Global reborn

By Guy norris in Rancho Bernardo

Even to the fully initiated, the next-generation Northrop Grumman RQ-4B Block 20 Global Hawk will be at first sight hard to differentiate from its well-known predecessor when it is unveiled in Palmdale, California in August. Yet, packed into its bulbous fuselage, this remarkable unmanned air vehicle will carry one of the most sophisticated surveillance payloads ever to take to the skies, offering a 50% improvement in sensor range and capability.

Today’s Global Hawk is also dramatically different in other ways from the slender white UAV that first rolled out almost a decade ago as a high-altitude, long-endurance (HALE) UAV advanced concept technology demonstration (ACTD) vehicle. Then the Global Hawk was an unproven, unknown quantity, with sceptics on all sides. Now, the vehicle is battle-hardened, a veteran of conflicts in Afghanistan and Iraq, and is widely considered a highly valued intelligence, surveillance and reconnaissance (ISR) asset of the US Air Force.

Launched by the US Defense Advanced Research Projects Agency (DARPA) in 1995 with seven demonstrator air vehicles ordered, the first Global Hawk rolled out at the (then) Teledyne Ryan Aeronautical site in San Diego, California in February 1997. Tentatively building up flight-control software experience, the team conducted a first flight at Edwards AFB, California a year later. The second air vehicle (AV-2) became the first Global Hawk to fly with a sensor payload in November 1998. Military utility trials began in 1999.

The same year also saw the loss of AV-2 to an erroneous “flight termination” test signal that had been sent from Nellis AFB, Nevada, while a high-speed taxi accident at Edwards AFB set back AV-3 in September 1999. Despite these incidents, the fledgling UAV went on to prove its utility in military exercises, including Operation Linked Seas 2000, during which the UAV made the first crossing of the Atlantic Ocean and performed the first demonstration of air vehicle control from another continent.

The events paved the way for 2001 to be a milestone year for Global Hawk. Passing its Milestone 2 review by the Defense Acquisition Board (DAB) in February that year, the Global Hawk entered its engineering, manufacturing and development phase in March 2001. The following month it became the first UAV to cross the Pacific Ocean on deployment to Australia and in May the programme was awarded the Collier Trophy for the year 2000.

“We knew we were doing well, but then came 9/11 and that changed the whole complexion of the programme,” says Global Hawk USAF programmes integrated product team leader and director George Guerra. “Suddenly the government wanted to know how quickly we could deploy, and two months later we were in Afghanistan with some of the people on the original team deploying with the vehicle.”

Thrown into operations, the Global Hawk’s baptism of fire did not come without casualties. AV-5, the vehicle that had flown to Australia, crashed near its Persian Gulf base on 30 December 2001 after suffering a flight control push-rod failure, while AV-4 broke up during a diversionary landing after experiencing an engine failure caused by a stuck fuel valve.

Arguably the biggest mission yet for the vehicle was the war in Iraq, when AV-3 was deployed to support operations in 2003. During the coalition’s approach on Baghdad, the aircraft flew virtually around the clock, amassing a 95% mission effectiveness rate and at one point flying 40 missions in a row with no reported system problems. AV-3, also known as “Old Faithful”, eventually returned to the USA for the last time from operations on 20 February 2006 after being replaced by two production RQ-4As. On completion of its flight, AV-3 had built up more than 4,000h of combat time, logging more time than any single high-altitude UAV.

Spiral development

By now the Global Hawk was not only an accepted element of the force structure, but was on a well-defined “spiral” development path, having been selected as a “transformational” weapons system in 2001-2. This was confirmed in February 2002 when the US Department of Defense approved a plan to develop the Global Hawk’s capability in phases and, at the same time, directed the USAF to support US Navy proposals to use the UAV as a testbed for its Broad Area Maritime Surveillance programme.

Spiral 1 was aimed at “operationalising” the Global Hawk technology demonstrator, improving communications, speeding mission planning and addressing obsolescence issues. The capability was introduced with delivery of the first production aircraft, which became known as Block 10 versions of the RQ-4A. The first low-rate initial production (LRIP) contract worth $101 million was awarded to Northrop by the USAF the same month to supply the first of these two air vehicles. Just under a year later a further $307 million contract was awarded for Lot 2 LRIP of four RQ-4As, followed in February 2003 by a further $185 million contract award to build two more vehicles for the USN to support maritime surveillance demonstrations.

The first production-standard RQ-4A (an LRIP Lot 1 air vehicle) was delivered to USAF in August 2003, while the first Lot 2 air vehicle (AF-3) flew on 1 July 2004 and was delivered to Beale AFB, California on 28 October 2004. Earlier in the same month the initial US Navy Global Hawk Maritime Demonstration aircraft – designated N-1 – flew for the first time. This was delivered to NAS Patuxent River on 28 March this year after a 10h coast-to-coast ferry flight from Edwards AFB. The second is to follow later this year.

The USAF, meanwhile, awarded Northrop a $147 million contract in March 2003 to launch the spiral development programme leading to the RQ-4B configuration. This was designed to improve air vehicle performance by increasing gross weight, endurance, electrical power and payload and became the basis for Block 20. Sensor range was to be increased by up to 50% with an enhanced integrated sensor system (EISS) comprising an improved electro-optical (EO), infrared (IR) and synthetic-aperture radar (SAR) package, while signals intelligence was to be provided for the first time with the addition of a full signals intelligence (SIGINT) LR-100 package called ASIP. The combination of reconnaissance and signal snooping provides what the DoD refers to as a “multi-INT” capability. Spiral 2 also became the baseline for the naval Global Hawk, which was to employ maritime modes for the Raytheon EO/IR and SAR sensors and an augmented electronic support measures payload.

A $30.1 million contract covering funding for long-lead LRIP items for four Lot 3 air vehicles was awarded in June 2003, with three of the four being designed to the new RQ-4B Block 20 standard. A further $50.6 million contract followed for long-lead LRIP items for four Lot 4 RQ-4B air vehicles in March 2004. This batch includes the first aircraft to be equipped to Block 30 standard under the planned Spiral 3 development, which includes an initial self-protection suite, simultaneous sensor image recording, satellite communications via Inmarsat and high-band signals intelligence.

These improvements pave the way, in turn, for integration of the active electronically scanned array (AESA) Multi-Platform Radar Technology Insertion Programme (MP-RTIP) radar during Spiral 4. The next-generation sensor is being developed by Northrop and Raytheon.

Including the seven aircraft making up the Block 10 RQ-4A fleet, funded production is expected to cover six Block 20s (AF-8 to 13), 26 Block 30s and 15 Block 40s. First flight of the initial Block 30 is due in 2007 and the first Block 40 will fly for the first time in 2008. Initial operational test and evaluation of the Block 20 is currently scheduled to start in late 2008.

Northrop says the average unit cost (AUC) of the improved Block 20 configuration vehicle is $29.4 million, while the recurring air vehicle cost (on an AUC basis) falls slightly to $27.1 million for both Block 30 and 40 variants. The sensor package proportion of the total cost rises, however, with the increased sophistication of the more advanced payloads for the later blocks – all of which rise by 455kg (1,000lb) to 1,360kg against the 910kg payload of the current RQ-4A/Block 10. Block 20 will cost a total of $45.9 million per unit when the $16.5 million cost of the payload is included. Similarly, Northrop says the $15.4 million full ASIP SIGINT package in development for the Block 30 will increase overall unit cost to $54.2 million when added to the $11.7 million EISS already lined up for this variant. The MP-RTIP radar-equipped Block 40 will carry a unit cost of $66.3 million, of which more than $39 million will be associated with the sensor.

To support this spiral development, Northrop has been challenged with some significant design changes to the basic RQ-4A. “We needed greater ability to carry more payload. This basically grows from 2,000 to 3,000lb, so with a 50% increase in payload capability the question is where do you put it?” says Global Hawk chief architect Alfredo Ramirez.

The result, when the Block 20 is compared side-by-side with the earlier model, is a Global Hawk on steroids. The fuselage is stretched by a total of 2.1m (7ft), the wingspan increases 4.5m, the V-tail is slightly taller and covers a 14% larger area, and the fuselage is significantly deeper, with a more pronounced forward radome housing, denser centre fuselage section and wider splayed, stronger undercarriage.

Fuselage extensions

The stretch combines extensions in the forward, aft and centre of the fuselage with a nose stretch where the radome is blended into the fairing surrounding the nose-mounted video camera used for monitoring approach and taxiing. Further extensions were made around the mid-body forward of the wing at fuselage station (FS) 295, and around FS 100, where a new pressurised section was added to the aft equipment bay.

Block 20 continues the conventional aluminium alloy-based monocoque construction of the fuselage established with the original design. Although Teledyne Ryan considered an all-composite structure for the DARPA contest, the massive non-recurring costs of tooling for the relatively limited expected production run was a big factor in sticking to conventional materials. Ramirez adds that “one of the biggest things is minimising risk, and to do that we have to protect the sensors and equipment from the external environment. The aluminium is used to create a Faraday cage effect to protect the electronics, which are shielded from the radiation of the SAR as well as high-frequency environments of the type of airfields we operate it from.”

The fuselage also supports the aircraft’s single Rolls-Royce AE3007H turbofan, which is mounted above and aft. “We looked at multiple engines, a lower engine location and bifurcated inlets before settling on this,” recalls former Northrop vice-president for Global Hawk business development, and UAV pioneer, Norm Sakamoto. “We liked the engine on top because, if it is single engined, it had to be either inside the airframe or outside. If inside, it occupies around a third of the internal volume and you lose all that space for equipment. The other option was to put it in a pod underneath, and we didn’t want to do that in case we had to belly land – and engines are very expensive.”

The engine was chosen because of its “good heritage” (it is developed from a common core used in powerplants for the Lockheed Martin C-130J and Bell Boeing V-22, as well as being virtually identical to the variant powering the high-altitude, high-speed Cessna Citation X business jet) and “because it had the best specific fuel consumption/thrust performance at altitude we could find at the time. Plus, they had a lot of experience that gave us confidence,” says Sakamoto.

Aft of the engine, mounted either side of the exhaust, the Block 20 retains the original Global Hawk’s distinctive V-tails. Set at the same dihedral angle of 50°, the tails have a greater span of 3.8m (3.5m on the -4A), and have an aspect ratio of 3.1, while covering an area of 4.5m2 (48.4ft2). The design was chosen because two surfaces were “cheaper and lighter than three”, says Sakamoto, and “because we wanted to minimise parts count”, adds Ramirez. “Although vertical tail volume is good for directional control, we needed to keep it away from the ground, and if we put in a conventional single tail, we needed some sort of fancy S-shaped exhaust. That was too heavy,” recalls Sakamoto.

Loads from the extended glassfibre-composite cantilevered V-tails, made by Virginia-based Aurora Flight Sciences, are absorbed into the aircraft’s broad “boat tail” aft fairing. This is also made of lightweight glassfibre, like the extended “turtleback” fairing covering the larger 1.21m-diameter Ku-band satellite communications antenna mounted over the forward fuselage of the Block 20. The satcom antenna was sited forward to avoid distortion effects on the inlet, and to provide a counterbalance to the engine, as well as to give the best uninterrupted sky-viewing angles.

Higher sweep

Aside from its larger span and proportionate increase in chord, the wing shares the same aerofoil section as the original. “It is the same aerofoil, and has the same aspect ratio [25:1], but the sweep has been slightly decreased to 5°,” says Ramirez, who adds that the original RQ-4A sweep was 5.9° [measured at the 25% chord point – see Global Hawk technical description, Flight International supplement January 2001] “added to ensure you got the best balance point for centre of gravity reasons”.

Other than the span increase, the other significant external wing-related changes include a distinct 2° dihedral and the addition of trailing-edge conformal pods to enclose the widely spaced gear. “The dihedral was added because we had, at certain fuel states on the Block 10, fuel balance considerations on the runway,” says Ramirez. “So with the dihedral, this stops the listing tendencies we can get sometimes on the ground.” When the Block 10 is fully fuelled, the tips sag down by 0.3m, while the Block 20 is expected to be roughly wings level at a similar loading.

Structurally the lightweight wing, made by Vought Aircraft Industries, is essentially identical to the Block 10’s, but differs in the way it fits together, and in some areas where it has been given additional plies to add strength and toughness. It is constructed almost entirely from carbonfibre-epoxy composites with some interspersed aluminium alloy ribs. Running through the wing are four I-section shear spars that bear a closer resemblance to shear webs than conventional spars.

Unlike the Block 10 wing, which is designed in sections (a 15m-span centre, two 10m outer panels and wingtip assemblies), the Block 20 wing consists of two large centrally spliced inner sections and two 3.05m tip sections. “Because we have just one big piece of composite structure spliced at the centreline it improves the aerodynamic efficiency because we don’t have external splices,” Ramirez says.

Main gear changes

To improve the stability of the broader-span Block 20 on the ground, the design team also moved the main gear outboard and made it slightly taller. The undercarriage legs now retract aft into the conformal pods blended into the undersides of the trailing edge, rather than tuck up into the bays in the lower fuselage. “The big benefit is that it frees space in the belly,” says Ramirez, who adds that, although the new gear is a single-wheel oleo design rather than the twin main-wheel unit of the Block 10, “the functionality is the same. The actuation for extracting and retracting the gear is stronger, but it’s the same configuration as the Block 10, so there’s a lot of reuse.”

The original tricycle gear configuration of the Block 10 evolved from Teledyne’s similarly proportioned, 25m-span Model 235 (Compass Cope R) UAV developed in the mid-1970s. In the case of the Block 10, the main undercarriage is an off-the-shelf unit taken straight from the Bombardier Learjet 45, while the nose gear is a two-position unit from the Canadair CF-5F. With the move to the larger Block 20 the main and nose wheels and tyres of the Lockheed Martin F-16 are used.

More power

To meet the additional power requirements of the Block 20, most of it for the SIGINT and expanded EISS and LR-100 package, an additional 25kVA generator has been mounted to the accessory gearbox. “The engine had a spare pad, which we’re using,” says Ramirez, who adds that the generator is an adapted Boeing F/A-18 unit. In its original Block 10 configuration the radar, with a peak power output of 3.5kW and weighing 290kg, requires 4.7kW of 400Hz power and 1.3kW of 28V DC power, while the EO/IR system, weighing 100kg, requires just over 0.58kW of 28V DC power. Although the equivalent details for the Block 20 are classified, the power output of the radar transmitter is known to be increased beyond 3.5kW, while the other changes to the EISS system will not affect power requirements.

As well as the new 25kVA AC generator, the power requirements are met by a 28V DC generator on the engine. This develops around 10kW of DC power. A further 8-10kW is generated by a hydraulically powered AC generator. The system includes an inverter to convert power from DC to AC, and a transformer/rectifier to go from AC to DC. Three batteries provide back-up for up to 1h. Overall, the Block 20’s upgraded AC power supply (at 120V/400Hz) provides up to 17.3kVA for the sensors and 7.7kVA for communications/infrastructure against 6.1kVA and 3.9kVA respectively for the Block 10. Similarly, the DC power supply (at 28V) at 400A provides 1.14kW to the sensors on the Block 20 against 0.58kW on the Block 10.

To help keep the avionics and sensor system modules warm at high altitudes and cool at lower altitudes, air temperature is controlled in a pressurised section of the fuselage. Monitored autonomously by a Honeywell environmental control system built to Northrop specifications, the system uses the aircraft’s own fuel as a heat sink.

Fuel is fed though tubing in the leading edge to the outboard tanks and gravity-fed back to the centre fuselage tank. Two pumps feed the fuel to the engine and excess fuel, which is pumped around the equipment, goes to a fuel/air heat exchanger. “At altitude we need to warm it, and we do this by dumping bleed air to warm the fuel itself. This then pumps around the compartment and warms it up,” says Ramirez.

With the increased sophistication of the payload, and the need to meet evolving systems requirements during the spiral development, Northrop Grumman has taken the opportunity on the Block 20 to introduce an open avionics systems architecture. “This is not only for the multi-INT, but also positions us for other configurations in the future,” says Guerra. The new configuration introduces a sensor management unit that serves as the interface to a gigabyte Ethernet databus, allowing the use of commercial off-the-shelf software.

Open systems

“This allows us to do two things, it gives us flexibility to interface to other payloads and allows us to separate the mission functions from the flight-critical functions. Before they were integrated through the flight-control system [FCS] computers. Now we have computers solely devoted to flight control and sensor management,” says Guerra. As on the Block 10, the Block 20 has a dual-redundant FCS controlled by two on-board flight control computers that receive constant input from the aircraft’s suite of navigation and air data sensors. This includes an inertial navigation system, inertial measurement unit and global positioning system. All provide input to update the flight-control computers, which are pre-programmed with a flight plan before departure. The UAV navigates via GPS waypoints, and has several built-in default modes that can be activated.

Once airborne, the flight is controlled and monitored by the launch and recovery element (LRE) – part of a suite of command and control and mission-planning equipment developed by Raytheon. Communicating initially to the UAV via a line-of-sight (LOS) common datalink (CDL), and then by Ku-band and UHF satcom, the LRE hands over to the mission control element (MCE) for the surveilance mission. Ku-band and CDL links are mostly used for data transmission, including threat information and UAV status, while UHF is mostly used for command and control.

The Block 20 will offer operators increased capability with its EISS and SIGINT systems. The SAR antenna is housed in a bulged fairing immediately aft of the nose gear, and provides real-time imagery of the ground in several formats. With a field of regard of ±45° either side of the aircraft in azimuth and +/-20° in roll, the Raytheon X-band radar can cover up to 138,000km2 (53,300 miles2) a day in search mode from a range of more than 200km (110nm) in the Block 20.

“We’ve now got a larger antenna and increased the power of the transmitter in the radar to increase the range,” says Ross Mohr, Raytheon capture manager for Global Hawk systems. The changes, which include software modifications, have been made “on time, and on cost, and are in the same operational mode as part of the Block RQ-4A releases with the same levels of robustness”, he adds.

Radar detection

In ground moving-target indication (GMTI) mode the radar can search up to 15,000km2/min, detecting any targets with a ground velocity of 4kt (7.5km/h) or more from a range of 100km. With a 10m-range resolution, the GMTI mode scans a 90° sector, and can be used to cover zones between 20km and 200km either side of the aircraft. Operators can hand off detected targets to the SAR spot mode for more detailed viewing. Up to 1,900 images per 24h sortie can be made in spot mode. Each spot covers 2 x 2km and has a range resolution of around 30cm. In SAR strip mode, the 600MHz bandwidth radar can cover a swath 10km wide with three different depths and a resolution of 1m.

The Block 20's enhancements allow a 50% increase in payload

The Raytheon-supplied EO/IR system, mounted in the chin of the Global Hawk, combines a Recon/Optical camera with a third-generation Raytheon IR sensor. The EO system uses a commercial, 1,024 x 1,024 pixel Kodak digital silicon charge-coupled device (CCD) camera, while the IR sensor has a 640 x 480 pixel 3-5μm indium antimonide detector derived from Raytheon’s common-module forward-looking infrared (FLIR) system.

Both EO and IR sensors are fed by a fixed 1.75m focal-length reflecting telescope with a beam splitter and 254mm reflecting mirror that forms the main area of improvement in the Block 20. “We’ve not changed the camera unit. The enhancement is a change in the material of the mirror, which can be more finely polished, and the introduction of a real-time precise focus capability,” says Mohr.

Neither system has the 6,000-plus pixel width needed to provide the required 1m resolution in a single exposure, so the telescope scans continuously sideways while an internal mirror back-scans to freeze the image on the sensor. This “step-stare” approach means the mirror returns to the start point every one-thirtieth of a second, while the small patches are assembled to generate a larger picture. “This means the system has a 30Hz capability, which is practically a video,” he adds.

The entire system uses a gimbal mount derived from a Raytheon AAQ-16 FLIR turret that can roll ±80° or move ±15° in pitch and yaw. Stabilised to 3mrad, compared with the more normal 20mrad, the system can cover up to 104,000km2 a day in wide-area search mode, or generate up to 1,900 4km2 spots in spot mode. Dual-band coverage is provided in the visible (0.4-0.8μm) and IR (3.6-5μm) wavebands. Imagery from the sensor suite is sent via the Ku-band link originally developed to transmit at up to 50Mb/s, though projected performance now ranges between 1.5Mb/s and 47.5Mb/s.

Radar system

Exact data for the Block 20 radar system is classified, but the performance is likely to be similar to the original Block 10 ISS in terms of throughput. In this system the imagery output is around 30Mb/s, and can be compressed to 8Mb/s at 2bits per pixel. The Block 10 EO has a raw data rate of about 40 million pixels/s at 8-10bits/pixel, or up to 400Mb/s and can be compressed to about 40Mb/s using JPEG techniques.

This all adds up to a formidable package that will becoming increasingly available as an ISR asset as RQ-4Bs are gradually added to the inventory over the decade.

Source: Flight International