Advances in Russian and Chinese Active Electronically Steered Arrays (AESAs)

Carlo Kopp

Clayton School of Information Technology Monash University, Clayton, 3800, Australia

Carlo.Kopp@monash.edu

Abstract—Russian and Chinese AESA technology remains largely unknown in Western literature. This survey explores recent advances in Russian and Chinese AESA designs operating in the X-Band, S/C-Band, L-band, and importantly, VHF-band.

Keywords— Active Electronically Steered Array, AESA, radar

I. INTRODUCTION A recent development of some importance is the emergence

of Russian and Chinese radars employing Active Electronically Steered Array (AESA) technology. This subject area has not been widely studied, in a large part due to incomplete or irregular disclosures in Russia, and especially China, but also the need to translate publications. Gaps in disclosures, whether intentional or otherwise, impose often the need to perform detailed forensic analysis of published materials. Such effort entails the need to develop numerical models of the designs, to establish likely performance parameters, or feasible bounds on performance.

At this time the most extensive open source forensic study of this subject area is a collection of technical reports and journal papers published by Air Power Australia, an independent privately funded military think tank, based in Australia. This paper comprises mostly a survey of these works.

II. ESTABLISHED RUSSIAN PASSIVE ESA TECHNOLOGY Established Soviet era X-band 5N63/30N6 FLAP LID S-

300PT / SA-10 GRUMBLE and 9S32 GRILL PAN S-300V / SA-12 GIANT/GLADIATOR engagement radars are PESA designs, developed to engage aircraft, cruise missiles, standoff missiles and tactical ballistic missiles. All three also shared the same design approach, using a optical space feed and transmissive primary antenna array of passive phase shift elements. First described by Barton, these designs used an elaborate dual plane monopulse feed horn arrangement, placed behind a lens assembly [1][2][3][4].

This feed arrangement was also adopted in the Soviet X-band 9S19 Imbir / HIGH SCREEN ABM acquisition radar, developed for the S-300V / SA-12 system. The Janus-faced S-band NIIIP 5N64/64N6 BIG BIRD battle management radar developed for the later S-300PM / SA-20A GARGOYLE is also a transmissive PESA [1][2][3][4].

While early US effort in airborne ESA radar focussed on bomber radars, the first Soviet airborne X-band PESA was the 1,700 element Tikhomirov NIIP BRLS-8B Zaslon or FLASH

DANCE pulse Doppler air intercept radar, developed for the large MiG-31 FOXHOUND interceptor. This aircraft had the challenging role of intercepting low flying US air, sea and ground launched cruise missiles. The Zaslon was built to concurrently guide four long range R-33 AMOS missiles against low RCS targets in ground clutter, and was the first volume production ESA fitted to a fighter aircraft. An innovative feature was that an L-band IFF interrogator PESA was embedded in the X-band array [1][5].

PESA technology continues to be used in a number of new production Russian designs, including the hybrid ESA Tikhomirov NIIP N011M BARS radar in the Su-30MKI/MKM FLANKER H fighter, the derivative N035 Irbis E radar in the Su-35S FLANKER fighter, the Phazotron Zhuk-MFS/MFSE PESA for the Su-33 FLANKER D naval fighter, the Leninets B004 multimode attack radar for the Su-34 FULLBACK bomber, modelled on the Westinghouse APQ-164, and the NIIP Ryazan GRPZ Pero PESA upgrade package for the N001VE FLANKER radars. The unusual Pero is a reflective PESA. The technology is also used in the X-band 9S36 engagement radar developed for the new 9K317 Buk M2 / SA-17 GRIZZLY SAM system, and evolved 9S32M GRILL SCREEN, 92N6E GRAVE STONE and 91N6E BIG BIRD designs [1][2][3][4][6][7].

III. RUSSIAN X-BAND AND L-BAND AIRBORNE AESA DEVELOPMENTS

The 1990s saw a progressive transition in the United States and EU to AESA designs in key applications, with Russia and China now following. While the new AESAs exploited much of the technology previously developed for PESA radars, they introduced fundamentally different transmitter technology [1].

NIIR Phazotron was the first Russian manufacturer to offer an X-band AESA in 2007, with the Zhuk AE for the MiG-35 FULCRUM fighter, soon followed by the competing Tikhomirov NIIP with a much larger AESA for the Su-27/30 FLANKER fighter, and the low observable Sukhoi T-50 PAK-FA [1][7][9].

A. Phazotron Zhuk AE Design Philosophy The Zhuk AE developed for FULCRUM production and

upgrades was the first Russian AESA radar to be disclosed publicly. The manufacturer, NIIR Phazotron, released a considerable volume of technical literature detailing the design philosophy and technology employed in this radar. This pre-production radar operates at the lower end of the X-band and

Illustrations and other material in this paper were sourced from Air Power Australia (APA), with permission. The author co-founded APA in 2004.

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Fig. 1. Zhuk AE on MiG-35 demonstrator at AeroIndia 2007 (RSK MiG).

Fig. 2. Zhuk AE configuration (NIIR Phazotron).

Fig. 3. Zhuk AE lattice pattern (RSK MiG).

has a lower TR channel count than Western radars of similar aperture size, yet delivers power-aperture performance superior to all but the very latest Western small aperture fighter radars. The Zhuk AE employs lower density liquid cooled quad channel transmit receive module packaging technology which is comparable to first generation of US AESA designs [9][10][11][12][13].

The stated design aim was to build a low sidelobe AESA with a maximum beamsteering angle of 70°. This is the basic problem in all AESA designs, insofar as grating lobes require element spacing of less than one half of a wavelength, while the resulting volumetric packing density presents heat transfer problems.

The power rating and PAE (Power Added Efficiency) of the drive transistor was considered a problem, with initial estimation at 6 to 8 Watts CW (12 to 16 Watts peak at 50% duty cycle). The small size of the aircraft and its limited power and cooling capacity were seen as serious constraints. The drive transistors are operated in A-class to provide best possible linearity, with a performance penalty in a design with an overall PAE of 22% to 25%. C-class operation was rejected due to its adverse impact on signal purity.

Phazotron stated, that the greatest difficulties were encountered in engineering the TR modules. The approach chosen from numerous alternatives was to integrate four TR channels into a single “quad” module. An interesting observation is that this is a scheme identical to that used for first generation AESAs by US designers in the late 1980s, followed by the TR “stick module” scheme used in early US production AESAs.

Extensive design tradeoff studies were performed, covering power aperture and range performance versus thermal load performance for average TR module power ratings from 1 Watt to 15 Watts. A major issue was beamsteering to 70°, as problems arose with sidelobes and projected aperture area beyond 60° of beamsteering angle.

Phazotron appear to be exploring digital beamforming techniques in what Chief Designer Dolgachev describes as a two stage processing scheme, with initial beamforming performed in the AESA, and additional beamforming in the digital receiver, downstream of the ADC stage. Adaptive nulling of mainlobe jammers is also raised as a benefit of the AESA design. Dolgachev also observed that a key factor in the design process was maintaining a focus on key performance parameters, and exploiting computational simulations extensively throughout the design process.

Single channel TR modules were rejected in favour of a “more thermally efficient 4 channel quad module design”.

The proprietary diamond lattice placement of radiating elements used in earlier NIIR Phazotron PESAs was rejected, as it presented difficulties in splitting the array cleanly into the multiple phase centres required for monopulse angle tracking, nevertheless the stagger in the elements still provides a robust diamond lattice pattern.

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Fig. 4. Zhuk AE TR Module (NIIR Phazotron).

Fig. 5. GaAs 2-bit phase shifter MMIC die (NIIR Phazotron).

Fig. 6. GaAs 4-bit phase shifter MMIC die (NIIR Phazotron).

The resulting module configuration is designed to carry RF signals along the shortest geometrical path between the array face and the feed, with coolant flow transverse (normal) to the antenna boresight.

The result of these tradeoff studies yielded the final placement of the radiating elements in vertical columns, each comprising an integer multiple of four elements to accommodate the TR module structure.

Fig. 7. GaAs Gain Controller MMIC die (NIIR Phazotron).

Fig. 8. GaAs buffer amplifier MMIC die (NIIR Phazotron).

Fig. 9. Packaged Gain and Phase Control GaAs hybrids for use in TR module construction (NIIR Phazotron).

Performance claimed for the final element placement was a first sidelobe at -30 dB, an average of higher order sidelobes at -50 dB, mainlobe width degradation of 4 dB at maximum

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beamsteering angle, and no grating lobes within the intended beamsteering angular range.

Computational simulations were performed to determine the quantisation increments for antenna TR channel phase and gain control. Five bits were found to be adequate for amplitude, and six bits for phase control. Each TR channel in the array is individually addressed.

The backplane feed uses an undisclosed radial waveguide design, rather than the segmented linear branched feeds seen in first generation Western AESAs and ESAs. A network of coaxial waveguide switches between the feed network and TR modules is used to manage phase centres and perform monopulse summing and differencing for angle track modes.

Modules and channels are independently addressed, evidently with two low order bits reserved for the channel, and the remaining eight high order bits for module addressing.

An exciter preamplifier stage was developed to boost the output from the master oscillator module to compensate insertion loss from injection into the antenna feed backplane. The liquid cooled amplifier module has four ganged amplifier chains with a peak power output said to be 20 Watts.

Power supply distribution to the TR modules presented similar problems with module 'pulling' during current drain transients, and was accommodated by the pragmatic expedient of attaching a large charge store capacitor on the main power bus near each of the TR modules.

Cooling was arranged by mounting each TR module on an integral frame cold plate, the latter being actively cooled by liquid flow. Heat is transferred from each MMIC or transistor into the base of the module, and then into the cold plate for removal. Phazotron have not disclosed the thickness of the cold plates or TR modules, but clearly the horizontal element pitch is the bounding constraint. Each TR module includes an embedded thermal sensor which forces a module shutdown if overheating occurs, and restart cannot occur until the module cools down. All modules are thermally compensated in amplitude and phase to ensure that the performance characteristics remain aligned regardless of temperature and operating frequency.

Dolgachev stated the following TR module parameters: average power of 5 Watts, transmit path gain of 34 dB, receive path gain of 30 dB, receiver noise figure of 2.5 dB, phase shifter control increments of 5.625°, amplitude control increments of 0.7 dB, dynamic range for amplitude control of 24 dB, overall PAE of 25% [10].

Phazotron stated that the existing Zhuk AE design was performing below its potential, since its processing was taken unchanged from earlier mechanically steered arrays and is thus not optimised to exploit the AESA.

A separate paper by Semyonov et al discusses in some detail the design of GaAs MMICs used in the gain control, driver and phase shifter blocks of the TR channel. These were packaged together in single 8 x 22.5 x 2.5 mm sized hybrid with a heat transfer optimised metal case. The 5-bit digitally controlled attenuator is a GaAs MMIC die which uses 50 Ohm/sq resistive film for resistor components. The active

components are Schottky transistors. High order bit stages are implemented in two 8 dB stages, for a total of 16 dB of controlled loss. The low order bit control stages are implemented as 1 dB stages. The total insertion loss of the controlled attenuator is 8 - 10 dB, with a stated RMS error of 0.5 dB between 4 and 11 GHz, and total attenuator bandwidth of 4 to 14 GHz.

The 6-bit phase shifter function was split between two GaAs MMIC dies. Phazotron stated that the shifters were intentionally built using a folded directional coupler design rather than switched filters. The four higher order bits, covering 180.0°, 90.0°, 45.0° and 22.5° shifts are implemented on one die, the two low order bits for 11.25° and 5.625° shifts on a second smaller die. This approach was chosen to obviate problems with device yield. The design has been proven to perform between 8 and 11 GHz with an RMS phase error of around 6°, i.e. one bit. To compensate for the insertion loss of the attenuator and phase shifter stages, an additional buffer amplifier was included in the hybrid design. This GaAs MMIC design provides 7 to 9 dB of gain between 8 and 11 GHz.

According to Phazotron, the performance of the hybrids proved initially below expectations, the intent is to transition to LTCC (Low Temperature Cofired Ceramic) and MCM-D (Multi Chip Module - Deposited) technology to get high production yields.

Taking a critical technical perspective on the Zhuk AE, it is a remarkable exercise in producing a viable design using a technology base which shows chronic underinvestment in key areas such as component packaging and MMIC fabrication. Technologically the Zhuk AE compares best to first generation US AESAs like the 1990s APG-63(V)2 design deployed in limited numbers on the F-15C fleet. The technology especially for module packaging is similar to late 1980s US developmental designs.

Notably, the Zhuk AE with 652 TR channels has between 50% and 70% of the TR channels of a comparably sized US radar, which is typically in the 900 to 1200 single TR channel module count class.

The low element count will be reflected in sidelobe performance, to the extent that a relatively sparse array like the Zhuk AE design is inherently much more sensitive to phase and amplitude errors in the array TR channels, compared to more dense arrays. This is difficult to assess accurately in the absence of performance data for the phase and amplitude error correcting mechanisms embedded in the array. If they perform well, this may not prove to be an problem, if not, sidelobe performance cited at -30 dB may be difficult to maintain.

Phazotron have not disclosed the taper function employed nor even alluded to such. The choice of taper function will influence aperture efficiency, sidelobe behaviour and phase front behaviour in the mainlobe. As it is one of the parameters applied dynamically to the TR channel gain settings, Phazotron's taper functions are likely to evolve over time. Beamsteering agility in terms of duration to switch modules has also not been disclosed, but given published data describing other Russian ESA designs, a figure of the order of 0.4 milliseconds can be expected.

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The Russian TR modules deliver around 5 Watts average power per channel. The peak power for the Zhuk AE has however been disclosed at around 6 kW which puts the per TR channel peak power at around 10 Watts, accounting for some taper function induced reduction in overall power output. What Phazotron have not disclosed is the headroom in cooling capacity provided by the TR module packaging and cold plate design, which will put the upper bound on TR channel output stage peak power ratings, but they do allude to growth potential.

Given the penchant of Russian designers to build as much headroom as possible into designs, this may not be a critical constraint to long term growth of the design using newer Gallium Nitride transistors.

The bandwidth of the AESA has not been disclosed, the critical bottleneck in any such design is usually in the phase shifter blocks, as GaAs MMIC amplifiers are inherently wideband. Therefore an estimate for the TR module of 2-3 GHz centred on 9.5 GHz will not be unreasonable, with potential for further growth with refinement of the gain/phase block MMIC designs and packaging. This is however not consistent with the array design.

The upper frequency bound of the AESA will be determined by element spacing and grating lobe formation, for the existing design this appears to be at 8.5 GHz assuming for a diamond lattice a 17.5 mm spacing in the horizontal plane. This suggests a usable bandwidth of around 1 GHz or less centred on less than 8.5 GHz, accuracy of measurement permitting.

The details of the radial feed have not been disclosed and this may further constrain usable bandwidth. The literature cites “16 centre frequencies” which if separated by 100 MHz bandwidth would suggest 1.6 GHz, feed permitting. This would put the centre of the band coverage at about 7.7 GHz which would be consistent with past Russian design practice.

B. Phazotron Zhuk AE Growth Phazotron stated, in 2007, an intent to scale up the Zhuk

AE for the FLANKER, in the manner of the Zhuk-27 and Zhuk-MSFE variants, using a 0.98 metre diameter aperture. If we assume that such a scaled up design uses exactly the same quad module technology as the Zhuk AE does, and an enlarged cooling plate and mounting frame, then the achievable performance will scale with the aperture size. For the 0.98 m antenna outside diameter, assuming a similar unused area around the emitter array, the total usable aperture diameter will be around 0.8 metres, and the element count will sit at around 1160. If we assume tighter placement and a 1.1 metre antenna outside diameter, as used in the Pero PESA, then the total usable aperture diameter will be around 0.95 metres, and the element count will sit at around 1630, or about the same as the Zhuk-MSFE PESA design.

With a peak power rating of 10 Watts/channel the latter yields a peak power of the order of 16.3 kW which results in a radar which outperforms the N011M BARS, APG-63(V)1, APG-71 and APG-79 in cited raw power-aperture product performance.

Fig. 10. Instrumented AESA prototype (Tikhomirov NIIP).

Fig. 11. AESA antenna mounting. This example is constructed using TR module sticks, using an arrangement similar to the BARS and Irbis E, including the slot radiators. This brochure image may be of a developmental antenna, as the example presented in the Vesti video uses the same style of circular dielectric radiator as the competing Zhuk AE/ASE series (Tikhomirov NIIP).

If Phazotron improve the TR channel power rating as they have stated an intent to do, then the results bear some careful consideration. Once Phazotron have engineered a “Zhuk ASE” with ~1630 TR Channels, then scaling up power aperture

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performance is only a matter of changing the TR Module design to use more powerful transistors, and improving the per module heat transfer performance in the AESA. Both of the latter represent fairly low risk incremental design changes.

Fig. 12. X-band TR Module stick. Of particular interest is that the feed networks are symmetrically split, permitting this design to produce dual plane monopulse sum and difference outputs from a stack of such sticks (Tikhomirov NIIP).

C. Tikhomirov NIIP X-Band AESAs In August, 2009, Tikhomirov NIIP were permitted to

publicly display the new AESA developed for the PAK-FA, and also a clear candidate for FLANKER retrofits, stating that the integration of an AESA into the FLANKER airframe would not present difficulties [15].

The large 0.9-1.1 metre diameter aperture provided by the FLANKER nose and radome design will be especially attractive to an AESA designer. This aperture size permits around twice as many AESA modules of similar size to most current Western designs, apart from the F-22A Raptor APG-77 and F-15C APG-82.

The implications of this are sobering, insofar as with modules rated at half the peak power of the current state-of-the-art, such a radar could provide about the same peak power rating as current upper tier US AESAs. The power aperture would thus be higher due to the aperture area being so much larger. With COTS derived modules of much higher peak power rating than current US military GaN HEMT technology, a future FLANKER AESA could have a very much higher Power Aperture Product figure [7].

In 2009, there were two candidate AESA designs for installation in new build, or retrofit into existing service FLANKERS. These radars are NIIR Phazotron's intended “Zhuk-AS/ASE”, scaled up from the Zhuk AE, and a derivative of Tikhomirov NIIP's new PAK-FA AESA.

Both radar designs would be based on the quad channel TR module technology first disclosed during the public release of the Zhuk AE. These X-band modules are now being mass produced on an automated line by NPP Istok, who are also planning S-band module production. Mostly Russian produced

GaAs components are employed. Cited capacity is sufficient for 50 AESA radars annually [7].

The Tikhomirov NIIP AESA design for the PAK-FA employs an antenna aperture very similar in size, if not identical, to the aperture of the N035 Irbis E hybrid PESA. The design is intended for fixed low signature tilted installation, rather than gimballed installation, and auxiliary cheek arrays are planned for, emulating intended F-22 placement. The design is claimed to have been integrated with an existing BARS/Irbis radar back end for testing and design validation purposes.

Public statements made in Russia claim 1,500 TR module elements. Counting exposed radiating elements on video stills of the antenna indicates an estimated 1,524 TR channels, with a tolerance of several percent. This is within 5% of the 2008 model for a FLANKER AESA, produced by Air Power Australia [7][15].

NIIP have publicly claimed detection range performance of 350 to 400 km (190 to 215 NMI), which assuming a Russian industry standard 2.5m2 target, is consistent with the cited 2008 model for a radar using ~10 W rated TR modules, which in turn is the power rating for the modules used in the Zhuk AE prototypes. This puts the nett peak power at ~15 kW, slightly below the Irbis E, but even a very modest 25% increase in TR module output rating would overcome this.

There are distinct differences between the AESA displayed by NIIP for Vesti, which has less depth and uses circular radiators, and the examples displayed at MAKS 2009 and depicted on brochures, which are constructed using TR module sticks and are several inches deeper. Until further disclosures are made, the final AESA configuration will remain uncertain.

The best strategy available to the Russian industry for reducing AESA cost is the export of AESA upgrades to the large global community of FLANKER users over the coming decade, emulating the US approach with this technology. Tikhomirov NIIP brochures state that the existing AESA would be the basis of AESA upgrade designs for the Su-27/30/35 FLANKERS.

D. Tikhomirov NIIP L-Band Leading Edge AESA An interesting parallel development to Russian X-band

designs is a Tikhomirov-NIIP L-band AESA intended for embedding in the leading edges of fighter wings and strakes, providing a dual role IFF and Counter Low Observable capability [16].

This design has clear potential to provide a genuine “shared multifunction aperture” with applications including search, track and missile midcourse guidance against low signature aircraft; Identification Friend Foe / Secondary Surveillance Radar interrogation; precision passive angle tracking and geolocation of JTIDS/MIDS/Link-16 emitters, IFF and SSR transponders, L-band AEW&C/AWACS radars and surface based search radars at long ranges; high power active jamming of JTIDS/MIDS/Link-16 emitters, satellite navigation receivers, guided munition command datalinks, L-band airborne and surface based search radars at long ranges [16].

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Performance modelling for a range of feasible configurations indicates the radar will deliver tactically credible search range performance for feasible TR module power ratings.

The volume, weight, power, cooling and cost penalties of installing an L-band search radar in a fighter have historically precluded the use of this band in such applications. An X-band or Ku-band radar provides for greater accuracy, and directivity, given the available installation geometries. The only reason to pursue the L-band is thus if it can do something which cannot be done easily in the X/Ku-bands. That is the ability to produce useful skin returns from targets, which are difficult to detect and track in the X/Ku-bands. Embedding an IFF/SSR function in the design simply increases the design payoff, as a single design can perform two functions, interleaving IFF/SSR interrogation messages with target search pulse trains.

Fig. 13. L-band AESA general layout (Tikhomirov NIIP).

Fig. 14. L-band AESA quad radiator element subarray (Tikhomirov NIIP).

Fig. 15. NIIP antenna control module for the L-band AESA (Tikhomirov NIIP).

The basic array design and its integration into the leading edge flap structure are well documented via a wealth of imagery produced at the MAKS 2009 event. Each array employs twelve antenna elements. Three quad TR modules each drive four antenna elements, in three subarrays, for a total of twelve elements per array. The linear array is embedded in

the leading edge of the wing flap, with the geometrical broadside direction normal to the leading edge. The leading edge skin of the flap covering the AESA is a dielectric radome which is conformal with the flap leading edge shape.

Fig. 16. NPP Pulsar quad high power L-band TR-module used in the L-band AESA design. Note the use of eight RF power transistors in the design (NPP Pulsar).

Fig. 17. The array geometry produces a fan shaped mainlobe which is swept in azimuth by phase control of the twelve TR modules, providing a 2D volume search capability (Kopp, Falkenberg).

As the array is only one element deep in height, the angular coverage it provides in elevation will be fixed, and determined by the vertical mainlobe shape of the antenna elements. The arrangement of the AESA produces a fan shaped beam which is swept in azimuth to cover a volume in the forward hemisphere of the aircraft.

Whether the AESA can actually sweep the full volume which is geometrically available depends primarily on the mainlobe shape and boresight direction of the antenna elements, which remains to be disclosed. As the imagery of the antenna elements conceals the internal structure under a dielectric cover, at best we can make reasonable assumptions about the design.

The most likely technology employed is that of a microstrip antenna with a dielectric foam or air gap spacer, forming a sandwiched block. This technology has been used extensively in L-band designs for communications and satellite navigation. This technology would also permit precise shaping of the mainlobe in both axes and control of element sidelobes.

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There is an inherent tradeoff in such a design. Elements with higher gain will impose restrictions on bandwidth, and in beamsteering angle. The latter is critical in this application, since wide beamsteering angles in azimuth dictate a wide radiation pattern in azimuth. The element mainlobe angular width must be greater than the maximum beamsteering angle, or significant loss in total array gain will occur as the AESA mainlobe is steered into the region where element gain falls off rapidly with azimuth angle. The physical alignment of the array is with the leading edge of the wing, at ~42° for the FLANKER airframe.

Two possible design strategies for the antenna elements. The first is to employ very low gain elements, to provide the best possible angular coverage. However, it also drives up the emitted power requirement for any given detection range performance, as the total array gain is reduced.

An alternate strategy is to sacrifice total angular coverage to increase total array gain, and thus maximise power-aperture product. If the AESA is intended to provide significant detection performance operating as a radar, this is the preferred strategy.

Both design strategies permit single plane monopulse angle tracking within a narrow angular volume around the nose of the aircraft, where a target is within the coverage of both the left and right wing mounted AESAs. This is an operationally acceptable arrangement as the precision angle tracking provided by monopulse operation is employed primarily for weapon targeting. This does not preclude performing single plane monopulse angle tracking within each of the AESA arrays, using the subarrays, but affords higher total gain, greater baseline, and detection range performance.

Several estimations of element gain can be applied. The first is the simple rule of thumb estimate of ~6 dBi per element. If we assume a more refined design, with a mainlobe of 80° in azimuth and 40° in elevation, and apply Barton's approximation, G = 30,000/(θaz x θel), the element gain is ~9.7 dBi. Finally, we might assume a dielectric lens or more aggressive microstrip design, or some combination thereof, with an mainlobe size in elevation of 20°, which yields, again using Barton's approximation, an element gain of ~13 dBi.

Sidelobe performance will be poor compared to X-band AESAs, due to the limitations inherent in a 12 x 1 element linear array.

Receive path noise figure performance should be excellent, due to the short feed between the TR module and antenna element, the potential for low loss integral directional coupler design, and typical transistor noise figures in the L-band of a small fraction of a deciBel.

Choices in PRF, CPI, duty cycles and pulse compression technique Tikhomirov NIIP remain to be disclosed. Public disclosures on X-band designs suggest that Barker codes may be in use for pulse compression. Open source data indicates that most operating modes in Russian pulse Doppler designs emulate those commonly used in US designs, with medium and high PRF modes commonly used.

Modelling of the radar's performance for a range of viable configurations shows detection range performance against 1 m2 class target to be tactically credible, especially for configurations with higher gain antenna modules and higher TR module average power ratings. This analysis is inherently limited by the poor availability of data covering the actual design, especially in terms of coherent processing parameters and other basic choices. By increasing dwell times and coherent processing interval durations, range performance could be further improved in this regime.

The Tikhomirov NIIP L-band AESA has considerable growth potential by virtue of the large size of the FLANKER airframe, permitting additional antenna elements, cooling and power. Growth options include: increasing the power rating of the existing TR modules, retaining conduction cooling; further increasing the power rating of the TR modules and introducing liquid cooling; improvements to antenna element design to increase gain; extending the arrays further along the wings, to add an additional one, or two, subarrays; addition of receiver arrays in the leading edge of the vertical tails to provide dual plane monopulse precision angle tracking capability for fire control purpose.

Increasing the array size to 16 elements improves power-aperture product for the existing design by almost 80%, by virtue of additional gain and transmit power. The use of more powerful TR modules provides for further improvements. The practical limit will be the available leading edge flap volume as the design progressively tapers toward the wingtips, and system constrains liquid cooling capacity.

Fig. 18. NRIET J-10B X-band AESA cited at ~1200 TR channels. An 8 element L-band IFF interrogator array is embedded (NRIET)[17].

IV. CHINESE AIRBORNE AESA DEVELOPMENTS To date, disclosures of substance on China's X-band AESA

technology, have been mostly scarce [1].

China has a highly active research and development community working in radar, especially in phased arrays. Researchers are resident in universities, research laboratories and industry, the latter primarily in NRIET (Nanjing Research Institute of Electronic Technology / No.14 Institute), and collaborations are common. The principal academic publishing platform is the Chinese language peer reviewed monthly journal “Modern Radar”, published since 1979, and covering

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topics from basic theory advances through to case studies of applications, including frequent surveys of non-Chinese technology advances.

The most substantial disclosure to date on Chinese X-band AESAs is a poster, likely authored by NRIET, providing some basic details on three developmental AESAs built for fighter aircraft applications [17].

A. NRIET J-10B X-band AESA Described thus: “The NRIET J-10B AESA is our 1.5th

generation design based on GaAs dual channel TR module technology, providing ~1200 TR channels”, the J-10B AESA is the first design to be publicly shown installed in a fighter aircraft, and may be an evolution of the 1152 channel demonstrator described by NRIET engineers Xia and Niu [18].

The airborne demonstrator employed a “linear subarray” configuration, with modules arranged in vertically aligned sticks [18].

Fig. 19. NRIET Flanker X-band AESA cited at ~1760 TR channels, employing dual channel GaAs TR modules. The upper right caption describes 16+1 coaxial outputs from receive path subarray feeds, the lower right caption the DDS technology exciter array, and the bottom caption the carbon fibre composite mounting frame (NRIET) [17].

Fig. 20. NRIET J-20 X-band AESA cited at ~1856 TR channels, employing dual channel GaAs TR modules. The captions translate to a.AESA layout; b.AESA element emitters; and c.3D MCM tile (NRIET) [17].

B. NRIET FLANKER X-band AESA Described thus: “Our 2nd Generation Direct Digital

Synthesis technology J-16 AESA employs a TR stick module arrangement, providing 1760 TR channels”, this AESA was developed for installation in the J-16 FLANKER, which is a dual seat long range strike variant of the Chinese built J-11BS FLANKER C fighter, modelled on the Boeing F-15E Strike Eagle. While developed for the J-16, the physical sizing of this AESA is implicitly compatible with other Chinese built

FLANKERS, i.e. the J-11B, J-11BS, J-11BH and navalised J-15 [19].

This AESA has 1760 TR channels, which is 6% more than the Russian NIIR Phazotron Zhuk MSFE PESA developed for the Flanker, or 8% more than the 2008 APA model for a Flanker-sized “Zhuk ASE” AESA. The array packaging appears similar to the Zhuk AE, but there is insufficient disclosed data at this time to conclude that the design is the result of a technology transfer from NIIR Phazotron. Some earlier NRIET planar array X-band radars were based on NIIR Phazotron technology, derived from the Zhuk-8II for the Shenyang J-8II FINBACK fighter. The use of 17 separate coaxial receiver feeds indicates the AESA is divided into multiple phase centres for monopulse operation, and possibly GMTI/MMTI operation. The backplane mounts an array of “Digital Direct Synthesis” exciter/driver modules.

C. NRIET J-20 X-band AESA Described thus: “Built in 2009, the J-20 AESA is our 3rd

Generation design, using eight channel four layer GaAs 3D Multi-Chip Module (MCM) technology, providing 1856 TR channels.”, this large AESA is the first Chinese design to employ a tiled TR module packaging scheme, evidently modelled on contemporary US AESA designs, but employing eight TR channels per tile, with 232 tiles in the array. The J-20 is a large “F-111-sized” delta-canard low observable supersonic cruise optimised fighter [17][20].

The cited number of TR channels is 12% higher than the Zhuk MSFE, 14% higher than the 2008 APA model for the “Zhuk ASE”, and 23% higher than the cited number for the Tikhomirov NIIP AESA. Only modest array repackaging would be required to produce a variant sized for a FLANKER airframe, given similar airframe nose cross section.

Fig. 21. An early production KJ-2000 AEW&C system. The dielectric panels on the dorsal radome indicate this is a three sided phased array, probably operating in the L-band and evidently influenced by the design of the Israeli Elta Phalcon system (Zhenguan Studio via APA).

D. Chinese Airborne Early Warning & Control AESA Developments AESAs penetrated into the global Airborne Early Warning

and Control (AEW&C) radar market during the 1990s. Israel's IAI/Elta developed the L-band EL/M-2075 Phalcon on a Boeing 707-320, later selling the demonstrator to Chile [1].

The same technology was sold to China, the order later cancelled under pressure from the Clinton Administration. The cancellation of the Israeli order led China to initiate the development of the KJ-2000 system, which is evidently modelled on the three sided EL/W-2090 L-band AESA previously offered to Australia, and has been supplied to the

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People’s Liberation Army Air Force (PLAAF), on the Ilyushin Il-76 CANDID airframe. The People’s Liberation Army Navy has been procuring the KJ-200, itself evidently modelled on the Swedish SAAB/ Ericsson PS-890 Erieye design [1].

At least three KJ-2000 prototypes were built, but there have been no substantive technical disclosures to date [22].

Imagery clearly shows three sided phased array dielectric radomes, and wingtip ESM fairings, on a rebuilt Il-76 CANDID airframe.

There has been some speculation that the PRC may be attempting to clone the Israeli Phalcon system using indigenous technology. Given that L-band radio frequency power transistors of suitable ratings are available commercially, cloning is feasible and entirely consistent with the long running PLA policy of concurrently developing indigenous products while importing foreign equivalents. An L-band array TR module design of suitable performance and configuration could be used for both the A-50 system and the smaller Y-8 design, sharing most of the system hardware and software. A least one image exists of a ground based antenna testing rig, built up as an AESA radome and mount on top of a mast on a larger building [22].

Specific performance parameters for this AESA, such as module count and peak power rating, remain to be disclosed. It is reasonable to speculate that these parameters would be very similar to the Israeli Elta design, to which the PLA had considerable exposure. Media reports suggested that in late 2009 four PLAAF KJ-2000 systems were operational, but it is not known what the actual status of the onboard mission systems is [1].

Fig. 22. KJ-200 “balanced beam” AESA arrangement (Zhenguan Studio via APA).

The second smaller AEW&C program has been labelled the KJ-200 or “Y-8 Balanced Beam” system. Installed on a late model Y-8F-600 airframe with Pratt & Whitney Canada PW150B turboprops and Honeywell avionics, the KJ-200 has been observed in the Nanjing area flying with a dorsal structure resembling the Erieye AESA system, as well as ventral radomes. With a similar payload/volume to the C-130A, a Y-8

with an Erieye style AEW&C system would be expected to be equivalent in performance and endurance to the C-130/Erieye proposals offered during the late 1990s [22].

V. CHINESE LAND BASED AESA DEVELOPMENTS Chinese search and acquisition radars are also seeing

increasing use of AESA technology. The Chinese S-band Type 305A / K/LLQ305A appears to be fundamentally influenced by the Thales/Raytheon Groundmaster series S-band GM200 and GM400 designs. The depth of the primary antenna and its structural frame is typical for AESA designs in this category, using a stacked modular feed network arrangement; this is well documented in a number of Russian AESA designs. The new Type 305A 3D acquisition radar is otherwise unique and does not resemble any other known Chinese radar designs. The stated use is for early warning and battle management against aerial and ballistic targets [1] [23].

Fig. 23. Type 305A phased array acquisition radar deployed (Bradley Huang via APA).

Fig. 24. Aft view Type 305A antenna, deployed (Chinese Internet via APA).

The rear face of the antenna frame is largely occupied with voluminous equipment housings, of similar depth to the antenna frame itself, and of equal height. These would be

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consistent with the installation of low voltage AESA power supplies, cooling equipment, receiver, and exciter hardware.

VI. RUSSIAN LAND BASED AESA RADARS The Russian Almaz-Antey / NNIIRT 1L119 Nebo SVU

and 55Zh6ME RLM-M/ME Nebo M/ME VHF-band three-dimensional Counter Low Observable search and acquisition radars are unique, globally [1].

A. Almaz-Antey/VNIIRT 67N6E Gamma DE The 67N6E Gamma DE is a long range L-Band 3D AESA

MTI search and acquisition radar intended to support interceptors and Integrated Air Defence Systems. It is claimed to detect and track aircraft, cruise missiles, precision guided munitions and tactical ballistic missiles at medium and high altitudes. Performance claims include azimuthal tracking accuracy of 0.17-0.2°, elevation accuracy of 0.2-0.3°, range accuracy of 60-100 metres, and detection range of 330-400 km against 1 m2 targets for the D1E variant [24].

Fig. 25. Almaz-Antey/VNIIRT 67N6E Gamma DE deployed , the upper half of the AESA is folded back when stowed (VNIIRT).

Gamma DE installations can be supplied with three different AESA module power ratings, yielding the D1/D1E, D2/D2E and D3/D3E variants. A single Gamma DE system comprises a towed antenna head trailer with the 1280 element 8 x 5.2 metre AESA on a turntable, a semi-trailer radar cabin with electronics and operator stations, and a dual redundant 16 kiloWatt diesel generator. An option cited for the Gamma DE is deployment of the radar head on the 24 metre 40V6M or 40 metre 40V6MD semi-mobile mast systems [24].

While publicly listed at this time on the Russian Ministry of Defense website, there is no evidence proving that any significant number of Gamma DE systems have been deployed by Russia’s armed forces.

B. Almaz-Antey S-500 Triumfator M / Prometey Radar Systems The new S-500 “Triumfator M” or “Prometey” anti-

ballistic and air defense missile system is intended to become the upper tier of Russia air and space defense system. While disclosures remain fragmentary, it is known that three ESA radars will be employed in the system. One of these is the NIIIP 91N6A(M) acquisition and battle management radar, which supplants the BIG BIRD series employed in the earlier S-300PS/SA-10, S-300PM/SA-20 and S-400/SA-21 missile systems, and similar in role to the US TPY-2. The X-band

76T6 and 77T6 ESA engagement radars are respectively optimised for aerial and ballistic missile targets, much like the 9S32 and 9S19 space fed PESA radars employed in the S-300V/VM/SA-12/SA-23 systems [25].

Fig. 26. 1L119 Nebo SVU deployed. At least one unit has been repeatedly photographed in Iran (NNIIRT).

Both the 76T6 and 77T6 are described as AESAs in the Russian media, including the respected Voenno-Promyshlenniy Kurier web journal, but official documents validating this have not appeared to date. The 91N6A(M) is also poorly documented this time, the best item being an animated illustration, and may or may not employ AESA technology.

C. Almaz-Antey/NNIIRT 1L119 Nebo SVU The VHF-band 1L119 Nebo SVU AESA, first disclosed

in 2001, was intended to replace the 1L13 Nebo SV BOX SPRING. The intent of this new radar was to produce a design capable of detecting and tracking Very Low Observable (VLO) and Low Observable (LO) aircraft designs. Like the Nebo SV, this development project was led by Igor Krylov at NNIIRT. Interviewed by Russian television in 2002, Krylov stated “We can see the Stealth [F-117A] as clearly as any other plane”[26].

The design rationale for the earlier NNIIRT 55Zh6UE Nebo U TALL RACK has been discussed in detail in Russian literature, but no such document exists for the Nebo SVU at this time. Therefore we can at best infer the reasoning of Krylov's NNIIRT development team, based on the observable or publicly documented features of the radar [26].

The Nebo SVU was the first ever VHF band AESA, with multiple Russian sources elaborating on the use of antenna array mounted TR modules. The similarity in array size, range performance, overall power consumption, operating frequency and general arrangement to the earlier BOX SPRING thermionically powered radar suggests that a peak power rating of the order of 120 to 140 kW should be expected. With 84 elements this indicates a per TR module peak power rating of 1.4 to 1.7 kW per module which is readily achievable with mature off the shelf technology. Russian datasheet tables claiming a “20 kiloWatt peak power” are not consistent with cited performance [26].

Commercial low cost VHF band MOSFET transistors rated at ~500W are available in the global market, therefore building a VHF band TR module rated at 2 kW with four ganged

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MOSFETs presents no great difficulty. With the low packaging density for a VHF AESA, it is clear that this did not present any obstacle for Krylov’s team.

What remains to be disclosed is how NNIIRT designed the beamsteering control of the antenna, as the claimed module providing this function is quite compact. At such a low frequency a Digital RF Memory (DRFM) solution might have been adopted, rather than a classical analogue delay line or phase shifter solution - if a carrier frequency of 150 to 220 MHz typically seen in Russian VHF radars is assumed, extant Russian DRFM technology would suffice [27].

The radiating antenna element design is a three element hybrid - a vertically polarised two wire 3/8 λ folded dipole with a single parasitic director, using additional support frame mounted reflector elements. Scaling vehicle dimensions yields estimation of the wavelength at ~2 metres with a symmetrical ~1 metre array element spacing. The choice of a 3/8 λ folded dipole permits tighter element spacing in the array. Gain is of the order of 3-4.5 dBi per element, but may be reduced by array coupling effects [28]

Grating lobe performance is interesting in this design. If we assume that electronic beamsteering is used mostly for precision angle tracking of targets near the boresight, grating lobes do not impose the burden they do in fixed X-band AESAs, and there is some flexibility in operating frequencies.

If electronic beamsteering is used for sector searches, with significant deflection angles, then grating lobes become a potential problem in the design, and the <1/2 λ element spacing rule limits the upper frequency of the design to around 150 MHz, with degraded gain in the 3/8 λ folded dipole imposing the lower limit on frequency agility. The range of measurement error in array geometry suggests the design was sized for larger deflection angles, so ±45° to ±60° off boresight is achievable, subject to aperture foreshortening, sidelobe performance limits, and the shaping of the hybrid folded dipole element mainlobe. Were the design limited to small off boresight steering angles, the element spacing could be greater.

With only 84 elements, the Nebo SVU uses a sparse array, so highly accurate calibration of module phase/delay and gain are absolutely critical to achieving the intended sidelobe control and accuracy [26].

Russian literature covering the Nebo SVU describes it as capable of detecting and tracking aircraft and ballistic missile class targets. The antenna can be tilted at least 17° in elevation, specifically for “ballistic missile acquisition” [26].

The antenna can also be mechanically rotated for aerial target acquisition, or pointed in a fixed direction to cover a specific threat sector. Using a circular sweep pattern the antenna is claimed to be limited to an elevation angle of 25°, but in its fixed azimuth/sector target tracking mode the highest beam elevation angle can be as high as 45° to 50°. If we assume the design is mechanically limited to a tilt angle of 17° this suggests an electronic beam deflection angle in elevation of ±28° to 33°. A similar bound thus applies to horizontal deflection angles, through commonality in delay/phase shifter hardware.

Russian documents state that the radar employs “Complete space-time digital signal processing”. This may be a poor translation of Space Time Adaptive Processing (STAP), or simply suggest the radar uses digital processing. At this stage the issue of STAP capability in the Nebo SVU remains unresolved, but it is a likely capability in this family of radars longer term. There are no obstacles, which preclude dividing the array into multiple receive path phase centres.

D. Almaz-Antey/NNIIRT 55Zh6ME Nebo M/ME In late 2008, details emerged of a new multiband 3D radar

system in development by NNIIRT, designated the “Nebo M”, radically departing from previous Russian designs [24][26].

Fig. 27. Rendering of Almaz-Antey/NNIIRT 55Zh6ME Nebo ME deployed. The VHF-band component is at the right of the image, the S/C-band component is at the left of the image, the L-band component in the foreground, and the data fusion system, in the background. All components are carried on high mobility vehicles. At least one hundred of these systems will be procured for the Russian Federation Air Defence Forces (NNIIRT).

Fig. 28. 55Zh6ME Nebo M RLM-ME VHF-Band Radar System, employing 24 x 7 3/8 λ dipole short Yagi elements (Vitaliy V. Kuzmin via APA).

The self-propelled Nebo M is a package of three discrete radars and a single processing and command van, all hosted on BZKT BAZ-6909-015 8 x 8 all terrain 24 tonne chassis, common to the S-400 / SA-21 missile system, and networked using Ka band links, likely the Luch M48 series [26].

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The integration of three radars, each operating in a discrete band, is novel. A track fusion system in the KU-RLK command post vehicle will be required, providing a capability analogous to the US Navy CEC (Cooperative Engagement Capability) system. CEC technology was previously developed for the Salyut Poima E naval track fusion system [26].

The improved and larger self-propelled Nebo M RLM-M system extends the ideas employed in the Nebo SVU design. Parametric modelling of performance suggests a 40 percent range improvement over the Nebo SVU if equal TR module power ratings apply. Angular error in azimuth is almost halved, further increasing the potential of the design for midcourse SAM guidance [26].

Fig. 29. 55Zh6ME Nebo M RLM-ME hybrid two wire 3/8 λ folded dipole elements (Vitaliy V. Kuzmin via APA).

Fig. 30. 55Zh6ME Nebo M RLM-DE L-Band Radar System (Vitaliy V. Kuzmin via APA).

The Almaz-Antey/NNIIRT 55Zh6ME Nebo ME was displayed publicly for the first time at the Ramenskoye Air Base centenary open day and air show at Zhukovskiy, near Moscow, August, 2012. Of the four components comprising the Nebo M multiband radar system, only the S/C-Band RLM-SE component was not displayed [26].

The RLM-ME system component appears largely identical to prototype hardware shown in poor quality imagery released some years ago, with 24 x 7 3/8 λ dipole short Yagi elements, common also to the earlier Nebo SVU demonstrators. The TR modules appear to be embedded in the structural beams. The compact cabin at the base of the folding antenna mast houses the hardware for beamsteering, exciters, and receivers. Coaxial cables from the elements to the central enclosure are discernable on the horizontal antenna beams [26].

Fig. 31. 67L6M/E Gamma S1M/E ESA Radar deployed (Said Aminov via APA).

The Nebo M RLM-DE L-band component general arrangement is similar to the Thales Groundmaster GM-400 series and VNIIRT Gamma S1E AESA designs, with TR-module enclosures mounted on the rear face of the antenna frame. Radiating elements are arranged in quad blocks, with four dipoles per block. There are 38 columns and 48 rows of elements, for a total of 1824 elements, with a 4:5 aspect ratio slightly favouring heightfinding performance. This number of elements would allow for very low sidelobe performance should a suitable taper function be employed [26].

Illustrations of the Nebo M RLM-SE S/C-band component show similarities to the S/C-Band 67L6E Gamma SE, a solid state ESA acquisition radar recently introduced into service. It is not clear from Russian literature whether the Gamma SE is a genuine AESA, a hybrid PESA, or PESA using distributed solid state power amplifier technology [1].

CONCLUSIONS Russian and Chinese AESA technology has yet to reach the

refinement and maturity of US, EU and Israeli technology. Reflecting many formidable technology base obstacles, extant designs frequently follow design strategies previously used by US and EU designers. In many respects Russian and Chinese AESAs closely follow established US design approaches, but

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with often unique adaptations to overcome a range of limitations in the respective technology bases [25][30].

A recurring theme observed in Russian designs is exploitation of globalised COTS (Commercial Off The Shelf) semiconductor device and software technology, to overcome critical gaps in domestic technologies.

Russian designs such as the multiband Nebo M/ME or L-Band airborne AESA display considerable originality, and present as unique innovations in radar design and integration.

Future advances in Russian and Chinese device technology will over time result in progressive convergence with Western technologies, in many key areas of AESA design.

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[27] TsNIRTI, “Digital RF Memory and Exciter”, FSUE “CNIRTI named after academician A.I. Berg”, 20 Novaya Basmannaya St., Moscow, Russia 105066, Available: http://www.cnirti.ru/catalog-11-24.htm

[28] J.D. Kraus, “Antennas,” 2nd Ed., McGraw-Hill, 1988, 11-39, 11-61. [29] C. Kopp, P.A. Goon, “Assessing the Sukhoi PAK-FA”, APA-2010-01,

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