AESA Radar – Required Not Desired

The US Air Force (USAF) has installed the Northrop Grumman AN/APG-83 Scalable Agile Beam Radar (SABR) on Air National Guard (ANG) Lockheed Martin F-16 Fighting Falcons.

For some years now, whether or not a fighter has an AESA radar has been a key discriminator, but the technology is increasingly being seen as an essential – the kind of ‘theatre entry standard requirement’ which a fighter aircraft will require if it is to be welcomed as part of a coalition.

As more air forces have started to operate Active Electronically Scanned Array (AESA)-equipped fighters, the technology’s advantages have become more and more obvious.

Traditional Mechanically-Scanned (M-Scan) radars rely on physically moving the antenna to steer the single radar ‘beam’. Some modern M-Scan radars can ‘interrupt’ their scan pattern to revisit an area of interest, but they rely on a gimbal, motor and other moving parts to move the array, introducing potential points of failure. The antenna has to be moved quickly and precisely, sometimes under high g-loads, and these mechanisms have to be complex and powerful, and thus heavy! M-Scan radars usually operate on fixed frequencies with little frequency agility and with no ability to operate in the air-to-air and air-to-ground modes simultaneously.

Passive Electronically Scanned Array (PESA) radar systems transmit one signal at a single frequency and then split it between different antenna elements to maximise its range and strength. PESA radars are generally able to scan large areas much faster than traditional mechanically-scanned radar systems, but with relatively poor accuracy, are heavier, and use a single transmitter giving a potential single point of failure and often some cooling issues.

AESA radar systems (sometimes known as E-Scan radars) use a phased array antenna, but one consisting of large numbers of individual solid state antenna elements, known as Transmit/Receive Modules (TRMs), each with its own amplitude and phase control. Each of these emits an individual wavefront, which can then converge into one or more ‘beams’, or plane waves. These beams are then steered electronically by delaying the frequency or ‘phase’ of the individual antenna modules. Improved levels of digital beamforming, should allow beams to be generated and steered using ever smaller groups of TRMs.

Because AESA radar systems have multiple TRMs rather than a single large transmitter like a PESA or M-Scan system, the AESA enjoys greater reliability. Many TRMs can fail and the radar will still operate, performance degrading ‘gracefully’.

AESA radars can simultaneously transmit multiple beams at multiple frequencies, allowing areas of interest to be monitored without interrupting the main scan pattern, and even allowing simultaneous operation in different modes – air-to-air and air-to-ground, for example. In a two-seat aircraft like the Boeing F/A-18F Super Hornet, this means that the pilot in the front cockpit could be undertaking an air-to-air task, while the weapons systems officer in the aft cockpit is simultaneously prosecuting an air-to-ground attack, with the radar supporting both.

With relatively little modification an AESA radar can also be used to send and receive large amounts of information at very high data rates. A trial using a Lockheed Martin F-22’s AN/APG-77 radar and a software-programmable modem demonstrated the ability to transfer a 72Mb synthetic aperture radar image in just 3.5 seconds at a data rate of 274Mbps. This would have taken 48 minutes using Link 16! Subsequently, data transmission rates of 548Mbps and data receive rates of up to 1Gbps were demonstrated. The creation of AESA systems that offer enhanced multi-function applications like these is likely to be a focus of future AESA programmes.

Precision and Accuracy

While PESA radar systems can sometimes scan larger volumes of airspace faster, AESA radars have lower signal losses and scan more precisely, providing an ability to detect smaller and low RCS targets such as micro-UAVs. Even more significantly, the AESA provides more accurate target tracking which allows them to provide a ‘weapons quality track’ at longer range.

Because AESA radars use multiple individual TRMs, capable of transmitting multiple beams, at multiple frequencies, they may be harder to detect and identify, and harder to jam.

AESA radars are typically small and relatively light weight, but cost more than PESA and M-scan radars, thanks in part to the number of TRMs required. Traditional ‘fixed plate’ AESAs also have a relatively limited field of view/field of regard, with electronic beam steering causing performance to fall off at higher azimuth limits. A small number of AESA radars incorporate mechanical repositioners as a means of offsetting this, including the Leonardo Raven ES-05 radar for the Saab Gripen E, and the Captor-E family of radars used by some Eurofighter Typhoons.

An alternative approach is to provide multiple arrays, with sideways- and even rearward-looking antennas to increase field of regard. Leonardo’s Osprey radar is offered with up to four antennas, offering full 360° coverage. It is fitted to Norwegian Leonardo AW101 Merlin helicopters, and to the AHRLAC Mwari, but not to any fast jet combat air platforms. This approach was talked about for the Sukhoi Su-57 – though this uses an NIIP N035 Irbis PESA radar and not an AESA. Two prototypes are fitted with a Tikhomirov NIIP N036 X-band AESA radar. The system also has two N036L-1-01 L-band arrays on the wing leading edge root extensions but these are used for IFF and EW, lacking the resolution and range for fire control.

Two AESA variants of the older Chinese KLJ-7A (Type 1478) radar have been linked with the new CAC/PAJF-17 Block-3, one of them fitted with a mechanical repositioner, and the other with two auxiliary lateral antenna arrays to increase field of view. This kind of system of distributed antennas seems likely to form the basis of next generation combat aircraft like the Tempest and the Franco German SCAF.

Service History

The first in-service fighter to use an electronically scanned array was the Russian Mikoyan MiG-31 ‘Foxhound’, which entered service in 1981, though its Zaslon radar used a PESA. Russia has remained wedded to PESA radars ever since, including the N035 Irbis for the Sukhoi Su-27SM3 and Su-35BM, and the NIIP N011M Bars for the Su-30MKI (though some sources suggest that the SM3 retains the original N-001V radar).

Though many view PESA radars as representing something of a technological ‘dead end’, they remain important, and capable. The superiority of Russian fighters in the war against Ukraine has been attributed to their armament – active radar homing R-27EA/EM and R-77 missiles, but their radars were perhaps even more of a factor. While Ukraine’s MiG-29s and ‘vanilla’ Su-27s had mechanically scanned 1970s vintage radar, Russia’s PESA equipped fighters could obtain ‘weapons quality tracks’ at much longer range, and could maintain a more robust lock-on, and then provide better support to missiles in flight.

They were also available when AESA radars were not.

But while PESA radars offer real advantages over older mechanically scanned radars, they are no match for modern actively scanned radars. This helps to explain why Dassault have changed from the Thales RBE2 PESA radar to a new version of the radar with an AESA, the RBE2-AA.

Japan’s Mitsubishi F-2 was the first operational military aircraft to feature an AESA radar (the indigenous Mitsubishi Electric J/APG-1) when it entered service in September 2000, beating the handful of US Air Force Boeing F-15Cs with Raytheon’s AN/APG-63(V)2 which entered service at Elmendorf in December 2000. The AN/APG-63(V)2 was an upgraded version of the basic mechanically scanned F-15 radar, fitted with a new AESA antenna and new IFF.

Next into service was the Northrop Grumman AN/APG-80, fitted to the Block 60 Lockheed Martin F-16E/F Desert Falcons delivered to the United Arab Emirates (UAE) from 2004.

It was always intended that the USAF’s new fifth generation fighters would be fitted with AESA radars, and the Northrop Grumman AN/APG-77 fitted to the Lockheed Martin F-22 Raptor formally entered service in December 2005 and remains an extremely impressive sensor. The original version had a 50:50 mix of 1,956 separate single-function transmit modules and receive modules, and provided full air-to-air and air-to-ground functionality (high-resolution synthetic aperture radar mapping, ground moving target indication and track (GMTI/GMTT), and automatic cueing and recognition).

Technology and modes from the APG-77 formed the basis of the Lockheed Martin F-35’s AN/APG-81, which Northrop Grumman describe the APG-81 as “the latest and most capable AESA in the world,” and say that it “provides unparalleled battlespace situational awareness.”

In addition to air-to-air and air to-ground capabilities, the APG-81 can also operate as an electronic warfare (EW) aperture using the multi-function array (MFA) to transmit powerful jamming signals with great precision. This allows the radar to be used for electronic protection (EP), and electronic attack (EA) enabling the F-35 to suppress advanced enemy air defences.

AN/APG-81 AESA Radar
Northrop Grumman claims that the F-35’s AN/ APG-81 radar is “the latest and most capable AESA in the world.”

In addition to its LO aircraft, the US decided that other frontline fighters would be equipped with AESA radar. Raytheon designed the AN/APG-79 for the Boeing F/A-18E/F Super Hornet, with an AESA antenna designing and supplying the X-Band phased array antenna. This was claimed to provide a near instantaneous and multi-target tracking capability. The new radar entered service in early 2007.

Raytheon used the same technology in its AN/APG-63(V)3 radar which was retrofitted into USAF F-15C/Ds and used for Singapore’s new build F-15SGs and Saudi Arabia’s new F-15SA aircraft. Raytheon delivered the first prototype APG-63(V)3 system in June 2006, and began work on an initial production order in October 2007.

European rivals were slower to adopt AESA technology. Dassault’s Rafale (operational since 2004) did not receive an AESA until 2014, and the small size of the Rafale’s nose means that this has only about 830 TRMs, or roughly half the number used in the Typhoon’s AESA radar, for example.

The Eurofighter Typhoon, labouring under the disadvantage of having probably the best M-Scan radar in the world, did not get an operational AESA until last year, with the delivery of ECRS.Mk 0-equipped aircraft to Kuwait and Qatar. Many believe that the new Captor-E may be the best fighter AESA in service, with its large array allowing a large number of TRMs, and with an innovative dual swashplate repositioner providing an unmatched Field of Regard. Other Captor-E variants are under development for European Typhoon operators. The ECRS.Mk 2 for the RAF’s Tranche 3 aircraft has little hardware commonality with earlier versions, with a new array and with a different type of repositioner (based on that of the Raven ES-05), and offers impressive new electronic attack capabilities.

Typhoon C-Captor
Flight trials of Royal Air Force Typhoons that are newly equipped with Leonardo’s European Common Radar System Mark 2 (ECRS Mk2) E-Scan radar will be conducted in 2023.

The ES-05 Raven (originally known as the Vixen 1000E) for the Saab Gripen E/F features an innovative roll-repositionable AESA antenna providing a full ±180º field of regard – or roughly twice what fixed arrays provide. The single-axis, single-jointed, rotating barrel-type antenna repositioner requires the use of innovative connectors between the antenna and the back end of the radar, and these were based on technology used in oil drilling.

Other AESA radar programmes are underway in India and South Korea. In India the Defence Research and Development Organisation (DRDO) and its subsidiary, the Electronics and Radar Development Establishment (LRDE), are developing an indigenous AESA radar, the Uttam, for future Tejas variants, and for planned Sukhoi Su-30MKI and Mikoyan MiG-29K upgrades. In South Korea Hanwha Systems has developed a largely indigenous AESA radar for the new KAI KF-21 Boramae fighter.

Upgrade Offerings

Perhaps the hardest fought part of the AESA radar market is that dealing with systems intended for retrofit to existing fourth generation fighters. Many of these aircraft remain in service, and still have competitive aerodynamic and kinematic performance, but lack modern sensors and connectivity. AESA retrofit can address these shortcomings, providing a cost-effective path for some air forces to leverage expanded capabilities and increased relevance.

At one end of the scale, the AN/APG-82 (previously known as the APG-63(V)4) was specifically designed for the modernisation of the USAF’s F-15E fleet, combining the antenna of the AN/APG-63(V)3, and the upgraded processor of the APG-79. The APG-82 is also used on the IDF/AF’s F-15I and the JASDF’s F-15J upgrades.

F-16 retrofits are perhaps the most lucrative market. The Raytheon Advanced Combat Radar (RACR) or AN/APG-84 was a scaled-down version of the AN/APG-79 (already operational on the F/A-18E/F Super Hornet and EA-18G Growler), and was selected for the original Republic of Korea Air Force (RoKAF) F-16 upgrade. This upgrade was subsequently cancelled, and the APG-84 has gained no real traction since then.

Raytheon’s APG-79 AESA radar mounted in the nose of an F/A-18 Hornet.

By contrast, the rival Northrop Grumman Scalable Agile Beam Radar (SABR) or AN/APG-83 has enjoyed greater commercial success, forming the basis of the latest Block 70 F-16 and the F-16V upgrade. Derived from the F-22’s APG-77 and the F-35’s APG-81, SABR is designed to fit into the F-16 aircraft with no power, structural or cooling modifications.

AESA radar
US Air Force Staff Sgt. Jackie Zheng, left, avionics specialist, 113th Wing, District of Columbia National Guard (DCNG), briefs Lt. Gen. Michael A. Loh, right, director, Air National Guard, on the radar being replaced by the Active Electronically Scanned Array (AESA) radar now equipped on F-16 Fighting Falcon aircraft assigned to the DCNG on Joint Base Andrews, Maryland, 9 June 9, 2022.

For the F/A-18 Heritage Hornet (F/A-18A-D) upgrade market, a scaled version of the AN/APG-79, the (V)4 is being offered. The (V)4 was flight-tested in May 2022 and is being used for an upgrade of USMC ‘Heritage Hornets’. The new radar uses Gallium Nitride (GaN) technology, giving longer range and enhanced performance. The new radar was chosen by Malaysia for an F/A-18C/D Hornet upgrade and for the newly developed FA-50 Golden Eagle.

Raytheon's APG-79
Raytheon’s APG-79(v)4 AESA radar has been installed on the US Marine Corps F/A-18C/D classic Hornet fleet.

The indigenous Turkish Aselsan MURAD AESA radar is being retrofitted to some 35-36 Block 30 F-16s as part of the Project ÖZGÜR F-16 upgrade, and is expected to be used for the new MMU fighter programme. Aselsan has said that it expects its new radar to enjoy a performance similar to the Northrop AN AESA used for the F-16V Viper upgrade and new-build Block-70/72 F-16s, but with better detection range and more precise targeting.

Though Sweden’s new Gripen-E uses the Leonardo Raven radar, Saab has developed a new GaN-based X-band AESA radar which it flew on a two-seat Gripen-D in April 2020, and which it has offered as an upgrade option for the Gripen-C/D.

Israel’s ELTA has produced the EL/M-2052 AESA radar primarily for the retrofit market, and has already sold the system to India for its DARIN III Jaguar upgrade, and for installation on the indigenous Tejas fighter.

Having ‘missed the boat’ with RACR (AN/APG-84), Raytheon’s next attempt at a lightweight, lower cost AESA radar is innovative and radical, and promises to be something of a disruptor. Raytheon Intelligence & Space’s PhantomStrike weighs in at less than 100 pounds (less than half the weight of other modern AESA radars), that costs 50 percent less while using 65 percent of the power of comparable radars. It achieves this by using a first-of-its-kind air-cooled design, GaN-power, and an innovative packaging of its digital receiver/exciter and processor.

The PhantomStrike is designed to be platform agnostic, able to be integrated onto any platform, including fighters, light-attack aircraft, rotary wing aircraft, unmanned aerial vehicles and even stationary platforms such as cell towers.

Raytheon’s new PhantomStrike lightweight radar weighs less than 100lb (45kg) and can be integrated onto a variety of platforms from light-attack aircraft and rotary wing to unmanned aerial vehicles and even ground platforms such as cell towers.

Raytheon seem to have delivered an AESA with the lightest ever form factor, and one that it says delivers the heavyweight performance needed for superior battlespace situational awareness.

Radars like PhantomStrike certainly offer the kind of cost/capability mix that will allow a real expansion in AESA capability, allowing smaller, lighter platforms to field a real AESA radar.

by Jon Lake