AIR DEFENCE RADARS
The Company’s first S-Band, high power, long range, defence radar.
During the Second World War, the ‘Chain Home’ and ‘Chain Home Low’ radar systems had served the nation well and by the end of the war UK Defence Radars were able to detect approaching aircraft, equivalent to the Canberra bomber, from a range of some 120 nautical miles. With the onset of the so-called ‘cold war’ and to counter a potential threat from enemy aircraft the MOD decided that Britain’s defence radars should be capable of detecting oncoming aircraft at a range of 250nm.
As a first stage, TRE, (later to become the Radar Research Establishment - RRE), configured an experimental radar set-up designated “Green Garlic”, which consisted of two Type 14 reflectors bolted end to end on a Type 7 turning-gear which also carried a 1.0 MW transmitter. Flight trials showed a very significant improvement over the then current standard early warning radars used as part of the ROTOR SYSTEM (see Appendix 2). This led to the formulation of a specification for the design and production of elements for a replacement High Power Defence Radar. RRE devised a system concept using a 75ft. wide by 25ft. high single curvature antenna fed by a slotted waveguide feed (linear array). The waveguide assembly was to be air pressurized to reduce the possibility of RF breakdown under power. A four-leg gantry would carry the turning-gear, which in turn would support the on-mounted antenna and linear array. Suspended from the turning gear would be a cabin containing the Transmitter and Receiver racks. The turning-gear, driven by four 50 HP DC motors, was to be powered by a 12 phase mercury-arc rectifier (colloquially known as the ‘Mekon’, taken from the Dan Dare Comic strip) housed in a small building at ground level underneath the rotating cabin. DC motors were to be used, rather than AC motors, as rotation of the cabin/ antenna assembly was to be variable between 4 and 6 rpm.
In 1951 Decca Radar was awarded a contract for the development and manufacture of the transmitter/receiver racks and also the manufacture of the antenna linear array. John Curran, a Company based in Cardiff was contracted to supply the turning gear and the four-leg gantry. Rendal Palmer & Tritton were the mechanical consulting engineers, with Starkey Gardner manufacturing the antenna. Decca Radar was to be responsible for the overall project management of the manufacturing, installation and commissioning programme.
Decca Radar designed and built the prototype models of the Transmitter and Receiver racks within a year and delivered them with the linear array, for the prototype TYPE 80 trial installation to Bard Hill, near the town of Holt in Norfolk. Already in place were the four-leg gantry, the antenna and the rotating cabin.
A small team of Decca Radar engineers, together with part time RRE staff installed all the equipment and got everything up and running within 4 weeks. Flight trials followed using a Canberra Bomber (the standard NATO calibration aircraft) and demonstrated that the desired performance had been achieved.
Initially, Mark 1 Type 80`s transmitted with a peak power of 1.0 MW at a prf of 270pps and a pulse length of 2.7 microseconds. Shortly after the Bard Hill trials had been completed the transmitter was enhanced to operate with a 2.5MW power output and would be designated the TYPE 80 MK3.
The first production Type 80 Mark 1 was installed at RAF Trimmingham on the Norfolk coast. After extensive flight trials the Type 80 was commissioned into service in 1954. This was followed by the installation of more Mark 1 systems at a number of RAF sites around the east and south coasts of the UK as part of the Linesman - Mediator system (See Appendix 2). At a later stage, most Mark 1 Type 80’s in service with the RAF were converted to the Mark 3 category. A number of Type 80s were also installed overseas, for example Metz in France, managed for NATO by the Royal Canadian Air Force. A quantity of four went to Sweden and one to the top of Mount Troodos, in Cyprus, for the RAF.
In 1993, a duo of retired Decca Radar senior engineers, who had been involved in the Type 80 product design, were invited to attend the “Switch Off” of the last operational Type 80 at RAF Buchan near Peterhead, Scotland. This equipment had given service since it’s commissioning in 1956, of some 37 years continuous operation. The formal ’switch off’ was attended by local dignitaries and senior serving officers, and was preceded by a fly-pass from a flight of front line RAF Aircraft.
Authoritative records show that the first five ‘production’ Type 80’s went into Trimmingham, Beachey Head, Bempton, Ventnor and St. Margaret’s, but the latter four do not show as such, on the operational listing of installations. It is also recorded that Wartling was the first operational Type 80 Mk.3.
As shown above the Hydra radar used a back-to-back antenna system similar to the DASR1, but with four over-lapping beams at different elevation angles with separate receiver channel for each beam. Both 30ft wide antennae were illuminated by a pair of horns rather than the single horn feeding each of the antennae of the DASR-1.
The rotating joint comprised two RF channels. In order that four sets of transmit and receive signals (relating to the four horns) could pass through the double channel rotating joint, it was arranged to use two 2.5MW S-band transmitters operating at different frequencies spaced 150Mhz apart. The power from each transmitter was divided into two. A waveguide configuration then arranged that each rotating joint channel, on transmit, comprised half of the transmitter power of the first transmitter plus half the power of the second transmitter. A pair of frequency separators above the rotating joint then allocated the RF energy to the four horns. In this way power was radiated at the first transmitter frequency from the lower beams on each side and the second transmitter frequency from the two upper beams. Each beam was therefore radiating a power level of 1.25 Mw.
The received radar returns followed a converse path into receiver head amplifiers, providing four separate received channels relating to the four beams. The ultimate video signals were then available for use on PPI displays.
SNERI (Societe Nouvelle Electronique et Radio Industries) had proposed a ground clutter suppression system designated ‘Air Target Indicator’ (ATI) with the selective blanking of near origin video signals from the lowest beam to eliminate ground clutter out to the range where the lowest beam is no longer giving responses from ground targets. ATI had some merits but did not completely eliminate ground clutter as well as could have been achieved by a ‘well-designed’ MTI equipment. At that time Decca Radar did not possess an MTI sub-system although development of MTI using ‘valves’ (and intended for use on the DASR1) was underway. Design effort, however, was diverted to the development of a transistorised MTI for use on the AR-1 airfield approach radar, alsounder development at that time.
A view of the ingenious but fairly complex waveguide configuration used to overcome the problem of passing four sets of RF signals through a double channel rotating joint is depicted.
In 1957 a prototype installation combining the Decca and French elements was set up at the SNERI test site at Limours near Paris. The object was to carry out a series of demonstrations to representatives of NATO countries (SHAPE) over a period of one week. NATO were shortly to go out to tender for the update of a major part of the NATO radar network. Following the demonstration, the DECCA-SNERI consortium submitted a bid but lost-out to the Marconi Company. The partnership of the French and British companies survived the setback for a while, with, for example, Hydra sales to Iran and Indonesia but Hydra never became an outright winner. Customer installations were powered both by TYPE80 and HF200/5007 transmitters.
The LC150 was designed to provide long-range Low Cover coastal warning of low flying aircraft or surface targets. The antenna and turning gear was usually, but not exclusively, mounted on a tower as shown. The horn fed antenna was capable of being tilted to suit specific operational requirements. The antenna width was 35ft. and the height was 10ft., giving a horizontal beam width of 0.3degrees. (This antenna was again one where the horizontal and vertical profile followed the same mathematical law and was therefore of the ‘orange peel’ type).
In order to fulfill its role of coastal surveillance, provision was made for the antenna’s rotation to be variable from 0.2 to 6 rpm, this in either direction, controlled by an operator at a 12inch fixed coil PPI display. The transmitter was a variant of the S-band ‘LOTUS’ Tx/Rx, stretched to 800kW. Pulse lengths were 1.0 microsecond at a prf. of 400 pps. or 0.3 microseconds at a prf. of 1000 pps.
There had been a development by SNERI design engineers to extract height information from a Plan Radar by the inclusion of a ‘Robinson’ feed device on the special antenna. (Project code name ‘Crusoe’). Height information was generally achieved by the use of a ‘so-called’ nodding height finding radar in conjunction with a plan-radar. The height finder used by the RAF during World War 2 was the Type 24 and later the American FPS6 or the British Type 13.The performance of the then in-service height finders was judged by the UK MOD to be inadequate for the ‘Cold War’ and this the management of Decca Radar saw as an opportunity to design and supply a replacement for all such equipments. Therefore, it was decided to go ahead with the design and manufacture of a new long-range nodding height finder, financed by the Company.
A prototype HF200 Radar (antenna, complete with Tx/Rx and HRI Display) was manufactured and installed at the rear of the Davis Road site to enable preliminary tests to be carried out.
The key parameters of the HF200 were:-
- Horn fed 35ft.high by 8 ft. wide.
- Horizontal beam-width of 3 degrees.(This antenna profile was one of ‘orange peel’ where the curvature in the vertical plane was to the same mathematical law as that for the horizontal).
- Vertical beam-width 0.75 degrees
- Elevation nodding range -3 to +33 degrees
- Peak power, 2.5 MW,
- Pulse length, 5.5 microseconds.
- Prf, 270 pps
Antenna nodding rate: Variable from 20 to 60 sweeps per minute.
The HF200 had some novel features as both the nodding action and rotational slewing were hydraulically actuated. The HF200 radar data was displayed on a Height Range Indicator (HRI). Height information was achieved by firstly identifying a target on the PPI of the ‘linked’ plan radar. The operator adjusted a bearing line so that it coincided with the echo from the chosen target. This action then applied a position control signal to the hydraulic systems. The aerial slewed to the target bearing. The reflector nodded over an angle sufficient to illuminate the target and a resultant short vertical line appeared on the HRI, this then, under operator control, was bisected by a curved cursor. The curved line was calibrated to take into consideration the earths curvature. The action on the HRI produced the elevation angle of the target. A combination of the elevation angle and range, by a simple algebraic computation, led to a height output. By pressing a button the measured aircraft height was sent to an analogue meter adjacent to the plan radars PPI.
Many HF200s were produced and installed by the company. RAF Boulmer, RAF SaxaVord, RAF Neatishead, RAF Bishops Court, 280 Signals Unit (Cape Gata and Mount Troodos in Cyprus) were among the RAF sites; with several having two HF200s on the same site. Other overseas sales of the HF200 were 3 to Finland, 3 to Sweden, 10 to Iran, deliveries going on into the late 1970’s, (via Westinghouse) 1 to Singapore and 4 to The Republic of South Africa. In total more than 30 HF200s were sold.
A significant number of HF200s were in service for many years, in fact, right up to the advent of ‘3D’ radars. There was an HF200 still in service at Saxa Vord at the time of the switch-off of the Type 80 at RAF Buchan in 1993. (A brief outline of the ROTOR and LINESMAN-MEDIATOR radar chains is given in Appendix 2). The contribution made by Decca Radar to the MASTER STATION system established the company as a major supplier of Defence Radars.
In 1975 the Plessey Company offered to the market a self-contained transportable convoy designated the TYPE 40/80-5 COMMAND AND CONTROL POST. Its function was to operate in conjunction with fighter aircraft and missile batteries to provide an autonomous Air Defence Capability. The convoy comprised: -
Antenna Unit. Transmitter Cabin.
Processing and Control Cabin. Communications Cabin.
Diesel Generators. Workshop Cabin
The primary radar, AR-3D, provided range, bearing and height information at 260nm.and 120,000ft, with the data then being used to initiate target tracks, as a basis for computer assisted fighter control.
The antenna system illumination of the reflector was by a linear (frequency sensitive) feed which, when fed by a pulse having ‘within pulse frequency sweep’, produced a pencil beam that scanned in elevation. The pulse had a frequency deviation of 140MHz and was 36 microseconds long. Thus all elevations would be radiated during each pulse period. The frequency sweep was non-linear, being lowest at the frequency corresponding to the lowest beam angles and increasing as the higher elevations were reached. This resulted in ‘rectangular-shaped’ radar coverage.
The transmitter delivered a peak power of 1.1 Mw. (10Kw mean) and took the form of a two-stage power amplifier, the first stage of which was a Travelling Wave Tube. The TWT was fed from a low level input signal supplied from a programmable solid-state sweep oscillator, which generated frequency-modulated RF pulses. The second stage used a grid-modulated klystron.
The Receiver and Signal Processing System
Signals received from the antenna, after amplification were down-converted to a first intermediate frequency and then split into 8 signal processing channels. Each receiver channel contained information, which related to coarse elevation data. The frequency of the received signal within a receiver channel provided the fine elevation information. To improve range accuracy, pulse compression techniques were applied to compress the pulse width to 100 nanoseconds. Digital MTI processing was applied to the IF channel of the lower elevation receiver. The MTI cancelled out returns from local ground clutter. The low elevation channel was then converted back to analogue to enable the analogue frequency discriminator to extract the fine frequency and thus elevation data. In later updates to the system a digital frequency discriminator was utilised which removed the necessity for digital-to-analogue conversion. Background averaging and moving window detection techniques were employed to achieve high detection accuracies. The combination of these processing sub-systems together with the coarse and fine elevation information was transferred to a plot extractor. The resulting aircraft track data (bearing, range and height) was passed to the displays in the Processing and Control Cabin. Track data could, if required be passed, for example, to a centralised command centre via the Communication Cabin.
A 14ft. IFF antenna was mounted above the primary radar antenna. The associated SSR/IFF equipment contained in the transmitter cabin was capable of interrogating and processing replies from suitably equipped aircraft. The processed output was passed to a plot extractor contained in the Processing and Control Cabin and thence to the consoles using Plessey Series 9 Displays.
Main Power Supplies
Power was provided by a diesel generator that also had a stand-by back up.
The principal sales of transportable versions were as follows:-
- Egypt (3),
- South Africa (10),
- Germany (1 for RAF)
- Quatar (2),
- Ecuador (2),
- Two went to Falklands in January 1983 followed by one in February 1984.
One of the principal limitations of AR-3D, for military purposes, was that the frequency/elevation law was fixed by the products mechanical design and it was thus impossible to provide frequency agility within the bursts. This was a factor leading to the evolution of the AR-320.
AR-320 was developed as a joint venture between Plessey Radar and ITT- Gilfillan, who were based in Los Angeles, USA. ITT Gilfillan provided the antenna and its pallet, and its transmitter, while Plessey Radar provided the signal processing equipment. In RAF service its was designated Type 93. The joint venture was seen as a way of gaining access to the NATO/USA market, although in practice this did not turn out to be the case.
The AR-320 was a long-range 3-D air-defence radar operating in E/F (S) band. Its antenna was a low side-lobe planar-array with frequency/phase-scanning. As with AR-3D it operated using within-pulse frequency scanning providing full elevation coverage during each transmitted pulse.
Two-way clutter suppression was provided in elevation and azimuth while maintaining a high data rate. A high average power two-stage transmitter was employed, also a high-gain pencil-beam antenna. Short compressed pulse, wide-dynamic-range CFAR (constant false alarm rate) receivers, frequency agility, and doppler processing were all key features. They combined to provide long-range detection and accurate high-resolution 3D plot information on each antenna scan, with high immunity to ECM. Combined with an on-board SSR, plots were output as primary, secondary or associated primary/secondary. A separate ECM assessment receiver continuously monitored the ECM environment, providing data for operator display and automatic control of ECCM functions. In addition, an on-board radar simulator, for operator-scenario training and system-performance monitoring formed part of the system. High penetration by BITE (‘Built In Test Equipment’) with high reliability facilitates, increased system availability.
The antenna provided phase control for accurate beam positioning in elevation at all frequencies together with elevation beam tilting as required. The mechanical rotation of the antenna extended this capability to include all azimuths. In addition, the azimuth frequency sensitivity of the antenna allowed electronic azimuth beam control, extending the coverage power, while tailoring it to the azimuth dimension. This allowed exploitation of additional ECCM techniques only applicable to this type of system in both the active and passive roles.
Summary of System Features
The elevation beam angle was frequency dependant. Frequency measurement of a returned signal provided the appropriate elevation angle. Phase control on the antenna allowed the beam-start angle to be positioned on a pulse-to-pulse basis in frequency-agile operation. The antenna exhibited azimuth squint over the agile band. The beam would randomly squint over the squint window and signal returns would be stored and processed in azimuth for plot detection. The narrow sidelobe pencil beam was scanned in elevation and azimuth on both transmit and receive, offering a number of advantages:
- High data rate – positional accuracy and track quality;
- Narrow beamwidth – ECCM performance and target resolution;
- Clutter suppression – two-way suppression for off-axis clutter; and Coverage shaping – in both elevation and azimuth. The ability to control the beam position in elevation and limited azimuth, on a pulse-to-pulse basis, being a valuable ECCM tool.
A nonlinear, frequency-swept pulse concentrated energy at lower elevation angles for long-range performance. Shorter MTI pulses were linear. Long-range pulses were interspaced with groups of MTI pulses. On reception, surface-acoustic-wave (SAW) equalizers generated short compressed pulses. The Operational modes available being fixed frequency, agile, pulse-burst agile, and automatic least-jammed frequency. A jamming monitoring receiver operated multiplexed with the active transmissions within each azimuth beamwidth. Operational control of the radar was performed in distributed microprocessors, such that further exploitation of system characteristics in response to changing threats was a relatively simple matter. The high-gain, low-sidelobe antenna and the high mean transmitter power combined with the above, provided ECM protection, good resolution, and accurate long range 3-D plot detection on each antenna scan.
The antenna group consisted of a primary array, a secondary radar (SSR) antenna, and a Sidelobe blanking (SLB) antenna co-located on a trailer mounted turning gear. Transmitted waveforms were generated at low power and amplified to feed the antenna via a high power duplexer.
Received signals were fed via the duplexer to a wideband multi-channel receiver, for both primary and side-lobe receiver systems. A two-stage intermediate frequency receiver was used with frequency agility performed on the first local oscillator with elevation beams formed at the second IF. After detection and plot extraction, signals were output for plot combination with the SSR returns.
Major functions included CFAR detection, sidelobe processing, plot extraction and ECCM processing, also jamming and clutter extraction for radar frequency programming and operator data. A dedicated processor provided control of the transmitted frequencies and waveforms to ensure that optimum performance was achieved under all operating modes and environments. Output signals controlled the transmitter signal generation and antenna beam positioning on a pulse-to-pulse basis.
The primary antenna, a 5-metre wide-band planar array, consisted of horizontal slotted waveguide arrays, which provided exceptionally low side-lobes over the operating frequency band. The arrays were end fed from a single serpentine slow wave feed, which provided the phase frequency slope to move the pencil beam through the elevation coverage when illuminated with a frequency swept pulse. As with AR-3D an elevation taper was required on transmission to hold the beam for longer at low elevations. Wideband directional couplers within the serpentine provided this taper. Each coupler fed a horizontal array via a high power ferrite phase shifter, which enabled phase control on a pulse-to-pulse basis to steer the beam position.
The purpose of the phase shifters was to enable frequency agile operation. The AR-3D system had a fixed relation between frequency and elevation. As a result the AR3D radar could not operate in a frequency agile mode, this limiting its ECM performance and resulting in poor elevation accuracy due to scintillation errors. To overcome this on the AR320, phase shifters were fitted to each waveguide element, which allowed the elevation profile to be maintained against changes in frequency. This facilitated full frequency agile operation, improving the ECM performance and also overcoming the effects of scintillation with the resulting improvement in elevation accuracy. It also had the additional benefit of achieving electronic tilt of the elevation coverage.
The horizontal array elements contained dual slotted radiators, with slot coupling distribution providing the designed azimuth taper. A radiation pattern monitor sampled each array for monitoring and testing purposes. A beam steering processor, mounted on the back of the array, converted beam commands to phase control signals for the phase shifters. An SSR antenna and SLB horn were mounted on the primary antenna.
The received pulse was processed in a wide-band First-IF chain, prior to pulse compression at the Second-IF. Amplitude and frequency detection was performed on the compressed pulse, followed by plot processing of the detected signals; this to provide output plots for display or message formatting. The ‘First-IF’ signals (500MHz) were then down converted to the Second IF in a frequency- channelised receiver, each channel maintaining wide dynamic range pulse compression, logarithmic detection and optimised bandwidth to maximise performance. The channels formed overlapping elevation beams. ‘Within-channel’ frequency discrimination provided the elevation data.
For moving target indicating (MTI), high stability coherent processing provided ‘I and Q’ outputs to an adaptive digital MTI canceller. The phase shift across the first delay was used to adapt the velocity notch to cancel moving clutter.
Logarithmic amplitude and frequency measurements from each of the IF receivers and the MTI cancellers were digitally processed in conventional LOG CFAR circuitry. An ancillary receiver and processor removed side-lobe returns. Partial plots were stored and de-squinted prior to azimuth detection and correlation. Distributed processors achieved primary plot correlation and detection, resolution of adjacent plots and extraction of the plot range and elevation and azimuth statements. Range and elevation statements were processed to provide height data using an atmospheric refraction model updated at regular intervals with meteorological data.
Central to the signal processing was the frequency programmer unit (FPU) controlling radar modes automatically, the main pulses and MTI bursts being organised into sequences for optimisation of performance under various operational scenarios. The coverage pattern for each pulse or pulse burst type was independently controlled by the FPU, to maintain uniform coverage and random frequency selection. Signal processing and display equipments comprised significant numbers of embedded processors, these being linked together on a local area network (LAN) allowing easy extension of equipment facilities. The diagram (Fig A) shows the parts of the radar as elements of the LAN, a different viewpoint from the normal presentation (Fig. B)
Initially the AR-320 proved to be unreliable in RAF service due to a combination of component failures and the scaling of spare parts. It was rarely available in high power. In 1995 it was almost withdrawn from service, but a major combined effort by the RAF, Siemens Plessey and ITT got the performance back to an acceptable level. The absence of BITE in the transmitter was a permanent weakness.
Modern air defence radars are presented with a dense electronic warfare (EW) threat. The opportunity to exercise radars against this threat is very limited.
A realistic scenario simulation injected into the radar provided the mechanism to analyse the radars performance. The AR320 simulator provided up to 60 minutes of play, enabling training of operational and technical personnel in radar performance assessment, this with the radar under simulated ECM threat conditions. RF or IF injection modules were available with signals mixed with real radar returns.
The Company had been very successful with their Watchman and 996 programmes, but this meant that it had committed almost all of its private venture funding, for research, development and bidding, for the next few years. The Long Range Air Defence market had been very profitable for the Plessey Radar Company, but the AR-3D was then both obsolete and expensive to maintain, similarly for the AR-320. A new product was needed that could utilise the transmitters and processors that the Company was developing on other programmes and therefore there was no option but to undertake the development of an advanced planar array antenna. Plessey management took a unique approach to this problem by assembling, within the Company, a top team of experts to conceive, design and cost a new three-dimensional radar. The company funding for this large-scale effort was to be zero, the design team being asked to work completely in their own time!! Arrangements were made to compensate this effort, the team agreed and the AR-325/Commander was born.
The following pages describe the progression from AR320 to the Commander radars AR325, AR327 and onto the solid-state system Commander SL. Elements of the systems tend to be the same, in that all have, for example, receiver down-converters, CFAR processing etc. Hence, to avoid a lot of similar text, the details for each system will not be included although it is hoped the text will explain where major changes were made. As a general rule improvements in detection have been gained as both stability of components and transmitter designs were enhanced. Techniques and technology improved allowing more complex algorithms and processing systems to be used. An example of the latter being the regular improvement in clutter maps as computer memory store capacity and speeds increased.
Towards the end of the 1980’s two separate design/development initiatives were taken on 3D radars. The first was the design of an ‘in house’ planar array to replace the ITT-Gilfillan antenna used on AR-320. The resulting system was to be known as the AR-3DP.
The second initiative adopted the new planar array antenna but moved on to re-design the transmission and receiver system. The system was to be designated AR-325 and was the first of the Commander Series of 3D radars. The Company secured an export contract to supply AR-3DP systems, but well before major design work got underway and certainly before any ‘metal was cut’ the contract was amended such that an up-graded system would be supplied. The product would be known as the AR-325.
There were a number of drivers in the move from AR-320 to COMMANDER-325. Cost was an important factor but advances in technology and signal processing enabled system performance improvements. The AR-320 had provided elevation coverage by using a frequency-scanned beam. This required the use of multiple receiver channels, each tuned to a different part of the frequency sweep, so separating the returns in elevation. The downside of this approach was that the receivers were expensive but also required high maintenance to keep the channels tuned. The approach taken for the design of the Commander 325 product was to reduce the receiver channels to only two.
The AR325 Commander Radar, like its predecessors, was a ‘long range’ air defence radar. It was a transportable system capable of being moved by land, sea and air with a deployment time of four hours. The main elements of the product comprised the Primary Radar Antenna with an ‘on mounted’ IFF antenna, a Radar Management Cabin containing primary and secondary transmitters, receivers and signal processing, together with the radar control and display system, and either one or two Generators. An additional cabin containing further displays was available to convert the basic Reporting Post into a Command and Reporting Centre (CRC).
Method of Operation
The radar transmitted a pair of pencil beams on every Pulse Repetition Interval (PRI), this being accomplished by transmitting two sub-pulses, one immediately after the other, at the start of each PRI. The sub-pulses had the same duration and within-pulse frequency sweep, but were centred on different radio frequencies (F1 and F2). One of the characteristics of an end fed planar array antenna is that of frequency dependent azimuth dispersion or ‘squint’. As a result two beams are formed with the same elevation angles but different azimuths, as illustrated below.
Azimuth diversity effectively doubles the transmissions per beam-width thus reducing the granularity of the illumination pattern. It also ensures that targets are illuminated with two frequencies, thus giving the detection advantages of independent samples. This is known as frequency diversity gain. Two independent receive channels were used to process the two beams through to the plot extractor, where the partial plots were realigned in azimuth and combined in the final detection processing.
The azimuth coverage was provided by mechanically rotating the antenna through 360 degrees at 6 rpm to give a 10 second data rate.
The elevation illumination was achieved by electronically stepping the pencil beams through a sequence of positions as defined by the software. In order to minimise the number of elevation positions required, the elevation beamwidth was electronically varied with elevation angle, it being narrowest at low angles where maximum sensitivity and clutter rejection were required and progressively broadened with increasing elevation.
In each beam position the inter-pulse period corresponded to the instrumented range appropriate to that elevation. This shortened dramatically as the elevation angle increased. In order to make full use of the high mean power available from the transmitter the transmitted pulse lengths were also varied with elevation angle to maintain the full transmitter duty cycle. This ensured maximum detection sensitivity in all beams. The flexibility in pulse length was available through the pre-programmed driver unit and pulse compression. These features combined to give a very efficient and flexible use of transmitted power.
The transmission patterns employed by the AR325 Commander were only made possible by the use of both digital waveform generation and digital pulse compression techniques. This move, from the historical analogue units, allowed multiple pulse lengths to be used and these could be changed on a pulse-to-pulse basis. This, together with the ability to change both pulse repetition interval and elevation, provided the designer with the flexibility to tailor the transmission patterns, thereby providing optimum coverage where it was required.
The AR325 Commander adopted the technique of phase mono-pulse, rather than amplitude monopulse, to compute target heights. This was new at the time but has since become the preferred approach, as conventional amplitude mono-pulse measurement techniques are not able to achieve the required accuracy and stability.
Target elevation is determined from the phase difference of signals received by the top and bottom halves of the antenna as illustrated below. By this method accurate height finding could be achieved down to the radar horizon on a single beam. This provides full 3-D performance against a low level tactical intruder. Amplitude mono-pulse techniques, as employed on previous generation radar systems, often failed to provide height measurements on low flying targets.
The AR320 transmitter did not naturally provide the flexibility required for multiple pulse lengths and variable pulse repetition intervals, so a new transmitter was required. This was achieved by adapting the transmitter used in the Type 996 Naval Radar system. This transmitter already provided the flexibility of operation but had insufficient output power for’Long-Range’ ground radars. The basic modules of the 996 transmitter were retained and the design was adapted and modified to provide the duty cycle (6%) and output power required for Commander. The final output stage remained as a Travelling Wave Tube.
Processing and Display
As with all of the 3D radars, full processing ‘on receive’ was provided. Both primary and secondary radars were independently processed through to the output of their plot extractors. The primary and secondary plots were then combined and passed to a tracker. Depending on the configuration required by a customer the system could output either plots or tracks.
A single local display was provided in the Radar Management Cabin. The basic configuration of the radar was as a reporting post, and for this a single display was sufficient. If additional displays, or a Command and Control configuration were required, then an additional cabin was available which provided the necessary display and communications equipment. The display provided for the presentation of raw video, plots and tracks, as well as providing all the facilities for setting-up and controlling the radar. Built-in test data was also available for, at any time, both monitoring the operational state of the radar and the presentation of fault data in the event of a failure. Diagnostic aids were always available to help the maintainer pinpoint faulty units.
From its design approach the AR325 provided a number of additional benefits. The major one, from an Air Defence point of view, was that it had a natural resistance to jamming or ECM, which was better than that of previous 3D radars. The radar used a wide transmission bandwidth and provided a frequency agile mode with random frequency selection. It used multiple pulse lengths and variable PRF, and in addition each pulse had a frequency sweep applied that was then matched to the pulse compression system on reception. Over and above this, every transmission consisted of two separate pulses at different frequencies, both frequencies capable of being independently agile. All of these facilities made the radar extremely difficult to jam with any degree of success.
Another benefit was that the ‘azimuth squint’ of the antenna, being dependent of frequency, provided an opportunity to increase detection at specific azimuths and / or elevations. This facility being known as ‘burnthrough’. By selecting the transmission frequency sequentially rather than randomly the transmission beam could be held at a specific azimuth and/or elevation for an extended period, thereby increasing the number of pulses transmitted at the selected area with a resulting increase in detection probability.
The AR325 was designed as a transportable system capable of being moved by land, sea and air (C130 or helicopter). The deployment time of the radar was not considered to be a critical parameter and therefore it was configured such that it could be deployed in a time of under four hours. The Antenna was designed to be symmetrical about its horizontal centre with a hydraulic folding mechanism that allowed the top half to be folded down for deployment. The on-mounted IFF Antenna was removed and transported separately.
- Output power: 134 KW Peak, 7.5 KW Mean
- Prf. Variable
- Pulse length: multiple up to 100 microseconds
- Frequency Band 2.7 to 3.1 GHz
- Number of elements: 60
- Azimuth beamwidth:1.4 degrees
- Elevation beamwidth: 1.5 degrees
Few systems of the AR-325 were sold due to increased operational requirements for mobility, shorter deployment times (i.e. 1 hour), and more stringent logistics demands (i.e. the impact of reliability, availability and maintainability on ‘life cycle costs’ were an integral part of the design stage and then measured in day-to-day operations). For the first time ‘availability’ had to be delivered at a fixed price in the radar supply contract. Typically the system availability would be assessed quarterly and the support cost paid only if the contracted availability was achieved.
These changes were the predominant drivers towards the adaptation of the AR-325 Commander into the AR-327 Commander, initially developed for the U.K. MOD as the T101 Radar.
Changes from AR-325
The first contract for AR-327 was received in the mid 1990’s and development commenced. The basic building blocks of the radar remained the same as AR-325, as did the method of operation. The biggest single change was to the Antenna that had to be reduced in size to achieve the mobility requirements. This was accomplished by reducing the number of waveguide elements (rows) from sixty down to forty, which involved a redesign of the whole stripline beam-forming system for both transmission and reception paths.
The resulting antenna could be folded with the IFF antenna in place, and was capable of both helicopter lift and C130 transportation as a single unit. The reliability of AR-325 was considered to be very good, but a major effort was applied to improving the maintainability of the AR-327 system. This involved design engineers focusing on accessibility as well as speed of removal and replacement of units most likely to fail. The results were very successful, as the system was well liked by both operators and maintainers wherever it was commissioned. Advances in technology mainly drove other system changes. Processors had become considerably faster and although still not initially fast enough for truly ‘real time’ processing, they were progressively increasing in processing power. Hence, functions like pulse compression could now be carried out on commercially available processors as opposed to custom built digital circuitry.
AR327 (RAF type 101) continued to sell successfully for several years and although the move to a solid-state version was considered on a number of occasions it was not until the year 2002/3 that the decision was taken to proceed in earnest. This coincided with the announcement of plans for a new Air Defence Radar procurement by MOD. It is probably worth emphasizing that the main drive towards solid-state was logistical. Solid-state modules are more reliable than the lump transmitters, but in addition, such radars could continue to operate successfully with a number of failed modules in place. This latter fact greatly improves the availability of the radar, as the system does not need to be taken out of service for each failure.
Another factor that affected the design of COMMANDER SL was the link with the Italian company Alenia. At the time of concept, the radar business was part of AMS (Alenia Marconi Systems) and it was agreed that there would be a Solid-State3D Radar development, but only one. A form of competition was held between the UK section and the Italian section of the company, with cost as one of the primary deciders. This ‘competition’ drove a number of the design decisions towards minimizing development costs and maximizing use of existing designs from either previous Commanders or other radars. In practice the majority of the RF Driver and Receiver units were taken straight from the latest Naval radar.
The change from a single lump transmitter to a distributed solid-state transmitter determined, to a considerable degree, what had to change within the system. As with the move to AR327 the basic principals of operation of the system remained the same but other areas to change were driven by technological advances. Such changes were to provide additional performance, along with cost and reliability benefits. The main building blocks of the radar remained unaltered, with the systems comprising:
An Antenna pallet supporting a Primary Antenna, with an ‘on-mounted’ Secondary antenna.
A radar Management Cabin containing primary receivers and signal processing, a secondary transmitter, receiver and processing plot/track extractor and a console providing the radar control display.
An optional additional generator was always available, as was an optional Command and Control Cabin with additional displays and radio communications.
Antenna and Transmitter
The basic approach to the transmitter design was that it would be a distributed transmitter mounted on the antenna with power delivered directly to each waveguide element through a high power duplexer. To achieve the detection performance with the power available for each row it was necessary to increase the number of waveguide elements. However, the over-all antenna size was kept the same as the AR327 with the additional six elements being accommodated by decreasing the spacing between the elements.
To retain the antenna central folding mechanism the transmitter assembly was designed as two, virtually identical units, for the top and bottom halves of the antenna. Hence each unit or TRIM (Transmit Receive Integrated Module) contained twenty-three transmit/receive rows together with the necessary distribution and control circuitry. The main structure of the antenna remained very much as for AR327 with minor strengthening to facilitate the handling of the additional weight of the transmitter and its power supplies. The photograph below is of the Antenna. The two units on the left side are the low voltage DC power supplies feeding the two TRIM assemblies. It is also worth noting that each TRIM has four fairly large fan ducts. The cooling system was one of the major design achievements, given that each assembly was dissipating several tens of kilowatts of wild heat.
At the same time the system was required to operate in the temperature range of -46 to +50 degrees centigrade, so there were situations when heating rather than cooling was required.
As with previous Commander radars, it was necessary to provide a power taper across the antenna face to generate the narrow pencil beam, previously achieved through the stripline power divider network. For the solid-state system each row was to have its own transmitter/receiver so the power taper had to be achieved within the row. A design was produced whereby the power amplifier unit (cassette) and the row design allowed for up to four cassettes to be fitted to any row. Each row of the transmitter contained a transmit driver unit, a receive unit and up to four cassettes. The outer rows of the antenna contained no cassettes but transmitted only the power from the driver. Progressively, rows had one, then two and finally four cassettes for the centre rows. In addition to this, two variants of the cassette were produced with differing output power. The combination of the above allowed the required power taper across the antenna face to be generated. The two TRIMs were effectively mirror images of each other as far as cassette population was concerned. Stripline combiners within each row provided the combination of the output power of the cassettes for transmission, with a stripline divider operating on the receive path.
Both transmit and receive paths for each row included phase shifters in order that the beam could be ‘pointed’ in elevation by controlling the phase relationship of all the rows.
A further feature of the cassette design was that it was produced with three or four (the difference between the two variants) identical amplifier circuits known as TPAMs (Transistor Power Amplifier Module). The reasoning behind this was to reduce the need for holding spare units. A smaller number of spare cassettes could be held as the units could be quickly and easily repaired by replacing the TPAM. This considerably reduced the cost of the spare parts holding whilst retaining the system availability. The BITE system was able to highlight failures down to an individual TPAM within each cassette.
Radar Management Cabin
The removal of the transmitter from the Cabin meant that the cabin layout could be rearranged. This was taken further by employing an external consultant to work with BAE to produce a cabin with a modern look and feel to it, a real step forward appreciated by all users.
As with previous generations of radar, technological advances offered some real benefits. In this case the advances in processor speed and configuration allowed the software design team to move from using extremely expensive custom processor boards to commercial processors. This also allowed far more of the processing and control to be carried out in the software, thereby again reducing the number of internal digital boards. The latter were still being used for the ‘real time’ front end processing, and the high-speed transfer of data to and from the antenna.
With the receiver front end mounted on the antenna, the amplified signals from the low noise amplifiers (LNAs) on each row were combined together within the TRIM and then carried through the rotating joint and across site by coaxial cables to the main receiver rack in the radar cabin. In the receiver rack the RF ‘received signals’ were down-converted with Sensitivity Time Control (STC) applied before direct digital conversion to In-phase and Quadrature components.
Four receiver channels, one pair for the upper half of the array and one pair for the lower half, had each processing signals from one of the two frequencies transmitted. This enabled phase monopulse height extraction to take place by using the antenna as a spatial filter. In order to give highly accurate phase monopulse height finding the receiver pairs were matched in phase and amplitude with a dynamic calibration correction used to remove any variations with temperature or frequency.
Signal Processing circuitry was rack mounted in the radar cabin. It consisted of a commercial, off-the-shelf, processor (COTS)- with software provided by BAE Systems Group- along with the Pulse Compression, Moving Target Indicator (MTI), Video Processor and Plot Extractor,. racks
The system used pre-programmed transmission beams for each elevation in the coverage, providing the appropriate number of illuminations for the detection process required at that elevation. The signal processing was optimised on a beam-by-beam basis for each transmission to provide the most advantageous improvement to each received signal.
All pulses were pulse compressed in a pre-programmed and flexible implementation in software to achieve low range side-lobes. The pulse compression was set to match each of the transmitted waveforms optimised for each elevation beam.
The adaptive MTI comprised a fixed ground filter followed by a phase adapting complex multi-pulse moving clutter filter. The adaptive MTI was configured in each mode to give a high degree of clutter rejection, appropriate to the elevation beam being processed. A mode was provided which allowed MTI processing to 256nm (474km).
The Video Processor contained the detection circuits including Constant False Alarm Processing (background averaging and clutter maps) and the ‘thresholding’ process, included a capability for generating a raw video output for the displays.
The first stage of the Plot Extractor carried out the position extraction in azimuth and elevation for each threshold crossing and then developed these into partial plots by range collapsing and azimuth ordering, each partial plot being labeled with a range, azimuth and elevation statement.
The second stage of Plot Extraction comprised three main functions, the Correlator, Resolver and Parameter Estimator, which combined the partial plots from all illuminations of the same target to provide one positional statement for each target. These target positions were then passed to the tracking facility for primary, MSSR and combined radar tracking and reporting.
The other area to benefit from the improvement in processors was the Radar Control and Display Sub-System (RCDSS). Commander SL moved on from the Controller System used in previous Commanders and adopted the Common Product New Generation (CPNG) system that was in development at the time. (A team led the CPNG development from the company’s establishment at Christchurch, also involving engineers from both Cowes and Chelmsford).
CPNG provided all the radar control functions as well as plot combination and tracking for local display.
From the Isle of Wight County Press, 5th March 2010 issue, Wg Cdr Mark Presley was reported as saying that the “Type 101 systems, deployed in Iraq and Afghanistan, were playing a key role in co-coordinating allied air operations,” these being part of some 20 systems operating around the world.