Airborne Sensor Sees Multitude of Threats

January 2007
By Henry S. Kenyon

The Defense Advanced Research Projects Agency (DARPA) Integrated Sensor Is Structure (ISIS) program seeks to develop airship-based airborne sensors capable of detecting low-flying aircraft and dismounted infantry from hundreds of miles away. ISIS is developing a variety of materials technologies to incorporate a large aperture radar sensor into a high-altitude robotic airship.
High-altitude airship-based radar will track small craft, people over many miles.

An experimental airborne radar system currently under development may improve tracking capabilities for hard-to-detect airborne and ground targets. The radar would be lofted aboard a robotic airship to float tens of thousands of feet over a region, providing comprehensive and persistent surveillance for up to a year. A key advantage of this high-flying system is that the radar antenna would be a part of the airship’s structure, turning the entire platform into a sensor.

The major technical function of the Defense Advanced Research Projects Agency (DARPA) Integrated Sensor Is Structure (ISIS) program is to explore potential solutions for difficult radar tracking issues. According to Timothy Clark, ISIS program manager, Arlington, Virginia, the most complex aerial targets for airborne radar to detect and track are ground-hugging cruise missiles and dismounted infantry.

Clark notes that in current radar systems, the smaller the aperture, the higher the power required for scanning. More power equates to better search capabilities but not increased tracking performance. Pumping power into a small antenna creates heat, requiring cooling systems that add weight and bulk to an airborne platform, but “there’s only so much space that you can set aside on a fighter for a radar,” he says.

Large radar apertures have many advantages over smaller systems, specifically in fidelity and transmission power. “As you grow the aperture, the amount of transmit power decreases, but your performance goes up enormously,” he explains.

DARPA scientists, exploring ways to produce large apertures capable of covering an extremely wide geographic area and detecting both air and ground targets, found that high-altitude airships would offer solutions to aperture and power issues because of their large size. The ISIS effort seeks to develop a very large aperture radar that uses little power.

Clark notes that high-altitude airships originally were designed to contain a small payload bay for sensors. However, engineers determined that a large aperture radar would be too heavy for an airship if it was installed conventionally. The decision to integrate the sensor into the airship’s structure led to the ISIS program.

Positioning sensors in an airship’s structure reduces weight. Designers are still mulling the choice of placing the aperture on the outside or the inside of the airship. “There are real advantages for putting it on the inside. The outside has some issues, so we’re moving away from that,” Clark says. One advantage of situating the aperture internally is that the system is protected from the environment. This placement also avoids problems such as bonding aperture material to the outside of the aircraft, which exposes it to the environment.

Two firms developing feasibility designs for the program’s first phase are Lockheed Martin, Palmdale, California, and Northrop Grumman, Baltimore. Clark notes that in the first phase, industry teams examined basic issues, including the system design, the aperture power necessary for the mission, the mass and the technology developments required to make the program viable. He explains that the design groups also are responsible for ensuring that the systems in development are on the right technical path. “We don’t want to be developing technologies that are going to peter out after five to 10 years and never reach the end goals that we’re looking for,” he says.

The high-altitude, or stratospheric, airship will possess a confluence of technologies that work well for airborne surveillance. It will have a large surface area, which is good for a radar system. The aircraft will be very slowmoving, which is advantageous for moving-target indicator radar. “It’s effectively a ground-based radar at 70,000 feet,” Clark says. Operating at altitudes between 60,000 to 75,000 feet, where winds are at a minimum, will allow an airship to keep station without using much power.

An ISIS radar on a stratospheric airship will be able to detect aircraft at a range of 373 miles. “If you were sitting over Baghdad, you’d pretty much see all of Iraq and into neighboring countries,” he says. Terrain such as mountains and ravines can create grazing angles—areas where terrain blocks radar scans and provides areas for ground units to hide—but the platform’s high altitude will mitigate such problems, he says. From 60,000 feet, ground tracking will be effective out to 186 miles.

The airship’s radar will have a dual system operating in the ultrahigh frequency and X bands. It will conduct wide-area searches for both air and ground targets, penetrate foliage and provide ground moving-target indicator functions. Clark notes that standard radar antennas cannot perform multiple functions because the antenna determines the system’s resolution. But the ISIS antenna will be so large that its resolution will be very good for tracking small targets.

The ISIS radar system will consist of groups of electronically scanned arrays built into the airship’s skin. The arrangement is designed to provide the airborne equivalent of a ground-based radar.
The radar will be an active, electronically scanned array system. It will be spaced to allow different parts of the aperture to scan and will be divided into subarrays that permit sufficient bandwidth during scanning. Clark notes that the designs provided by both subcontractors allow the system to group subarrays independently. This facilitates the rapid creation of a variety of apertures for aerial and ground surveillance.

The ISIS program is intended to operate without in-theater ground support. The aircraft will launch from the United States and stay aloft for 10 years, moving to where it is needed across the globe. The program is designed to keep the airship on station for one year. “If I wanted to park it over Baghdad for a full year, regardless of what the winds aloft were like, it [the airship] would be sized for that,” he says.

Management of such airships would resemble satellite control because the spacecraft are operated to conserve fuel for maneuvering, Clark explains. Likewise, stratospheric airships will preserve fuel by remaining stationary and by moving only to avoid storms large enough to affect the stratosphere. Because the radar’s ground footprint will be so large, the aircraft could be moved many miles out of a storm’s path and still maintain coverage of an area. “The goal is not to be tethered to the ground in any way—virtually or physically,” he emphasizes.

The stratospheric airship envisioned for ISIS will be 500,000 to 1 million cubic meters in size, many times larger than current airships. The size will depend on how ambitious the contractors are with their designs, he says. Several potential airship designs are possible, ranging from a traditional cigar shape to a lenticular, or lens, shape. Clark notes that while volume provides lift, the airship’s shape will affect issues such as aerodynamic drag, placement of solar arrays and the radar apertures.

A major design challenge for airships is the relationship between mass, volume and power. “The mass drives volume, and the volume drives the power, which leads back to the mass,” Clark says.

But not much can be done about volume. Hydrogen and helium are the two most effective lifting gases, while a vacuum-based system would be heavier because it would require stiffening the structure. “We are focusing on mass and driving things out of that. It turns out that power is important because it has a secondary effect on the amount of mass,” he says.

Clark explains that less power required for the airship means less area that must be covered with solar panels, allowing the aircraft to reflect more heat during the day, reducing pressure from the day-night cycle of expansion and contraction. This temperature savings permits designers to relax hull material requirements, allowing the hull to be thinner.

For the mass of the hull aperture material, the current state of the art is 400 grams per square meter. But the material’s ultrahigh molecular-weight polyetheline fibers tend to elongate in such a way that they can rupture over time. This elongation is caused by manufacturing anomalies that create amorphous noncrystalline areas in the fibers’ crystalline structure.

Eliminating these imperfections allows the material’s mass to be reduced to 100 grams per square meter, ensuring a 10-year lifetime while maintaining 85 percent of the material’s strength. Clark adds that this weight also factors in protective coatings to counter ultraviolet radiation, ozone and large temperature swings. He notes that at 60,000 feet, the normal temperature range is between –40 degrees during the day and –80 degrees Celsius at night. “That means the hull material has to remain pliable down to at least –90 [degrees Celsius]. You don’t want it freezing and becoming brittle on you,” he notes.

At the same time, the material’s melting point must be relatively high to permit other equipment, such as the antenna aperture material and solar arrays, to be bonded to it. Clark explains that anyplace the material is bonded to another substance or anywhere the component itself will reach a high temperature, the material must not lose its strength and begin to deform.

The present state of the art for the aperture’s mass is 20 kilograms per square meter of antenna. The ISIS program estimates about 1,600 square meters of projected aperture or about 6,000 square meters total across the airship. But a material weighing 20 kilograms per square meter will be too heavy. “We are looking for an order of magnitude change in that, around 2 kilograms per square meter,” he says.

Clark notes that there are two different approaches for a lightweight aperture. Contractors Raytheon Company, El Segundo, California, and Northrop Grumman, Baltimore, are each developing unique materials concepts. In the next two years, both companies must demonstrate a square meter of aperture bonded to the hull material that is less than 2 kilograms per square meter, and the aperture must consume less than 5 watts per square meter in receive mode. Clark explains that because of the frigid temperatures at the airship’s operational altitude, it is important to keep the aperture at a constant temperature to allow it to scan in any direction. One way to achieve a steady temperature is by keeping the low-noise amplifiers continuously turned on; however, they must be able to draw practically no power when they are in receiving mode.

Powering the airship is another challenge. Clark notes that no fully regenerative power system has been designed for airships. The separate Extremely High-Altitude Airship program was looking at a fuel-based solution because the energy density for collection and storage was not available. DARPA approached Lockheed Martin in Denver to develop the power plant. In two years, the goal is to demonstrate 400 watt hours per kilogram of delivered overnight power divided by the entire mass of the power system. This mass includes solar arrays, cabling, fuel cells, electrolyzers and storage tanks. Clark believes that no one has ever attempted this kind of energy conservation. “We’ve got a minimum [requirement] of 400 watt hours, and that’s just a design path to 800 watt hours a few years away,” he says.

The technology phase of the ISIS program will end sometime during the first fiscal quarter in 2008. The current set of contracts was awarded in April and May of 2006. If the program continues to meet its minimum operational requirements and still fits within the system’s concept, DARPA will continue to move the effort to a demonstration phase. The demonstration aircraft will be a one-third scale version of the airship and capable of remaining on station for several months.

The demonstrator will not have a full-scale radar, but it will have enough of an aperture to prove that it can calibrate and compensate for scanning and form beams for transmission and receiving. Clark adds that DARPA is in discussions with the military services to continue research on radar processing and transmission modes to make the airship a testbed. If the program moves forward without any major setbacks, Clark predicts a test flight in 2010 or 2011.

Web Resources
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