Secondary Surveillance Radar - Developments To Address The Deficiences

Developments To Address The 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 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 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.

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. 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.