The original IFF (Identity Friend-or-Foe) system was designed during WW2 as an integral part of the primary radar system. Receivers to detect the primary radar signal were fitted to the aircraft. Once detected in the aircraft the signal was amplified and retransmitted, on the same frequency, so that the ground radar received an extra signal to the ‘skin paint’ of the aircraft. The displays showed the ‘skin paint’ plus, at a slightly longer range, the IFF paint.
Subsequently, IFF was improved to employ discrete transmit and receive frequencies and to work independently of or synchronously with the primary radar.
This version of IFF was recognised as being extremely important in the world of air transport. For civil applications it was called SSR (Secondary Surveillance Radar), or ‘Beacon’ in the United States.
Plessey Radar’s involvement in IFF/SSR commenced in the mid 1960’s with the need to add IFF to radars such as the AR-1. This work was carried out at Tolworth before moving to Addlestone and eventually Cowes.
The earliest IFF/SSR systems were created by buying-in units from other manufacturers such as Cardion (later to become part of the Siemens/Plessey Group), Cossor, Whittaker and Burroughs. Presentation of the IFF data was on digital readouts adjacent to the radar displays. Selective target decoding was activated by means of ‘light pens’.
With the advancement of techniques the IFF/SSR data, which included aircraft identity and height, was decoded as a digital output and presented directly on the radar display with a tag line to the primary radar ‘skin paint’. Further improvements, including primary radar plot extraction, enabled better integration of data from the primary and secondary radar sources with functions such as ‘code-call sign conversion’, tracking etc. becoming the norm. Because this data was now in digital form it could be sent over telephone lines to remote locations.
In this period Plessey began to design and manufacture more of the system itself, including antennas (hog-trough, large vertical aperture and MSSR), interrogator/receivers and processors. Plessey also developed a very skilled software capability for code call-sign conversion, tracking and planning for airways. This software aspect is covered in Chapter 12.
IFF/SSR systems designed and supplied by Plessey included the S100, S200 and the RSM 970 from Cardion.
The S100 system, with the exception of the antenna (normally obtained from either Cossor or Cardion with 7ft, 14ft or 28ft aperture depending on the operational requirement) comprised entirely of Plessey designed and manufactured units.
The main elements were the:
Interrogator/Receiver Type PTR 821 which was a solid-state unit providing all the interrogation modes (1,2,3/A, B C, D) required for IFF/SSR in both civil and military usage.
Digital Defruiter Type PDD100 overcame the majority of spurious responses. Such responses occurring because all aircraft with transponders within the coverage would respond, thereby causing many garbled replies that had to be cleared.
Secondary Radar Plot Extractor type PSX100, would accept the defruited signals, decode them and give a digital output of identity and height codes suitable for narrow band transmission.
Secondary Radar Display Processor Type PDP100 processed the incoming decoded signals and allowed each air traffic control operator to select the aircraft targets of interest to their sector of operation. Target selection was usually made at the display by means of a ‘light-pen’ or (at a later date) a ‘rolling ball’.
The S200 system was effectively an update of the S100 and employed very similar items of equipment.
At the radar head the PIX 200 cabinet contained an Interrogator/Receiver type PTR826, a Secondary Radar Extractor type PSX 200 and the narrow band line drive equipment.
At the operations site there was the rack Type PPS 200 which contained a PDP 11 computer to perform the signal processing functions. There were three key peripherals to assist in its processing; An ASR33 Teletype, a high-speed tape reader, and a visual display unit (rather like a personal computer).
The visual display unit allowed data to be entered onto the system and facilitated the conversion of the aircraft identity code to its call sign. Presentation on the operational display units would then be automatic. The converted call sign was included in a label, which had aircraft height and very often, the aircraft destination.
S200 was available in both single and dual configurations. A typical single channel S200 configuration is shown in the following diagram.
There were a number of later improvements in IFF/SSR Systems. One of the most important was the introduction of the ‘Large Vertical Aperture’ antenna to replace the standard ‘hogtrough’. The LVA provided a better aerial-pattern, which allowed some selective interrogation thereby reducing the amount of ‘fruit’ arising from broadcast interrogations. (See the picture of WATCHMAN in chapter 6.10)
For much of the period under consideration Cardion was, except for antenna supply, a competitor. Cardion became part of the Siemens group and brought to the portfolio their own SSR system called the RSM700, which was sold in large numbers. Close collaboration then followed between the Cardion and Cowes based engineering design groups.