Stratospheric Aircraft Ready to Soar

December 2007
By Henry S. Kenyon

Zephyr is a high-altitude, long-endurance unmanned aerial vehicle (UAV) designed to
operate as a reconnaissance or communications relay platform. The solar-powered aircraft can remain on station for several days.
High-flying unmanned platform offers possibility of airborne radio relays and long-term sensing.

A solar-powered, high-altitude robotic aircraft may soon provide warfighters, scientists and first responders with imagery, sensor data and extended communications links. The lightweight, long-endurance airplane is designed to remain on station, many thousands of feet over a region, for weeks or months at a time.

Unlike most unmanned aerial vehicle (UAV) efforts, the Zephyr program was designed to support a private endeavor—an attempt to break the manned balloon altitude record in 2003. This effort saw two balloonists seeking to ascend to 132,000 feet, a height reached only by rocket-powered experimental aircraft and the space shuttle. The balloonists asked the Farnborough, United Kingdom-based firm QinetiQ to design a lightweight UAV to fly around the balloon and film it as it ascended. The record-breaking mission was never launched, but Zephyr was built.

Since then, the program has evolved from a publicity-oriented vehicle into a surveillance and communications relay platform to meet intelligence, surveillance and reconnaissance mission requirements for the United Kingdom’s Ministry of Defence (MOD), says Paul Davey, QinetiQ’s development director for Zephyr. The company also is negotiating with the U.S. Defense Department to develop and use the aircraft. “Its prime roles are surveillance and communications relay from high altitude,” he shares.

Designed to operate at altitudes in excess of 60,000 feet and to rely on solar power, Zephyr must be as light as possible. With a frame made of a carbon fiber composite material covered by a Mylar skin, the UAV weighs only 66 pounds. Although it has a 59-foot wingspan, its light weight allows it to be manually launched by three people. Davey notes that Zephyr does not require a fixed runway, only a flat surface roughly 25 yards long for the ground crew to run along and loft the aircraft into the prevailing wind. At the UAV’s groundstation, a pilot navigates the aircraft to a safe altitude before activating the autopilot. Once in the air, the groundstation’s navigational computer directs the aircraft to travel on a pre-determined course set by waypoints.

Zephyr is designed to fly continuously for months at a time. Recent flight trials have tested the aircraft’s electric power system, which uses solar cells to collect power and stores the energy in rechargeable lithium sulfur batteries for use at night. The flight tests over the White Sands Missile Range in New Mexico demonstrated that the aircraft can fly through two 24-hour day/night cycles. “We’re at the point where we can fly an aircraft on solar power during the day, store power into the battery and then discharge that power during the night to keep the aircraft flying,” Davey explains.

Zephyr’s solar power is provided by thin-film photovoltaic cells bonded to the top of its wing. The solar cells are conformal to the wing structure and do not interfere with the aerodynamics. The solar array generates 1.5 kilowatts of power and weighs about 3.5 pounds. Davey notes that the collected solar power runs the engines, recharges the batteries and services the UAV’s systems and the payload. “We’re not power limited. The array can generate more power than we can use at certain times,” he says.

Davey believes that the White Sands demonstrations are the first time a solar-powered unmanned aircraft has operated to its full capabilities. During the most recent tests in July, Zephyr nearly broke an unofficial endurance record by flying for 54 hours at 58,355 feet. Davey notes that the QinetiQ team was not seeking to break a record when it undertook the trials. “We weren’t really prepared for the whole process of trying to have the records officially accepted,” he says.

The existing official UAV endurance record is 30 hours, held by Northrop Grumman’s Global Hawk. However, Davey adds that the Boeing Company’s Condor UAV flew nonstop for some 58 hours several years ago and that this event also was unrecorded. “There are various unofficial records, but the official record is only 30 hours, and obviously we surpassed that by some distance. But it is important to note that the Israelis and Boeing have flown UAVs for very long durations,” he says.

One of Zephyr’s mission goals is to serve as a high-flying communications relay. The UAV’s high operating altitude places it well above the jet stream and most weather and commercial air traffic. Other firms are attempting to achieve this capability with unmanned airships. Davey explains that airships normally carry larger payloads than UAVs, but adds that airships also are more expensive to operate. Zephyr is designed as a lightweight, low-cost alternative to larger, heavier platforms. Compared to the thousand pounds that an airship can potentially loft, the lightweight UAV has only 5 to 6 pounds available for payload.

Constructed of carbon fiber composites, the Zephyr UAV can be hand-launched by three people. The lightweight aircraft can reach altitudes in excess of 60,000 feet.
Although the aircraft is constrained by a small payload, it has flown several communications relay missions at White Sands. Davey notes that a relatively simple relay unit can transmit ground radio communications to other ground-based radios over a large distance. He maintains that the relay and surveillance tests conducted to date have been successful.

Zephyr has a self-contained payload system. An electro-optical sensor has been tested for reconnaissance applications. The sensor package has its own integral global positioning system for navigation and target location. QinetiQ also is examining the use of a synthetic aperture radar (SAR) system for Zephyr. But Davey says this development is still at a tentative stage. However, he notes that QinetiQ engineers believe they can design a lightweight SAR of 11 pounds or less.

Zephyr flies on a pre-programmed mission track. New waypoints can be uploaded to the aircraft while it is in flight. Davey explains that this automated flight capability leaves the ground control personnel free to operate the payload. The firm is developing a desktop simulator for operator training. “They [the operators] can point the payload wherever they want while the aircraft flies on autopilot. That’s what we were doing in the recent trials,” he says.

Over the last several years, Zephyr has flown three sets of trials, with the most recent ones held in July. Based on results from the recent trials, QinetiQ engineers are modifying the UAV with the goal of extending the aircraft’s flight endurance to several months. Davey says that flight trials will continue through 2008 with the aim of transitioning to pre-production status by 2009. The company will then begin work with a production partner to manufacture the aircraft in volume. Although he cannot comment about the current status of the contract with the MOD, he shares that the firm is examining steps to transition to what he refers to as an equipment program.

Besides military applications, a Zephyr UAV has been provided to a Belgian research agency. The aircraft will be used for high-altitude remote sensing. Davey adds that the Belgian scientists will produce their own sensor package for their UAV.

QinetiQ has a joint capability technology demonstration with the U.S. Army, with trials scheduled for the summer of 2008. Davey notes that Zephyr’s underlying solar cell, high-endurance technology is being used in similar programs, such as the Defense Advanced Research Projects Agency’s (DARPA’s) Vulture UAV effort. He says that QinetiQ is hoping to participate in the Vulture program because Zephyr meets key DARPA platform requirements for low-weight, high-endurance, solar-powered systems and lightweight communications systems.

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