Martello is a family of phased array radar systems developed by Marconi Electronic Systems in the 1970s and introduced operationally in the early 1980s. They provided long-range early warning capabilities but also had the accuracy needed for interception plotting and "putting on" of other weapons systems like surface-to-air missiles. The name comes from the Martello towers that provided defence in earlier years.
A key feature of the new design was its solution to measuring altitude. Earlier mechanically scanned 3D radars used multiple feed horns in a vertical stack, but this was difficult to make in a mobile form; when moved they would go out of alignment. Newer phased arrays used electronic phase shifters to sweep up and down to measure angles, but this required expensive electronics. Martello used fixed phase shifters to produce a pattern of eight stacked beams, recreating the multiple feed horn pattern in a small box that was inexpensive to implement.
The system was initially offered to the RAF, whose Linesman radar network was the subject of much concern over its survivability due to its fixed locations and single control centre in London. It was suggested it should be replaced as quickly as possible by a mobile, distributed system. Around the same time, NATO was beginning the process of upgrading their Europe-wide radar network, NADGE, and their basic requirements were similar to the UK's. Martello was conceived to fill both requirements.
The first S713 was introduced in 1978 and entered RAF service in 1982 as the AMES Type 90. Changes to the NATO specification led to the S723, introduced in 1984 and entering service with the RAF in 1986 as the AMES Type 91. The S723, and product-improved S743 version found a number of international buyers. At least 22 systems of the S700 family have been produced through 2003. The Lockheed Martin AN/TPS-77 has replaced the Martellos in the UK, as the AMES Type 92.
History S600 Marconi was the leading supplier of ground-based radars in the UK for many years, mostly working with designs developed at the Royal Radar Establishment (RRE) and its predecessors. These generally used large radars for 2D scanning to a plan-position indicator (PPI) display and separate height finding radars for measuring target altitudes.
In the early 1960s, the company carried out several industry studies to better understand the future market. A new product concept emerged from this process, a long-range air-scanning radar to provide battlefield overview and early warning, what the British Army called a tactical control radar. The key feature was mobility; the design had to be able to be pulled by Land Rovers, slung under a Sea King helicopter, and fit within a single C-130 Hercules aircraft. The design also intended to use the same components to serve different roles, including air traffic control and fixed-place systems.
It combined a traditional rotating radar scanner for detection and measurement of bearing with one or more height finder radars to measure altitude. A unique feature was that the main scanner optionally mounted two antennas back-to-back and could be operated in the S band and L band at the same time. Advanced moving target indication (MTI) systems were also included to suppress clutter.
This concept emerged as the Marconi S600, which was first announced in May 1967 and demonstrated at the 1968 Farnborough Air Show. Orders began to arrive almost immediately, and over the next several years the company sold 74 systems worth over £100 million to 15 countries around the world. This was a huge success, especially given the £2.5 million cost of developing the system.
3D scanning In the late 1960s a similar study started to look for new product lines to supplant or even replace the S600. This process produced several new requirements; like the S600, the system should be mobile or at least transportable, that it have improved jamming resistance, and that the era of the 2D radar and separate height finder used in the S600 was over and that the new design had to have a single 3D antenna. Normally a search radar's beam is shaped like a fan, very narrow from side to side in order to accurately determine bearing, and very wide top to bottom in order to catch any aircraft no matter its altitude. If one desires to accurately measure altitude with the same radar, the beam will need to be narrow in both directions, producing a "pencil beam" that has to be scanned in both directions.
Marconi had worked on 3D systems in the past, notably the Orange Yeoman radar of the early 1960s. These used multiple feed horns to generate a series of pencil beams, each at a different vertical angle. The waveguide network and horns were complex and had to be accurately aligned during setup and maintenance, and a version robust enough to be mobile would be difficult to build. Another concern was the feedhorns generated side lobes with the first lobe on the order of -20 to -25 dB of the main beam. This meant that radar jammers were picked up multiple times as the radar rotated, both in the main beam and the sidelobes.
Since that time a number of new solutions for 3D scanning had emerged. The primary one used phased arrays and shifted the frequency of the radar to produce a beam that could be steered entirely in electronics. However, using contemporary electronics, this was very expensive and was still being used only in high-end designs. One could also achieve the same basic goal by careful arrangement of the waveguides and antennas, a concept known as "squint", such that changing the frequency caused the entire beam to rotate vertically. This was being used for civilian radar systems like air traffic control, but for military use this system resulted in any particular aircraft always seeing the same frequency, which make it easier to jam. Among squint-steering's notable successes was the Plessey AR-3D.
In 1965 by the Royal Radar Establishment began development of an advanced 3D concept as the Storage Array Radar, or STAR. In 1970 the RRE gave Marconi a contract to develop a prototype of a commercial version of STAR, using an S600 transmitter with a new phased receiver that could be offered as an upgrade to existing S600 systems. Ultimately the system instead demonstrated that the concept was too costly to be economically attractive. This early work continued to be supported through the 1970s.
New concept The majority of the cost of the STAR system was the complex signal handling system. This had to store the received returns over the entire period of the transmission pulse and then correlate the returns in that signal using a series of variable signal delays. In the new concept, the variable delays of STAR would instead fixed and produce a series of fixed-angle vertical lobes. Horizontal scanning would be achieved as it had in the past, by rotating the entire antenna array.
Such a system can steer the beam vertically by delaying the signal as it is sent to the elements, and this phased array radar concept was being actively researched, especially in the United States. However, this required each element to have its own transmitter as well as receiver, and in the era of electronics made of individual transistors, these were very expensive. In contrast, Marconi's concept sent out a single fan-shaped beam as in STAR, and required only the receivers to be individual to each element.
After conversion to the IF, the signal from each element was sent into a series of fixed delays, the "beam-forming network". These mixed the signals from different sections of the array, creating outputs that were sensitive in certain directions. The end result was a series of outputs, nine in the original design, eight of which were aimed in a different vertical direction while the ninth was an all-sky beam used for early detection.
The aimed outputs were identical to those created by the separate physical feed horns in a system like Orange Yeoman, but implemented completely in electronics in a box about the size of a minibar. The system could be re-purposed to provide more vertical coverage or change the coverage pattern by replacing this box, with no changes to the antenna. Altitude information was extracted as it had been in Orange Yeoman; comparing a received pulse's strength in adjacent outputs allowed the vertical angle to be measured with some degree of accuracy. Such systems are known as "stacked beam radars".
Linesman and UKADGE
While development on new 3D systems continued, the Royal Air Force (RAF) was in the final stages of the installation of its latest radar network, Linesman. Linesman had been designed in 1958, in the era when the NATO response to any attack by the Warsaw Pact would be widespread use of its overwhelming superiority in air power to deliver tactical nuclear weapons against high-value targets. Any attack on the UK was assumed to be by bombers carrying strategic-sized hydrogen bombs, so no attempt had been made to harden the radar sites or the single centralized command center, L1 - if there were bombs falling, the defence had failed and there was no point trying to protect it further.
By the late 1960s, the USSR was reaching some level of parity in both tactical and strategic weapons, and the idea that any war in Europe would be met by the early use of nuclear bombs was obsolete. If the war was going to stay conventional, the USSR could easily risk an attack on Linesman with no fear of triggering a nuclear response. The RAF had long complained that the L1 station was highly vulnerable to any form of attack, even a truck filled with explosives, and the shore-side location of the radars made them easy to attack by low-flying aircraft. They repeatedly demanded that control of the interceptor aircraft take place at the radar stations rather than L1, ensuring that a single attack would not destroy the entire network. Even more worrying was that the data was communicated over microwave relay, which opened it to the possibility of being jammed, rendering the entire network useless.
Other changes through this period including the new "plot extraction" systems that were highly effective at picking out moving aircraft, especially when they combined the information from more than one radar. By digitizing this data early in the process, it could be passed from site to site using modems on conventional telephones lines, which would provide much higher security and some level of redundancy. The pan-European counterpart to Linesman, NADGE, had semi-automated this process, but Linesman lacked the ability to read this data and plots being forwarded from NADGE had to be entered manually by voice telephone calls.
Starting in 1972, the government redirected the money originally earmarked for upgrades to the Linesman system to be used to replace it as soon as possible with a new network known as UKADGE. Marconi joined a consortium with Hughes Aircraft and Plessey which won the contract to supply the networking systems, with Marconi supplying over 200 of the standardized operator terminals based on their Locus 16 computers.
Rampart Marconi saw the introduction of the UKADGE system as an opportunity to make practical use of their stacked-beam design. They proposed a new radar with the same performance as the Linesman radars, but in a form that was semi-mobile, or "transportable" in UK parlance. Backup systems could be stored away from the radar stations and then set up rapidly in the event the station was attacked. Chain Home had only survived attacks by the Luftwaffe due to the availability of backup radar systems that could be put into operation within hours, and Marconi noted their design could offer the same capability.