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AESA does it...or does it?

Posted 20 September 2019 · Add Comment

Leading Arab air forces have been among the vanguard of air arms that have enthusiastically embraced active electronically scanned array (AESA or E-scan) radar technology for their next generation of fighters. Jon Lake takes a detailed look at the technology.

The jury is probably still out on the total benefits of AESA radar over the more traditional mechanically scanned (M-scan) radars.
Some believe that AESA radar is the single most important innovation in new-generation fighter aircraft. Most aircrew view it as being a ‘must have’, and are fulsome in their praise for the new technology, and persuasive and articulate in outlining its benefits.
However, others do not share their enthusiasm.
As far back as 2012, a US Navy Super Hornet pilot from VFA-81 told me that, for some missions, he would prefer to take an aircraft with the M-Scan AN/APG-73 radar than one fitted with the AESA AN/APG-79 – his unit then having a mix of aircraft fitted with the two radar types.
More recently, in November 2018, an experienced Luftwaffe Typhoon pilot told me that he hadn’t found himself “in one situation where I wanted or needed E-scan”.
He highlighted the excellent performance and capability of the M-scan radar on the Eurofighter and stressed that he would rather have the EuroFirst passive infrared airborne track equipment infrared search-and-track (PIRATE IRST), which was absent on German Eurofighters, than a new radar.
He would also rather augment the existing radar with a Litening laser designator pod (LDP) to give a passive long-range visual identification capability.
Contrast that with some UK Royal Air Force pilots with experience of operating over Syria viewing M-scan radar as being on the verge of complete obsolescence, since mechanically scanned radars exhibit an inherently greater vulnerability to jamming and suffer from an inability to fully exploit the performance and capabilities of new weapons, including the Meteor BVRAAM.
Whatever the arguments, there’s no doubt that the Middle East has embraced AESA technology.
The UAE Air Force’s Block 60 Lockheed Martin F-16E/F Desert Falcons had a Northrop Grumman AN/APG-80 AESA radar when they were delivered from 2005, making it one of the first air arms to operate AESA-equipped fighters.
Saudi Arabia’s new F-15SAs and upgraded F-15SRs are equipped with Raytheon’s APG-63 (V)3 AESA radar, as are the F-15QAs ordered by Qatar.
The Rafales supplied to Egypt and now being delivered to Qatar, are also AESA equipped, using the Thales RBE2-AA, an AESA derivative of the original passively scanned RBE2.
Kuwait’s newly-ordered Super Hornets and Bahrain’s F-16V Fighting Falcons will be equipped with E-scan radar in the form of the Raytheon AN/APG-79 and Northrop Grumman AN/APG-83 scalable agile beam radar (SABR), respectively.
Also, Kuwait, Qatar and probably Saudi Arabia, are set to receive Eurofighter Typhoons equipped with the next-generation Leonardo Captor-E AESA radar, which is equipped with a mechanical re-positioner to give improved angular coverage and performance.
The Eurofighter Typhoon’s M-scan Captor-C radar was originally engineered with a lightweight antenna, big, powerful motors and robust gimbals, which allow it to be repositioned very rapidly and with great precision. It could intersperse the search pattern with ‘loopbacks’ to hit high-priority targets, rather than being stuck in a constant raster scan pattern like many conventional mechanically scanned radars.
Some have claimed that the Captor-C is the world’s most advanced and most capable M-Scan radar, and that, as such, it may offer some advantages over some older and smaller E-scan radars.
When Typhoon was being conceived, Euroradar believed that AESA was not really ready but that commercial RF technology would make it a less costly and more efficient upgrade option, or as a new radar for late production aircraft.
Conventional M-scan fighter aircraft use a moving antenna, which is repositioned mechanically in order to ‘steer’ the radar beam. This requires complex and powerful mechanisms to move the antenna quickly and precisely, sometimes under high g-force.
Traditional M-scan radars produce a single radar beam and usually operate on fixed frequencies – often with relatively limited frequency agility. There is no ability to simultaneously operate in both air-to-air and air-to-ground modes.
AESA radars also use a single transmitter/receiver, making them potentially vulnerable to unreliability – with a number of ‘single-points-of-failure’.
The first in-service fighter to use an electronically scanned array was the Russian Mikoyan MiG-31 ‘Foxhound’, but its Zaslon radar used a passive electronically scanned array (PESA), also known as passive phased array.
This still had a single transmitter but it was connected to a fixed antenna with multiple antenna elements or transmit/receive modules (TRMs). These allowed the radar beam to be steered electronically by ‘phase shifting’, using timing differences between the signals from each element to form and steer the radar beam without needing to physically move the antenna array.
A PESA radar was able to scan a volume of space much quicker than a traditional mechanically scanned radar, and was able to generate several beams, allowing a degree of simultaneous multi-tasking.
A number of PESA fighter radars were produced, including the N035 Irbis for the Sukhoi Su-35BM, the NIIP N011M Bars for the Su-30MKI and the Thales RBE2 for the Dassault Rafale.
The RBE2, in particular, demonstrated an impressive ability to operate in air-to-air and air-to-ground modes simultaneously, and was able to dispense with the heavy and complex hydraulic drives required by mechanically scanned radars, with consequent serviceability and reliability advantages.
But PESA radars came with considerable disadvantages, too. They had modest range capability and, with a transmitter that represented a potential single point of failure, many applications were also heavy and prone to cooling problems. PESA radars are today most often viewed as representing a technological dead end.
The first fighter radar to use AESA technology was the Mitsubishi J/APG-1, fitted to the indigenous F-2 tactical fighter, which entered operational service in 2000, five years before the F-22 and F-16E/F.
Early AESA arrays tended to be heavier than equivalent mechanically scanned planar arrays and their drive systems required more cooling and electrical power. However, the lack of moving parts significantly improved reliability, and reduced costs of ownership, while performance and operational advantages were compelling.
Most AESA radars use a fixed antenna, consisting of a matrix of multiple solid state TRMs, each of which effectively functions as an individual antenna capable of generating and radiating its own independent signal, producing pulses on different frequencies and producing multiple beams simultaneously.
These modules operate at relatively low power, obviating the need for a large high-voltage power supply.
Because each module operates independently, the failure of a single TRM will not have a significant effect on overall system performance, and even the failure of a number of modules will not prevent the radar from operating, but will rather result in a ‘graceful degradation’ of performance.
The AESA radar antenna array can be divided into sub-arrays that are capable of adaptive beamforming, including the generation of multiple independent beams. These beams may be interleaved, allowing the radar to support multiple simultaneous radar modes, including real beam mapping, synthetic aperture radar (SAR) mapping, sea surface search, ground moving target indication and tracking and air-to-air search-and-track.
These multiple modes can be operated near-simultaneously, providing a significant improvement in combat capability compared to legacy M-scan radars. This means that in a two-seat combat aircraft like the Boeing F/A-18F Super Hornet, the pilot in the front cockpit could be undertaking an air-to-air task, while the weapons systems officer in the aft cockpit is prosecuting an air-to-ground attack, with the radar supporting both.
An AESA radar will typically make a raster-type scan but using a thin pencil beam – giving a narrower and more elongated main lobe than a typical mechanically scanned radar. Because it can scan at a very high rate it can produce more power in this more ‘dense’ lobe, and with reduced latency.
Latency is further reduced because a track can be updated without having to wait for the next ‘sweep’ of the antenna. Instead, an AESA radar can prioritise and scan around the first detection or ‘hit’ and build track information. It also gives a higher quality track at range due to the shape of the beam, providing an accurate enough picture to support a wingman’s weapons via data link.
Alternatively, one beam may be ‘frozen’, or can be kept pointing at a target continuously rather than returning to that target with each raster scan, while other beams scan elsewhere.
Keeping the beam still during acquisition improves resolution and allows multiple targets to be tracked continuously, or for spatially separated targets to be tracked without any degradation in performance.
With an AESA radar using multiple beams, there is no need to rely on a single beam following a set raster scan pattern, or even to use that single beam to interrupt its scan to conduct a ‘high-density update’ on a contact.
An AESA radar will typically change its frequency with every pulse, generally using a random sequence, making it harder to jam. The simultaneous use of multiple frequencies makes life harder for radar warning receivers and electronic surveillance (ES) systems.
AESA radars can also spread their signal emissions across a wider range of frequencies, which makes them more difficult to detect over background noise. The fact that an AESA does not have a fixed pulse repetition frequency makes it a low-probability of intercept (LPI) radar – ‘stealthier’ than conventional fighter radars.
An AESA radar can also use adaptive power management, using no more transmitting power than is needed to obtain the required information for each individual target – resulting in a further reduced probability of the radar’s signals being intercepted by hostile forces.
AESA antennas can also be used to create high-bandwidth data links between aircraft and other equipped systems, and advanced AESA radars offer a useful electronic attack capability.
Many current AESA radars are lighter than early models, and are better able to work with existing aircraft power and cooling. Some now represent something close to a plug-and-play upgrade option.
However, most do require some increase in cooling and electrical power generation capacity, and often some strengthening of the forward fuselage.
The latest AESA radars represent something of a hybrid – adding a mechanical repositioner to a conventional E-scan array. This increases the radar’s angle of regard from about +/- 60° on each side of the centreline to greater than 90°. This allows a fighter to turn away or ‘crank’ harder after launching a beyond-visual-range (BVR) missile, making it less vulnerable to a return missile shot, while still providing mid-course updates for the missile fired.
Leonardo’s ES-05 Raven radar for the Saab JAS39E/F, and the same company’s Captor-E for the Eurofighter Typhoon, are both equipped with a repositioner.
China is also understood to be working on an AESA radar using a similar repositioner.
Russia lags behind the west in AESA technology, but is developing E-scan radars for the new Sukhoi Su-57, and for the MiG-35.
Future AESA radars may incorporate additional conformal arrays to increase angular coverage, such antennas having been considered for the Su-57 and for the Rafale, among other types.
Leonardo has shown the way forward with its Osprey surveillance radar, which uses up to four fixed conformal arrays to provide up to 360° azimuth coverage for a range of rotary- and fixed-wing platforms.
 

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