Chapter Fourteen




14.3. STORK


14.5. SIGMA





It is believed that Electronic Warfare (EW) was first employed as a preliminary to the battle of Jutland. In the days preceding 31st May 1916, Admiral of the Fleet Sir Henry Jackson deployed coastal radio direction finders to successfully detect German Fleet movements. The changes in the apparent angles of arrival of radio transmissions from the German Fleet were very slight, but Sir Henry dared to move and deploy the British fleet on the basis of this new information source with great success. This was an early example of Electronic Surveillance Measures. (ESM)

The British and American authorities immediately co-operated on developing new techniques leading to further more extensive deployments in the Second World War (WW2).

During the Battle of Britain the Germans developed and deployed an extensive network of short wave (200 KHz to 900 MHz) navigational stations in northern France. Navigation beams were directed over London and aircraft with quite rudimentary receivers would simply follow the beams to their targets. This was very successful, the system was known as Lorenz. After considerable study the British deployed a counter measure called ‘Meaconing’. Combinations of receiver and transmitter stations were deployed 5 to 10 miles apart. The receiver picked up the German beam and sent the signal via landlines to the transmitter, which re-transmitted the beam, thereby masking or distorting the original beam to great effect. Many German planes got lost and landed at British fields. This was an early example of Electronic Counter Measures (ECM) deception.

Both sides in WW2 were frantically developing aircraft detection systems. The operational requirements and performance specifications for civil and military radars were similar and at times equipment was used in both roles. Scientists and engineers were working to maximise the detection ranges of such radars. In a hostile situation there is a need to be able to spoil (reduce) the radar performance so that attack intentions cannot be easily interpreted. One of the very early methods of spoiling radar performance was, in WW2, the use of ‘Window’(USA name) and ‘Chaff’(U.K. name). These two passive devices would show on the radar screen as extended echoes. Dropped by ‘scout’ aircraft and drifting on the wind, metallic paper strips (of a length relative to the radar’s wavelength) obscured aircraft radar reflections. This had some success but was very dependent on weather and needed aircraft to deploy the chaff. There quickly grew the need for electronic active jamming, which could be switched on to interfere with the target radar receiver and reduce the detectability.

Germany countered by deploying more complex signal patterns, whereupon the British further countered with more complex ECM. So the Electronic War with ever increasing counter /counter moves had begun and has continued with great complexity ever since.


Plessey Radar undertook the development of ground based electronic warfare capabilities in the following areas:

  • Electronic Counter Counter Measures (ECCM) - the means by which a ground radar can protect itself from jamming.
  • Electronic Counter Measures (ECM) - the means of jamming a ground radar after detecting its parameters.
  • EW Command and Control Systems - for collecting data on the overall electronic environment; deploying and activating the ECM.


During the early 1970’s Plessey Radar was successfully supplying highly complex Air Defence and tracking radars. Potential adversaries were developing ECM systems aimed at blinding or confusing those Air Defence radars. Plessey engineers therefore developed a number of techniques built into their radars to nullify or reduce the ECM threat. These ECCM features included:

  • Narrow beam antennas with low side-lobes
  • Constant false alarm rate receivers (CFAR)
  • Wideband frequency agile coherent systems
  • Sophisticated digital signal processing

It became apparent that, world wide, there were many older generation Air Defence radars in service, supplied by other manufacturers that had no ECCM capability and were therefore wide open to ECM jamming threats. This situation was seen as a significant market opportunity. A range of packaged receivers were then developed, employing techniques used in the Company’s own radars, which could be sold for retrofit to any radar to improve performance against jamming. These ECCM receivers were designated the R405 and R505 series (see Chapter 15.2 for detail)

A need also arose to be able to test these ECCM measures and train operators in their use. This led to the development of a range of low power ECM Simulators. Following a number of successful ECCM contracts, the Company gained follow-on contracts to update display/ processing and other radar system elements.


Plessey Radar became firmly involved with specialist EW users and it was but a short step to enter the ECM market. During the product development phase extensive modeling and trials were undertaken in conjunction with RSRE at Malvern. The wider bandwidth requirements of EW demanded a different approach and new ranges of component technologies. A wide spread of component suppliers were contacted to develop such components. For example, the widest band, high power CW Travelling Wave Tube Amplifiers were drawn from the U.S.A airborne programmes. A particular problem for ground-based systems was excess path loss over free space, especially in wet hilly terrain. However, systems deployed in the Middle East proved very effective, particularly when part of an overall Command and Control system.

Photo of High Power CW Travelling Wave Tube Amplifier

Developing a family of High Powered Mobile Jammers, capable of operating against pulsed, agile and continuous wave radar transmissions provided ECM. These were deployed by EW Commands to detect, locate and disrupt or jam an addressed radar. The mobile jammers had built-in ESM receivers locating, identifying and recording the victim radar characteristics. The direction was measured and the radar position calculated by triangulation. The optimum jamming transmissions, to disrupt the radar, were programmed and when required, transmitted at high power. The ECM systems had a number of modes that could be selected to counter specific targets or to deceive an opposing electronic intelligence system. The system offered spot or barrage jamming, noise, pulse and deception modes for tactical misinformation. Receiver sensitivity typically allowed the jammer to operate at a range of 100km from an addressed radar.

It is essential that ECM equipment can be rapidly deployed and then very quickly dismantled and moved, as once a jammer has radiated it can be detected and receive a homing missile. Deployment of the equipment would typically take ten to fifteen minutes, with dismantling taking a similar time. The ECM equipment was therefore designed to be housed on a long wheelbase vehicle such as a Land Rover. This gave room for two operators, with a portable power supply generator, and cables, stored in the rear of the vehicle during transit, and each system rack was mounted on slides for easy removal and maintenance. The receiving and transmitting antennae were mounted on a folding mast. The antennae were adjustable in azimuth and elevation for pointing towards the victim radar. (More detail is provided later in this chapter).


It became apparent, after experience with larger EW Commands, that a sophisticated Command and Control System was required to collect and interpret ELINT (Electro-magnetic Intelligence) data from radar detection units and COMINT (Communications Intelligence) data from radio detection units and provide an up-to-date status of the ‘Electronic Environment’ prior to initiating multiple deployments of mobile jammers. These systems had an operational modelling capability, providing electronic environment mapping and simulation. This enabled real time simulation of the impact of jamming in reducing the victim radar’s coverage. Further, by taking into account the topography of the operational zone, it could be shown how the local terrain obstructed radar coverage penetration. This was vital, as active jamming provides a positional beacon to the enemy.

EW Command and Control systems were configured with two, separately located, parts. The first part, termed the ‘data filtering sub-system’, received all the intelligence ‘field data’ (over landlines or radio) where it was filtered and collated in a sub-system database. This sub-system was strategically located in a trailer mounted mobile cabin, with the option of helicopter lift for redeployment. Sophisticated software displayed the received data on display consoles and attempted to identify and categorise the data source. The operator would use his EW knowledge and experience to verify or modify the software conclusions and transfer the filtered data details into the sub-system database. In major systems there would be additional sub-systems. The second part of the system was the Electronic Warfare Operations Centre (EWOC). It comprised multiple system display consoles, was located in a more permanent base area and was the hub of the overall system. The EWOC system database regularly interrogated the sub-system database computers via landline and radio.

EW Commanders were able to use the software facilities mentioned above to conduct multiple warfare scenarios on the EWOC computer display screens, simulating real-time battlefield conditions prior to deploying and activating the ECM jammers. Coordination of the operation was tightly controlled by the EWOC and the ECM jammer units were urgently withdrawn to avoid reprisal attack. Mobility was the key. The system information was also presented in local language along with their specific military labels, this to describe friendly and enemy radars, weapon types and concentrations. The design and build of the EW Command and Control systems was at Addlestone and Chessington (where programmes such as Leopard and Cheetah were notable) whilst design and build of ECCM and ECM was at Cowes (where Goshawk, Merlin, Raven and Fox were key programmes).


The Type R405J is an example of a ground-to-ground mobile ECM system. It included an ESM receiver to detect and analyse the characteristics of an addressed radar and ECM equipment to provide various types of jamming. The receiver system, with its processing and display unit, analysed received signals and displayed radar parameters in terms of pulse length, prf, dwell time, radar antenna rotation rate and accurate bearing. The transmitter/receiver control unit received inputs from the processing unit and selected the waveforms required by the transmitter. The types of counter measures selectable included CW (spot frequency), Swept CW (band-width controllable), EM Noise (band-width controllable) and Pulse (prf and pulse length controllable). There was also the capability to produce false targets in both range and velocity. An azimuth deception facility was included in both the ECM and the false target channels of the transmitter. As the R405J was designed on a modular basis, various types of ECM were included to meet particular user requirements.

The R405J was supplied to cover any one of four frequency bands: 1GHz to 2GHz, 2GHz to 4GHz, 4GHz to 8GHz and 8GHz to 12GHz.In some cases the frequency coverage was extended to 16GHz.The receivers were front end frequency swept units with RF and IF filtering, which provided precise frequency analysis. The selected jamming signal sitting on the victim radars spectrum was amplified up through the high power broadband travelling wave amplifier and transmitted through a high gain directional antenna, focusing a typical ERP of 50dbw on the victim radar.

A coastal version of the R405J was designed to combat the threat from enemy sea borne radars and covered the frequency bands 4GHz to 8GHz and 8GHz to 16GHz.In this case the equipment was fitted into an air conditioned transportable shelter which carried the antenna on a folding mast. For transit purposes it was carried on a prime-mover vehicle and was fitted with mechanical jacks to enable its removal and lowering to the ground for semi-static operation. Technical data for the antenna, receiver, transmitter, display, power supply and environment is listed below.

Photo of


Polarisation 45degree linear
Nominal mid-bandwith: Receive Transmit
Band L 12degree 8 degree
Band S 10degree 6degree
Band C 6degree 3.5degree
Band X 4.5degree 4.5degree


  • PRF: 50pps to 10,000pps
  • Pulse Length: 0.1µs to 5µs
  • Sensitivity (noise power in 1MHz bandwidth at receiver input, excluding antenna gain and feeder losses: Not greater than –82dB in mid band
  • Receiver sweep rates Min.20ms across band Max 20sec across band
  • Detection threshold level Nominally set 12dB above noise
  • AFC lock in range :t40-1Hz


  • Frequency: Tracking input signal to 2MHz relative to the received frequency
  • Types of ECM:
    • (a) Pulse CW on frequency
    • (b) Swept CW
    • (c) FM noise
    • (d) Pulse on frequency
    • (e) AM modulation
  • Transmitter output stage: TWT amplifier
  • Mid-band ERP
    • L Band 50.4dBW
    • S Band 52.1dBW
    • C Band 51.5dBW
    • X Band 47.5dBW


  • Frequency: 8-digit display on frequency counter to within 500Khz when receiver is locked to received radar.
  • PRF 3 decade display
  • Pulse length 3 decade display
  • Antenna rotation rate: 4 to 60 rpm 3 decade display
  • Dwell time µsec 3 decade display
  • Radar azimuth 0 to 360degree 3 decade display


  • Voltage 220v +/- 6%
  • Frequency 50 Hz
  • Source 5Kva Petrol Generator


  • Operating temperature 0 degree C to 50 degree C
  • Storage temperature -40 degree to +70 degree
  • Relative Humidity 90% max
  • Mechanical 6g shock
  • Wind Speed:-
    • Survival 130 Km/hr
    • Stall 90 Km/hr


The S-band R405 jammer was deployed at St Catherine on the far south of the Isle of Wight and targeted on a radar on the Cowes site, at a range of some 25 km. The trial was a technical trial, not operational and the victim radar had only minimal ECCM fitted. The radar was frequency coherent with pulse length discrimination and digital MTI, although from the photographs the MTI appears to be off. The radar was probably representative of radar performance of that era. Later Air Defence radars would have more ECCM fitted reducing the jamming effect.

Photo of Fig 1 and Fig 2

Fig.1 Swept CW with 10MHz deviation - The CW jamming signal was locked to the received radar and constantly swept with a 10MHz deviation. The antenna main beam can be seen south noting that the receiver is saturated. Most side lobes are saturated and the swept effect can be seen as a series of ‘race track’ images.

Fig.2 As with Fig.1 but with jammer ‘look through’ - Look through pulsed the jammer off, enabling the jammer operator to monitor the radar whilst jamming. This reduces the energy in the antenna side lobes.

Photo of Fig 3 and Fig 4

Fig.3. Noise with 10MHz deviation - Broadband white noise with a 10MHz deviation saturates the radar receiver through 360 degrees.

Fig.4. Noise with look through

Photo of Fig 5 and Fig 6

Fig. 5 anf Fig. 6

Fig. 5 Pulse jamming 10KHz prf and 50-microsecond pulse width. The antenna main beam is saturated and much reduced effect in far out side lobes.

Fig. 6 As 5 with look through.

Photo of Fig 7 and Fig 8

Fig. 7 Single Target
The jammer is synchronized to the radar rotation and prf and flies a dummy target pulse towards the radar by introducing, a time delay at the set target velocity. The photograph is time lapsed, the target can be seen flying down the main beam.

Fig. 8 Multiple Targets
Introducing a delay on the antenna main beam enables targets to be generated at offset angles assuming the jammer pulse is powerful enough to break into the antenna side lobes. A target can be seen flying down the main beam and other azimuth off-sets, although the photograph is not so clear.


In the mid 1960’s the UK Ministry of Defence (MOD) conceived a Space Communications System that would become known as SKYNET. In response to the need for this new communication technology, Plessey Radar established their Space Communications Division, led by Dr. Kenneth Milne, at Cowes on the Isle of Wight.

SKYNET (The name is an extraction from ‘sky communications network’) was to be different in two respects from its American equivalent, the IDCSP (Interim Defence Communications Satellite Programme). Firstly, the SKYNET satellite would be in a fixed orbit, rather than being allowed to wander like the US satellite. Secondly, equipped with a transponder it would have the ability to function with two channels of communication active at the same time, this giving the satellite greater flexibility in using two types of earth station at once.

The operational deployment of SKYNET in 1970 completed an MOD programme, which began in 1962. It was described by Air Vice Marshall Les Moulton, the AOC of No.90 (Signals) Group of the RAF, responsible for operating the system, as “representing a very exciting and new future in space communications”. The origin of SKYNET, as explained at a briefing at RAF Medmenham, in September 1969, by Gp Capt Frank Padfield (responsible for the product at MOD) went back to 1963-64 when, as he put it, “we were looking for answers to British operational needs”. In 1964 the decision was made that the communications satellite should be in stationary orbit (as opposed to the USAF concept) over the Indian Ocean. Gp Capt. Padfield emphasised “how remarkable it was that the system had been brought into operation just 3 years after the system definition had been finalised”. It was the Signals Research and Development Establishment (SRDE) at Christchurch, that undertook a detailed systems study (completed by the end of 1966) to determine exactly the configuration of the SKYNET system.

SKYNET comprised two satellites over the Indian Ocean (one active and the other being used as a standby) and nine earth stations, of which the principal stations were at RAF Oakhanger (near Alton Hampshire), Cyprus and Singapore. These all had 40ft.dishes and 20Kw transmitters. The other stations were at Bahrain (in the Persian Gulf) and Gan (in the Indian Ocean). These stations were known as SKYNET V and had 20ft. dishes, 5Kw transmitters and were air transportable. There were two additional air-transportable stations, which could be lifted by Hercules aircraft, and two sea-borne stations, each having a 6ft. dish and 5Kw transmitter, which were initially installed aboard HMS Fearless and HMS Intrepid.

It was the design of the sea-borne element that Plessey Radar’s Space Division contributed to the SKYNET programme, but they also undertook the manufacture of some 40ft. dishes, although these were not to our Company’s drawings and were installed, typically at RAF Oakhanger and Chilbolton. Early in 1966, the Admiralty Surface Weapons Establishment (ASWE), Portsmouth, placed a contract for the development of a Naval Experimental Satellite Terminal (NEST). This was a joint programme between ASWE and Plessey Radar at Cowes.

ASWE provided a gun mounting with azimuth and elevation drives, a stabilisation system using Ferranti gyros and the ship interface design. Plessey Radar was to design and produce the transmitter-receiver system including a 6ft. (1.8m) dish, the monopulse tracking feed, the tracking receivers, extended threshold FM demodulators and the baseband system. The 20Kw CW klystron transmitter was mounted on the antenna, with the power supply for the klystron in a cabin adjacent to the antenna mount. A heat exchanger to cool the klystron and the waveguide, together with a pressurised air system to prevent waveguide RF breakdown, were also mounted close to the antenna. In order to ensure that the receiver noise and temperature were maintained as low as possible, a Mullard parametric amplifier, cooled with liquid helium, was fitted as the head amplifier. An additional cabin contained the tracking receivers, modulators/demodulators, base-band and control equipment.

The NEST equipment was designed from scratch at Plessey Radar Cowes, where it was manufactured and tested and then fitted into HMS Wakefield in early 1967, all within a 13 months time frame. This was a remarkable achievement by all concerned.

Prior to installation on the ship, the complete system was tested at Cowes, utilising a dummy satellite transponder, provided by SRDE from their test works at Christchurch, and mounted on the cliffs at Alum Bay on the Isle of Wight. Luckily the Christchurch facility could be seen from the Alum Bay site. Trials on HMS Wakefield were successfully completed in the summer of 1967 and a further contract, for the development and supply of Production Systems, known as SKYNET V (ship-borne terminals) was concluded, with the installations going on to HMS Intrepid in 1968 and HMS Fearless towards the end of 1970. The SKYNET V terminal was also known as UK/SCC 001. Its purpose was to provide simultaneous two-way communication with other SKYNET stations, and with stations using IDCSP satellites.

The SKYNET V sub-system incorporated transmission and reception equipment together with satellite acquisition and tracking facilities. The station consisted, essentially, of two self-contained compartments relying on a minimum of ship support services. The antenna mounting incorporated a 6ft. parabolic reflector that handled the circularly polarised communication and tracking signals in the frequency band 7.25 to 8.4Ghz. The power supplies to the station were 440v, 3phase, at 60Hz and 55Kva; 115v, 3 phase, at 60Hz and 8Kva also 115v, single phase, 400Hz at 1Kva. Throughout the 1970’s, until the small ‘SCOT’ terminals became available, the two Plessey built SKYNET V terminals were moved between RN capital ships deployed to the Middle/Far East, thus ensuring reliable communication with the UK at any time. The ships that received the equipment were, typically, HMS Intrepid, HMS Fearless, HMS Hermes and HMS Ark Royal.

During the project programme a completely new design was configured for the pedestal stabilising and steering of the antenna. To predict both pitch and yaw, servo/gyroscope control systems were employed with an accelerometer cluster as reference. Dry air and cooling water systems were also provided to guard against the waveguide run breaking down under full RF power.

The station facilities included:¬
a) Frequency Division Multiple Access (FDMA) capability multiplexed as:-
Mode 1- one speech channel plus 3, 75 band frequency shift keyed (f.s.k.) telegraph channels. Mode 2 - 6, 100 band plus 3,75 band f.s.k. telegraph channels.
b) Terminal Equipment Capability:¬
Limited to 3,75 band transmit/receive channels at the Main Communications Office (MCO) and 1,75 band transmit/receive channel for an engineering link at the Satellite Communication Control Office (SCCO).
c) Satellite ‘Power-down’ link power sharing between the various Skynet terminals.

Transmission Characteristics:

  • a) Input - baseband signals
  • b) Modulation - frequency modulation (fm)
  • c) Frequency - synthesised carrier 7.9-8.4 Ghz
  • d) Radio Frequency Power Source - 0-100mW (phase locked klystron oscillator)
  • e) Power Output - 0.1-5.0 kW

Reception Characteristics

  • a) Inputs - communication and beacon signals
  • b) Radio Frequency Range 7.25-7.3 Ghz (processed by a monopulse comparator)
  • c) Amplification -
    • 1) Sum signal amplified by a low noise parametric amplifier with phase locked klystron local oscillator.
    • 2) Azimuth and elevation tracking signals amplified by low noise tunnel diodes.
  • d) Intermediate Frequency Range 45-95 MHz

The station consisted of two major units:-
1) The Mount Support Cabin (MSC, also known as Antenna Power Supply Office, Satellite Communications) - which included the antenna mount carrying the 6ft dish and monopulse feed, the receiver head amplifiers and the high power transmitter tube. The cabin was 12ft 6in long, 8ft wide, 7ft high and weighed some 8.5 tons.
2) Satellite Communications Control Office (SCCO) – containing the transmitter power supply, the tracking receivers, communications receivers, modems and control equipment. This cabin was 16ft 6in long, 8ft wide, 8ft 9in high and weighed some 13 tons.

The antenna mount rotated ± 270° from Ship’s Head in azimuth and covered -25°to +115° in elevation. The antenna beam was 3° wide at –3dB, with side-lobes at least 15dB down.


Although the Decca/Plessey Radar Company had initiated ‘private venture’ investment in the Sat. Coms. business, their first product, while gaining them considerable knowledge, publicity and respect did not secure them a foothold in the world market. The Company marketed their Sat Coms Ground Station as the Type 45, (taking its nomenclature from the diameter of its 45ft. dish.). The system was conceived to facilitate communication between pilot and ground control and also provide flight passengers with conversational links to the ground. The mechanical construction of the dish used the Company’s unique GRP backed sprayed metal reflecting surface technique, where a number of segments were used to encompass the 360degrees, each being 22ft in length and 50sq.ft in area, with their profile accuracy held to within one sixteenth of an inch of the required overall dish profile.

Photo of Satellite Communication Antennae

The performance of the high-gain, high efficiency antenna developed by Plessey Radar owed much to the corrugated waveguide feeds developed by the company, where side-lobe performance was outstandingly better than conventional feed horns. Such feeds were supplied with integral monopulse tracking facilities and manufactured using techniques in which Plessey Radar were specialists.

Photo of SKYNET V (Installed on HMS Fearless) NEST (Installed on HMS Wakefield) The SKYNET Master Ground Station, 40 foot diameter dish installed at RAF Oakhanger and the 45 foot Sat-Coms Dish, of Metal Sprayed manufacture, installed at Cowes.


In the early 1960s Decca Radar was awarded a contract to design and build a long pulse transmitter for use with an experimental programme to be controlled by NATO/SHAPE in Holland. This consisted of a low power TWT feeding a high power output TWT. The output stage was water-cooled. The main parameters being set out below:


  • Transmitter Band-width: 2900 -3100 Mhz
  • Maximum Pulse Length: 31 microseconds
  • RF Output: 100KW peak, 1 KW mean

The transmitter could operate in a multi-mode format as follows:-

  • Single- Pulse: 20 microsecond, 500 pps.
  • Double- Pulse: 210 microsecond pulses at 500pps,

The single or double mode format could be selected from an external modulation trigger unit.

A phase measuring system was also built as part of the programme to measure the phase modulation performance.


Both of these products were designed in the early 1960’s for use at the UK’s rocket flight testing range at Woomera, Australia, where the Black Knight and Black Arrow products were being evaluated.

The projects were UK Government sponsored and were co-ordinated by RAE at Farnborough.

TANDEM – was a ‘Range Control’ radar, produced as a variant of the DASR-1. It was containerized in an ‘AIRTECH’ Cabin (the start of a good relationship between the Companies).

Tandem was one of the last projects completed at Davis Road No. 2 before the group moved to the Isle of Wight.

Photo of inspection of cabin.

‘STARFISH’ was a research project, where after the design phase RAE had the production quantities manufactured by an enterprise other than Decca Radar.

The requirement was to track rockets returning to earth (remembering that launch and return were almost vertical). Tracking was essential to retrieval both for analysis and keeping the range clean. The principle of operation was to ‘flood’ the area with RF and through a significantly high number of receivers, (arranged in a cruciform configuration at ground level) a number of the receivers would provide reference to the spent rocket’s location.

14.5 SIGMA


Early in 1992 SIGMA was taken on to the market by the Radar Company, then operating under the Siemens banner. It was primarily a product of Roke Manor design but mainly for QA Controls within an approved manufacturing unit. SIGMA was moved to Cowes where it gained engineering design support while in production.

SIGMA was a product that could measure and store the dynamic radar signature of ships, aircraft, land vehicles, chaff and background clutter. It was integrated with Epsilon RCS Prediction Software. (Storing such profiles of all friendly and hostile weapon platforms resulted in a very sophisticated form of IFF).

SIGMA incorporated two independent radars: Measuring Radar obtained and extracted reflection data and Tracking Radar acquired and maintained a lock on the target. An in-built CCTV system allowed visual acquisition and identification of targets. An additional surveillance radar provided further target acquisition and general assistance. Powerful Signal Processing of measured data gave a real-time display, but more complex analysis would be done off-line with hard-copy results. The equipment comprised three parabolic dishes and was housed in an air-conditioned trailer, but could be deployed to fixed sites or ship born platforms. Operation was controlled by advanced menu-driven software. Additional displays provided real-time monitoring of key parameters.

Photo of SIGMA

SIGMA – A Multi-band pulsed radar

System Data:

  • Wide-band operation – dual-polar 8 to 18GHz - Extendable to 3GHz and 35GHz (S-band)
  • Multiple Measuring Modes – Synthetic high-resolution target signature analysis
  • Target Tracking – In-built two-axis monopulse tracker. Variable servo bandwidth.
  • Computer controlled operation – Automated calibration procedures. Auto-step frequency and polarisation.
  • Colour graphics presentation.
  • Tx. Power (RCS Mode) 1KW. High-resolution mode 40W
  • PRF 7KHz up to 100KHz
  • Polarisation – horizontal or vertical preset.
  • Beamwidth - 1.3degrees to 1.5degrees maximum.
  • Range – greatest 5,728 Metres.


In the 1970’s, at Upminster in Essex, the Plessey Company had developed an Environmental Sensors business and a decision was made to move the product line from Upminster to the Isle of Wight where it would head-up through the radar company. River ‘Flow Meters’ and Depth Gauges were major selling lines along with such sensors as Rain Gauges. Complete Air Sensing stations were constructed to embrace anemometers, min/max temperature thermometers and humidity detectors, each with a recording mechanism.

Plessey Radar (at Cowes) branched out into the Environmental Sensors Systems Business of which the following are examples of integrated systems.

Water Quality Stations - These took samples of water from rivers, passing the sample through a series of sensors and logging the results. These were succeeded by EDASS.

Environmental Data Acquisition Sub-System (EDASS) - This was a mini-computer based system primarily used for weather stations with a variety of sensors. Data was recorded and transmitted to remote central stations. Systems were supplied to UK authorities, also to Saudi Arabia for use in the Empty Quarter desert of Saudi Arabia.

Electromagnetic River Gauges (EMRG) - These had a large 48 V coil set into the riverbed with voltage probes set into the riverbanks and controlled by a minicomputer. The moving conductor, the river, generated a voltage across itself as it moved through the vertical magnetic field. This gave the velocity, while a depth gauge gave the volume of water flowing. Ultrasonic River Gauges (USRG) - An array of ultrasonic transmitters and receivers was set into the two riverbanks to pass beams across the river at an angle of 45degs. The equipment measured the Doppler shift and thus the velocity of the river as the water flowed past the sensors. Depth gauges gave the volume of water flowing.

Laser Radars - Plessey Radar, experimenting with a number of gas and solid state LASERS for visibility measurement produced a number of products, among them the Slant Visual Range Monitor (SVR) previously mentioned in the book and Point Visibility Meters (PVMs) - These were designed to replace the existing optical sensors that had required long base lines. The PVM used semiconductor solid-state lasers to measure the dispersion of the beam caused by the aerosol particles of mist or fog. A system was produced for ASWE and smaller units were produced for use, typically, on motorways and oilrig helicopter landing pads. The roadside PVMs suffered with the problem of spider webs. Its production was transferred to a Plessey subsidiary in East Anglia.

Ceilometers - These were designed to measure the cloud height and replace the existing scanning light beam systems. The ceilometer consisted of a pair of Cassegrain telescopes mounted side-by- side, one for the solid-state infrared laser and one for the semiconductor diode receiver. The difference between infrared light and visible light systems meant that they gave differing results depending on the cloud type. A number of manufacturers were producing similar systems and all suffered the same problems. The ceilometer business was eventually transferred to the US distributor.

Photo of Point Visibility Meter and Ceilometer


When Laser Designation was in its infancy, but considered an emerging threat in the battlefield, ‘Plessey Research’ at Caswell developed high sensitivity laser diode technology and the Cowes research team developed Optical-focusing devices to detect designation. Technology demonstrators were built and detection principles successfully tested with Defence Research Organisations. Although the technology was successful, for operational reasons the product was not, as there was insufficient reaction time available to counter the threat after detection. However, the technology was carried forward by UK MOD into developing Alerting Receivers for detecting that ground laser designation was occurring on high value targets, bridges etc. This provided a degree of warning to alert anti-aircraft response against airborne attacks. Plessey’s involvement ended with the success of the demonstrators.



FEATURES:- Providing instant warning of a variety of threats.
Detecting ruby and neodymium lasers.
Discriminating between rangefinders, designators and Infra Red searchlights.
Gives instant reading of bearing to within 15° (3° option)
All weather, day and night, reliable operation.
Very low false alarm rate
May be mounted on any vehicle


The Plessey Laser and IR Search Light Detectors were available in two versions. The first, an indirect detector, which provided instantaneous warning of laser radiation, even when the detector was not directly illuminated. The second version was a comprehensive sensor, which incorp¬orated the indirect detector and in addition provided the approximate location of hostile lasers and IR search lights.


The indirect detector, being sensitive to pulsed laser radiation scattered by the natural atmospheric aerosol, was able to detect the use of laser designators and rangefinders even when these were not directly incident on the detector. A single unit mounted on the super¬structure of a vehicle thus provided warning of a laser beam incident on or passing close to any part of the vehicle from any direction. The equipment was designed to operate in all levels of ambient illumination and over all types of terrain. Under typical clear atmos¬pheric conditions rangefinders and des¬ignators could be detected at ranges up to 10 Km.

The presence of local concentrations of smoke or dust increased the sensi¬tivity by providing an enhanced scatter¬ing medium. A narrow-band optical filter was incorporated to transmit only ruby and/or neodymium laser radiation and to give excellent discrimination against solar glints and other back¬ground effects. Further electronic filter¬ing contributed to an overall false alarm rate that was extremely low.

The total package consisted of two parts, the detector head, which required an unrestricted 360-degree field of view, and the remote display unit mounted inside the vehicle. Power was drawn from the vehicle's own supply.


The comprehensive sensor incorporated the indirect detector described above and, in addition, included detectors to indicate instantaneously the bearing of the source of radiation to within a 15 degree sector when illuminated by the hostile beam. The equipment was designed to operate in all levels of ambient illumin¬ation and over all types of terrain. Under typical atmospheric conditions range¬finders and designators could be detected at up to 10 km range and infrared searchlights up to 2 km.

The directional detector was sensitive to any pulsed laser radiation in the band 400-1100nm during the day. At night when ambient illumination was low it could be made sensitive to continuous wave radiation, thus allowing IR searchlights to be detected.

An optional addition to the basic pack¬age was the inclusion of a detector unit, which indicated the direction of the source to within a 3° sector when brought to bear in the approximate direction. The total system consists of a head unit mounted on a vehicle's superstructure and a compact remote display and control unit mounted inside the vehicle. Power was drawn from the vehicle's own supply.


The detector heads were designed for mounting on the superstructure of any vehicle. The remote display unit, connected by a single cable, gave audio and visual warning of a detected threat and indicates whether a single pulse (rangefinder) or train of pulses (design¬ator) had been received. In the case of the comprehensive detector, the display unit also gave warning of the detection of IR searchlights and indicated the bearing of the radiation source to within a 15° sector. An optional addition provided bearing accuracy to 3° on rotation of the detector head to bring the reference direction to the indicated sector. The directional detector head had a field-of-view encom¬passing 360° in azimuth and +45° to ¬-10° in elevation, thus allowing detection of both ground-based and airborne laser systems. A built-in facility enabled the function¬ing of the system to be instantly tested. Installation, setting up and operation were very simple. Maintenance requirements were minimal.


  • Indirect Detector
  • Spectrum coverage: ruby (694nm) and/or neodymium (1060nm)
  • Detection coverage: hemispherical
  • Directional Detector
  • Spectrum coverage: 400 - 1100 nm
  • Detection coverage: azimuth field 360°, vertical field 55° Bearing accuracy 15° (3° option)
  • Power supply requirement: 28 volts DC nominal tol¬erance band 22-32 volts DC
  • Operating environment: all weather mobile system adaptable for use on any type of platform


In volume terms probably our best selling product.

Radars with effective Moving Target Indication (MTI) have the advantage that they can distinguish and track objects such as aircraft in flight against a background of fixed ground clutter. A disadvantage is the consequent inability to detect and display fixed landmarks that would otherwise make an operator aware of the general geography and the presence of obstructions. A requirement arose in the 1960s for a device that could be fixed at strategic locations to provide targets detectable by the MTI processor. Initially used to delineate the borders of runways this device was dubbed the 'MTI Marker'. Essentially it was a stationary object that imposed Doppler shift on the radar return so that it appeared to the MTI as a moving object and yet stayed in the same place. A simple microwave switch that changed the phase of the reflection by 180 degrees on every alternate pulse provided the required modulation of the radar return. This switch consisted of a PIN diode mounted in a ridged waveguide, biased on and off at half the radar's PRF, this by a multivibrator powered from a Fencer battery. To give this modulated reflector some gain and directivity the waveguide feed was mounted at the focus of a 1-metre diameter parabolic dish.

The picture is of the Runway Marker Prototype

The whole assembly was moulded in polyurethane foam in the shape of a cone with the feed at the apex and its circular parabolic base metal-sprayed to form the reflector. The MTI marker was a successful and popular Plessey product, designed in the late 1960’s yet it was still in production in the 1990’s.

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