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RX12874 - Passive Detection

All warfare is based on deception - Sun Tzu (The Art of War)

RX12874 Passive Detection receive only head (photo - Dick Barrett)Passive Detection was a lesser known component of Linesman system and easily the most fascinating part but then, as an ex PD tech, I am a bit biased! This receive only S-band "head" was known as the "HSA" or "High Speed Aerial" and it was a part of the RX12874 Passive Detection anti jamming equipment at RAF Boulmer in Northumberland. The following article is extracted from "Forty Years Of Marconi Radar From 1946 to 1986", "GEC Review", vol 13, No.3, 1998 with the kind permission of the Editor. It is the only account I have seen in the public domain that describes the system:

"Passive detection was one of the most fascinating and innovative developments in air defence technology since the war, and yet one of the least publicised. The air defence requirement in UK in the early 1950's was for a system alongside the main conventional radars which would be able to detect and locate individual 'jamming' targets in a mass raid, the majority of which were also jamming, with the same discrimination in range, bearing and height as the 'active' radar. A brilliant concept by George Clarke at the Royal Aircraft Establishment in Farnborough, skillfully analysed and proven feasible on paper by Norman Bailey at TRE in Malvern, started the research and development programme.

In essence, jamming signals were to be detected at each end of an extended baseline, of the order of a hundred miles. One signal was transmitted to the other end of the baseline, and the two signals compared in a 'correlator'. A correlator is a device which will give an output when two identical signals (including white noise) are fed into the device in the same phase, that is, without any time difference between them. Therefore, after compensating for the delay in transmission, a variable delay can be introduced into one signal until correlation is achieved. Rapid laboratory trials were carried out to prove that a suitable correlator could be designed. A test link was set up between Great Baddow and Great Malvern to demonstrate that correlation could be achieved after a signal had been carried on a microwave link with repeaters. The whole project carried the codename WINKLE.

The majority of the development work for the project was carried out by Baddow Research staff. In 1956, a link was rapidly set up between a test site at Bard Hill in Norfolk and an RAF station at Bempton in Yorkshire and a programme of detailed trials began. Initially a static test jammer was placed on a tower mid way along the baseline, which enabled a great deal of experimental work to be done, improving correlator design and moving towards the practical implementation of a working passive detection system. Historic tests with four closely-spaced jamming aircraft showed that Clarke and Bailey had got it right. The full system had to be automatic with a high data rate and a very low false-alarm rate, which meant a massive increase in the amount of equipment needed.

The final design involved signals detected at one end in the conventional radar, and at the other end by a specially designed 'high-speed aerial'. This scanned a fixed angle in the horizontal plane once for each beamwidth of the main radar; it was mounted on turning gear so that the fixed angle of scan could be moved for optimum coverage. Correlation when a target was at the coincidence of the two beams achieved the objective by giving its position with the same accuracy and data rate as the radar. The positional data on jamming targets was three-dimensional because the main radar had a 'stacked-beam' configuration and passive detection was installed on each beam. This is of course a considerably simplified description. Substantial (for that time) computer power was involved in each installation, to extract data reliably in parallel with the main radar . Operational tests proved that Passive Detection fulfilled all its requirements. The full system listened in silence for many years as a vital part of the UK air defences."

Reproduced from: "Forty Years Of Marconi Radar From 1946 to 1986", GEC Review, vol 13, No.3, 1998 with the kind permission of the Editor.

The full article is available here or on line at: http://www.marconi.com/media/gecjournals/gecreview/v13n3/p172.pdf


RX12874 Passive Detection receive only head, Type 84 in background (photo - Neatishead Air Defence Museum)The system was a form of triangulation and two computers were used to determine the position of radar jamming aircraft. A third computer known as the TC or "Test Computer" could take over the role of one or other of the two computers should one fail. Each computer had a 4 Kb ferrite core store, the logic gates were constructed from discreet components - not an integrated circuit in sight whilst the diodes and transistors used in the system were germanium technology.

Operational programmes were kept on a drum storage device whilst other software could be loaded from 5 hole paper tape. The software language was called "Panec" - a source of delight once Douglas Adam's play "A Hitch Hiker's Guide To The Galaxy" made it on to the radio. The "High Speed Aerial" at R.A.F. Boulmer (pictured at the top of this page) was remotely controlled from RAF Staxton Wold in Yorkshire, we at Boulmer had similar heads at Staxton Wold, Yorkshire and Dundonald Hill, near Ayr, Scotland, remotely controlled from Boulmer. The Staxton Wold head was shared between Boulmer and Neatishead.

Samples of radar jamming were taken from the remote heads and the local Type 85 radar. The system gains were manually adjusted to apply the optimum level of signal to the processing equipment. Time delays were applied to the local signal to compensate for the propagation time of the remote signals along the link paths and the remote samples were compared with the local sample for correlation's between the remote and local signals. Correlation's were processed by the computers and the resulting data was displayed as a symbol representing the position of the jamming aircraft on the Fighter controller's PPI screen. The controller could then reduce the radar gain on his display to suppress the jamming signal and vector a fighter on to the jammer using IFF paints and the symbol representing the jammer.  The HSA antenna could rotate at 4 rpm or sector scan, both modes synchronized to the Type 85, or it could operate on one of four fixed bearings, two on either side of the baseline, known as "near look" and "far look". A wave guide switch in the form of an organ pipe arrangement in the enclosure in front of the HSA reflector scanned the antenna beam width 360 times per second.

The Passive Detection Theta/Phi disply (double left click here to get a better view) (sketch - Dick Barrett)An unusual feature of this equipment, from an Air Defence radar technician's perspective, was that it was operated by the technicians who maintained it. A thorough grasp of the technical principles involved was required; the complexity of the system, its unique method of operation and its unusual "q/f" (Theta/Phi) vertical raster display meant that the system could not be operated by the regular "Scopie's", they just couldn't get their heads around it!

When the correlations were displayed on the Theta/Phi display they appeared as a cluster of many dots. The technician then adjusted two sets of gain controls to reduce the number of dots to two or three dots (in order not to lose the jammer completely due to signal scintillation) and each dot resulted in the generation of a symbol representing the jamming target's position on the fighter controller's display. One of the gain controls operated collectively on all the sectors, whilst the other allowed individual sector gains to be adjusted.

The dynamic range of the system was excellent; strong jamming signals such as those radiated by the USAF B52's were easily attenuated and were relatively unaffected by signal scintillation, making the operating technician's job quite easy, whilst weaker signals could be followed down to the system noise floor, which corresponded with that of the Type 85 radar. 


I am grateful to Steve Long for getting in touch with him. Steve was a Deputy PDSO at L1 (R.A.F. West Drayton) from 1980 to 1981 and he has written on PD operations from his perspective as follows;

"I was interested to read about the PDS from you technicians viewpoint so offer the following as a comment on the same system from a PD Officer's (yes, an operator who actually understood it {but sadly has forgotten most of it over the years}) viewpoint:

You are right about the complexity of the system and also about the fact that the operators at Staxton Wold. Boulmer and Neatishead couldn't gain access to it - local operators weren't trained to operate it anyway and there was no mechanism for the technicians to pass detected targets to the operators other than by voice. This was intentional! Although there was much argument to the contrary, the system was never intended to be operated by technicians on those sites either - it was intended that the technicians purely provided local technical control on behalf of the PDSO (PDS Officer) at RAF West Drayton! Each of the radar sites fed their signals into the Air Defence Data Centre at West Drayton where there was not only equipment to control all (but one) baselines simultaneously but also the mechanism to disseminate the detected jammers to the CRCs or CRPs as tracks. Every time the system was exercised there was a political struggle between the local technicians (who believed that the system was their's) and the PDSO at West Drayton (who knew it was his!). To be fair, most of the Fighter Controllers at West Drayton didn't understand the system either but there was one man who made the system his own, Flight Lieutenant Fred Pitcairn-Hill - Popcorn-Bill to his colleagues. He understood how the various baselines combined to produce the full system which was significantly more capable than any one site and its baseline alone. The technicians on the radar sites were hampered in that they could only access a single baseline at any one time whereas West Drayton could access all (but one) of them simultaneously. Boulmer was slightly different to the other sites in that it controlled two baselines although it could only access one at a time - and could only feed data from one at a time to West Drayton as well. Their main baseline was between them and Staxton Wold - Dundonald Hill was rarely used but was available should the Scots ever decide to jam the English! When West Drayton operated the PDS it produced 'centroids' - tiny circles - on the radar tubes at the ADDC which were tracked by the 'scopies' just as though they were ordinary tracks, the PDSO liased with local site 'techies' to eliminate the majority of the spurious centroids but the trackers and computers at West Drayton did the rest. The trackers put the letter J onto the resulting PD tracks to show that they were jammers and they were then transmitted to Neatishead, Boulmer and Staxton via the RPDS - the Recognised Picture Dissemination System - and to Strike Command at High Wycombe using the Higher Formation Display in the early days and ERAPDS - the Enhanced Recognised Air Picture Dissemination System - in the latter days. A curiosity of the system was that the greater the power of the jamming, the easier it was to use - when the RAF Canberras jammed it during a Coffee B (jamming against a single site) or Coffee C (jamming against one or more PD baselines) exercise the system needed to be worked hard to track the aircraft, but when a B52 jammed it the system almost tracked it itself."

Steve Long (Flt Lt Retrd Deputy PDSO 1980-81)


 

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Updated 05/03/2002

Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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