AMES Type 85
The AMES Type 85, also known by its rainbow code Blue Yeoman, was an extremely powerful early warning and fighter direction radar used by the Royal Air Force as part of the Linesman/Mediator radar network. First proposed in early 1958, it was eleven years before they became operational in late 1968, by which time they were already considered obsolete. The Type 85 remained the RAF's primary air defense radar until it was replaced by Marconi Martello sets in the late-1980s as part of the new IUKADGE network.
In the 1950s the RAF deployed the ROTOR reporting network, and later improved this system with the AMES Type 80 radar. While these were being built, the carcinotron radar jammer was tested against it and found to completely blank out its display. At first, it was feared the carcinotron would render all long-range radars useless, but over time a number of new concepts emerged to deal with this threat. Among them was the Blue Riband radar, which used a dozen 8 MW klystrons that randomly changed frequencies in order to overwhelm the jammer signal, and broadcast through an enormous array of four wide antennas running on railway tracks.
The introduction of the ballistic missile implied future attacks would likely be by medium range ballistic missiles, not strategic bombers. The need for a comprehensive anti-bomber system was questioned, and the high price of the Blue Riband made it a target for outright cancellation. In response, in 1958 a new design was built by combining the electronics from the Blue Riband with a smaller antenna originally developed as an upgrade for the Orange Yeoman radar. The result was the still-prodigious Blue Yeoman design, which proved so promising that it was further upgraded using the larger antenna from the AMES Type 84. The resulting Type 85 was declared operational at three sites in 1968.
By this time the entire Linesman concept had been called into question, as the radar sites and unhardened centralized command centre would be trivial to destroy even with conventional weapons. Funding for future upgrades to the system was instead directed to replacing it as soon as possible. Type 85 remained in service through the 1970s and into the early 1980s, when it formed part of the new UKADGE system. The Improved UKADGE replaced the Type 85 with a number of smaller and more mobile radars so that backup systems could be placed off-site and then rapidly brought into service if the main radars were attacked. The Type 85s went offline some time in the 1990s.
History
ROTOR
In the early 1950s the threat of nuclear attack by the Soviet Union led the UK to build an extensive radar network known as ROTOR. ROTOR initially envisioned two phases, the first using upgraded World War II radars like Chain Home, and then from 1957, these would be replaced by a dramatically more powerful radar known as the Microwave Early Warning, or MEW. A key part of the concept was a set of six Sector Control Centers where data from all of the radars would be sent to produce the Recognized Air Picture of the surrounding area.As ROTOR was just getting started, in 1951 the Telecommunications Research Establishment began experimenting with new low-noise crystal detectors that improved reception by 10 dB, and new cavity magnetrons of roughly 1 MW power. Combining these together on a lashed-up antenna, they were able to detect bomber aircraft at hundreds of miles range. A production version of this "Green Garlic" prototype would be available years before the MEW. MEW was turned into a long-term development project and spun off to Marconi Wireless. Green Garlic was rapidly developed as the AMES Type 80 and deployed beginning in 1954, with the initial network operational the next year.
It was soon realized that the system, with minor upgrades, had the optical resolution needed to guide interceptor aircraft to targets even at very long range. This task formerly required dedicated ground controlled interception radars with special antennas that provided the required resolution. Upgrades to the Type 80 would allow this task to be combined with the EW role, eliminating the need for separate radars. At the same time, a new 2.5 MW magnetron became available, increasing range beyond the original versions. These Type 80 Mark III's led to many changes in the ROTOR layout as the centralized control rooms were removed and the entire battle from detection to interception was instead handled directly from the radar stations themselves. Ultimately, after several changes in plans, the system emerged with nine Master Radar Stations and about another twenty radars feeding data to them by telephone.
Carcinotron
In 1950, engineers at the French company CSF introduced the carcinotron, a microwave-producing vacuum tube that could be rapidly 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 rapidly 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 fit it to a Handley Page Hastings named Catherine, testing it against the latest Type 80 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.
MEW
While ROTOR was being installed, the original MEW design at Marconi was still being worked on. With the RAF's immediate needs filled by the Type 80, the requirements for the MEW had been modified to produce a much more capable design. The resulting specification called for a 10 MW L-band klystron and an advanced moving target indication system.Calculations suggested that a carcinotron could produce about 10 W of signal on any given frequency. The 10 MW klystron transmitter would produce 11 W of return signal at 200 nmi, thereby overpowering, or "burning through", the jamming. Unfortunately, the klystron proved to be a problem and was only able to reach 7 MW on occasion. In 1958, the decision was made to abandon it and replace it with an experimental 2 MW L-band magnetron that had been fit to a radar at Bushy Hill in 1956. It was ultimately improved to 2.5 MW.
The MEW worked in the L-band at a 23 cm wavelength. This makes it much less sensitive the effects of Mie scattering off rain and ice crystals, meaning L-band radars are much more effective in rain or heavy cloud. The downside to the longer wavelength is that optical resolution is an inverse function of wavelength, so by operating at about three times the wavelength of the Type 80's 9 cm meant it also had three times less resolution. Some other radar would still be required to accurately guide the fighters.
Blue Riband
With the failure of the MEW's original klystron, in 1956 the RRE began development of a new radar in partnership with Metropolitan-Vickers. Given the rainbow code "Blue Riband", the design goal was simply to "produce the largest, most powerful radar that could be deployed in the ADUK." Blue Riband would overwhelm any possible carcinotron design, while also providing enough accuracy to directly guide interceptors. Further, they highly desired the system be a 3D radar so the separate height finders could be eliminated; height finders were often as expensive as the primary radars and time-consuming to operate.Magnetrons are somewhat odd devices in that they produce a powerful microwave signal in one step, and the frequency of the microwaves they produce is a function of the physical dimensions of the device and cannot be changed after manufacture. In contrast, the klystron acts purely as an amplifier. Given multiple reference signals, say from crystal oscillators, the klystron can amplify any source within a bandwidth of about 100 MHz, beyond which its efficiency falls off. Thus, by moving to a klystron it was possible to change the frequency of the signal with every pulse by connecting it to a series of different source signals.
To jam such a signal, the carcinotron would have to broadcast across the entire 100 MHz band, thereby diluting the signal to the point where it could no longer overpower the radar's pulses. Due to the radar equation, the energy of the radar's pulses falls off with the fourth power of range, so having enough power to ensure the carcinotron could not keep up at long range meant the output had to be huge. Blue Riband solved this problem by mixing the signal from multiple klystrons together, two or four depending on the model, and then broadcasting the resulting 8 MW signal.
Blue Riband planned to use the output of a dozen transmitters, each with two or four klystrons feeding a single feed horn with a degree vertical angle. The twelve horns produced a beam that was 6 degrees high in total, and the vertical angle of the target could be estimated by comparing the strength of its signal in adjacent horns.
To match the resolution of the Type 80, the antenna had to be wide enough to focus the signals into a similar degree wide beam. The downside to such a tightly focused pencil beam is that the beam sweeps past targets very rapidly as the antenna rotates to scan the sky. In the case of the Type 80's pulse repetition frequency of 250 pulses-per-second and its rotation speed of 4 rpm, this meant only 3 to 5 pulses would hit any given target as the beam swung past it. This leads to a relatively low blip-to-scan ratio, and if even a few of these pulses are jammed, the target might disappear. To solve this problem, Blue Riband proposed mounting four antennas in a square, meaning the entire sky would be scanned after it rotated 90 degrees. This allowed the rotation to be slowed to rpm, thereby greatly increasing the number of "paints".
Meeting the resolution goals required a parabolic reflector that was. Four of these together produced an enormous system, so large that there was no way it could be mounted on existing bearing systems. They ultimately settled on the solution used by the diameter Lovell Telescope at the Jodrell Bank Observatory. This runs on a modified railway roadbed with multiple sets of bogies carrying a huge triangular framework. For the Blue Riband, they adopted a somewhat smaller version with a diameter with six bogies carrying a framework on top that acted like a flat turntable.
The twelve transmitters would be buried in the centre of the assembly. Their power was fed to the antennas through a series of twelve rotating wave-guides, something that didn't exist at the time. Two possible waveguide designs were trialled, one at the RRE and another at Metrovick.
During development, a possible way to build the system with a single rotating wave-guide was presented. This fed the antennas a single signal through a vertically oriented slot antenna, and used an effect known as "squint" to move the beam up and down. Squint causes the signal to change angle when its frequency changes. By setting the dozen klystrons to different frequencies, squint would cause each one to exit at a different angle. This concept was abandoned when it was pointed out that steering the beam using the frequency meant any one aircraft would always be hit by the same frequency, which made the jammer's job much easier.
Another concept that was raised was to use only two antennas mounted back-to-back and use separate sets of a dozen feedhorns on both. One would be set to a beamwidth of 0.4 degrees covering the horizon, and the other 0.6 covering higher angles. This provided higher accuracy on the horizon while also increasing the total vertical coverage from 6 degrees to 12. In total there would be twenty-four transmitters. It does not appear this design was pursued.
A contract for the new klystrons was sent to EMI near the end of 1957. By this time the concept was to have each of the transmitters tuned to a different 100 MHz bandwidth, with the set of all twelve covering a band of 500 MHz, beyond which the receivers also began to fall off in sensitivity. By connecting the transmitters at random to the feedhorns, the frequency hitting any given target changed with every pulse, forcing them to jam the entire 500 MHz band in a form of barrage jamming.