Fighters face the Aesa revolution: since 2000, a small number of fighter aircraft have been flying with Active Electronically Scanned Array (Aesa) radars. To date, these have all been American, but design teams around the world are working to develop their own Aesa fighter radars.

In the traditional architecture for a fighter radar, the radio-frequency power is generated in a transmitter. The antenna is either a parabolic dish or a flat-plate array, and is steered mechanically. In the first electronically scanned radars to enter service, the radio-frequency power was still generated in a transmitter, the output power of which was passed via a power-dividing network to an antenna consisting of an array of phase shifters (usually based on ferrite).

Passive electronically scanned array (Pesa) technology was used in the first electronically scanned fighter radar to enter service--the N007 Zaslon ('Flash Dance') developed by V Tikhomirov NIIP for the Mikoyan MiG-31 Foxhound. First deployed in 1983, this 450-kg I/J-band radar had been improved by the addition of a new data processor which enhanced the range and ECCM performance. Further variants have offered improved processing and a larger antenna, but it is not clear if these models have been adopted for service.

At the time when work was begun by Thomson-CSF and Dassault Electronique on the RBE2 radar of the Dassault Rafale, the virtues of electronic scanning were appreciated, but Aesa technology was seen as a high-risk endeavour. The design team opted for a Pesa solution known as Radant, in which the beam would be steered by two radio-frequency lenses operating in elevation and azimuth respectively.

Russian designers have invested heavily in Pesa technology. Phazotron-NIIR's Kopyo radar was originally developed with a mechanically scanned flat-plate antenna, but the Super Kopyo-PH is a follow-on derivative incorporating a phased-array antenna assembly plus digital signal and data processing.

In its basic form, the Phazotron-NIIR N010 radar uses a mechanically scanned antenna. It was tested aboard MiG-29M prototypes during the early 1990s, and the Zhuk-8-II and N010M Zhuk-M variants are reported to have been supplied to China for use in the F-8-II and FC-1 fighters. Two Pesa versions have been noticed the Zhuk-MSF Sokol (could have been offered to China as an upgrade to the Su-27 and Su-30MKK) and Zhuk-PH.

India's Su-27MKI aircraft uses the V Tikhomirov NIIP N011M Bars, a phased-array version of the mechanically scanned planar-array N011 originally developed for the Su-27M, but probably abandoned. The 100-cm-diameter antenna is a Pesa unit, since the radar is known to incorporate a Chelnok travelling wave tube transmitter that gives the set an average power output of five to seven kW.

V Tikhomirov NIIP's Osa (Oca) I/J-band multi-mode radar is a 120-kg phased-array set offered for new build and retrofit applications. Reported to have been flown in a Mikoyan MiG29UBT two-seat combat trainer/ground-attack aircraft, it can simultaneously track up to eight air targets or two ground targets and allows the simultaneous engagement of up to four air targets.

V Tikhomirov NIIP sees the potential of upgrading existing mechanically scanned radars with a phased array variant. It offers its Pero antenna assembly in 105-cm and 75-cm-diameter forms for use on the N001 Myech ('Slot Back') radar carried by the Sukhoi Su-27 'Flanker' and the N019 radar of the MiG-29 'Fulcrum' respectively. No orders have been announced, but China has evaluated a prototype.

The problem with a Pesa solution is that RF power is lost in the power distribution network, phase shifters, circulators and the transmit/receive (T/R) switches used to alternate the antenna between its transmitting and receiving tasks. When the radar is receiving, these losses reduce the signal's strength before it reaches the low-noise amplifier input section of the receiver. As a result the overall sensitivity of the radar can be reduced by the order of 10 to 15 dB.

During the 1990s, the development of monolithic microwave integrated circuit (Mmic) technology based on gallium arsenide material allowed a new approach to electronic scanning in which the antenna consists of an array of transmit/receive modules, each containing a transmit power amplifier, a low-noise receive preamplifier, a phase shifter and a T/R switch. Since these are located close to the radiating element signal losses are minimised. Since the phase shifter is operating at a low power level, it can use solid-state integrated-circuit technology rather than ferrite, while the signal distribution and combining techniques can use inexpensive printed-circuit technology rather than waveguides.

By the late 1990s this technology allowed the creation of I/J-band active electronically scanned arrays (Aesa) suitable for use in fighter radars. The first Aesa radar to enter service was the Raytheon Space and Airborne Systems AN/APG-63 (V)2. Selected to be retrofitted to 18 US Air Force F-15C Eagle interceptors based at Elmendorf Air Base, Alaska, it was fielded in 2000. The remaining F-15Cs still carry the mechanically scanned AN/APG-63(V)l.

No radar programme has done more to push the development of Aesa technology than the effort to develop the Northrop Grumman/Raytheon Systems AN/APG-77 for the F/A-22 Raptor. Prototype hardware began flight tests in a flying testbed, and in November 2002 F-22 aircraft 4004 made its maiden flight with an AN/APG-77 installed. Production of Lot 1 radars started in 2002, and the radar is now in service (no picture could be supplied by the manufacturers due to confidentiality reasons, but oddly enough there is one on their website).

Originally conceived as a classic air-superiority fighter, the aircraft's mission was switched to multi-role before it had entered service, and its original F-22 designation was changed to F/A-22A. In 2004 Northrop Grumman announced that it had begun flight testing an AN/APG-77 version that incorporated a high-resolution ground-mapping mode.

The radar is designed to avoid compromising the aircraft's stealth capabilities. It has sophisticated air-to-air and air-to-ground modes and is reportedly capable of detecting a one sq/metre radar cross-section target at a range of 190 km with a single radar 'paint'. There is also a forward-looking high-gain passive mode that will allow emitting targets to be tracked without betraying the Raptor's presence.

Non-co-operative target recognition is presumably based on an ultra-high-resolution mode with a resolution of about 30 cm at long range. In the world of airborne reconnaissance this class of resolution is d considered good enough to allow the identification of specific types of ground vehicle, so it would be sufficient to allow the shape of a target aircraft to be deduced accurately enough to allow its identity to be determined from an onboard library of potential threats.

The F/A-18E/F Super Hornet entered service with the Raytheon Space and Airborne Systems AN/APG-73 mechanically-scanned radar, but the latest Super Hornets carry the company's AN/APG-79 Aesa radar.

This is made up of an active array antenna, a receiver/exciter subsystem, an open-architecture Common Integrated Sensor Processor (Cisp) based on commercial off-the-shelf PowerPC technology, a Motion Sensor Subsystem and a Radar Power Supply. Improved air-to-air modes are intended to allow target engagements at very long ranges, and to reduce aircrew workload. It is said that the AN/APG-79 provides between two and three times the air-to-air detection range of the earlier radar and allows higher-resolution tracking of significantly more targets. The air-to-ground modes can be interleaved and include long-range high-resolution ground mapping. Its synthetic aperture radar (Sar) mode has three times the resolution of that offered by the APG-73 and can overlay ground moving target indication (GMTI) tracks on the Sar image.

The US Air Force plans to upgrade its F-15Es with the APG-63(V)3 Aesa radar, which combines an updated and enlarged version of the AN/APG-79 antenna and its associated power supply with hardware from the APG- 63(V)1. However, there have been reports that this scheme might be shelved in favour of a different radar--either a further-developed APG-63(V)4 combining the -63(V)3 Aesa antenna with more hardware from the AN/APG-79 or a derivative of the AN/APG-77.

Singapore has ordered the AN/APG-63(V)3 for its F-15 force, and this radar, or the -63(V)4, could be candidates for future upgrades of existing F-15s. Israel and Japan are both potential customers.

Northrop Grumman Electronic Systems developed the AN/APG-80 Aesa radar for the F-16E/F fighters ordered by the United Arab Emirates. This has a greater detection range than the mechanically scanned AN/APG-68 that is fitted to most current-production F-16s, and can track up to 20 targets simultaneously or up to six with the accuracy normally associated with single target tracking. For strike missions it has an automatic terrain-following capability plus an ultrahigh-resolution Sar radar mode.

Once deliveries of production radars were under way the company began flight trials of new operating modes. These probably include GMTI and enhanced Sar/automatic target cueing modes.

The latest Aesa fighter radar under development in the USA is the Northrop Grumman Electronic Systems AN/APG-81. This will share module technology with the AN/APG-80 and developed versions of the AN/APG-77. Since the nose of the F-35 is smaller than that of the F/A-22A, the size of the antenna and thus the number of T/R modules it contains will be constrained, therefore the radar is expected to have about two-thirds the range of the AN/APG-77. However, its air-to-air detection capabilities will be much better than those of today's F-16C and all but the latest F/A-18s.

The F-35 avionic suite features a high level of integration and will be datalinked to sources outside the aircraft. This will allow the radar to be cued towards a target or a location specified by another onboard or external sensor.

Much emphasis is being placed to air-to-ground capability (the F-35 was originally known as the Joint Strike Fighter). For example, the Sar terrain mapping function is intended to have a higher resolution and larger area coverage than what is available from current fighter radars. An inverse synthetic aperture radar (Isar) mode will allow the identification of surface vessels.

In addition to acting as a multi-mode radar, the AN/APG-81 will provide a range of other functions, including active and passive EW and datalink communications.

Having developed the mechanically scanned Captor radar for the Eurofighter Typhoon, Euroradar--a consortium consisting of Selex Sensors & Airborne Systems, Eads Defence Electronics of Germany, Galileo Avionica of Italy and Indra of Spain--is now working on the Caesar (Captor Active Electronically Scanned Array Radar), which is planned for installation on Tranche 3 aircraft.

Flight trials of the new radar started in February 2006 using a Bac-111 flying test-bed. The first flight in a Typhoon had not yet taken place when this article was written, but is expected in the near future.

The Caesar is not an all-new radar, but will team a new active array antenna, antenna control unit and an adapted power supply with the receiver and processor of the existing radar to create a set that combines the advantages of electronic scanning with the functionality and modes of the original radar. The lower life-cycle costs resulting from the switch to Aesa is expected to more than cover the cost of the retrofit.

Selex also offers the Vixen 500E. Made up of two main line-replaceable units and based on cots-based processor cards, it can scan through plus or minus 60[degrees], offering single-target track, track while scan against more than ten targets, range while search and velocity search air-to-air modes, slewable, vertical and Hud scanning modes and a boresight mode. In the air-to-ground role its modes include real-beam ground mapping, Sar mapping with a resolution of less than three metres, ranging, sea-surface search and track and beacon interrogation.

The Vixen 500E is being proposed for the Korean Aerospace A-50 attack version of the T-50 Golden Eagle jet trainer. A slightly larger Vixen 750 version is being offered for use in lightweight fighters.

The technology used in the Caesar and the Vixen 500E was developed in Europe, says Selex. For example, the T/R modules were developed by United Monolithic Semiconductors in Ulm, Germany, and are being manufactured by Eads's Ulm facility. Since there is no US content in the two radars, future exports cannot be directly constrained by the US government.

France took a similar approach as the Eurofighter nations, opting for an Aesa solution that could be used to upgrade the existing RBE2, having made sure that enough power and cooling was available in the basic radar to allow a switch to Aesa at some time in the future. A Demonstrateur de Radar a Antenne Active (DRAA) prototype based on US technology was test-flown in 2002, and this led to the launch of a Draama (DRAA modes avances) based on European technology in 2004. The resulting configuration is expected to more than double the detection range and provide improved performance at high angles from the centreline.

Elta Systems announced its EL/M 2052 active phased-array fire control radar at the 2005 Aero India exhibition in Bangalore, a marketing debut that preceded the start of flight testing aboard a Boeing 737. Aimed at the fighter upgrade market, the radar will weigh between 130 and 180 kg, depending on antenna size (which is constrained by the dimensions of the aircraft's nose limitations). It can operate in air-to-air, air-to-ground and air-to-sea modes simultaneously.

Air-to-air modes include multi-target detection and tracking, multi-target ACM and high-resolution raid assessment, with up to 64 targets being tracked simultaneously. Air-to-ground modes are based on Real Beam Map, Doppler Beam Sharpening and Sar, and offer GMTI and ground moving-target tracking (GMTT). An Isar mode can be used to identify sea targets.

Current models of the Saab (formerly Ericsson) PS-05/A radar for the JAS-39 Gripen use a mechanically scanned antenna, a configuration that will be retained with the planned Mk 4 version. The Mk 5, expected to enter service around 2012, will have an Aesa antenna, but this will retain a degree of mechanical scanning in order to achieve high angles from the aircraft's centreline. Initial flight trials to explore the concept used an Aesa antenna obtained from Raytheon, but the source for the T/R modules to be used in the definitive design has not yet been chosen. Other technologies for the Mk 5 will be drawn from Sweden's Nora (Not Only Radar) technology demonstrator programme in order to create a multi-functional sensor able to handle active and passive radar, electronic warfare and communication roles- a concept originally known as Eira (Ericsson Integrated RF Avionics).

Japan is traditionally tight-lipped regarding its indigenously-developed weapon systems, so very little is known about the Aesa radar that Mitsubishi Electric and the Japan Defense Agency's Technical Research and Development Institute have developed for the Mitsubishi F-2 support fighter. It is reported to use an antenna made up from 808 T/R modules and to offer air-to-air, air-to-ground and navigation modes. Other features include look-down/shoot-down, track-while-scan, multiple target track and terrain avoidance.

India is shopping for a new multi-role combat aircraft, and wants this to have an Aesa radar. By 2009, India's Su-30MKI fighters are expected to be equipped with the new N035 Irbis radar, which may be the first Russian-developed Aesa radar to enter service.

One candidate for the Indian requirement is the new MiG-35 derivative of the MiG-29, which is being offered with the Phazotron-NIIR Zhuk-AE/FGA35. Due to begin test flying this year in FGA29 technology demonstrator form, this is an Aesa radar with a 575-mm-diameter antenna array made up of 680 T/R modules. The FGA29 configuration draws heavily on hardware from the mechanically scanned Zhuk-ME radar, raising the possibility that export users of the latter will be able to have their radars upgraded to the -AE standard.

The definitive FGA39 is expected to have a redesigned 'back end' and an antenna made up of more than 1000 T/R modules, giving a 200-km range against a five-sq/metre target. It is intended to track up to 60 air targets simultaneously and engage up to six.

Russia has now embarked on the design of its next-generation fighter, the Sukhoi Pak FA (Future Aviation Complex for Frontal Aviation). V Tikhomirov NIIP has been tasked with developing its radar, but although the aircraft is not due to enter service until around 2010 or 2012, some reports claim that early examples will have a Pesa radar, with Aesa following at some time in the future.

Such a conservative solution seems unlikely--today's 'savvy' customers want Aesa. That's hardly surprising. At last year's Farnborough Air Show, Selex compared the advantages of Aesa over older technology with that between DVDs and VHS videotape movies. It's a good analogy--when did you last watch a VHS videotape movie?