

The FAA engaged Lincoln Laboratory of MIT to further design the system. Frequent meeting between the two countries resulted in Lincoln Laboratory producing a series of ATC Reports defining all aspects of the new joint development. Noteable additions to the concept proposed by Ullyatt was the use of a 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 can identify down to a particular aircraft. The Lincoln Laboratory report ATC 42 gave details on the proposed new system.

The two countries reported the results of their development in a joint paper. This was followed at a conference at ICAO Headquarters in Montreal at which a low power interrogation constructed by Lincoln Laboratory successfully communicated with a upgraded commercial SSR transponder of UK manufacture.

Comparison of the vertical beam shapes of the old and new antennas

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 showning 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 selective address.

Antenna

The problem with the existing standard "hogtrough" antenna was due to the energy directed towards the ground which was then reflected up to interfere 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. An example of such an antenna is shown at the head of this article.

Monopulse secondary surveillance radar

Antenna main beam with difference beam

As explained above 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 beam of an SSR antenna to which has been added a "difference" beam. The produce the main, or "sum" beam the signal is distributed horizontally across the antenna aperture. This feed system is divided into two equal halves and then added to produce the original sum beam. However it is 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 used widely today. Mode S reply pulses are deliberately designed to be similar to replies to the existing mode A and C interrogations 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.

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 relies. The process is presented in ATC-78, one of the reports from the US DABS program. 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.

	Standard SSR	Monopulse SSR	Mode S
Replies per scan	20 - 30	4 - 8	1
Range accuracy	230m rms	13m rms	7m rms
Bearing accuracy	0.08° rms	0.04° rms	0.04° rms
Height resolution	100 ft	100 ft	25 ft
Garble resistance	poor	good	best
Data capacity (uplink)	0	0	56 - 1280 bits
Data capacity (downlink)	23 bits	23 bits	56 - 1280 bits
Identity permutations	4096	4096	16 million

The MSSR replaced most of the existing SSRs by the 90s and its accuracy provided for a reduction of separation minima in en-route ATC from 10 nautical miles (19 km) to 5 nautical miles (9.3 km)

MSSR resolved many of the system problems of SSR as only changes to the ground system were required and the existing transponders installed in aircraft unaffected. It undoubtedly resulted in the delay of Mode S.

MSSR can reduce garbling in multi-radar environments.

Mode S

Mode S interrogation, short and long

Mode S reply, short and long

Mode S All-Call interrogation

A mode S interrogation comprises two 0.8µS 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 1.25µS, synchronising the transponder's phase detector. Subsequent phase reversals indicate a data bit of 1 with no phase reversal a bit of 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 = 16.125µS, mainly used to obtain a position update, or long, P6 = 30.25µS, if n 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 1µS interval is divided into two parts if 0.5µS pulse occupies the first half and no pulse the second half then a binary 1 is indicated. If it is the other way round then it represents a binary 0. In affect the dated is transmitted twice, once 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 on its position. It is expecting a reply and if it receives one it checks the remainder from the parity check against the address of the expected aircraft. If this check fails 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 24µS, 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 which 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 specific aircraft using that aircraft's unique address. This normally 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 interrogate an aircraft its address must be known. To meet this requirement the ground interrogator also broadcasts All-Call interrogations. To a mode A & C transponder this looks like a conventional interrogation and will start the reply process on receipt of pulse P3. However a Mode S transponder will abort this process 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 giving rise to unnecessary interference. Instead a form of short Mode S interrogation can be used which can contain an indication of the ground interrogator transmitting the request. "If you have already told me that you are there please do not tell me again".

TCAS

Main article: Traffic collision avoidance system

ADS-B

Main article: Automatic dependent surveillance-broadcast

See also

• Air Traffic Control Radar Beacon System, all encompassing description.
• Automatic Dependent Surveillance-Broadcast, Free Flight enhancement.
• Automatic Independent Surveillance-Protocol, Free Flight enhancement with ATCRBS transponder TSO corrections.

External links

• Radar Basics