The advent of production Active
Electronically Steered Array (AESA) radar antennas represents one of the
most important, if not the most important development in radar
technology since the 1940s. With unprecedented reliability, superior
performance and typically of the order of a one thousandfold improvement
in beamsteering speeds, this technology will transform many aspects of
air combat and strike operations.
The idea of an electronically steered antenna is not new.
Early warning radars used for the detection of ballistic missile attacks
have exploited this technology widely since the 1960s. The US Navy's
SPY-1 Aegis radar, developed during the 1970s to defend carrier battle
groups against saturation attacks by Soviet cruise missiles, is perhaps
the best known electronically steered antenna design in operational use.
The B-1B Bone has flown since the 1980s with an AN/APQ-164
radar, fitted with an electronically steered array. The B-1A Batwing
also exploits this technology in its AN/APQ-181 multimode attack radar.
Both of these radars can be used for terrain following flight, as well
as surface attack modes. The Soviet MiG-31 Foxhound also carries this
technology in its SBI-16 Zaslon air intercept radar.
The important and recent development in electronically steered
antennas is the solid state X-band (centimetric wavelength) AESA, built
using Gallium Arsenide chips. This technology is now being retrofitted
to in-service fighters and will be a standard production component in
the F-22A and F-35 series fighters, and most likely a downstream
production/upgrade item for late model F-16C, F/A-18E/F, Typhoon and
Rafale fighters.
Active Electronically
Steered Arrays - A Primer
To best understand the importance of the AESA it is useful to
explore the limitations of conventional mechanically steered antennas,
and the first generation of passive Electronically Steered Arrays
(ESA).
The basic purpose of any microwave radar antenna on a fighter
(or bomber) is to focus transmitted microwave power into a narrow beam,
or receive reflected microwave power from targets (or terrain), again
within a narrowly focussed beamwidth. Targets are found by steering the
antenna repetitively through a programmed pattern, to search a volume of
sky or a surface footprint. The antenna transmits the energy, which
travels out to the target, reflects and is then received by the antenna.
For an antenna to be useful it must therefore not only be capable of
launching and receiving microwave power, but must be steerable precisely
and preferably very quickly.
The one bit of good news in antenna physics is the
reciprocity theorem which says that the radiation pattern of an
antenna when transmitting power is of the same shape as its reception
pattern.
In an ideal world the antenna produces an absolutely sharp
beam - the radiation pattern is for instance conical, and all energy
transmitted is within that cone, and all energy detected by the antenna
is also within that cone. In the real world this of course does not
happen, the radiation pattern leaks outside the main lobe of the beam,
creating what are termed sidelobes. A good measure of antenna quality is
how small the sidelobes are eg ten times (10 deciBels or dB), one
hundred times (20 dB), or one thousand times (30 dB) below the mainlobe
in magnitude.
The first generation of microwave fighter radar antennas were
mechnically steered concave reflectors, colloquially termed dish
antennas. This is the basic technology the RAAF still operates in the
F-111's AN/APQ-169 and -171 radars. These antennas have several
drawbacks - they are expensive to fabricate to high accuracy, tend to
have fairly large sidelobes, and also have a frequently large radar
signature when illuminated by a hostile radar - as all concave
reflecting cavities do.
By the 1970s the state of the art shifted to mechanically
steered planar array or slotted array antennas, an example being the
AN/APG-65/73 radar in the RAAF's F/A-18A. Planar arrays achieve their
focussing effect not by reflection as concave antennas do, but rather by
manipulating the individual time delays into a very large number of very
simple slot antennas, arranged in a planar array panel. By using a
cleverly designed and oft complex network of microwave waveguides on the
rear surface of the array, a designer could produce the desired fixed
beam shape and do so with much smaller sidelobes compared to a concave
reflecting antenna. As the antenna is a flat plate, it tends to act
like a flat panel reflector to impinging transmissions from hostile
radars and thus has a lower radar signature than a concave antenna.
While the US focussed on planar arrays, the Europeans and
Soviets deployed a number of Cassegrainian reflector antennas on fighter
radars, as these performed better than concave reflectors but were
cheaper to design and fabricate than planar arrays - the design and
manufacture of the complex feed networks on the rear face of any array
antenna is still considered to be somewhat a black art.
Planar arrays provided important gains in beam quality but due
to the need to mechanically point them they remained slow to steer and
also suffered the same reliability problems as concave antennas. The
complex mechanical gimbal arrangement and servomotors used to drive such
antennas suffer from wearout, and the cyclic mechanical loads on the
antenna proper can also induce material fatigue failures over time.
Airborne radar designers covetously eyed the electronically
steered antenna technology used on ground based radars and by the 1980s
this technology found its way into airborne radars, some examples noted
earlier. An electronically steered antenna of this ilk is designed with
an individually electronically controlled device behind each antenna
element, which can manipulate the time delay or phase of the microwave
signal passing through it. With a computer controlling each element in
unison, the beam direction and its shape could be digitally controlled,
within a matter of milliseconds or tens of milliseconds.
The first generation of such antennas used the signal phase
as the controlling parameter, typically using ferrite core devices for
this purpose. Therefore such antennas were known as phased arrays. A
typical design could resemble a conventional planar array, but with a
layer of digitally controlled ferrite core phase modules
inserted between the antenna array elements and the microwave feed
network on the back of the antenna. As the antenna contained only what
engineers term passive components, these antennas are also known as
passive phased arrays or passive electronically steered arrays. The
Russian SBI-16 Zaslon, Phazotron Zhuk Ph and NIIP N-011M are good
examples, as are the AN/APQ-164 and -181 mentioned earlier.
This generation of Electronically Steered Arrays permitted
unprecedented beam seering agility compared to mechanically steered
antennas, and very large reductions in antenna radar signature if well
designed. Beamsteering agility in turn permitted the important
capability of interleaving radar modes. An ESA can timeshare multiple
and diverse modes, a good example being shared concurrent operation
performing both terrain following and surface mapping for weapon
delivery.
However, the antenna did nothing to enhance either the
reliability or the efficiency of the radar. The high power Travelling
Wave Tube in the transmitter remained, causing its traditional share of
reliability woes, while the complex interconnections required to connect
the digital control signals to the wirewound ferrite cores was an
additional burden. Since the ferrite cores introduced signal losses in
both the receiving and transmitting directions, these antennas were less
sensitive than their mechanically steered predecessors, and required
more powerful microwave tubes to drive them.
The US DoD recognised the need for a better antenna technology
more than two decades ago. A new technology, using the phased array
concept but with a miniature transmitter and receiver in each antenna
element, was seen to be the answer to the limitations of existing
technologies. Known as active phased arrays or AESAs, these antennas
became the holy grail in the radar community - for reasons yet to
become fully apparent.
The enabling technology for AESAs is the Gallium Arsenide
Microwave Monolithic Integrated Circuit (GaAs MMIC)
or microwave circuit on a single chip. GaAs MMICs would permit the low
cost mass production of AESAs, with high reliability and repeatability.
Gallium Arsenide is however a finicky material to make chips
from and it took almost two decades for the fabrication technology to
move from expensive botique manufacture to industrial strength mass
production. Today this technology is being put into cellphones,
broadcast satellite receivers and TV sets. The author recalls a
development project in 1984 where he has not permitted to use a GaAs low
noise transistor in a $100k piece of high speed communications equipment
- too expensive, Carlo, find a cheaper way to do this!. A decade ago
the GaAs component market was dominated by military buys, which today
comprise only around 2% of the total market volume.
The problems in producing a digitally controlled solid state
AESA were evident very early - cost, density and power handling would be
critical. All of these factors have contributed to the relatively late
deployment of the technology in operational aircraft.
The basic building block of any AESA is the Transmit Receive
Module or TR Module. It is a self contained package making up one AESA
antenna element, and contains a low noise receiver, power amplifier, and
digitally controlled phase/delay and gain elements. Digital control of
the module transmit/receive gain and timing permits the design of an
antenna with not only beam steering agility, but also extremely low
sidelobes in comparison with passive ESA and mechanically steered
antennas.
Two other important benefits are derived from this design
approach. The first is an very important improvement in antenna noise
behaviour, since the TR module's low noise receiver is within the
antenna itself. Typically this yields between a two and fourfold
reduction in receiver thermal noise, in turn contributing to improved
radar sensitivity and thus detection range, all else being equal.
The second important benefit is a result of the transmitter
power stages being distributed across hundreds or over a thousand TR
modules, rather than being concentrated into a single transmitter tube.
As a result, failure of up to 10% of the TR modules in an AESA will not
cause the loss of the antenna function, but merely degrade its
performance. From a reliability and support perspective, this graceful
degradation effect is invaluable. A radar which has lost several TR
modules can continue to be operated until scheduled downtime is
organised to swap the antenna. Other beneifts also accrue - a
classical design with a high power tube must carry the transmitter
power to the antenna through a pressurised waveguide, and power will be
lost in the antenna. Transmitter tubes require highly stressed high
voltage high power supplies, which also tend to be unreliable. Since
each TR module only handles several Watts of power, fed from a low
voltage supply (several Volts rather than kiloVolts), it can be designed
for much lower electrical stress levels.
As a benchmark, typical conventional fighter radars have Mean
Times Between Failure (MTBF) of around 60 to 300 hours - AESA radars
push the MTBF into the 1,000 hours or better class. Rather than several
repairs annually to the radar, the AESA will see the radar needing
repair only once every several years of operation. If we assume an
annual flying rate of around 200 hours, on average the AESA needs to be
repaired once every five years! From a support costs perspective, this
means much reduced cost of ownership for fighter fleet operators who
transition to the technology.
An AESA becomes a combined transmitter, low noise receiver and
beamsteering package, providing high beamsteering agility, very low
radar signature when illuminated and extremely low sidelobes, all
digitally controlled. With digital control of TR module gain, power
management which is vital for reduced or low probability of intercept
(RPI, LPI) operation becomes relatively easy to do. Beamsteering agility
also facilitates reduced or low probability of intercept scan patterns.
In many respects an AESA is a fighter radar designer's dream
device, since it not only vastly improves performance and functional
capabilities, but does so with improved reliability and complete digital
control of antenna/transmitter functions. Over the life of an AESA
radar, progressive refinements in many aspects of antenna behaviour can
be incorporated through incremental software upgrades. Software
programmable AESAs at this time largely implement digital equivalents of
established antenna beam shapes, scan patterns and sidelobe behaviours.
Over time with proper intellectual effort, further improvements are
possible.
Are there any drawbacks to the AESA? Two issues are of key
importance with this technology.
The first item of interest is power dissipation. Due to the
behaviour of microwave transistor amplifiers, the power efficiency of a
TR module transmitter is typically less than 45%. As a result, an AESA
will dissipate a lot of heat which must be extracted to prevent the
transmitter chips becoming molten pools of Gallium Arsenide -
reliability of GaAs MMIC chips improves the cooler they are run.
Traditional air cooling used in most established avionic hardware is ill
suited to the high packaging density of an AESA, as a result of which
modern AESAs are liquid cooled. US designs employ a polyalphaolefin
(PAO) coolant similar to a synthetic hydraulic fluid. A typical liquid
cooling system will use pumps to drive the coolant through channels in
the antenna, and then route it to a heat exchanger. That might be an air
cooled core (radiator style) or an immersed heat exchanger in a fuel
tank - with a second liquid cooling loop to dump heat from the fuel
tank. In comparison with a conventional air cooled fighter radar, the
AESA will be more reliable but will require more electrical power and
more cooling, and typically can produce much higher transmit power if
needed for greater target detection range performance (increasing
transmitted power has the drawback of increasing the footprint over
which a hostile ESM or RWR can detect the radar).
Another issue of concern with AESAs is the mass production
cost of the TR modules. With a fighter radar requiring typically between
1,000 and 1,800 modules, the cost of the AESA skyrockets unless the
modules cost hundreds of dollars each. With early module builds yielding
unit costs of around USD 2,000 the cost penalty of using an AESA over a
conventional design was prohibitive. The good news in this respect is
that the ongoing trend has been downward, in a large part as the
production engineering of the modules and MMIC chips has improved.
Having an enormous commercial market for similar MMIC chips has yielded
important benefits.
The longer term technology trends for AESAs are clear - a
progressive cost reduction as volumes increase and production matures,
with concurrent refinements in digital antenna control techniques
improving the capabilities of the antennas.
At this time the US are leading the pack by a large margin in
AESA technology, with the EU and Israelis trailing. The Russians remain
in the passive AESA domain but this will change as commercially
available GaAs MMICs proliferate. The Russians have a robust track
record in passive ESA design and the only obstacle to an AESA equipped
Su-30M is the availability of suitable chips in volume.
Current AESA Programs
A number of AESA programs are currently under way, some as new
build radars for new fighter designs, some as retrofits to existing
aircraft.
The first generation of AESAs to field will be the L-band
(decimetric) radars used in the Israeli Phalcon derivatives, and
importantly the RAAF's Wedgetail MESA radar which is expected to be used
in the new USAF MC2A (Multi-sensor Command Control Aircraft) E-3 AWACS,
E-8 JSTARS, RC-135 Rivet Joint replacement. The lower L-band operating
frequency of these radars permits the use of older transistor
technology, giving this class of AESA about 5 years of market lead
against the X-band fighter AESAs.
The E-8 JSTARS is a candidate for an X-band AESA replacement
of its existing passive ESA radar, used for high resolution mapping and
surface target detection. It is likely that the planned JSTARS AESA will
be used on the MC2A - the MC2A variant to be used to replace the
JSTARS/AWACS combo will be a 767 airframe carrying both radars (this is
a potential growth path for the Wedgetail, by the addition of a
JSTARS/MC2A derivative radar under the forward fuselage).
US sources indicate that the RQ-4 Global Hawk is likely to see
an AESA upgrade later in the decade, to provide increased range and
radar optimisations for accurate ground target tracking, and airborne
target tracking
The fourth AESA to field is likely to be the 1,100 element
Raytheon AN/APG-79, formerly AN/APG-73 RUG III, on the USN's new
F/A-18E/F fleet. This AESA is a block upgrade to the existing AN/APG-73
series. Whether the whole F/A-18E/F fleet receives the radar has not
been disclosed, but given the longer term life cycle cost benefits this
could become a long term priority for the USN.
The F-35 JSF, if it proceeds to plan, will field with a 1,200
element AESA radar using a similar architecture to the F-22's AN/APG-77.
While little has been disclosed in the way of design details, this radar
is likely to resemble a scaled down and less capable F-22 radar, with a
strong optimisation for strike roles.
Other players are entering the market. Work continues on the
European AMSAR AESA for the Eurofighter Typhoon. Public data suggests a
1,500 element design although this might be optimistic given the size of
the Typhoon. An AMSAR derivative is a likely retrofit option for the
Rafale.
Israel's Elta has published datasheets on a range of X-band
GaAs MMIC chips which would be suitable for an AESA but as yet no
disclosures of system level products have been made.
Reports also suggest that the Russian radar houses are working
on AESA designs, although details remain very sketchy. Any Russian
design would have to make use of imported GaAs MMIC chips as Russia's
industry lags severely in this area. A likely outcome is that COTS GaAs
MMICs would be adapted for a Russian design, as the export controls on
high volume X-band satellite transceiver chips are likely to be
unenforcable over the coming decade. A suitcase of GaAs MMIC chips makes
for a lot of AESAs.
It is not inconceivable that we may see a robust number of
AESA retrofits over the coming decade to established teen series fighter
fleets in the West, as the investment in this technology returns a large
payback in medium and long term support cost reductions. With large
fleets of the F-15C/E and F-16C in USAF and export client service,
possibly several hundred aircraft could be retrofit candidates.
What impact does the AESA have for Australia? Clearly any
aircraft considered for A6K must have an AESA - anything less is simply
an unnecessary drain on fighter support budgets.
In terms of retrofits, the remaining life cycle and increasing
strategic irrelevance of the F/A-18A HUG make it a poor candidate for an
AN/APG-79 AESA retrofit later in the decade. The F-111 would be an
excellent AESA retrofit candidate especially since the support cost
reductions and reliability gains over the 1960s AN/APQ-169/171 suite
would achieve a major impact in life cycle costs in the 2015-2020
timescale. As recent testimony by LtGen Mueller to a parliamentary
committee indicates, the principal issue in F-111 life-of-type will be
the support of the remaining 1960s generation aircraft systems in the
post 2010 period, given that the RAAF is now retrofitting AMARC F-111F
wings to gain significantly more airframe fatigue life.
Current trends are that the AESA will supplant conventional
mechanically steered radars in all production fighter applications over
the coming decade, and there are good prospects for partial fleet
retrofits across the several thousand teen series fighters likely to
remain in service over the next 2-3 decades. The only issue with the
AESA will be securing near term funding for retrofits, given that the
support cost payback may take several years to be seen. Given the
tremendous combat capability gains resulting from the AESA, the case for
retrofits is very robust if the aircraft is to remain in service for
more than a decade.
In summary, the AESA will have a revolutionary impact over the
coming decade, and smart players should be now exploring how to best
exploit this pivotal technological development.