Secondary surveillance radar

Secondary surveillance radar is a radar system used in air t'raffic control, that unlike Primary S'urveillance 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 the Mode that an interrogation pulse code corresponds to, by transmitting a pulse telegram containing, e.g. the identity code as 4 digit octal number in Mode A, the aircraft's altitude from the barometric pressure sensor of an aircraft in Mode C and a unique 24-bit address and further information in other Modes. SSR is based on the military Identification Friend or Foe initially standardized in the U.S. and later adopted by NATO '', however first IFF systems date back to World War II. SSR Mode A, C and S are compatible with IFF Mode 3A, C and S. SSR Mode B and D are not specified in ICAO Annex 10 any more for use today. Additional SSR Mode S based systems are ADS-B, TCAS, Multilateration Systems and satellite based surveillance using ADS-B messages.

Overview

Primary radar

The rapid wartime development of Primary S'urveillance Radar had obvious applications for Air Defense for the detection of approaching enemies as well as Air T'raffic Control as a means of providing continuous detection of surveillance of air traffic disposition. Precise knowledge of the positions of enemy aircraft allowed AD to guide fighter aircraft towards unfriendly aircraft, while for ATC it permitted a reduction in the normal procedural separation standards which in turn promised considerable increases in the efficiency of the airways system as well as during approach to air fields for Ground C'ontrolled Approach by using Precision A'pproach Radar. ^{7.8, p.248 ff.}
PSR can detect and report the position of any object that reflects a sufficient amount of the transmitted radar energy back towards a PSR Radar sensor. Depending on the PSR design moving and/or stationary objects, e.g. aircraft, ships, birds, rain-clouds, other weather phenomena, land features or man made objects, can be detected. For air traffic control purposes this is both an advantage and a disadvantage. The advantage is that targets do not have to co-operate, they only have to be within its coverage and be able to reflect radio waves, the disadvantage a PSR Radar Sensor can only measure the slant range distance between the PSR sensor and an object. The azimuth is derived from the azimuth the antenna is pointing to. Specialized 3D-Radars allow within limits the detection of the aircraft height. While this allows to detect the position of the targets relative to the PSR sensor, it does not allow to identify if they are friend or foe, or which aircraft has been detected.
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 as primary means surveillance as backup in case a detection of aircraft by SSR failed, e.g. because the SSR detection failure / complementary system to secondary surveillance radar, for cases when aircraft is not equipped with SSR, has a defective transponder, or when the transponder was intentionally manipulated or destroyed.^, ^,

Secondary radar

The need to be able to identify aircraft more easily and reliably led to another wartime radar development, the Identification F'riend or Foe systems, which had been created as a means of positively identifying friendly aircraft from unknowns. The first systems, Mark I and Mark II, were limited to receiving a transmitted pulse and retransmitting it after amplification. Transponder were only activated when ordered to.
Starting with the Mark III system the reply was transmitted in a separate frequency band, which required a receiver for this band in the Radar equipment.
In 1943 a joint British-American project of the United States Naval Research Laboratory under the lead of Vivian Bowden developed the IFF Mark V, which was adopted for mass production under the designation United N'ations Beacon . The IFF Mark V system already used the L-Band in the frequency range between 950 MHz to 1150 MHz. After further development it became to Mark X, but still was a very basic system, which used 12 channels with a frequency separation of 17 MHz between the channels, but did not yet allow individual identification for an aircraft.
Further development led to the IFF Mark X system with Mode 1, Mode 2 und Mode 3 which used pulse coded interrogations to specify the Mode and pulse coded replies that provided 64 individual identifications for aircraft. The U.S. specifications were provided in 1952 to NATO for adoption and use of the system by NATO states.^, The X was intended to be a placeholder for a at a later time to be determined final designation, but would later become the number 10 from in the roman numeral system. As a consequence new versions were then called Mode XI and Mode XII. IFF Mark XI was however only used for a short time for the use of coded interrogations and replies^AN/APX-35 ^Transponder and nearly directly to IFF Mark XII. The addition of the Selective I'dentification Feature to provide selective identification of aircraft, is the equivalent to the later by ICAO specified SPI ''pulse and was provided in 1959 by the U.S. to NATO states.
Based on IFF Mark X the International Civil Aviation Organization began at the 5th ComDiv Meeting in Montreal 1954 with the standardization of the SSR System. ^{p. IV-4 ff.} An important decision was the selection of 1030.0 MHz for the interrogation- and 1090.0 MHz for the reply-frequency to ensure a interference free coexistence to the in 1950 adopted first Distance Measuring Equipment, which operated in the band 960 MHz to 1215 MHz. Interrogations were transmitted on 10 channels between 936.5 MHz und 986.0 MHz und Replies between 1188.5 MHz and 1211.0 MHz. Therefore SSR had sufficient frequency separation to the first ICAO DME system from 1950.
The declassified TACAN standards were used by ICAO to define today's DME/N, which utilized however the complete frequency range between 962 MHz to 1213 MHz in 1 MHz-steps. DME/N was standardized by ICAO in Annex 10, edition 6.^{Nr. IV-4} To avoid interference between DME/N, TACAN and SSR, the DME/N channel were the interrogation-, the reply- or both frequencies were within the receiver bandwidth of SSR interrogator and transponder receiver were restricted from use, when a land has SSR equipment in operation. ^S.3-1 Further parameter like e. g. SLS were adopted by the 7th ComDiv Meeting in 1962.
The ICAO SSR system Mode A to D interrogations, differ from IFF Mode 1 and 2 only in the use of a different pulse pair separation between the interrogation pulses. The IFF Mode 1 pulse coded reply and later the initial SSR Mode A reply used only 2 Octal numbers and therefore provided only 64 different ID codes for aircraft identification. ^Nr. ^3.9.3.2 IFF Mode 2 and SSR Mode A used 4 Octal numbers that allowed for 4096 different ID Codes to identify aircraft. ^{Nr. 3.8.6.2.1}
While the ICAO designation for the SSR system Mode A to D in the U.S. it is mostly referenced as the Air T'raffic Control Radar Beacon System. From the 4 interrogation Modes A to D, today only Mode A for Identification and Mode C for the barometric air pressure from the aircraft transponder remains standardized by ICAO for international use. Civil and military aircraft need to be fitted with operational Mode A and C capable transponder to allow detection by SSR- and IFF-interrogator. The transponder is a combined radio receiver and transmitter, which receives its interrogations on 1030 MHz and transmits the replies with the requested information for a Mode on 1090 MHz. The target aircraft transponder replies to signals from an interrogator by transmitting a coded reply signal containing the requested information for the used interrogation Mode.
While 4096 ID-codes for identification seemed sufficient at the beginning, the increased number of aircraft that are equipped with an operational transponder and the increasing number of AD, civil and military ATC, which each required their own batch of ID-codes out of the 4096 available ID codes, soon overcame the supply needed for day to day control. The U.S. FAA started with the development for a more refined SSR system that would allow for future growth, e. g. additional systems to help avoid airborne collisions. During the Development of the new Mode in the U.S. the system was called DABS.
It would take however until 1987 until the new developed Mode was also accepted, standardized and published by ICAO in Annex 10, Volume I, Amendment 67. The new Mode was dubbed Mode S for Selective and was foreseen to allow each aircraft to have its own 24 Bit address. Furthermore, Mode S would allow to selectively interrogate only one aircraft or all aircraft, to transmit additional information relevant for ATC and later a system to avoid airborne collisions, and to enable use as Data Link between Mode S capable aircraft transponders and ground interrogators.
Each state was allocated a number band of 24 Bit addresses for allocation to civil and military aviation. However, while during standardization of Mode S addresses with 24 Bits seemed sufficient to provide all civil Commercial Aviation'' and military aircraft with their own Id, todays increasing number of General Aviation aircraft, including e.g. glider, equipped with Mode S-capable transponder, the growing number of UAVs flying in Mode S airspaces, and the growing use for identification of ground vehicle equipped with a Mode S ADS-B-capable transponder via an airport based MLAT systems, e. g. with more than 300 vehicles per airport, already shows the limits of the 24 Bit address range, which could not have been foreseen during definition of the Mode S system.
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 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 re-calculate 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 messages to prevent "spoofing" by the enemy, and is used on many types of military platforms including air, sea and land vehicles.