A virtual antenna array, composed of multiple satellites sharing information, is one solution, and engineers at the Georgia Tech Research Institute (GTRI) have studied this approach. For the past three years, they have identified problems, developed solutions and ultimately concluded -- along with other research teams elsewhere -- that the concept has merit.

Now, the U.S. Air Force Research Laboratory (AFRL) is planning a flight experiment, dubbed TechSat 21 and scheduled for launch in 2006, to demonstrate a formation of three lightweight, high-performance microsatellites.

The formation will operate together as a "virtual satellite" with a single, large radar-antenna aperture.

Not only will the microsatellite formation find ground- based, moving targets, it can be configured for a variety of imaging, sensing and communications missions -- something not possible with a single, large satellite.

Missions include: precision geolocating; single-pass, digital terrain elevation data collection; electronic protection; single-pass, synthetic aperture radar imaging; and high data-rate, secure communications. The benefits of a microsatellite formation also include unlimited aperture size, greater launch flexibility, higher system reliability, easier system upgrade and low-cost mass production.

"If this system works, it will be unique," says lead researcher Bill Melvin, a senior research engineer in GTRI's Sensors and Electromagnetic Applications Laboratory. "There is no other concept like this."

While the concept has merit, TechSat 21 is still facing a number of hurdles, including properly timed wireless linking between the microsatellites, methods for calibrating system errors and reliable signal processing.

A team of four GTRI engineers, including Melvin and senior research engineer Daniel Leatherwood, continues to address these issues with algorithm development and modeling and simulation studies funded by AFRL. They are also helping with plans for experiments when the trio of microsatellites is launched in 2006.

AFRL researchers will gather data, including measurements to predict the system's performance using simulation code developed by GTRI engineers. Also, GTRI researchers are developing algorithms for processing that data, either in space or on the ground.

"The experiments will give us an opportunity to collect data for an extended period of time, not just for one day or one week," Melvin says. TechSat 21 will provide enough data to determine the data-sharing coherence between satellites, the timing requirements for communication and target detection capability, he predicts.

"Whether there is a residual military value from this experimental system will be revealed during the experiments," he adds. "Pressing needs may affect its missions."

Experimental systems have been deployed in the past when military needs arose. For example, during the Gulf War, the U.S. Department of Defense deployed JointSTARS, an airborne battle management system to conduct ground surveillance and support attack operations. JointSTARS detected the famous Iraqi retreat and guided the response of military commanders in the field.

"We want to automate as many processes as possible because the system will be gathering so much information," Melvin explains. "Certainly, the detection stage and switching between modes will be automated. Of course, the user could override the system."

Various technological improvements are making these and other TechSat21 capabilities possible. Advances in sensor technology, antennas, satellites, electronics and digital computing, as well as lighter-weight and more durable components, are contributing.

"Because of these advances, we can implement advanced algorithms and dream up new approaches that weren't even possible five or 10 years ago," Melvin says. "Technology is the driver of this project."

Given that, technology has placed GTRI engineers in the driver's seat. Leveraging their extensive experience in advanced signal processing and antenna modeling, they generated results for the AFRL within a year of the program's onset, and AFRL responded by increasing GTRI's role.

"We have focused on understanding the problems and developing suitable techniques to address them," Melvin explains. "Through modeling and simulation, we have determined the best possible performance for the system for ground moving target indication. So we know the concept has merit."

Only physics, not technology or research effort, will limit what TechSat 21 can do, he adds.

"TechSat 21 is a very non-traditional approach in radar," Melvin says. "There are a lot of doubting Thomases in the sensor community. So that makes it a challenge and makes it fun. We want to prove we can do it. Our task will be to convey to others that it can be done."

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