RX12874, also known as the Passive Detection System (PDS) and by its nickname "Winkle", was a radar detector system used as part of the Royal Air Force's Linesman/Mediator radar network until the early 1980s. Winkle passed out of service along with the rest of the Linesman system as the IUKADGE network replaced it.
Winkle was developed in the late 1950s to counter the carcinotron, a radar jammer so effective that it was initially believed it would render all long-range radars useless. Winkle used a network of stations to listen for carcinotron broadcasts, and combined the information from them to track the jammer aircraft as effectively as a radar could.
The system was based on a series of High Speed Aerial (HSA) installations and AMES Type 85 ("Blue Yeoman") radars. Both were used as receivers; the Type 85 was used primarily to measure the time of arrival of the signal, while the HSA rapidly scanned horizontally to extract a bearing. Information from HSAs and the Type 85s was combined in a correlator that used triangulation and time-of-flight information to determine the location of the jammer-carrying aircraft.
Once the location was determined, it was manually input into the interception controller's displays as if it were a normal radar return, distinguished only by its small circle icon instead of a single dot. Operators could decrease the Type 85 receiver sensitivity while the radar passed that location, so that the jamming did not obscure the display at nearby angles. Combined with identification friend or foe (IFF) signals, this allowed a fighter aircraft's signal to remain visible and interceptions could proceed as normal.
History Carcinotron
In 1950, engineers at the French company CSF (now part of Thales Group) introduced the carcinotron, a microwave-producing vacuum tube that could be tuned across a wide range of frequencies by changing a single input voltage. By continually sweeping through the frequencies of known radars, it would overpower the radar's own reflections, and blind them. Its extremely wide bandwidth meant that a single carcinotron could be used to send jamming signals against any radar it was likely to meet, and the rapid tuning meant it could do so against multiple radars at the same time, or sweep through all potential frequencies to produce barrage jamming.
The carcinotron was revealed publicly in November 1953. The Admiralty Signals and Radar Establishment purchased one and fitted it to a Handley Page Hastings named Catherine, testing it against the latest AMES Type 80 radar late that year. As they feared, it rendered the radar display completely unreadable, filled with noise that hid any real targets. Useful jamming was accomplished even when the aircraft was under the radar horizon, in which case other aircraft had to be to the sides before they were visible outside the jamming signal. The system was so effective that it appeared to render long-range radar useless.
ROTOR The Type 80 was a key part of the ROTOR system, a comprehensive radar and control network covering the entire British Isles. The Catherine tests suggested that the system would be rendered impotent before it was even fully installed. The Royal Aircraft Establishment (RAE) immediately began developing their own carcinotrons for the V Bomber force under the name Indigo Bracket, while solutions to the jamming problem for the RAF's radars were studied.
The first consideration was that the carcinotron provided a relatively weak signal, on the order of 5 kW. When used in barrage mode, this was diluted to perhaps 5 to 10 W per MHz of bandwidth. Due to the radar equation, at long range this was still much stronger than the reflection of the multi-megawatt signal from the radar itself. As the jamming aircraft approached the station, there was some point where the radar began to overpower the jammer, the "self-screening" or "burn-through" point. A very powerful transmitter would increase the range where this occurred. Further improvement could be gained by tightly focusing the beam to put as much power into the reflected signal as possible.
The Royal Radar Establishment (RRE) began the development of such a system with Metropolitan-Vickers (Metrovick) under the name 'Blue Riband'. It was assumed that a jammer could produce as much as 10 W per MHz across the entire S-band. Through the use of twelve 4.5 MW klystron transmitters broadcast through an enormous antenna system, the Blue Riband produced 11.4 W per MHz of reflected signal at , thereby overpowering the assumed threat. To force the jammer to spread out its signal across a wide band, the radar randomly changed frequencies with every pulse, across a 500 MHz bandwidth.
Changing strategies Throughout this period, there has been an ongoing debate about the usefulness of air defences. The introduction of the hydrogen bomb meant a single aircraft could destroy any target, and the higher speeds and altitudes of bomber aircraft meant the bombs could be dropped from further away. By 1954, the Chief of the Air Staff had concluded that close defence was useless, and began plans to remove anti-aircraft artillery from the defence. By December, planners believed the only practical role for air defence was to protect the V-force while it was launching. In keeping with this role, over the next few years the number of radar stations and fighters continued to be reduced as the protected area contracted around the Midlands.
The 1957 Defence White Paper shifted priorities from manned bombers to missiles. The only way to defend against a missile attack was deterrence, so it was absolutely vital that the V-force survive. This meant that any attack, whether by aircraft or missiles, would require the V-force to launch immediately; the interceptor defence could not guarantee their survival even in the case of an all-bomber attack, and could do nothing at all in the case of missiles. By the end of 1957, the idea of any defence of the deterrent force had been abandoned; a bomber attack simply implied missiles were following. Now the bombers would launch to staging areas after receiving any credible threat. The need for the long-distance coverage of Blue Riband disappeared.
One new role did emerge. Since the attack would likely come from missiles, the Soviets might attempt to jam the Ballistic Missile Early Warning System (BMEWS) radars by flying aircraft far offshore and using a carcinotron against the relatively narrow band of the BMEWS. They might mask a bomber attack on the V-force bases the same way by jamming the ROTOR radars. Such jamming would require the launch of the V-force while the nature of the threat was determined, and repeated spoofing of this sort could quickly wear out the aircraft and their crews. A system for detecting such an attack and countering it was considered valuable.
This role would not require the massive Blue Riband and led to the "Blue Yeoman" concept, combining the electronics of the Blue Riband with a smaller antenna originally developed as an upgrade for the Orange Yeoman radar. Associated Electrical Industries took up production of this system as the AMES Type 85. As these still had long range, only nine were needed to cover most of the UK. Over time these plans were repeatedly scaled back, eventually producing a system known as Linesman with three stations covering only the southern portions of England, protecting Bomber Command's bases and the BMEWS radar.
Correlation radar In 1947, the Royal Aircraft Establishment (RAE) was handed the task of developing guided missiles, taking over from a previously diverse group of efforts. Several engineers from the RRDE were sent to the RAE at Farnborough Airport to aid the design of the tracking and guidance systems. Among the group was George Clarke, who had worked on the LOPGAP missile guidance system but was more interested in advanced radar development.
In 1949 Clarke invented a novel sort of identification friend or foe (IFF) system that did not have to be triggered by a pulse sent from the ground. Instead, each airborne IFF would broadcast signals at random times. This avoided a problem seen in densely trafficked areas where the interrogation pulse from the ground IFF transceiver would generate so many transponder replies that they would overlap in time and interfere with each other. In Clarke's system the transponders naturally sent out replies spread out in time, making it much less likely they would overlap.
In a traditional IFF, the time between sending the interrogation pulse and its reception allowed the range to the transponder to be determined. In Clarke's system, the receiver did not know when the signal was sent, and could no longer use this method. Instead, the signal would be received by three antennas, and using a device known as a "correlator", better known today as a matched filter, pulses from a single IFF broadcast could be picked out of the many possible returns. By delaying the signals until they lined up in time, the difference in time that it took the signal to reach each of the antennas was extracted. The difference between any two antennas results in a continuum of possible locations along a hyperbola. By making similar measurements between all the stations, A-B, B-C and C-A, three such hyperbolas are constructed, which theoretically intersect at a single point, but more typically form a small triangle due to inherent inaccuracies. The idea was not picked up for development.
Later that year, Clarke proposed a new missile tracking and guidance system based on the same basic technique. Due to short flight times, a missile tracking system would want to detect the target as rapidly as possible, but as radars of the era were mechanically rotated, there was a limit to their scanning rate. Clarke proposed using a single large "floodlight" transmitter and three receivers placed at the corners of a baseline triangle. The signal reflecting off of any object in the area would be converted into a location in the same way as the IFF system. All of the targets within the floodlit space could be located simultaneously and continually. A review of the concept suggested there were too many unknown factors to begin serious development, and Clarke was moved to a group working on radar countermeasures.
Winkle
In 1951, Clarke proposed a yet another system based on the same principles, this time as a way to track aircraft carrying jammers. The RAE considered the concept and suggested there were three possible ways it could be used; the first was the three-station concept of Clarke's missile proposal, the second used angular measurements from two widely spaced antennas for simple triangulation, the third used two antennas to find one hyperbola using Clarke's method, and an angular measurement from one of the two stations to intersect with it.
While the two-angle solution might appear to be the simplest, it suffers from a problem when there is more than one jammer in an area. Against a single jammer, the receivers both receive the signal and measure its angle relative to their station. When these angles are plotted on a map, they intersect at a single location. If there are two jammer aircraft in the area, both stations will produce two angle measurements, one for each aircraft. When these are plotted on the shared map there will be four intersections; two of these hold aircraft, the other two are "ghosts". A third aircraft increases this to nine points and six ghosts, and so on. The RAF desired a system able to deal with mass raids, so this solution was not appropriate.