Airborne sensor platforms--the force multipliers.

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In 1940, during what Churchill termed the "Battle of Britain' (Germany's Luftschlacht um England), the defenders' ground-based radar was crucial in optimising the use of interceptors and thus in ensuring Britain's survival. Today the major nations depend heavily on surveillance and tracking radars mounted on airborne platforms, not only for use against aircraft, but increasingly against ground targets.

Airborne early warning and control (AEW&C) aircraft were initially developed to overcome the low-altitude gap in the coverage of surface-based radars. Today such aircraft are also valued for their ease of deployment, responding quickly to needs wherever they arise.

The first aircraft designed specifically for AEW&C was the twin-turboprop Grumman (now Northrop Grumman) E-2 Hawkeye, equipped with the 7.3metre dorsal radome and a General Electric APS-96 radar. The E-2A entered US Navy service in 1964, just in time for the Vietnam War.

The E-2C Hawkeye II with APS-120 radar entered service in 1973. The 23.6-tonne E-2C carries two pilots, a radar operator, a mission commander and an air control officer. It can remain four hours on station at 500 km radius, cruise-climbing from 25,000 to 31,000 ft. Aside from the US Navy E-2C fleet of around 75, the aircraft has been exported to Egypt (seven aircraft), France (three), Israel (four, now retired), Japan (13), Singapore (four) and Taiwan (six). The Mexican Navy has acquired three ex-Israeli E-2Cs.

By 2010 all US Navy E-2Cs will have been brought to the current Hawkeye 2000 production standard, which includes eight-blade Hamilton Sundstrand pro pellers and modernised cockpits. The first Hawkeye 2000 was delivered in 2001. Possible developments for export include a flight refuelling probe and a land-based version with fuel in the outer wings.

Digressing briefly, the Hawkeye's radar has also been applied to eight Lockheed P-3s operated by the US Department of Homeland Security, the Orions providing much greater endurance and crew space. In 2007 the Pakistan Navy requested three additional P-3Cs for similar conversion under FMS contract.

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The next E-2 generation will be the E-2D Advanced Hawkeye. In 2003 Navair awarded Northrop Grumman the $1.93 billion contract for the SDD (system development and demonstration) phase, which includes two test aircraft and is to lead to deliveries in 2010 and initial operational capability in 2011. The first E-2D flew on 3 August 2007, and the second three months later.

Lockheed Martin has received a $ 413.5 million SDD contract for the E-2D's APY-9 UHF radar, with a mechanically-rotated, electronically-scanned antenna. The E-2D will also have a new cockpit, with displays allowing either pilot to function as a fourth mission system operator. The US Navy plans to have 75 E-2Ds by 2020.

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Awacs

The first jet-powered AEW&C aircraft was the 152-tonne Boeing E-3 Awacs (Airborne Warning And Control System), which flew in 1972, based on the 707-320B airliner. Its Westinghouse (now Northrop Grumman) APY-1/2 radar has a radome 9.1 metres in diameter and 1.8 metres deep. Maximum detection range is over 375 km.

Deliveries of the E-3 Sentry to the US Air Force began in 1977. The US Air Force bought 34 E-3s, and 18 were ordered for Nato (to be based at Geilenkirchen in Germany), five by Saudi Arabia, seven by the UK and four by France. Around 65 E-3s are currently in service.

The US Air Force and Nato E-3s had Pratt & Whitney TF33 engines, but these are being replaced by the same company's quieter and less thirsty JT8D-219s. The British, French and Saudi aircraft are equipped with CFM International CFM56-2s, and have refuelling probes in addition to dorsal receptacles. The E-3 cruises at around 30,000 ft, and has an unrefuelled endurance of over eleven hours. The latest cabin layout provides for up to 19 mission consoles.

All export E-3s are being given avionics upgrades, aimed at the US Air Force Block 40/45 standard. Aside from radar and computer improvements, the modules include Link 16, Global Air Traffic Management (Gatm), the Northrop Grumman AAQ-24 (Laircm) and Boeing/UTL AYR-1 electronic support measures.

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In 1991 the 707 production line closed, and Boeing transferred Awacs marketing to the 767-200ER, which has over 50% more cabin floor area, and flies higher and further. The result was the 175-tonne 767-27C, powered by two General Electric CF6-80Cs, giving a cruise-climb from 34,000 to 40,000 ft. It provides 13 hours on station at 550 km radius.

Four 767-27C Awacs were purchased by Japan under the designation E-767, and deliveries began in 1998. The E-767 cabin has nine consoles and provisions for five more, but the basic design allows for up to 19.

A-50

The Soviet Union lagged behind in AEW&C. It is claimed that the Vega NPO design bureau started work on the Shmel (Bumblebee) radar in 1965, for installation by Beriev on an Ilyushin I1-76MD aircraft. However, the resulting A-50 entered service only in 1984, about six years after the E-3. It is believed that around 26 were built and that perhaps a dozen are still in service with the Russian Air Force.

The A-50 is a 190-tonne aircraft with four Rybinsk D-30 turbofans, although these are to be replaced by Perm PS-90As. It has four-man flight decks and eleven systems operators. Although larger than the 767 Awacs, it reportedly provides only four hours on station at 1000 km radius, due to its heavy avionics. It cruise-climbs from around 26,000 to 33,000 ft. The 210tonne Beriev A-50U is an upgraded version (export designation A-50E) with Shmel-M radar.

Russian Air Force A-50s were operated over the Black Sea and the Caspian Sea during the 1991 Gulf War to monitor coalition aircraft and Tomahawk missile flights. In 1999 India declined an offer of three A-50Es, but in the following year paid $1.9 million for a Russian A-50 to take part in its military exercises. In 2000 China turned down an offer of six A-50Es for one billion dollars.

Erieye

To recap, most AEW radars have an antenna with a wide reflector dish rotating inside a large, heavy radome. However, it is now possible for an array of transmitter/receiver modules to produce a narrow radar beam that can be steered by introducing phase differences between the individual energy pulses. Such electronically scanned linear arrays can (for example) be applied conformally to aircraft sides, or installed as a 'plank' below or above the fuselage.

The first major example was Sweden's Erieye, developed by what is now Saab Microwave Systems (seen piggybacking an Embraer 145 in the above photograph). In 1992 the company was contracted to supply Erieye PS-890 radars for use on six twin-turboprop Saab 340Bs of the Swedish Air Force. The PS-890 consists of 192 modules in an eight-metre array. The resulting S 100B Argus entered service in 1997. It was used only for AEW, raw radar information being downlinked to the ground. Two Argus systems have now been upgraded to S 100D standard with three operator stations to provide AEW&C, with Link 16, Have Quick II radios and Nato IFF Mode 4.

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Thailand has reportedly ordered one S 100B (presumably ex-Swedish Air Force), with an option on another. Pakistan has ordered five larger Saab 2000s with Erieye radars and five workstations. The first was shown at Farnborough in 2008. Sweden may eventually have six similar aircraft. Saab has exhibited a model of the Eads-Casa C-295 with integrated Erieye, believed to be for Poland.

Brazil chose to have the Erieye radar mounted on the Embraer EMB 145SA, a 20.6-tonne twin-jet aircraft with five workstations, six hours endurance at 280 km radius and an operational ceiling of 30,000 ft. Five are operated under the designation E-99. Brazil operates its aircraft as part of the Sivam Amazon surveillance programme. When they were introduced in 2002, Brazil's illegal-flight detection capability immediately shot up from 38% to well over 70%. Mexico has purchased one similar aircraft and Greece four.

The Erieye primarily gives coverage through 150[degrees] on either side. It can detect large aircraft at up to 450 kin, ships at 370 km, fighters at 350 km and cruise missiles at 150 km. Targets outside the primary arcs can be detected at reduced range by its side-lobes.

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A Chinese copy of the Erieye has appeared on a Shaanxi Aircraft Y8 (An12), designated K J-200, which first flew on 8 November 2001. A second flew in early 2005, but in the following year crashed due to icing.

Phalcon

The Elta Systems Phalcon (Phased Array Conformal) radar system attaches large electronically-scanned arrays to the fuselage in the form of lateral 'scabs' and more conventional dishes at either end. A six-array system was shown in model form at the 1987 Paris Air Show, based on a Boeing 707. Reports indicate that at least two Israeli and two South African 707s have some Phalcon elements. The Chilean Air Force Boeing 707-385C Condor, with Phalcon antennas in a bulbous nose radome and two front fuselage scabs, had its maiden flight in 1993. It appeared at the show in Paris in the same year, and was delivered in 1995.

In the mid-1990s Elta signed a contract with China for a number of Russian-supplied A-50s to be fitted with Phalcon radars. In 2000 the contract was cancelled due to US pressure, and Israel paid $ 350 million to China in compensation. It appears that the first aircraft (without Phalcon) was recovered by China via Russia in 2002, and first flew as the K J-2000 with a Chinese AEW radar on 11 November 2003. Four further KJ-2000s were produced by converting China United Airlines I1-76MDs. Two KJ-2000s were delivered in 2005, and four are now believed to be stationed in the Zhejiang Province. It may be noted that an Awacs version of the Shaanxi Y8 with 'flying-saucer' radome flew in 2006 and was marketed in Pakistan, but without success.

In 2004 Elta signed a $1.1 billion contract with India for three Beriev-supplied A-50s (probably originally earmarked for China's Phalcon programme) with Perm PS-90A engines, to be fitted with EL/M-2075 radars, EL/L-8312 ESM and EL/K-7031 Comint equipment.The radar uses a fixed triangular array of electronically-scanned antennas in a dorsal radome of eleven metres diameter. Deliveries of what Beriev terms AI50EIs are to be completed by 2010, and India may buy three more.

Elta has recently turned its attention to special-mission forms of the Gulfstream G550, including a Conformal AEW (CAEW) version for domestic use, with the EL/W-2085 radar antennas in scabs on the fuselage sides. The first CAEW appeared at Farnborough in 2008. Israel is expected to have at least three, designated 'Eitam' (Sea Eagle), and Singapore has ordered four for delivery between 2008 and 2010.

The G550 was selected for its operational ceiling, although the radar installation reduces this from 51,000 to 41,000 ft. It can remain on station for nine hours at 185 km radius.The dual-radar installation takes the form of S-band in the nose and tail, and L-band in the fuselage sides. The G550 has six operator stations, and its radar is said to detect fighter-size targets at up to 370 km.

737 AEW&C

The top of the current production range is the Boeing 737 AEW&C, which combines the 737-700IGW airframe and the Northrop Grumman Mesa (Multi-role Electronically-Scanned Array) radar. The Mesa has a 10.7-metre dorsal antenna system with side-looking arrays and smaller ones looking forward and aft, giving 360[degrees] coverage. The 737 AEW&C has a gross weight of 77 tonnes, and (with under-floor auxiliary tanks) can remain eight hours on station at 500 km radius, climbing from 30,000 to 40,000 ft. The baseline version has a two-man flight crew and six mission specialists, although up to ten consoles can be provided. It has an in-flight refuelling receptacle and a probe is available as an option. The 737 AEW&C is said to cost less than half the $ 400 million of a 767 Awacs.

The 737 AEW&C was selected by Australia for its Wedgetail programme in 1999 and six aircraft are now under contract. The first for the RAAF flew in 2004, and two aircraft with interim training capability are scheduled for delivery in July 2009. They will receive an electronic warfare system in early 2010. All six will be delivered with full operational capability by the end of 2010.

In 2004 Turkey signed a contract for four 737 AEW&C aircraft under the Peace Eagle programme. In 2006 South Korea signed for four to meet its EX requirement under the Peace Eye programme. The first is due for delivery in 2011 and the remainder in 2012.

Rotary-Wing

Britain's shipping losses in the 1982 Falklands/Malvinas conflict led to a demand for ship-based rotary-wing AEW&C assets, the Royal Navy having earlier relied on support from US Navy E-2Cs. The installation of a retractable Thorn-EMI Searchwater search radar on the Westland Sea King resulted in the 9.7tonne AEW2A with two operator stations and 3.75-hr endurance at 370 km radius. Some 13 aircraft were converted to this standard. A similar installation is also used on three Sikorsky SH-3Hs of the Spanish Navy. The latest in the Sea King-series is the ASaC7 (Airborne Surveillance and Control Mk 7) with the Thales UK Cerberus mission system including the Thales Sensors Searchwater 2000AEW radar.

Planned retirement of the Sea King ASaC7 is slipping into the 2020s, but it will eventually be replaced on Britain's two Future Carriers under the Royal Navy's Maritime Airborne Surveillance and Control programme. This will probably be achieved by installing the AsaC7 Cerberus system on either the Bell Boeing V-22 or the AgustaWestland AWl01. The Italian Navy already operates four AWl01 Mk 112s in this role with Galileo Avionica MM/HEW 748 radars. There is reported to be some interest from the US Navy and Marine Corps in developing a V-22 with AEW radar.

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Russia's Kamov Ka-31 has a Niir-developed E801M 'Oko' (Eye) radar feeding a six-by-one-metre rectangular antenna, which is mounted underneath, and scans mechanically in azimuth and electronically in elevation. For landing and take-off it is retracted to lie against the bottom of the fuselage. The 12.5tonne Ka-31 has a two-man crew, radar data being downlinked to the ship. It has an endurance of 2.5 hr and can detect fighters at 115 km and surface vessels at 250 km. Since 1999 the Indian Navy has ordered a total of 15 Ka-31s for use on its frigates and aircraft carrier, to provide not only an AEW facility but also targeting for Brahmos missiles.

Ground Surveillance

Most AEW&C aircraft have the ability to detect and track surface vessels. However, finding and tracking comparatively small ground vehicles requires a synthetic aperture radar (Sar) with moving target indication (MTI).

The use of Sar/MTI radars was pioneered by the single-seat Lockheed U-2 series, but the combination of such a radar with a multi-station command and control centre came only with the US Air Force's Northrop Grumman E-8 Joint Stars (Surveillance Target Attack Radar System).

The E-8 was first used in the 1991 Gulf War to monitor Iraqi Army movements and search for Scud launchers. The US Air Force now has 17 E-8Cs, all operated by the 116th Air Control Wing based at Robins AFB, Georgia, with a detachment at A1 Udeid AB, Qatar, operating with the 7th Expeditionary Air Combat and Control Squadron. By September 2008, E-8Cs had exceeded 40,000 combat-related flight hours in over 3650 sorties.

The 152.4-tonne E-8C is based on a pre-used Boeing 707-300 airframe, with a Northrop Grumman APY-7 radar, a 7.3metre phased-array antenna under the front fuselage and 18 operator stations. The antenna can be turned mechanically to scan on either side, covering a 120[degrees] sector with a range of up to 250 km. The E-8C has an unrefuelled endurance of nine hours and a ceiling of 42,000 ft.

In 2008 Northrop Grumman contracted Pratt & Whitney and Seven Q Seven to launch the re-engining of E-8Cs with JT8D-219s. Plans call for the whole fleet to have the JT8D by early 2014. It is anticipated that with avionics upgrades the E-8 will remain in service beyond 2070.

Joint Stars allows the Pentagon to manage a conflict anywhere, at short notice and with limited ground facilities. As such it goes beyond European needs and in 1998 Nato consequently produced its own Alliance Ground Surveillance (AGS) requirement. This developed into a 23-nation 3.3-billion [euro] programme aimed at fielding four Northrop Grumman RQ-4B Global Hawk Block 40 drones with MP-Rtip (Multi-Platform Radar Technology Insertion Program) radars, four Airbus A321s with six-nation Tcars (Transatlantic Cooperative AGS Radars) and 36 ground stations. However, in 2007 some key nations withdrew funding and the programme was rehashed as a straight buy of eight off-the-shelf RQ-4B Block 40s.

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Britain declined to join the AGS programme, preferring to develop its own Astor (Airborne STand-Off Radar) solution based on the Raytheon Systems Sentinel R1.The 45.3-tonne Sentinel uses the Bombardier Global Express airframe and a derivative of the Asars-2 radar employed in the U-2, with a 4.3-metre ventral antenna. It has an endurance of eleven hours and a ceiling of over 40,000 ft. The Raytheon Dual-Mode Radar has an active electronically-scanned array (Aesa) that produces a beam which is mechanically scanned in elevation and produces radar images to 300 km. The crew consists of two pilots, a mission coordinator and two image analysts. The Sentinel is not intended for battle management.

The $1.75 billion Astor development and production contract was signed in 1999, and the first Sentinel was delivered to No 5 (Army Cooperation) Sqn at RAF Waddington in January 2007. All five aircraft were to be delivered by end-2008. There are suggestions that the unit may be redesignated No 5 Joint Sqn, since it includes personnel from the other British services. It is widely expected that one Sentinel will be deployed to Qatar for operations over Afghanistan during 2009.

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ACS 

The US Army's Aerial Common Sensor (ACS) programme is intended to provide timely intelligence of ground threats, and was originally designed to replace both the 21-tonne Bombardier RC-7B and sixtonne Raytheon RC-12. The requirement was to carry a six-tonne payload, including an EO/IR turret, Sigint equipment and a Sar/MTI radar to track targets at 300 km. The aircraft was to have an endurance of ten hours and a speed of 650 km/h.

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In August 2004 Lockheed Martin was awarded an $ 879 million contract for the SDD phase of ACS, which included five RC-20s based on the 22-tonne Embraer Emb-145RS.The latter (designated R-99) is already used by the Brazilian Air Force in the ground surveillance, comint and elint roles, with equipment that includes Sar/MTI and a multi-spectral scanner.

However, it soon became clear that the Emb-145 was too small for the US Army's emerging sensor payload, and in January 2006 the US Army terminated the contract. User needs were then reassessed, while Northrop Grumman was awarded a contract to upgrade 33 RC-12 Comint/Elint aircraft to RC-12S standard, capable of serving beyond 2020.

Interestingly, Embraer recently disclosed that it was envisaging a study that would enable the firm to offer ready-to-integrate aircraft based on its larger platforms, like the RJ-170.

The revised ACS requirement is based on four incremental production standards, with Sar/MTI radar, Comint, Satcom, Manned-Unmanned teaming, four workstations and battle command facilities from the outset. Elint, EO/IR and modular sensors will be added with the later increments.

The ACS programme is expected to attract bids from teams led by Boeing, Lockheed Martin, Northrop Grumman and Raytheon, basing proposals on either the Gulfstream G550 or the Bombardier Global Express. It is hoped to award the six-year SDD contract (to include five fully functional aircraft) in the first quarter of 2009. Increment One aircraft are to be fielded in 2016, and Increment Four after 2020.

Israel already operates at least five special mission Gulfstream G550s, which may well have the Elta EL/I-3150 Multimission Airborne Reconnaissance and Surveillance System (Mars2); an 'all-in-one' system that combines Sar/MTI radar with Comint and Sigint facilities.

Finally, there is sudden interest in intelligence, surveillance and reconnaissance (ISR) versions of the Hawker Beechcraft King Air 350ER for use in southwest Asia. Following criticism of US Air Force 'foot-dragging' in deploying ISR assets, the service launched Project Liberty, a crash programme to acquire three RC-12 squadrons, totalling 37 aircraft. Equipment will reportedly include the L-3 Wescam MX-15i EO/IR turret and the General Atomics Lynx II Sar/MTI. Five such aircraft are being supplied to Iraq under FMS, and the UK has reportedly purchased four King Air 350ERs to be equipped by Raytheon Systems. The US Army is already using an ISR Constant Hawk version of the Shorts C-23 in Iraq.