Secondary surveillance radar

SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II; therefore, the two systems are still compatible.

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.

[citation needed] The International Civil Aviation Organization (ICAO) is a specialized agency of the United Nations headquartered in Montreal, Quebec, Canada.

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.

[7] 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 the standards specified in ICAO Annex 10.

[citation needed] ARINC (Aeronautical Radio, Incorporated) is an airline run organisation concerned with the form, fit and function of equipment carried in aircraft.

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 the benefit of SSR derived data.

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.

An aircraft detecting P2 stronger than P1 (therefore in the sidelobe and at the incorrect main lobe bearing), does not reply.

[7] A number of problems are described in an ICAO publication of 1983 entitled Secondary Surveillance Radar Mode S Advisory Circular.

However, as airspace became increasingly congested, it became important to monitor whether aircraft were not moving out of their assigned flight level.

As air transport expands and more aircraft occupy the airspace, the amount of FRUIT generated will also increase.

[9] 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.

[9] The ground antenna has a typical horizontal 3 dB beamwidth of 2.5° which limits the accuracy in determining the bearing of the 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.

To protect against errors a simple parity system was proposed – see Secondary Surveillance Radar – Today and Tomorrow.

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.

[16] 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.

[18] The two countries reported the results of their development in a joint paper, ADSEL/DABS – A Selective Address Secondary Surveillance Radar.

[14] This was followed at a conference at ICAO Headquarters in Montreal, at which a low-power interrogation test by Lincoln Laboratory successfully communicated with an upgraded commercial SSR transponder of UK manufacture.

This necessitated a vertical dipole array suitably fed to produce the desired shape.

A monopulse receiver[15] was developed early in the UK Adsel programme and this design is still used widely.

[1] The following table compares the performance of conventional SSR, monopulse SSR (MSSR) and Mode S.[19] 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 10 nautical miles (19 km; 12 mi) to 5 nautical miles (9.3 km; 5.8 mi)[21] MSSR resolved many of the system problems of SSR, as changes to the ground system only, were required.

[9] 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.

This form of modulation provides some resistance to corruption by a chance overlapping pulse from another ground interrogator.

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.

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.

[9] ICAO Annex 10 Volume III, Chapter 5 lists the contents of all those currently allocated.