Inside Unmanned Systems

APR-MAY 2018

Inside Unmanned Systems provides actionable business intelligence to decision-makers and influencers operating within the global UAS community. Features include analysis of key technologies, policy/regulatory developments and new product design.

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32 April/May 2018 unmanned systems inside SPECIAL REPORT NASA TECHNOLOGY "which showed the potential for small satellites to maneuver and inspect others, and following this, I worked on PalmSAT, a very small satellite that could be used in swarms." At Surrey, Underwood began de- veloping a concept for satellites to dock together using electromagnets. In 2009, he presented this work in California, where he met Pellegrino, and "AAReST was born," Underwood recalled. A AReST will test some of the au- tonomous rendezvous, docking ma- neuvers and optical technolog ies needed for GOAT and other future modular satellites. It w ill possess two mirror segments that can detach from the core satellite and use butane thrusters to maneuver themselves onto new locations on the spacecraft. Electromagnetic clamps will help each segment dock. The "active" mirrors that A AReST uses each consist of thin smooth glass bonded to ceramic shells that are piezoelectric—that is they change me- chanically in response to electricity. A AReST's camera senses the shape of the mirrors and applies an electric field to alter their shape if needed, Pellegrino said. One diff iculty the researchers faced in manufactur- ing these mirrors was the miniature high-voltage electronics they needed were not commercially available, "and we had to develop them ourselves," Pellegrino recalled. To permit autonomous behavior, the researchers investigated two different kinds of proximity sensors—one us- ing a modified version of the commer- cial Softkinetic DS325 LiDAR camera, and the other employing a strategy Underwood developed involving infra- red LEDs and machine vision tech- niques. Although they found the LiDAR system worked well in subdued light, it was prone to getting blinded by the intense sunlight encountered in space, Underwood said, while the infrared LED-based system could work "even when the sun was in the field of view of the camera." In flight, AAReST will use a combination of both systems to give the best estimates of position and range. AAReST's control algorithms run on Raspberry Pi Compute modules, and a wireless radio link will connect its core satellite with its mirror segments so the core satellite can record and re- lay telemetry data during the mirror segments' autonomous maneuvers. "This link also allows commands to be sent from the CoreSat to the adaptive mirrors carried by each MirrorSat, so that their shape can be adapted to give a sharp focus in different configura- tions," Underwood said. Developing the electromagnetic docking system was much more dif- ficult than expected at the beginning, Underwood recalled. "The concept seems simple, but we found that ac- curately modeling the magnetic inter- actions between all the components proved to be very difficult," he said. To best test prototype docking sys- tems, the scientists relied on virtually frictionless air-bearing tables similar to those used for air hockey games to simulate how spacecraft might drift toward each other in the micrograv- ity environment of space. However, they found that with the initial tables they used, no matter how careful they were, the air jets in these tables gener- ated turbulent currents that disturbed the docking procedures. They later de- veloped their own air-bearing tables consisting of carbon strips with micro- scopic pores that can provide uniform airf low across their surfaces. The final version of the AAReST test bed is currently under development at the University of Surrey in England, the California Institute of Technology in Pasadena and the Indian Institute of Space Science and Technolog y in Valiamala. The spacecraft is due for completion in December, with a launch expected sometime in 2019, Underwood said. Autonomous orbital rendezvous and assembly missions can have many po- tential applications besides space tele- scopes. Asteroid mining could excavate elements rare on Earth's surface—a single 500-meter-wide asteroid might hold nearly 175 times Earth's annual platinum mining output, according to research from MIT. Satellites that collect solar power in orbit could beam that energy down to Earth in the form of microwaves, Underwood said, with- out facing the obstacles that ground- based solar power farms do, such as nighttime or clouds. Other possibilities include spacecraft that could repair ailing satellites, Underwood said, or remove dangerous space junk. Orbital assembly may indeed be the future of space. NASA is now examin- ing "the potential of in-space assembly and manufacturing for future NASA missions," Pellegrino said. SMALL-SATELLITE TECHNOLOGY CAN BE USED TO SENSE OUR WORLD AND TO LOOK BEYOND, OUT INTO THE SOLAR SYSTEM.Ó Craig Underwood, head of the planetary environments group, Surrey Space Centre Ò

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