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Persistent airborne surveillance and reconnaissance over a specific area of interest is becoming recognized as a significant capability that is highly valued but difficult and expensive to attain using existing assets. Current and evolving communications architectures require persistent over-the-horizon (OTH) communications connectivity, greater communications bandwidth, and intelligent routing that cannot be affordably and rapidly accommodated by existing assets.

Military planners have articulated that this demand for persistence far exceeds the existing and planned capabilities of conventional airborne and satellite systems. Current fossil-fueled aircraft, manned and unmanned, require complex operations and numerous flights to maintain a continuous presence. Because of the lack of true affordable persistence, it is extremely difficult to conduct forensic intelligence operations. Satellite coverage at most latitudes depends on orbiting satellites with known revisit times, permitting the enemy to hide and avoid detection. Satellite bandwidth is at a premium and many surveillance, reconnaissance and communications systems cannot be optimized due to higher-priority communications conducted via satellite. A further limitation of satellite bandwidth results from operating at high orbits.

A system that could provide persistent and affordable coverage at the “near space” altitude of 65,000 feet could dramatically reduce logistic costs associated with current aircraft operations, eliminate the inherent bandwidth limitations of satellites, and seal the gaps and seams of both. The subsystems required to field such affordable persistent intelligence, surveillance and reconnaissance (ISR) and high-bandwidth communications systems and their respective development statuses are shown in Table 1.

If the benefits of persistence in the stratosphere are so obvious and everything but an appropriate platform is available now, what's the problem? The first challenge is simply staying there. Figure 1 shows that the air pressure in the stratosphere is less than 10% of the air pressure at sea level, which is a significant aerodynamic and propulsion system problem. The second challenge is staying there while keeping station over the desired area. Specifically, 60-knot to 100-knot winds must be regularly overcome in order to stay over specific areas of the world. The third challenge is affordably staying there over a specific area for extended periods. Even a platform that could remain airborne for more than 24 hours would still incur financial and logistical penalties when providing persistence in the stratosphere. True persistence in the stratosphere would require a much greater endurance at much lower operating costs.

A cursory examination of these issues quickly concludes that the solution is not a conventional manned or unmanned air-plane. One option might be an unmanned lighter-than-air vehicle when one can be developed and deployed that can fly faster than the 60-knot to 100-knot winds regularly experienced in the stratosphere and the ground-handling problems of these large vehicles can be solved. Another option is an unmanned aircraft when one with efficient propulsion systems and low Reynolds number aerodynamics can be developed and deployed that can affordably carry a reasonable payload for more than a week in the stratosphere.

Pursuing the latter option, AeroVironment (AV) has been involved for the past two decades in the development of extreme persistence, high altitude long endurance (HALE) unmanned aircraft systems (UASs) and the key technologies required to make these unique vehicles practical operational systems in the stratosphere. Six years ago, AV determined that customer requirements dictated that the first operational HALE UAS would have to use a liquid hydrogen fuel propulsion system due to the high specific energy content of liquid hydrogen. Since that time, AV has focused its development efforts on the additional key technologies, primarily a lightweight liquid hydrogen tank and ultrahigh-efficiency electric generation and motor drive systems, required to deploy practical HALE UASs in the stratosphere. During the development process, AV aircraft have broken many world records for high altitude flight.

The last of the key technology work was completed two years ago, and practical liquid hydrogen-fueled HALE UAS flight and ground operations were demonstrated last year. AV's efforts have resulted in the detailed design for the Global Observer (GO) UAS shown in Figure 2.

AV's GO UAS, carrying up to 1000 pounds of payload at 65,000 feet for more than one week continuously between landings, can eliminate current and future surveillance and communication architecture “gaps and seams.”

Existing surveillance systems that can be carried include the following types: electro-optic/infrared (EO/IR), synthetic aperture radar (SAR), communications intelligence (COMINT), signals intelligence (SIGINT), and/or measurement and signatures intelligence (MASINT). Existing communications systems that can be carried are capable of providing multi-Gigabit per second total throughput, multiservice interoperability, and/or equipment to enable future DoD Internet protocol-based systems, as well as other network-centric protocols in development. To accommodate these various payloads, the GO has been designed for plug and play” compatibility with multiple payload types, configurations and open architecture.

No known alternative technologies can cost-effectively provide this endurance and capability now in the stratosphere. Stratospheric airships may one day meet the requirement and carry heavier payloads when propulsion systems and extremely large vehicle technology can advance to the point where airships can keep station in high winds, often in excess of 60 knots to 100 knots above 60,000-foot altitudes. The global wind data in Figure 3 shows the probabilities of wind speeds at 65,000 feet at various latitudes. This chart shows that because the GO UAS has a loiter speed for maximum endurance of approximately 110 knots, it can station-keep in more than three sigma winds, or more than 99% of the time, at all latitudes and still attain its predicted performance.

The extreme endurance of the GO UAS is provided through a practical propulsion system that uses liquid hydrogen as a fuel. Ground handling, the storage of liquid hydrogen fuel on board the aircraft, and optimal use of hydrogen for the propulsion system are technologies successfully proven by AV with the flight tests of the GO prototype in May and June of 2005. The unmanned GO prototype aircraft shown in flight in Figure 4, the world's first liquid hydrogen-powered UAV, operated entirely on liquid hydrogen fuel and was flown under manual control and autonomous waypoint navigation.

The status of the other enabling aircraft and propulsion technologies required to field an operational GO UAS are shown in Figure 5. The red “high altitude” line shows that GO UAS aircraft and electric propulsion technologies have been flown numerous times in the stratosphere and were used in many world-record flights, including the flight of the Helios UAS to 96,863 feet. The blue “long endurance” line shows the various hydrogen storage and propulsion systems used in the GO UAS that have all been successfully tested in actual and simulated operational environments of 65,000 feet.

AV developed and tested a full-scale air compression system, multiple hydrogen power plants, and a lightweight liquid hydrogen tank for the GO UAS. As part of the Lockheed Martin team contracted to develop the Missile Defense Agency's High Altitude Airship, AV has developed and demonstrated the world's highest-power ironless core electric motor, whose characteristics include part load efficiencies greater than 98% and the highest torque-to-weight ratio of any known ironless core motor. Similar motors will be used on the GO UAS.

Telecommunications and remote-sensing capabilities have already been demonstrated on Japanese/NASA/AV test flights. The world's first high-definition TV and 3G-mobile high-altitude relay applications from 65,000 feet were demonstrated using payloads developed by Toshiba and NEC and off-the-shelf HD TV sets and mobile handsets.

The extreme endurance of the GO UAS results in a very low operational tempo and low operational costs with a minimum logistics tail. This ensures an affordable system in the operations phase. Conventional airborne (manned and unmanned) systems with relatively low endurance (one to two days) have high operational tempo that require significantly more manpower and operations funding due to many more take-off/flight operations/landing cycles. Figure 6 shows flights and tons of fuel necessary for a scenario requiring a 24-hours-per-day, 365-days-per-year persistence in the stratosphere with 1500 nautical miles distance from take-off to the area of interest.

Projected benefits in an application enabled by the extreme duration and fuel efficiency of AV's GO UAS vs. a typical fossil-fueled UAS are summarized below. These benefits are based on the same mission described above that requires a 24-hours-per-day, 365-days-per-year persistence in the stratosphere with 1500 nautical miles from take-off to the area of interest.

• 6x longer flight duration (over one week vs. just less than 1.5 days);

• 2x fewer aircraft plus payloads for continuous station-keeping (two vs. four platforms);

• 8x fewer take offs and landings (60 vs. more than 500 flights per year);

• 50x less fuel consumed (less than 100 vs. more than 4000 tons per year); and

• 4x lower life cycle cost (annualized acquisition plus operations costs).

The GO's significantly lower projected system and operating costs over conventional fossil-fueled UASs result in cost advantages even when taking into account differences in payload carrying capacities — specifically the typical GO payload capacity of up to 1000 pounds vs. fossil-fueled UAV capacity of 2000 pounds to 3000 pounds. Using a theater bandwidth gap filler communications relay application as an example, GO's cost per Mbps is a fraction of the cost per Mbps of satellite alternatives, enabling the offloading of limited satellite bandwidth to address other priorities.

For normal military operations the air-craft, ground support systems, and operations will be easily conducted from a site with a normal runway (150 feet × 5500 feet). Hydrogen generation and liquifaction systems are commercially available for on-site production of liquid hydrogen. Alternatively, liquid hydrogen can be easily shipped to most locations in the world. (More than nine million tons of hydrogen are produced per year in the United States alone).

The ground control/communication station uses a variant of systems currently being used for other military UASs. Hangar space is easily satisfied with commercially available low-profile structures. The aircraft would normally take off and climb to altitude while cruising to the area of interest, and then maintain orbit until another aircraft replaces it approximately a week later. See Figure 7 for a typical GO UAS mission scenario.

After landing, minimum maintenance is anticipated due to the simplicity of the composite airframe, electric motors, and other systems as compared to conventional aircraft. Most systems will be repaired by replacement and the failed system sent to a depot maintenance area for repair.

A GO UAS can supplement and complement current assets and capabilities. Of particular note is the fact this capability can complement existing UAS operations, such as Global Hawk or Predator, by acting as the over-the-horizon relay for the data being transmitted by these assets or as a gap-filler ISR platform when needed by operational commanders. Since the GO UAS will accommodate rapid reconfigurable payload swap out, this is just one of many examples of what will be possible when the GO UAS is deployed.

In addition to the obvious military mission scenarios and capabilities discussed and shown previously, the GO UAS holds tremendous commercial potential and benefits for private and other DoD and non-DoD governmental applications including:

• persistent, global, loitering capability in the stratosphere for defense and homeland security missions;

• low cost, rapidly deployable telecommunications infrastructure and GPS augmentation;

• hurricane/storm tracking, weather monitoring and wildfire detection/support; and

• environmental monitoring, agriculture optimizing and aerial imaging/mapping capabilities.

In the early days of the aviation and satellite industries there were many who questioned the value of these paradigm shifting systems and capabilities. Eventually, the significant value of these systems was recognized and capitalized on by those with vision and imagination. The Global Observer UAS is a paradigm-shifting system that will allow the regular use of the stratosphere for many critcal and high-value government, civil and commercial missions and applications. Many of the unique capabilities enabled by such a system are yet to be imagined. As the development and deployment of the Global Observer UAS moves forward, there will be opportunities for the members of the defense electronics industry to use their imagination and capitalize on this paradigm shift that will result in the routine use of the stratosphere for these capabilities. Soon, designers of military systems will be seriously thinking about what they could do with a mobile, 12-mile-high tower.

Ted Wierzbanowski is the managing director at AeroVironment. A graduate of the Air Force Academy, he is a retired USAF Colonel who was the first USAF X-29 test pilot and who also worked on many other advanced technology aircraft during his military career.

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