Unmanned Flight
Search, Find, Report and (Maybe) Strike!
Weary of dependence on assets owned by the US services (and on their willingness to share information), the leading Euro-Nato nations are now taking action to develop and procure independent means of ground surveillance and battlefield management. Rather than aiming for Nato-wide equipment commonality, the watchword now is interoperability.
Roy Braybrook, Doug Richardson

The sensor platform options under consideration include manned and unmanned (UAV) systems, certainly to be used in a combination 'system of systems', and probably to be associated with ground-based command and control facilities. Within the foreseeable future, manned sensor platforms may also be used to give precise guidance to comparatively 'dumb' low-cost munitions, while some unmanned sensor platforms will have autonomous means to strike at ground targets or designate them for attack by weapons launched from other platforms. The ultimate aim is to cost-effectively engage both static and mobile targets by day or night, regardless of weather conditions, with minimal risk to aircrew and to civilians in the area. By the next decade, this task is expected to involve the use of directed-energy weapons, carried initially by manned aircraft and later (as laser and microwave devices are miniaturised) by UAVs (Unmanned Aerial Vehicles).


AGS

Nato interest in Airborne Ground Surveillance (AGS) was sparked by the US Air Force's dramatically successful use of two Northrop Grumman E-8A Joint Stars (Surveillance Target Attack Radar System) development aircraft in the 1991 Gulf War. In 1992, Nato's Defence Planning Committee expressed a need for an Alliance (later Airborne) Ground Surveillance (AGS) system. Defence ministers endorsed a recommendation from the Conference of National Armament Directors (Cnad) in 1995 for a "minimum essential owned and operated core capability, supplemented by interoperable national assets". The Nato Staff Requirement for an AGS capability was formally approved two years later.

Artist's impression of a Fokker 100 trials aircraft with the Sostar-X antenna based on gallium-arsenide technology. The Sostar consortium represents France, Germany, the Netherlands and Spain. (Eads/Dornier)
In 1999, the Cnad welcomed the formation of groups of like-minded nations to fund the development of individual AGS options. The first such group was based on the Northrop Grumman Multi-Platform Radar Technology Insertion Program (MP-Rtip), i.e., the upgraded radar being developed for Jstars. It consists of Belgium, Canada, Denmark, Luxembourg, Norway and the United States. The group is known by the acronym Natar - Nato Transatlantic Advanced Radar. In addition to the industries of those six Nato nations, the Northrop Grumman proposal involves work for companies in the Czech Republic, France, Germany, Greece, Hungary, the Netherlands, Poland and Turkey. It is nonetheless admitted in the company's presentations that the "window of opportunity" for European industry to participate in the design and analysis of major radar elements will have disappeared by the start of 2003. The first MP-Rtip radar is scheduled to fly in 2005.

The second such group is the five-nation Sostar consortium representing France, Germany, Italy, the Netherlands and Spain. Sostar was founded in February 2001 as a joint venture by Eads/Dornier, Fiar and Thales Airborne Systems, each of which own 28 per cent, together with Spain's Indra 11 per cent and Fokker Space 5 per cent. In December 2001, the new company received an € 85 million contract to develop a European all-weather ground surveillance demonstrator radar named Sostar-X (Stand-Off Surveillance and Target Acquisition Radar). The contract was awarded by Germany's BWB (Bundesamt für Wehrtechnik und Beschaffung) on behalf of the five governments. The aim is to develop an advanced radar with an active electronically-scanned array (AESA) based on gallium-arsenide technology, and scaleable to allow installation on transport aircraft, business jets, medium helicopters or high altitude UAVs.

Sostar is to provide detection and tracking of slow moving targets, high-resolution imaging and target classification. It is planned to begin tests in 2004 with the radar and a three-metre antenna mounted on a Fokker 100 regional jet provided by Stork Aerospace and operating from Woensdrecht. The aircraft will accommodate three workstations and will have a datalink to transmit information to a ground station for processing, evaluation and distribution.

The Natar and Sostar groups report to Nato's AGS Support Staff Cell (SSC), and it is possible that at a later stage these two efforts may be combined. In April 2000 Northrop Grumman and Dasa (now integrated into Eads) signed a memorandum of understanding to evaluate potential business alliances in key technology areas such as ground surveillance and high-altitude long-endurance UAVs. The Sostar programme might be characterised as a catch-up effort by Europe, which is undoubtedly lagging behind Northrop Grumman and Raytheon MP-Rtip technology. If the AGS programme is to proceed on a transatlantic basis, Europe needs to be able to negotiate with America on much more even terms, since both France and Germany insist on producing a system that can be exported without the dead hand of Washington on international sales.



 The Northrop Grumman E-8C Joint Stars is the current leader in ground surveillance and battlefield management. Potential future developments include re-engining and the MP-Rtip radar. (Northrop Grumman)

The principal European manufacturers already have some experience in the air-to-ground radar field. Thales support includes the ground surveillance radar of the French Army's Cougar-based Horizon system and the Slar-2000 reconnaissance pod for the Mirage. Eads/Dornier is responsible for the Sarto Tornado Sar/MTI reconnaissance pod, the Sar for the CL-289 reconnaissance drone upgrade, the Awards (All-Weather Airborne Reconnaissance Drone Sensor) for low-speed UAVs and the Atlas (Airborne Topography and Land-use Assessment System) SAR. Thales and Eads/Dornier have jointly developed the Saris radar pod for combat aircraft, business jets and UAV applications, and the Sword radar for the CL-289. Italy's Fiar, part of the Avionics Systems and Equipment Division of Alenia Difesa, is better known for its combat aircraft radars (over 300 Grifo radars have been ordered), but it is also responsible for the Creso ground surveillance radar flown on the AB-412 trials helicopter.

The Natar proposition, based on the MP-RTIP radar, is that Nato would own and operate an aircraft and ground-station system capable of supporting two geographically separated Combined Joint Task Force (CJTF) sized operations simultaneously. The proposal states that, by adopting Natar, all Nato nations will have the necessary unrestricted access to ground surveillance data for decision-making in the first phase of the crisis. To deal with two simultaneous crises is estimated to require twelve fixed-wing platforms and 48 ground stations to ensure continuous coverage, although the Natar group accepts that on-going improvements in aircraft reliability and endurance may well reduce these numbers. If a contract were to be awarded to the Natar group by mid-2004, the first production aircraft and ground stations could be delivered in 2010. If it were decided (as seems likely) that Nato planning would be based on dealing with only a single crisis, the acquisition cost for six Natar aircraft (currently seen as modified Airbus A321s with twelve airborne operators) and 24 ground stations would be in the region of $ 2.5 billion. It is noteworthy that Northrop Grumman still envisages the aircraft platform as housing command and control facilities, and thus being larger than a business jet.


Jstars

Turning to existing national programmes within Nato, the US clearly leads with the Northrop Grumman E-8C Joint Stars, based on the 707-300 series aircraft equipped with 18 workstations, a number that may be increased to 22 as a result of operational experience in the 1990s. Deliveries of the E-8C to the 93rd Air Control Wing at Robins AFB, Georgia, began in March 1996, and initial operational capability was declared in December 1997. Procurement for the US Air Force is running at one aircraft per year, with funding for the 17th production aircraft (and a down payment for the 18th) requested in the budget proposals for FY2003. Northrop Grumman is also updating ten Block 10 aircraft to Block 20 standard, incorporating commercial off-the-shelf (Cots) technology. The first such aircraft was handed back to the Air Force in February 2002. The US Air Force initially planned a fleet of 19 E-8Cs, including the two development aircraft (upgraded to production standard), but this is clearly going to be exceeded.



 The Natar (Nato Transatlantic Advanced Radar) group proposes the use of the Northrop Grumman MP-Rtip radar in an aircraft of Nato's choosing, such as this converted Airbus A321. (Northrop Grumman)

The E-8C downlinks radar imagery to a Common Ground Station (CGS) based on a Hummer vehicle. Joint Stars has demonstrated the ability to pass target information directly to strike aircraft such as the F-15E, B-1B and AH-64. The E-8C is the subject of a series of production standard improvements, which will culminate in the Block 50, introducing the MP-Rtip radar with simultaneous Sar and MTI facilities.

The E-8C is participating in the Darpa-funded Amste (Affordable Moving Surface Target Engagement) trials, in combination with a Northrop Grumman-operated BAC One-Eleven equipped with the F-35 radar, and other sensor platforms. The objective is to guide seekerless, low-cost (< $ 50,000) munitions within ten metres of mobile targets such as ballistic missile launchers and long-range Sam systems. The tests employ GPS-guided weapons such as the Boeing Jdam or the Lockheed Martin On-target Weapon, Long-range (Owl), which are given more precise guidance based on data fusion from two precise airborne radars.

Looking beyond the current B707-based E-8C, there has been talk of supplementing the Joint Stars system with additional platforms, once the scaleable Northrop Grumman MP-Rtip radar becomes available. These platforms are expected to include modified business jets, capable of cruising well above the 32,000 to 35,000 feet normally used by the E-8C, and thus providing much longer radar range and reduced shadow effects in mountainous terrain. Although the current emphasis is on upgrading the radar, the US Air Force has been talking for several years of replacing the current Pratt & Whitney JT3D engines of the E-8C with the CFM56, Rolls-Royce BR700 or P&W JT8D-200, allowing it to cruise at up to 42,000 feet. The B707 airframe is also expected to be replaced by a 767-400 in the form of the MRC²A (Multi-Role Command & Control Aircraft), which will also replace the E-3 Awacs and the EC-135 Rivet Joint Elint platform. The MRC²A is expected to appear first in ground surveillance form around 2010, and to be used in conjunction with a Northrop Grumman RQ-4A Global Hawk equipped with the basically the same MP-Rtip radar.

Recent engine tests of the X-47 Pegasus enabled the Pratt & Whitney JT15D to be autonomously spooled up, brought idle and finally shut down once all the parameters were checked. Expected to make its maiden flight later in 2002, the Pegasus Ucav is being developed on company funds to demonstrate the aerodynamic qualities required for an autonomous operation from an aircraft carrier. (Northrop Grumman)

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Astor

The British Astor (Airborne Stand-Off Radar) requirement is being met with a Raytheon Systems (RSL) project based on the Bombardier Global Express equipped with a development of the Raytheon Asars-2 radar used on the U-2S, and accommodating three work-stations. The Global Express will cruise at 50,000 feet and have an endurance of eleven hours. The Astor requirement reportedly emphasised Sar performance rather than MTI capability. In 1999, RSL was awarded a $ 1.2 billion British contract which included the provision of five aircraft and eight ground stations (six tactical stations based on Steyr vehicles and two operational level ground stations deployed in air-portable containers). Service entry is scheduled for the third quarter of 2005 and the complete system is to be delivered by 2007.

The first aerodynamically representative Global Express began trials from Bombardier's flight test centre in August 2001, and the first real Astor (RAF serial ZJ690) was delivered to Raytheon's Greenville facility in Texas in January 2002. It is due to fly fully equipped in late 2003 and to be delivered to RAF Waddington in 2004. Raytheon is marketing a similar system under the designation Gsars (Ground Surveillance Airborne Radar System) with various options, such as a choice of ground station vehicles. The company appears to be looking primarily at potential customers in the Middle East. Reports suggest that the Royal Air Force hopes to upgrade Astor around 2010 with a radar that will leapfrog MP-Rtip technology.



 The Astor (Airborne Stand-Off Radar) aerodynamic validation aircraft, a Bombardier Global Express with fuselage fairings representing the radar and communications facilities, on delivery to Raytheon last August. (Bombardier)


Helicopters

High-flying fixed-wing aircraft can quickly scan large areas, but their imagery will often need to be supplemented by other sensor platforms, investigating radar shadows produced by hills and looking at areas of special interest in more detail and over a longer period. The Thales Target MTI radar was successfully operated on a Super Puma helicopter during the 1991 Gulf War, and is now used on four Cougars employed by the French Army. The Horizon system is marketed under the name Battle Scan for platforms such as the Mi-17, UH-60 and EH-101. The other European helicopter system is the Fiar Creso radar, which has been tested on an AB-412.

The French Army Horizon system employs four Eurocopter AS 532L Cougar helicopters equipped with the Thales Target MTI radar. It can process 4000 targets simultaneously and designate at 150 km. (Jérôme Deulin)


UAVs

The Northrop Grumman RQ-4A Global Hawk HAE (High Altitude Endurance) UAV is undoubtedly the most important current type. It first flew in 1998 and the formal development phase began in March 2001. It has seen operational service in Afghanistan. The RQ-4A has a maximum endurance of over 35 hours, or 24 hours within a 1200 nm radius. It has been flown to an altitude of more than 65,000 feet, and currently has a payload of 900 kg. The full payload (by Raytheon) combines electro-optical and infrared cameras providing still imagery (not video) and a Sar with an effective range of over 100 nm. As in the cases of the Shadow 200 and Fire Scout UAVs (discussed below), the operator simply inserts navigation co-ordinates rather than flying the aircraft. The US Air Force plans to buy 51 production RQ-4As. Two Block 5s have been ordered in the FY2001 budget and three are requested (at a cost of $ 170.8 million) for FY2003. The new budget also includes preliminary funding for a US Navy version, which would make it easier to bring production to an economic level of around ten per year. The US Air Force has been considering a stretched Block 20 with a payload in the 1350 kg class and possibly two engines.

The US Air Force's General Atomics RQ-1A Predator first flew in 1994 and has seen operational service over Bosnia, Kosovo and Afghanistan. In the last case it has laser-designated targets for manned aircraft, fed video imagery directly to AC-130Hs and attacked ground targets, using Hellfire laser-homing missiles. The Predator has a maximum endurance of over 24 hours, or 14 hours in a 400 nm radius. It has a payload of 200 kg and is normally operated at 15,000 feet. The Predator has electro-optical and infrared video cameras, and can simultaneously carry Sar. The US Air Force has purchased twelve systems, each with four air vehicles. Unlike the Global Hawk, it is flown manually, and is operated only in military-controlled airspace. From FY2002, when 16 were purchased, all Predators have been built with a laser designator in addition to the two cameras. The US Air Force has recently formed a third RQ-1A squadron in response to the demands generated by the fighting in Afghanistan. Some 22 aircraft have been requested for the US Air Force in FY2003, and six have been ordered by the Italian Air Force. The turboprop-powered Predator-B, which first flew in February 2001, can cruise much higher (up to 45,000 feet), and can carry a heavier load of Hellfire or Lockheed Martin Low-Cost Autonomous Attack System (Locaas) missiles.



 The RQ-4A Global Hawk began engineering and manufacturing development in 2001, but has already been used operationally over Afghanistan. A US Navy version is under consideration. (Northrop Grumman)

The IAI/TRW Hunter is the US Army's UAV workhorse, having been deployed to Kosovo three times. Around 40 airframes survive, and the Army expects to continue using the Hunter for several years. It is to be replaced by the AAI RQ-7A Shadow 200, which is much smaller and more easily deployed, three fitting inside a C-130. Following the acquisition of eight RQ-7A systems, each with three air vehicles, the US Army has recently ordered five more.

The US Navy and US Marine Corps use the IAI/TRW RQ-2A Pioneer for training, but the Navy has stopped funding the new Northrop Grumman RQ-8A Fire Scout helicopter, apparently on the grounds that the operational requirement was wrong. A much more ambitious rotary-wing UAV is the Frontier Systems A160 Hummingbird, which first flew in January 2002 under Darpa funding. It is hoped that the A160 can be developed into a production aircraft for the US Army, with an endurance of around 40 hours and a range of 2000 nm.

A fixed-wing UAV of comparable performance is Australia's Aerosonde, which was the first unmanned aircraft to cross the North Atlantic, covering more than 1750 nm in 26 hours 45 minutes.

European UAV developments have been less dramatic. In May 2201 it was announced that the Eads Eagle (derived from the IAI Heron TP) had been chosen for the French Air Force's medium-altitude, long-endurance UAV role. Eads is now offering the Eagle in the Australian Army's JP129 competition. The Sagem Sperwer-LE is a long-endurance derivative of the UAV chosen by the Danish, Dutch, French and Swedish armies. This new version first flew in December 2001.



 The Raytheon Systems prototype Astor tactical ground station (TGS), based on a Steyr 6 x 6 Improved Mobility Vehicle. The British Astor system will have six such vehicles, two larger ground stations, and five aircraft. (Raytheon)


Ucavs

The first trials of UAVs using air-to-ground missiles were carried out by the US Air Force during the Vietnam War, when modified Ryan Firebees were tested with Maverick and other lightweight weapons. Interest in the concept of an unmanned combat air vehicle (Ucav) has increased recently, due to the successful use of a Hellfire-armed Predator in Afghanistan and international sales of the IAI/MBT Harpy radar killer, which is essentially a small cruise missile.



 The General Atomics Predator B, which first flew in February 2001, is a turboprop development of the piston-engined RQ-1A Predator, offering a much higher ceiling and heavier warload. (GA Aeronautical Systems)

The current perception is that it will be many years before Ucavs can replace manned fighters in air combat, but stealthy examples could begin to take over much of the ground attack role (especially defence suppression) around the end of the present decade. The leader in this field is again the US Air Force, which is aiming to fly the first Boeing X-45B before the end of 2004. The X-45B is an enlarged derivative of the X-45A technology demonstrator, with an internal weapons bay sized to house six small diameter bombs. The first of two X-45As should have flown by the time these words are published. Development of the Block 30 X-45B with directed-energy weapons is expected to begin in 2008.



 The Northrop Grumman RQ-8A Fire Scout, based on a Schweizer 330SP with a Rolls-Royce 250 turboshaft engine, has lost US Navy funding, but development may continue for the export market. (Northrop Grumman)

The US Navy is also interested in UCAVs, but reportedly wishes to combine strike capability with long-endurance surveillance. The service is scheduled to award preliminary contracts to Northrop Grumman for the X-47 and/or Boeing for the X-46 in the near future. The US Army plans trials with the Northrop Grumman Bat anti-armour munition on the Hunter UAV, and hopes in the long term to develop an unmanned combat aerial rotorcraft (UCAR), such as the Boeing Canard Rotor Wing or a derivative of the A160 Hummingbird.



 Illustrating the likely trend in stealthy Ucavs for strike duties, the Boeing X-45A technology demonstrator is expected to lead to an enlarged production model (X-45B) flying in 2004. (Boeing)


Man-portable UAVs

The concept of a lightweight UAV that will enable a soldier to see over the next hill clearly has attractions, but it also has equally obvious problems, such as operator skill level, visibility, vulnerability to winds, and ease of recovery. The idea was pioneered by the AeroVironment Pointer, but thinking is now directed toward an even smaller UAV that can be carried in the soldier's backpack.



The stealthy Northrop Grumman X-47A Ucav was designed purely for the strike role, but the US Navy insists on combining this with surveillance, which will clearly mean considerable redesign. (Northrop Grumman)

The possibilities are illustrated by the 56.5-gram AeroVironment Black Widow with its 15 cm wingspan and which first flew in 1996. It carries a miniature camera and a transmitter, providing a real-time video downlink. Other companies working in the micro air vehicle (MAV) field include MLB and BAE Systems. The US Marine Corps' somewhat larger Dragon Eye programme is currently being competed by AeroVironment and BAI Aerosystems. Dragon Eye is envisioned as weighing around 2.0 kg and having a span of approximately 1.15 metres. The objectives include a one-hour endurance, a ten-kilometre radius, and a speed of 65 km/hr.


Synthetic Aperture Radar

Electro-optical and infrared sensors are badly affected by weather. This can be a particular problem in winter, in fog-shrouded mountain valleys or in littoral environments. The pollution-generated haze created over most large cities can also reduce EO/IR performance.

A radar sensor offers all-weather performance, but in its basic form has a much lower resolution. To obtain a high resolution, the antenna must be physically large, so that its size is many times that of the wavelength. Since wavelength is inversely proportional to frequency, the resolution of an antenna of fixed dimensions improves as frequency rises, but always remains relatively poor compared with that of an EO or IR sensor.

Synthetic aperture radar (Sar) technology improves resolution dramatically. It is a technique which exploits the aircraft's forward motion to gather radar data and simulates the effect of an antenna several hundred metres in width. This simulated antenna is known as the synthetic aperture, and its extended length provides a very high resolution.

As the aircraft flies past the object being observed, the range between the antenna and the target changes, so this effect must be compensated for before an image can be formed. In a description of the principles of Sar, Sandia National Laboratories also notes that, "for fine resolution systems, the range and azimuth processing is coupled (dependent on each other) which also greatly increases the computational processing".

Developed as a payload for the General Atomics Predator medium-altitude endurance (Tier II) drone, the Northrop Grumman AN/ZPQ-1 Tactical Endurance Synthetic Aperture Radar (Tesar) was used over Bosnia from March 1996 onwards. This is a 75 kg J-band Sar operating in strip map, spot map and MTI modes. In strip map mode, the radar observes terrain parallel to the flight path or along a specified ground path. Resolution depends on range and swath width, and can vary from 0.3 to 1.0 metres. Spot map mode covers 800 x 800 metres or 2400 x 2400 metres. In MTI mode, moving targets are overlaid on a digital map.

The US Air Force has purchased a General Atomics Lynx Sar for evaluation on the turboprop-powered Predator B. The Lynx is the result of a collaborative venture by Sandia National Laboratories and General Atomics. The two teamed in 1996, with General Atomics funding Sandia which had already developed an Sar, to create an enhanced version, and to deliver two prototype units together with licenses and manufacturing information which General Atomics would need in order to manufacture the unit.

The Lynx weighs 52 kg, and can provide real-time images of near-photographic quality in day or night conditions, and through clouds, rain or fog. It has a range of up to 85 km depending on weather conditions and imaging resolution.

A three-axis gimbal mounted within a 48.3 cm diameter radome carries an offset-fed dish antenna, a travelling-wave tube with a peak power of 320 W, a high-voltage power supply and an inertial measurement unit. This hardware draws less than 550 W of electrical power, while an associated electronics assembly requires a maximum of 500 W.

As the resolution of small Sar systems falls to between 0.3 to 0.15 metres or below, the stability of the transmitter becomes a limiting factor. Advances in TWT (travelling wave tube) technology were the key to developing the phase-stable transmitters needed to allows Sar/GMTI (Ground Moving Target Indicator) systems to produce images with resolutions down to 0.1 metre.

A transmitter of this type developed by dB Control is used in the Lynx Sar, allowing the radar to produce 0.1 metre resolution images of scenes which are 25 km away and obscured by clouds and light rain. "Without a doubt, the high quality of the RF power provided by dB Control's Ku-band transmitter enables us to produce fine-resolution Sar/GMTI images," says Jon Lathrop, Lynx Sar Program Manager for General Atomics.

The Lynx has a moving-target indication (MTI) mode, and can use a technique known as coherent change detection to recognise very small changes in a scene by taking two images of the same location, but at different times.



The US Army hopes to develop a Ucav with Vtol capability, possibly based on this Boeing Canard Rotor Wing (CRW) concept, using a reaction-drive rotor/wing that is fixed for high-speed cruise. (Boeing)

The two images are then matched up and compared pixel by pixel. Where changes have occurred, the pixels in the two images will no longer correlate. In some cases this will be due to minor natural movements such as trees blowing in the wind, but the technique can be used to detect man-made changes as small as vehicle tracks in the soil.

Future upgrades could include an enhanced ground MTI, inverse Sar mode for imaging of seaborne targets, interferometric Sar for three-dimensional imaging and automatic target recognition.

To equip the US Army's planned fleet of AAI Tactical UAV (TUAV) systems, Northrop Grumman is developing the Ku-band TUAVR, a lightweight - around 30 kg - derivative of the Tesar that equips the Predator. The long-term goal of this programme to field a radar with a range 3 to 10 km in Sar mode and 3 to 14 km in MTI mode. The same radar is also a candidate to equip the US Navy's new Northrop Grumman RQ-8A Fire Scout vertical takeoff and landing UAV.



 In the Kill-Assist All-weather Targeting System, a Lynx radar (shown mounted on a helicopter) measures the exact GPS-derived position of a time-critical mobile target and passes this information to a GPS-guided munition. (General Atomics)

The Northrop Grumman RQ-4A Global Hawk currently carries a Raytheon (originally Hughes) sensor suite which teams EO and IR sensors with a Sar whose gimballed antenna scans from either side of the vehicle. Resolution is 1 metre in wide-area search (Was) mode, and 0.3 metres in spot mode, while the MTI mode has a minimum detectable velocity of 7.5 km/h. The definitive Sar sensor will be a new variant of the Raytheon Electronic Systems (formerly Hughes) Asars-2 carried by the Lockheed Martin U-2S reconnaissance aircraft.

Lockheed Martin's J-band (16.0 to 16.5 GHz) Astar pulse-Doppler radar has a range of 2.5 to 17.5 km in clear air, or in the presence of smoke and dust. This falls to 11.5 km in conditions of heavy fog or rainfalls of up to 4 mm/h. It uses a pulsed, phase-coherent waveform to measure Doppler frequencies, and target returns whose Doppler data differ from the range-Doppler co-ordinates of the surface clutter (indicating target motion) are processed to indicate target type.



 Developed for the French Ministry of Defence, the Thales Airborne Systems Swift is a four to eight GHz Sar which uses a fixed sideways-looking antenna. (Thales Airborne Systems)

The current status of the AIL AN/APS-144 J-band pulse-Doppler MTI radar is unclear. Developed under the US Army's Airborne Reconnaissance Low (ARL) programme, this started flight tests more than a decade ago, but no applications have been reported.


European Developments

European industry had developed its own Sars for UAV applications. The Thales/Eads Dornier Sword (Standoff all-Weather Observation and Reconnaissance Drone) weighs 30 kg, and was designed to replace the existing optical payloads of the CL-289 UAV. It is a J-band Sar/MTI sensor covering a swath width of approximately 2000 metres, generating high-resolution Sar imagery in real time, and medium-resolution MTI imagery in near-real-time. A 49 kg Eads Dornier version known as Awards (All-Weather Airborne Reconnaissance Drone Sensor) uses much of the same hardware, but adds an onboard real-time processor, plus antennas which are mechanically steered in elevation and electronically-steering in azimuth.

Astrium is marketing its Quasar modular Sar for UAV applications. Based on technology originally developed by Matra Marconi Space, the radar is being offered in 4 to 8 GHz, 8 to 12 GHz, and 12 to 18 GHz variants, all of which would have Sar strip-map, spotlight imaging, and MTI modes.

By 1994, Ericsson is expected to test-fly a version of its Coherent All Radio Band Sensing (Carabas) VHF band radar in a UAV. The system will operate at 20 to 90 MHz, for Sar imaging, and 60 to 90 MHz for MTI mode. The basic system has been extensively flight-tested on a manned aircraft.



 Dassault's experimental validation Petit Duc Ucav had its maiden flight on 18 July 2000. It has an empty weight of 35 kg for a maximum take-off weight of 60 kg. (Dassault/Aviaplans)

IAI Elta's EL/M-2055 operates in strip mode for area search and detection, in high-resolution mode for spotting, and in MTI mode with wide-area Doppler beam sharpening surveillance for tracking moving targets. This 56 kg system has been demonstrated on the Searcher II, may have been installed in the Hunter and has been fitted to the Indian Defence Research and Development Organisation (DRDO) Nishant UAV.


See Through

Inevitably, new roles and operating modes are being studied for Sar systems. Systems operating at low radar frequencies (one GHz or below) can penetrate foliage, detecting targets which are invisible to EO and IR sensors. Under some soil conditions, they can even see into the ground. Schiebel Technology has promoted its Camcopter rotary-winged UAV as a potential aid to landmine detection and clearance. The company has described how a ground penetrating Sar tested on fixed-wing platforms can scan between 10 and 40 acres of terrain per hour, using an oblique angle technique that maximises the energy transmitted through the surface, reducing surface backscatter and improving the visibility of shallow-buried mines.

Another promising development involves fusing Sar data with that from other sensors. During trials conducted in 1997, a Lockheed Martin Small Tactical Synthetic Aperture Radar (STacSar) was installed in a surrogate Unmanned Airborne Vehicle (UAV) along with an L-3 Communication Systems - East Ultralight elint (Electronic Intelligence) receiver-processor subsystem. During missions flown over the Harquahala Plain near Phoenix, Arizona, the integrated system showed an ability to accurately identify and locate potential targets in real time more efficiently than individual systems. During searches for an RF emitter positioned in an unknown location by US Department of Defense personnel, the elint sensor dramatically reduced the search time to find a potential target and direct the aircraft or UAV over a target for imagery collection. "By improving and integrating several technologies, we can find the needle in the haystack in record time, resulting in dramatically improved mission effectiveness and UAV survivability", says L-3 Communication Systems - East president Greg Roberts.

Under a programme funded by the US Office of Naval Research, Metratek is evaluating the feasibility of offloading much of the Synthetic Aperture Radar electronics and data storage from the UAV, placing these in a ground station that can use inexpensive commercial off-the-shelf equipment to store and process the data. Up to 80 per cent of the major subsystems of the radar could be relocated in the ground station, says the company. Moving the functions traditionally handled by costly avionics into the ground station would reduce the volume, weight and power-consumption of the airborne hardware, and reduce the cost of the hardware which would be lost in the event of a UAV crash.

The concept relies on the UAV remaining in communication with the ground station, so is best suited to UAV's that remain within line-of-sight of the ground station. In the Processing and Control Station demonstrated by Metratek, GPS data from the airborne platform is used to steer high-gain dish antenna towards the aircraft, minimising data link transmitter power and eliminating an expensive range and angle tracking subsystem.



 Image-sharpening techniques incorporated in the Lynx radar provide a high-definition image of this group of main battle tanks. (General Atomics)

At a time when defence spending is tending to shrink, the Sar market seems relatively stable. In its Airborne Sar Forecast for 2000 to 2009, the Teal Group predicted that the total market for reconnaissance and surveillance Sar systems will remain at about $ 600 to $ 700 million per year from Fiscal Year (FY) 2000 to 2009. UAV Sar funding was forecast to more than double between FY00 and FY09, from $ 42 million to $ 104 million, and the report predicted that the Raytheon Hisar will become the largest UAV-related programme. However, group senior electronics analyst Dr David Rockwell, warned that "UAV Sars will still never capture more than 17 per cent of the total market.''

 

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