Chapter six




6.3. TYPE MR 75


6.5. MR100






6.11. WATCHMAN ‘S’


Photo of London/Heathrow – The worlds first permanently equipped Airport with Millimetric (Q Band 40GHz) ASMI

During 1952-3 Decca’s ‘Radar Lab’ designed and supplied a radar, which significantly increased take-off and landing capacity at London’s Heathrow Airport, the product was also installed at Paris/Orly, Rome and Milan by providing supervised direction to pilots of aircraft moving on the ground. The radar had a scan rotation rate of 70rpm with a prf of 15 KHz. The system finally had a fixed coil display from 0.5miles to 2.5miles with an off-centering facility.


The 424 Airfield Control Radar was designed in 1952 to meet an RAF requirement for an ‘Approach Radar’. It was designated by the RAF as the ACR7D (note that Cossor also produced a radar to meet the requirement and their equipment was designated the ACR7C. It is thought that only 6 systems of this type went into service). Operating at 3cms wavelength (X-band) the first version used a 14ft. single curvature antenna fed by a ‘hoghorn’. Later versions (including the Circular Polarisation variant) used an antenna curved in both planes, but each plane following the same mathematical law, which in the profession is known as ‘an orange peel reflector’ (as against a ‘double curvature’ reflector which follows differing mathematical laws in the vertical and horizontal) and was fed by a horn. The antenna was mounted on a transmitter box (housing 2 transmitters), which in turn was mounted on a turning gear (shades of the Type 159 marine radar from which it was derived). The 20Kw transmitter operated with a pulse length of either 0.1 or 0.5 microseconds at a prf of 1000pps. The PPI displays were of the 12inch rotating coil type similar to those used in the Type 45 Marine Radar. Both mobile and transportable versions were produced.

The mobile version was mounted on a ‘Karrier’ (Rootes Group) ‘Gamecock’ chassis, which had quite small road wheels to minimise the overall height. Two models only were produced. One painted in red and white stripes as a demonstrator for the UK market, it became a general trials vehicle. The other, painted yellow for export demonstrations, was eventually sold to Argentina.

The transportable version used an aluminum cast pedestal, with outriggers giving stability to the antenna and turning gear. The Displays, the Control Panel and the two Receiver Units were mounted on a folding table in the operations room/cabin.

Photo of Static 424Mk2 with ‘Orange Peel’ Antenna and Mobile 424 Antenna

Photo of Type 424 Mk.1 with hoghorn fed antenna at Akureyri in Iceland

In the mid 1950’s a 424, of the Airfield Control type, was sold to the Vickers Aircraft Company and was installed at their test airfield at Wisley in Surrey.

TYPE 424 Airfield Control Radars were supplied to over 45 Civil and Military Authorities, a quantity of 22 in the UK and others to countries such as India, New Zealand, Ceylon and Brazil. Total sales were approximately 125.

Photo of Typical 424 Operations Room Layout

6.3. TYPE MR 75

In 1955, system integration work began at the Radar Development Laboratory at Davis Road (No.1) on a new product aimed at the Terminal Manoeuvering Radar market. The operational requirement was for coverage out to some 75nm. The resultant radar was launched as the MR75 and was classed as Medium-Range ATC radar. The MR75 mainly brought together elements from existing products.

The antenna was of the 14foot, horizontal aperture double curvature (cosec2) design, originally produced for the 424W Mine Watching Radar. The turning gear, which was to rotate at 15rpm, was from the same source. The Transmitter/Receiver, designed at the ‘White City’ under the code name ‘CROCUS’, was to be an element of a completely new design operating with a peak power of 250kW and housed in a cabinet of unique configuration. The ppi display was a new 12inch rotating coil unit.

The prototype system was installed in a towable cabin and underwent trials, at Blackbush Airport, conducted by a team from the Ministry of Transport and Civil Aviation (MTCA). The weather during the trial was hostile and the MTCA did not purchase the product.

Anti clutter measures were limited to a logarithmic receiver with ftc (fast time constant) and swept gain. At X-band (3cm) clutter was always a problem (there was no circular polarization available to reduce such clutter but this technique was under development at the time in the ‘Radar Dev’, Antenna and Microwave Laboratory). Six MR75 systems were sold to Canada and one or two to Iceland but where the demonstration model ended up remains uncertain.

Photo of MR75 Rotating Coil Display and MR75 Antenna


In the mid 1950s an agreement was signed between Decca Radar and Societe Nouvelle Electronique et Radio Industrie (SNERI) to develop and market a range of surveillance radars for civil and military applications. SNERI specialised in the design and manufacture of back-to-back antennae mounted on a turning–gear that incorporated a double channel S-band waveguide rotating joint. Decca Radar’s contribution to the partnership was the supply of Transmitters, Receiver Systems and PPI Displays.


The first system jointly developed by the two companies would come to be known as the DASR-1.It was to be aimed, mainly, at the Air Traffic Control (ATC) market. Two 650 KW S-band transmitters, of the ‘LOTUS type, were used, each alternatively feeding one of the two 25 ft. wide back-to-back antennae reflectors. The combined radar coverage was designed to provide detection of approaching aircraft out to a range of 120nm and up to a height of 30,000 feet The problem of presenting returning signals on a PPI display from alternate radar transmissions, was solved by using two diametrical opposed radials to show the radar responses, each radial representing radar returns from each of the two antennae. A view of the antenna system is shown above.

The first sales were to Sweden in the late 1950’s with installations at Arlanda airport near Stockholm, a second at Torslanda near Gothenburg and a third at Bulltofta airport near Malmo. These were followed by sales to Venezula, Singapore and Indonesia with UK installations at Clee Hill (for CAA/NATS) and at RAE Llanbedr.

An early three-dimensional radar (code name ‘CRUSOE’) was produced, using a single slowly rotating SNERI antenna with a special (Robinson Horn Adaption) horn feed to cause the beam to nutate.

6.5. MR 100

Photo of MR 100

The MR100 was a variant of the DASR-1, employing just the high beam of that Antenna. It was designed and built in the winter of 1957 and installed at Vickers-Armstrong Test Airfield at Wisley in Surrey during the summer of 1958. Operating at S-band, it was powered by a derivative of the ‘LOTUS’ transmitter operating with a range of some 120nm. This radar was specifically designed to provide the air traffic controllers visibility of aircraft undergoing test flying (eg.prototypes of the Viscount, Vanguard, Valiant, Swift and Scimitar aircraft).

It also gave the controllers at Wisley visibility of both the civil air traffic going in and out of Heathrow and the military aircraft going in and out of Farnborough.

The conclusion at that time was, with the MR100, Vickers at Wisley were operating “the best equipped private airfield in the country and possibly in the world”.


In 1959, the MOD was already conducting trials of a radar system against their own performance requirement specification and they were not content with the results. The Decca Radar Company was given 10 months to produce a competitive radar system; a product that was to become known as the AR-1 achieved this. The AR1 was configured by bringing together elements of existing radars. The antenna and turning gear design was taken from the AWS-1. A variant of the ‘LOTUS’ Transmitter/Receiver would be produced to power the system with a peak power of 650 KW. operating at S-band, with a 1.0 microsecond pulse length and a prf of 700 pps . The standard system was configured with two transmitters operating in frequency diversity (alternatively, in a single transmitter configuration). The receiver utilised a travelling wave tube RF amplifier from English Electric. The double curvature antenna was designed (cosec2 pattern) to give coverage out to 70+nm with a maximum height of 40,000 ft., (using a calibration aircraft such as the Canberra or commercial airliner of equivalent radar surface area). The antenna rotation rate was 15 rpm and circular-polarisation, at S-band, was introduced for the first time.

The AR-1 was designed to meet both military and civil airfield approach control requirements with many different display configurations, utilising the new Mk.5 Fixed Coil Display.

Photo of AR – 1

The AR- 1 radar became an international success with sales reaching 108 systems, of which 32 were to the U.K. MOD for use on RAF and Royal Naval stations in the UK and overseas. Much of this success can be attributed to the introduction of the transistorised Moving Target Indicator system - a world wide first into production. Details of both MTI and Display technology employed on the AR-1 are given in Chapters 14 and 12 respectively.

Photo of The ‘Lotus’ Tx/Rx updated to employ a TWT Receiver and a STALO for MTI Operation

Photo of Project ‘Patsy’ – the AR-1 Mobile Configuration

Photo of ‘Mobile’ AR-1 Transmitter and Display Cabins


Photo of ACR430 Display and ACR430 Antenna

The ACR430 radar was designed in the mid 1960s as a replacement for the Type 424 radar to meet the requirements of local area surveillance and aircraft approach sequencing. It comprised an antenna mounted on a turntable capable of selectable rotation rates of 20 or 40 rpm. Two X-band transmitters were utilised, each delivering a peak power of 50KW with selectable pulse lengths of 0.1 microseconds or 0.5 microseconds at prf’s of 3200 pps or 1000 pps respectively. The output of the transmitter/receivers, fed a single reflector via a twin horn assembly providing a two-beam coverage. One horn illuminated the reflector to give a cosec2 pattern for general surveillance; the other horn partially illuminated the reflector to produce a pencil beam for ‘Approach’ radar surveillance. Circular polarisation was used to reduce the effects of rain clutter.

Plessey Radar had for some time been using a technique for manufacturing complex waveguide components by spraying metal onto a mandrel and supporting the resulting structure using glass reinforced plastic (GRP). After the curing of the two- part mix resin, the mandrels were extracted.

Traditionally, radar reflectors had been manufactured using metal alone. Where the reflector is double curved, as with the ACR430, the manufacturing operation became expensive, so it was decided to apply the Company’s GRP expertise in the manufacture of the ACR430 reflector, thereby reducing costs significantly.

Extensive tests were carried out at the Cowes site to ensure that the required dimensional accuracy of the reflector could be maintained during manufacture. It is believed that this was the first time the method had been used for the construction of radar reflectors. The same manufacturing method was also adopted at a later stage for a 45 ft. Communication Satellite Ground Station Antenna and the Dagger/Rapier Radar Antenna.

A significant number of ACR-430 systems were sold, as were its design derivative Coastal Defence Radar Type CDR-431, dealt with under the Defence Radar Section of this book.

An update package was designed for the ACR-430, comprising an analogue MTI, along with a higher performance display sub-system that was sold to enhance a great many of the existing ACR430 installations worldwide.

Webmaster's note: We have a copy of the Sales Manual for the ACR430. This gives a lot more of the technical details and operation of the radar. To view and/or download please click here.


If the AR-1 radar had outstanding success in the 1960’s and WATCHMAN was its equal in the 1980’s then AR-15 was the radar of the 1970’s. Some 54 systems were sold to existing and new customers as each expanded Civil Airport and Military capabilities. When observing the AR-15 radar antenna or the transmitter/receiver cabinet, little difference from the AR-1 could be detected, but coverage was enhanced to almost 80nm and 50,000 feet. The AR-15 brought to the market a much improved system performance (inclusive of stability and reliability), the parametric amplifier noise factor would not be worse than 2.5dB (FET Amplifier). A tuneable/motorised Magnetron was employed. Transistorised ECCM Receivers were optional. EHT generation was Solid State where the glass enveloped rectifiers had been designed out. Digital MTI used double cancellation, obtaining a 33dB sub-clutter visibility ratio. Digital pulse-to-pulse integrator video processing ensured outstanding clarity on the displays.

Photo of a variant LOTUS Tx, Modulator Cabinet and RF Head

The AR-15 was subjected to a ‘value engineering’ assessment, which resulted in a number of important benefits: the parallel (vertical) tower easing flexibility in providing various tower heights. The turning gear was reconfigured and the cabin at the top of the tower was designed out (this configuration can best be seen in the Watchman photograph, (page 42) for the new design was carried forward there).

Within the marketing cycle of the AR-15 the AR15-2 variant introduced ‘twin-beam’ technology. The second, or high beam, was designed as a receive only beam pointing 5degrees above the main beam, it was switched in range using a common receiver for short range targets, significantly reducing ground clutter due to its upward pointing. The horn generating the second beam was mounted under the main feed horn.


(Also inclusive of the AR51, ‘Routeman’ and ‘Pegasus’ L-Band Transmitters for China).

Although decades apart, to do justice to the family of L-Band Radars, their details have been consolidated in this chapter. Viewing the radars skyline images no difference could be detected between them but their transmitter technology and signal processing techniques were a lifetime apart.

12th December 1967 was the date when pictures were first seen on the radar display of an AR5.

Previously both Decca Radar and its successor Plessey Radar had almost exclusively used S-band for ATC and Defence surveillance radars. The AR5 was the first product for which the company had used L-band. The longer wavelength of 23cms, compared to S-band (10cms) produced lower returns from precipitation such as rain, hail and snow. This was a very useful operational feature, particularly for radars deployed in countries where medium to high precipitation levels could be expected.

The antenna was 50ft.wide and 18ft high, resulting in a horizontal beam width of 1.2 degrees at half-power points and a vertical beam of 1.2 degrees with cosec2 profile for high coverage. The reflector and the double feed combination directed most of the transmitted energy to provide long range low cover but also provided a high level cut-off coverage up to 30 degrees. The antenna rotation rate varied from 4 to 14 rpm depending on customer requirements.

The AR5 transmitter used a tunable magnetron delivering a peak power of 2MW. The prf and the pulse length could be selected from the wide range offered, in order to meet a specific operational requirement. Pulse length: in the range 2 to 5 microseconds and prf in the range 750pps to 250pps with up to a seven period stagger.

Photo of AR5 at Nairobi and AR5 at Clee Hill Shropshire.

The pictured installation of the AR5 at Clee Hill would typically monitor en- route air traffic from Scotland in the North to the Paris region of France. The dual AR5 installation at Burrington, North Devon, would monitor the first or last 300+ miles of Concord’s flight in and out of Heathrow to and from New York.

Installations of the AR5 in the 1970’s were typically Clee Hill, Burrington (2), Dublin, Nairobi, Dubai, Brunei, Abu Dhabi and Bahrain.

Photo of The AR5 Transmitter

After the best part of two decades the product AR5 was updated to AR51. While the Antenna/reflector remained unchanged it was the signal processing, displays and transmitter where the enhancements were engineered. Subsequent customer orders each required specification variations and so what started as AR51 became the ‘Routeman’ family of products. The Transmitter enhancements were an ‘up-sizing’ of the Watchman technology.

Photo of The Routeman Transmitter

Variations within the Routeman family of products extended beyond the transmitter design where, for example, some would employ MTI systems and others AMTD (Adaptive Moving Target Detection).


The Company had a long history in the worldwide ATC market, the foundation to their superb export marketing skills. The RAF were looking to replace their 38 airfield surveillance radars and this, therefore, represented a ‘must win’ opportunity for the Company. The WATCHMAN radar, under company private venture funding, was in development and represented a major step forward in ATC radar performance. The new radar featured a coherent transmitter, advanced digital signal processing and a completely new display system.

The Company’s main design and supply competitor had developed modernised traditional Air Surveillance Radar with a magnetron transmitter. This was not a threat from a radar performance point of view but it would come at a lower cost, a factor that the MOD always weighted heavily.

The Plessey Radar management team decided to counter this threat by bidding its own low cost, magnetron based, system using many parts common to other radars. Secrecy was essential to keep the competitor from discovering the Plessey strategy. Within the Company complete secrecy was maintained, this to the point that even the corporate management did not know of the second proposal until they were required to finally review the bid and approve the cost implications. In the event the customer was surprised to receive two proposals.

The postscript to this story is that the second ‘low cost’ radar nearly won; it had deliberately been priced very keenly and the MOD was always keen to save money. Fortunately the WATCHMAN proposal had a classified section illustrating the performance of the two radars when being jammed by a Soviet airborne standoff jammer (the ‘Click’). The dramatic loss of range coverage with the magnetron system convinced the RAF to go for the WATCHMAN and the MOD was happy that Plessey offered the best value for money.

The Watchman radar was developed in the early part the 1980’s as a successor to the AR1 and AR15. It was decided to invest in the design of an up-dated Airfield and Approach Radar, which would take advantage of the significant advances in solid-state technology and microwave power sources developed in the last decade. The product would be known as WATCHMAN.

Photo of Watchman Antenna with on-mounted LVA (Large Vertical Aperture) SSR Antenna at Heathrow

The WATCHMAN antenna has the following characteristics: A double curvature antenna (an improved, spot-welded replica reflector of the AR-1) providing an azimuth beamwidth of 1.5 degrees and an elevation cosec2 shaped beam backing-up to 30+degrees. The reflector has two feeds displaced vertically, providing dual beams. The upper feed operates in transmit/receive mode whilst the lower feed is ‘receive’ only. The vertical feed displacement produces high and low elevation beams. The antenna rotates at 15rpm.

An S-band transmitter uses a ‘coupled-cavity TWT’, which delivers 60KW peak power to the upper of the two feeds. The received signal from the lower feed is amplified by a low-noise FET RF amplifier forming part of the antenna assembly and is connected to the main receiver by a low-loss elliptical waveguide.

The outputs from both the high and main beam Field Effect Transfer (FET) RF amplifiers are passed to the Signal Processing Cabinet, which includes both digital MTI and plot extraction, thence via modems to a two-core fibre-optic link to the Display Interface Unit. This unit provides outputs to a number of ‘Series 9’Autonomous Displays (manufactured by Plessey, Addlestone). Multiple use displays could vary from 3 up to 16, depending on operational requirements. Watchman was usually supplied for static installations but mobile versions were available, and the product could also be supplied in an air transportable configuration.


In 1982 a production Watchman was unveiled at Farnborough. In the same year the MOD placed an order for an initial 30 systems and the first equipment delivery from that order was to the RAF at Lyneham. The MOD ordered Watchman for all RAF and UK Royal Naval Air Stations, also for the research facility airfields at Boscombe Down, Farnborough and RAE Bedford.

Plessey, in 1987, were contracted by the Civil Aviation Authority to supply a WATCHMAN for their Cromer site in Norfolk, to control traffic to and from the North Sea oil fields. Following this, there was an order for a further 6 more Watchman radars for the CAA. There were several orders from the UK non-state airports such as Leeds/Bradford and Teeside. Many orders from overseas flowed in, for example from Ghana, China, Spain, Oman, Dubai, Bahrain, Finland, Switzerland, Saudi Arabia (both fixed site and mobile systems) and India. The USA also placed an order for a Transportable Watchman system (this for performance assessment) but it spent its life monitoring drug trafficking into the Caribbean.

More than 150 Watchman systems were sold and installed before the manufacture of Watchman ceased at Cowes. The root cause of this termination of production was the take-over of the Cowes element of Siemens-Plessey by British Aerospace (1998), and the transfer out of the Watchman manufacturing rights to the Finmeccanica/ Alenia Marconi joint ventures.

Photo of Internal and External views of a Watchman Transmitter.

During the early 1990’s solid-state (SS) power amplifiers were experimented with as a replacement for TWT’s. Using a multiple set of combined SS amplifiers had the advantage of power modules being ‘hot replaced’ when the transmitter exhibited progressive or solid failure – and the transmitter life could be extended indefinitely. Watchman proved to be its own worst enemy for the outstanding reliability of the TWT system meant that its main customers (the RAF) own cost benefit analysis of the offered SS transmitter concluded that if the TWT was not left in stand-by for long periods then the SS transmitter was not cost effective and therefore they showed little interest in a SS upgrade. Further development work was undertaken as the transistor supplier proved unreliable and the redesign of ‘pulse compression’ and signal processing was enhanced to accommodate the much longer pulse of the solid-state transmitter over the higher duty TWT. It was some 10 years before a true solid state Watchman was introduced into the product range.

6.11. WATCHMAN ‘S’

Finally in the summer of 1994 Siemens/Plessey unveiled a solid-state transmitter version designated WATCHMAN (S). A prototype was built and tested at Cowes and displayed at the Farnborough Air Show of that year, but it only went into limited production.

Photo of Coverage Diagram Height in thousands of feet and Range in nautical miles

Elements of the Watchman ‘S’ system, for example the CRP (Carbon Reinforced Plastic) Antenna, the most accurately reproduced reflector ever produced at the Cowes site, were installed at the airport on the Isle of Man. Full systems were manufactured at Cowes and today give total customer satisfaction whilst meeting the highest operational standards on both of the Islands of Jersey and Guernsey. The CRP antenna was the first designed solely by CAD, cut by a numerically controlled machine and tested on the Cylindrical Near-Field Test Facility at Cowes. (See CNTF page 138).

Photo of  WATCHMAN ‘S’Operational in Jersey, WATCHMAN ‘S Solid State Transmitter and WATCHMAN ‘S’CRP Antenna with On-mounted LVA SSR Antenna

The picture of the Watchman ‘S’ Transmitter displays the modular design approach to the transmitters power generation circuitry, which overcame the difficulty of power handling, insulation and maintainability prevalent in prior generations of radar transmitters.


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