AESA Radar – Pan-Domain Multi-Function Capabilities for Future Systems

A.Farina, P.Holbourn, T.Kinghorn, L.Timmoneri

Selex ES
A Finmeccanica Company

Abstract • Airborne Maritime Surveillance,

Active Electronically Scanned Array (AESA) radar technology is
now relatively mature, with numerous systems in service on air,
• Point and Area Defence,
land, sea and space platforms. There is an increasing drive to • Electronic Protection Measures (EPM),
expand the multi-functionality of systems both within traditional • Electronic Surveillance (ES),
radar frequency bands and, increasingly, by the adoption of
wideband or multi-band technologies. These developments
• Target tracking ,
present major technical and engineering challenges in • Missile guidance,
implementing optimised solutions within cost and installation • Target classification,
constraints. Major enablers are a modular, scalable approach • Counter- Rocket, Artillery and Mortar (C-RAM),
which limits non-recurring expenditure, and a drive to re-use
technologies in as many applications as possible. Selex ES • Ballistic Missile Defence (BMD).
operates across the land, sea and air domains and has developed
a ‘design once use many times’ design philosophy which has The development of a multi-function AESA radar
already delivered significant benefits. This paper describes the represents a formidable engineering challenge which will
current state-of-the-art in AESA radar systems in Selex ES, and test the capabilities of any radar design house, requiring
considers future developments, especially wideband multi- access to a wide span of state of the art technologies,
function systems and common technical approaches across the including Monolithic Microwave Integrated Circuit
land, sea and air domains. (MMIC) design and fabrication, microwave packaging,
Key Words: AESA radar, multi-function, wideband
antenna design, real time software and advanced signal
processing, within a framework of rigorous systems
engineering. The costs of ab initio development are such
INTRODUCTION that there is increasing pressure to seek common solutions
across a wide range of application areas, and to find ways
to reduce development expenditure through re-use and the
Selex ES is a new industrial grouping that brings together,
adoption of commercial technologies where possible. This
amongst other things, the established expertise in airborne
approach has been successfully used on current generation
AESA radar in the UK part of the company (formerly
AESA radar systems; we expect this trend to continue into
Selex Galileo) and ground/naval AESA radar in the Italian
the future.
part of the company (formerly Selex-SI). Historically, the
naval work started a little earlier (1986) [1] than the
This paper describes some current generation AESA radar
airborne part (1991) but the state-of-the-art across the new
technologies for airborne, ground and naval applications.
company is now similar and well-established. Current
We then go on to consider future developments, especially
products include the Seaspray range of X-band airborne
wideband multi-function systems [2], and examine the
surveillance radars; the naval European Multifunctional
different challenges and constraints in the airborne, ground
Phased Array Radar (EMPAR); the Vixen range of
and naval fields. We then go on to highlight the similarities
airborne multi-function fighter radars; the Kronos range of
and differences in the potential technology solutions for
C-band naval and ground-based multi-function radars; and
these diverse application areas.
at the smallest end, the PicoSAR surveillance radar for
Unmanned Air Vehicles (UAVs).

Multi-function AESA radars can perform simultaneously AESA RADAR ARCHITECTURE

the detection and tracking of a large number of targets in a
dynamic and automatic way adapted to the operational Traditional mechanically scanned radar architectures have
environment, by the use of intelligent radar resource tended to result in point designs with limited opportunity
management. By exploiting the architecture of sub-arrayed for common modules and re-use. In contrast the AESA
antennas (where groups of radiating elements share the architecture, and the intrinsic attributes of the technology,
same digital receiver) AESA radars can provide lends itself to a ‘design once use many times’ approach to
simultaneous digital beamforming and adaptive create a modular, scalable architecture based on a set of
cancellation of jamming and interference. With these common modules, e.g. Transmit/Receive Modules (TRM),
capabilities, the AESA radar can fulfil many operational radiating elements, antenna sub-assemblies, Direct
missions, including: Current-Direct Current (DC-DC) converters, RF down
convertors, processor and software modules. We have
adopted this approach to create a comprehensive portfolio
• Air Surveillance (Airborne and Surface-based),
of AESA radar systems for a wide range of fast jet,
• Ground Surveillance (Synthetic Aperture Radar maritime patrol aircraft, helicopter, UAV, naval and
(SAR), and Ground Moving Target Indicator ground applications. Whilst we are still short of complete
(GMTI)), commonality across the whole product range, this ‘design

978-1-4673-1127-4/12/$31.00 ©2013 IEEE 4

once use many times’ philosophy has significantly reduced results in some restrictions on the azimuth scan limits
the development costs required to realise a broad product compared to the elevation scan limits, but it is a cost-
portfolio and, in production, enables the products to be effective engineering solution. The lower power density
offered at a competitive price. allows forced air cooling to be used. The scalable and
modular nature of the design concept is clear from these
Critical to this ‘design once use many times’ approach is photographs.
setting the right sub-system functional, performance and
interface specifications to ensure that the sub-systems can The TRM lies at the heart of any AESA radar. Whilst
re-used in many different applications. Setting appropriate function and performance is important, price is everything;
specifications for the common sub-systems requires careful the price challenge requires a TRM price of rather better
consideration of all potential system configurations and than $1000 each in volume quantity. What we have today
operational modes. This requirements analysis then leads to is the result of many iterations of design over the last 10 to
the definition and allocation of sub-system functional, 15 years. For most applications our preferred TRM
performance and interface specifications. architecture is the ‘common leg’ architecture which is
characterised by a switching network to commutate the
AIRBORNE AESA CONFIGURATION digitally controlled phase and gain elements between the
AND SUB-SYSTEM DESCRIPTION Tx and Rx paths. A typical TRM, currently in volume
production, is shown in Figure 4.
Figure 1 illustrates the physical realisation of a typical
AESA for airborne fire control radar applications in terms TRMs are primarily a design for manufacturing and design
of what is generally called a ‘brick’ architecture in for unit production cost challenge. A pragmatic and
accordance with the disposition of the TRMs [3]. In this knowledgeable approach is required to set the key design
implementation the TRMs are configured as planar parameters and their acceptable range of variation to ensure
rectangular hybrid packages and are mounted on the that system performance is achieved, whilst yield at TRM
surface of a ‘plank’, a complex multi-layer printed circuit level is acceptable. This leads naturally to adopting a
assembly, which incorporates the radiating elements, local statistical approach for acceptance, which establishes
power conversion, digital control signal distribution and individual and batch acceptance thresholds for TRM
RF manifold to form a linear array. The planks are then manufacture. Establishing a close partnership with a
stacked and are retained within a stiff mechanical frame. commercial high volume Gallium Arsenide (GaAs)
Each plank is bonded on to a light weight mechanical foundry is a critical factor in hitting the performance and
structure which includes a liquid cooling channel; a cooling price targets.
manifold within the frame distributes liquid coolant to each
plank. A separate electrical manifold, parallel to the front Whilst the radiating element may appear physically simple,
face of the array, interconnects the planks at the rear and its electromagnetic design is subtle, and optimising the
provides a further level of power, control and RF design for its intended application is non-trivial. Over the
distribution. The rear radio frequency (RF) distribution years we have modelled and tested most types of radiating
network in conjunction with the planks performs the RF element, including printed patch, round wave guide, square
distribution and analogue beam forming function for wave guide and various notch and slot designs. A good
receive and transmit. Typically Sum, Azimuth (Az) and design requires iteration and depends on extensive
Elevation (El) monopulse difference and a number of electromagnetic (EM) modelling and construction of test
receive only guard channels will be implemented. The CTI pieces for evaluation in near field and compact antenna test
(Control and Timing Interface) and RFIF (RF Interface) ranges. Our experience concluded that a notch radiating
provide the digital and RF control interfaces respectively element is a low cost design that can support efficient
between the array and rest of the radar system. broadband operation at wide scan angles with a physically
short compact implementation which is compatible with
For airborne fire control radar applications a fully filled the plank architecture and design implementation.
array, supporting 2-D electronic scanning, is implemented
with the TRMs and radiating elements arranged on an Dynamic and adaptive beam steering and beam shaping are
equilateral triangular /2 mesh. For a 60 cm diameter array key differentiators between AESA and Mechanically
this requires ~ 1000 TRMs at X-band and ~ 30 planks of 4 Steered Array (MSA) radar systems. These new degrees of
or 5 different lengths. Note that for some applications, e.g. freedom can confer considerable operational benefit. The
maritime surveillance, it may be convenient to thin the implementation of the beam steering function must support
array in 1 or more dimensions, by mapping a single TRM these features. Requirements include:
to several radiating elements.
space stabilised raster search patterns,
Figure 2 shows a small airborne fire control AESA radar cursive beam control (to support look back and confirm
with a typical plank. This is a fully filled array, with full and other data adaptive detection and track strategies),
2-D electronic scanning, and is liquid cooled. By way of separate Tx and Rx beam shape, and
comparison, Figure 3 illustrates an airborne surveillance instantaneous beam pointing with minimal latency.
AESA, with its associated plank; in this instance the AESA
is sparsely filled in azimuth, i.e. 1 TRM is mapped to 4 A distributed architecture for beam steering and beam
radiating elements, and is fully filled in elevation. This control has been adopted whereby:

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 the radar processor generates periodic steering commands Technically, airborne radars must be robust to operate in a
in array coordinates very severe range of environmental conditions. In terms of
distributed beam controllers Field Programmable Gate performance, there are many variables but look-down air-
Array + Non-Volatile Random-Access Memory (FPGA + to-air radar can stretch technical performance to the limit.
NV RAM) on each plank calculate and communicate
individual TRM gain and phase settings; this calculation is Airborne radars, typically, have adopted higher operating
dependent on many parameters, including plank and TRM frequencies (usually X-band or Ku-band), accepting the
calibration data, frequency, beamshape and operating more significant atmospheric attenuation as the price of
temperature. achieving better angular resolution and accuracy from a
physically constrained antenna aperture.
The power supply sub-system is very definitely non-trivial
in the context of an AESA radar. The AESA radar poses a By contrast, naval systems are typically far less constrained
low voltage high current challenge which demands that in terms of size, power and cooling; there is some pressure
best engineering practice be applied to power distribution, on mass due to the need to mount the radar as high as
earthing, safety interlocks, condition monitoring and Built possible in the ship. These systems are procured in very
In Test (BIT). A typical 1000 TRM airborne fire control low quantities, so the scope for economies of scale is low.
AESA might require >1000A at 9 – 10V and this is
supplied via a multi-stage redundant modular architecture. Naval systems have typically adopted lower frequency
bands (L, S and C-band) as the atmospheric characteristics
at low-level are much more benign. This leads to the need
NAVAL AND GROUND BASED AESA for larger antennas but this is not too much of a problem.
The ready availability of power and cooling supports
CONFIGURATION
significantly higher power operation and thus longer range;
although due to the inverse fourth power law relationship,
Figure 5 is a pictorial view of a typical mobile ground
the benefits of higher power operation are less than might
based multifunctional radar scenario while next Figure 6
be expected.
illustrates the same view for a naval application.
So the question arises, to what extent can common
Figures 7 and 8 report two multi-function systems recently
technical solutions be adopted across this diverse range of
developed and delivered by Selex ES to respond to the
applications? Clearly wideband technologies would be
missions presented in the previous figures.
advantageous, as systems could be readily configured to
operate in different ways for different applications.
The Kronos NV system is the subject of figure 7; it is a
Economies of scale could clearly be of benefit, provided of
naval AESA radar equipped with approx. 800 TRMs,
course that common solutions are not over-engineered to
suitable for installation on ships of around 400 tons [4]. Its
meet the superset of all possible requirements.
main missions are point defense, air and sea surveillance
and gunfire support.
FUTURE DEVELOPMENTS
The AESA system called MFRA (figure 8) is equipped
with more than 2000 of the same TRMs used for the The development of AESA radars are driven by the need to
Kronos NV; it has been developed for the following main respond to new operative missions. Two examples are
missions: Tactical Ballistic Missile (TBM) defence and Counter-
Rocket & Artillery Mortar (C-RAM) systems [5]. The new
 threat identification, requirements of these functions call for fast initialization
 ‘full volumetric’ surveillance, and tracking of highly manoeuvring targets together with
 multi target tracking with different priorities enhanced resolution and accuracy capabilities to correctly
 missile up-link, and predict the launch and the impact points of the two threats.
 adaptive ECCM (Electronic Counter Counter The design of AESA radars for these requirements implies:
Measures) function.
 conception of new operative modes and
waveforms including the need of working with
the antenna in ‘stare mode’;
AIRBORNE AND NAVAL SYSTEMS  new techniques for antenna design to minimise
the number of active modules while achieving
The vast majority of airborne systems must meet stringent remarkable sidelobe level and antenna scanning
constraints on size, mass and power consumption. Most capabilities;
airborne systems are procured in relatively large numbers,  development of more efficient TRMs;
so there is also a great deal of pressure on cost. Rarely,  innovative strategy of thermal management;
however, are the production numbers sufficient to fully  new concept for analogue receiving chain; and
realise the benefits of mass-production techniques.  modern computational architecture based on, for
example, Graphics Processing Units (GPUs).

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Other drivers for the continuous improvement of AESA similar). By contrast in airborne radar both power and
radars are: cooling are generally in short supply, and in most cases are
insufficient to make much use of the potential of GaN for
 the need of rationalise the equipments on board of higher power.
a ship thus concentrating in one mast the radar,
communication and Electronic warfare (EW) However GaN offers a distinct advantage for wideband
sensors; this initiative is known as integrated operation. Because of the higher power density, less
mast approach. transistors must be paralleled up to deliver any given
 the requirements for contributing to the Space power compared to GaAs. The result is that the output
Situational Awareness programmes where it has impedance of a GaN amplifier is significantly higher, and
been understood that phased array radars are the this makes the design of the necessary matching networks
main ingredients for a meaningful approach to an to the antenna (whose characteristic impedance is much
integrated solution. higher) much easier- especially for a wideband design.
Thus GaN is a valuable enabler for such systems. The issue
The impacts of the above mentioned functions and the however is cost; as a (currently) relatively niche
corresponding new developments will be detailed in the technology, it is much more expensive than GaAs, and it is
full paper. only likely to become affordable if large scale commercial
applications of the technology become widespread.
FUTURE TECHNOLOGIES
True time delay beam steering is the holy grail for
The more advanced current generation systems already wideband arrays but a technical solution is elusive. The use
offer a significant multi-function capability, notably the of optical signal distribution has long been heralded as a
combined surveillance, tracking and fire control potential breakthrough, but this approach is limited by the
capabilities of systems such as the naval Kronos family and restricted dynamic range of such systems; a barrier that
the airborne Vixen family. These systems employ a wide much research has failed to move significantly. Electronic
range of radar techniques, adaptively scheduled and delay line solutions are too lossy and inefficient to be
managed, to deliver capabilities that previously required a attractive, and cannot realistically provide adequate time
multiplicity of systems each optimised for an individual delay for large antennas.
task. The one real limitation of these current generation
systems is their fairly limited frequency coverage; Vixen, Another approach is to adopt element level digital signal
for example, is restricted to X-band, whilst Kronos is synthesis and element level digitisation and this has
restricted to C-band. achieved some success in low performance antenna designs
using Silicon Germanium (SiGe) devices. However
Convergence of radar, Electronic Warfare (EW) and achieving the spectral purity and jamming resistance
communications functionality within a broad-band multi- needed for high performance radar is another matter
function RF aperture is a clear future market trend, as is the entirely, and would necessitate perhaps two orders of
adoption of multi-band radar techniques. There are magnitude reduction simultaneously in each of volume,
however some significant engineering challenges to the weight and cost (with no relaxation in performance) in the
realisation of such systems, including: receiver and synthesiser functions compared to current
practice. This is a tall order to say the least, more so when
RF and array EM design, it is realised that the necessary filtering functions (which
true time delay beam steering, are dependent on fundamental physical phenomena) are
broadband efficient power generation, crucial to the necessary performance.
• wideband compact receivers, and
However good engineering options to deliver wideband
• wideband signal synthesis.
arrays exist. For example, it is convenient for many reasons
Some of these difficulties can be overcome by the use of to divide an AESA into sub-arrays, and by adopting digital
clever engineering design, but some would undoubtedly control at that level, together with more traditional
benefit from fundamental technology breakthroughs. analogue control within a sub-array, an effective
engineering compromise can be achieved.
An excellent example is the advent of Gallium Nitride
(GaN) power devices as an alternative to the GaAs devices CONCLUSIONS
widely used today. One of the key original motivations for
the development of GaN is its higher power density- Multi-function AESA radar systems are now a mature
typically 5x GaAs. Whilst at first the advantages of this technology, with a wide range of systems in service in the
look obvious, a more detailed examination suggests air, maritime and land domains. The fundamental
otherwise, at least in certain applications. In ground and modularity of AESA radar architectures naturally lends
naval radar prime power and cooling are generally readily itself to a design once, use many times philosophy to
available, so the challenge in the adoption of GaN is reduce the costs of non-recurring engineering and to enable
finding thermal management techniques which can deal a range of systems to be created in a relatively short time.
with the higher power density in the array (in general the Existing systems have already made significant progress
power added efficiencies of the two technologies are towards this ideal, and this trend is continuing. However

7
there remain important engineering reasons why different
technical solutions are sometimes required in different
application domains- for example the availability (or AUTHORS
otherwise) of power and cooling.

The next significant advance in multi-function systems will Alfonso Farina, FIEEE, FIET,
be the advent of wide RF bandwidths, encompassing two FREng, FEURASIP
or more traditional radar bands and expanding system
functionality to include electronic warfare and Alfonso Farina joined Selenia, now
communications. Key challenges here include wideband, Selex ES, in 1974, where he has
high efficiency power devices; wideband array designs; been a Manager since May 1988. In
and true time delay beam steering. Fundamental his professional life, he has provided
technology advances combined with more sophisticated, technical contributions to detection,
increasingly digital antenna architectures, are addressing signal, data, image processing, and
these challenges and wideband, multifunction RF systems fusion for the main radar systems
will start to emerge as mainstream products in the conceived, designed, and developed
relatively near future. These systems will support a in the Company. He is the author of
dramatic shift in the capability of military platforms, over 500 peer-reviewed technical
enabling smaller, lower cost platforms to take on roles that publications and several books. He
previously required much larger and more expensive is the Recipient of the IEEE 2010 D.
systems - a true force multiplier . Picard Medal for Radar
Technologies and Applications.
REFERENCES
Paul Holbourn, BSc (Hons), MSc,
[1] M. Cicolani, A. Farina, E. Giaccari, F. Madia, R. PhD, MIEE, CEng, FREng
Ronconi, S. Sabatini, Some phased array systems and
technologies in AMS , IEEE International Symposium on Dr Paul Holbourn has spent his
Phased Array Systems and Technology, 14-17 October career working in the field of
2003, Boston (Ma). Invited Paper. airborne radar, He is Chief
Technical Officer for the Air and
[2] A.M. Kinghorn, Where next for Airborne AESA Space Systems Division of Selex
Technology? , IEEE International Radar Conference, ES. Responsibilities include product
Rome 2008 (reprinted in IEEE A&E Systems Magazine, strategy, technology and new
Nov 2009, pp16-21). product development.

[3] P. Holbourn, The Engineering Challenge of Airborne Tony Kinghorn, MA (Cantab),

AESA , IRSI India 2011 (Bangalore November 2011). FIEE, CEng, FREng

[4] A. Cetronio, M. D Urso, A. Farina, A. Fiorello, L. Tony Kinghorn has spent his career
Timmoneri, M. Teglia, Phased array systems and working in the field of airborne
technologies in SELEX-Sistemi Integrati: state of art and radar. He is Chief Technical Officer
new challenges , Invited as plenary talk, Phased-Array for RF Systems in the Radar and
2010 Symposium, Boston (USA). October 2010. Advanced Targeting line of business
in Selex ES. Responsibilities include
[5] W. A. Kuhn, W. Sieprath, L. Timmoneri, A. Farina, technology strategy and advanced
Phased array radar systems in support of the Medium technology development.
Extended Air Defense System (MEADS) , IEEE
International Symposium on Phased Array Systems and Luca Timmoneri
Technology, 14-17 October 2003, Boston (Ma). Luca Timmoneri received his
doctoral degree in Electrical
Engineering from the University of
Rome, Italy, in 1989. He then joined
Selenia, now Selex ES, where he is
now Head of the System Analysis
Group. He is the author of several
articles and co-author of three
tutorials on Adaptive Array and
STAP submitted to the IEEE
International Radar Conferences in
1995, 1999 and 2003.

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 Figure 1.
Airborne
AESA
Configuration

Control &
Timing
Power Interface
Supply

RF Manifolds Frame

Array face strips Cowl Planks Motherboard RF Interface

Figure 2. Airborne Fire Control AESA Figure 3. Airborne Surveillance AESA

9
Figure 4. X-Band Transmit Receive Module

Figure 5. Typical Naval Multifunction Radar

Figure 6. Typical Mobile Terrestrial Multifunction Radar

10
 Figure 7. Kronos NV

Figure 8. MFRA

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