Laser-Based System Could Expand Space-to-Ground Communication
A new research project announced recently as a collaboration between the Georgia Institute of Technology and satellite communications provider Xenesis could help open the bottleneck that now limits the flow of data from Earth-orbiting satellites to ground stations.
The project will miniaturize, space-qualify, and test a laser communications transceiver that could dramatically expand the bandwidth available for downlinking information from the growing number of satellites — and future constellations of space vehicles — in low Earth orbit. Xenesis licensed the technology from NASA's Jet Propulsion Laboratory (JPL), and will work with Georgia Tech and JPL to mature it for use as a primary communication system for satellites as small as CubeSats.
The NASA Technology
The JPL-developed system is a compact, low-cost laser communications transceiver that surpasses the severe spectrum-allocation and bandwidth limitations of conventional radio-frequency communication systems. The innovative design reduces complexity, size, mass, and cost by using readily available flight-grade parts for the compact optics assembly and high-capability electronics assembly. JPL's laser communications transceiver can uniquely and inexpensively satisfy the high-bandwidth communications needs of Earth-orbiting spacecraft.
The laser communications transceiver comprises two primary modules: an optics module and an electronics/laser module. The optics module includes a 5-cm-diameter telescope; a two-axis, coarse-pointing gimbal; monitoring sensors; and thermal control. The electronics module includes a transmitter, processor, controllers, and power conditioning.
Keeping optical uplink rate modest and emphasizing downlink, the high-bandwidth downlink transmitter uses coarse wavelength-division-multiplexing for operation at four 2.5-Gb/s channels (a total data rate of 10 Gb/s). Applying this technique enables the use of larger active-area photodetectors at the ground station, which reduces the atmospheric scintillation/turbulence effects on the received beam. These effects are further reduced with forward-error correction and deep-interleaver codes.
A compact laser communications transceiver with a single transmit/receive aperture has been built using components with traceability to flight qualification (i.e., a flight-qualified version is commercially available). The transmit downlink wavelengths fall within the standard C-band telecom grid of EDFA fiber amplifiers (1530 to 1560 nm).
Mark LaPenna, CEO of Xenesis, compared the benefits of the planned space-based network to the jump in performance from terrestrial dial-up connections of the 1990s to today's high-speed broadband services.
“Xenesis recognizes the need for a global communications revolution, and we plan to empower space with an optical product called XenHub,” LaPenna said. “Through this architecture, any company, mission, or global operator on the ground or in space will be able to compete on a level playing field for the first time since Sputnik.”
“We expect to significantly add to the total bandwidth of information that we can get down from space, and the more bandwidth we have, the more information we can exchange and the more value we can get from satellite networks,” said Brian Gunter, an assistant professor in Georgia Tech's Daniel Guggenheim School of Aerospace Engineering, who will be leading the project.
Gunter's lab has experience with small satellites and will apply that expertise to the project with Xenesis, which signed a $1.2 million contract to support the work. Georgia Tech's contribution will be to miniaturize the original JPL technology, update the control software, space-qualify all the hardware, and test the improved system from space — likely from the International Space Station.
“With all of the satellites that are going into space, everything from Cube-Sats to major satellites, there is more information being generated than can ever be downloaded,” said Dennis Poulos, chief technology officer at Xenesis. “Most of today's systems depend on radio frequency downlinks, and there is just a limited amount of bandwidth available for use.”
Laser-based systems can expand that bandwidth to beyond 10 gigabits per second, Poulos said. In addition to boosting bandwidth, optical systems can use smaller antennas, use power more efficiently, and provide better data security.
Though it is subject to interference from clouds, the laser system will benefit from producing a narrow beam that can travel farther than comparable radio frequency transmissions at the same power level. The initial focus will be space-to-ground communication, though the system could also be used for cross-linking communication between satellites. The small antenna size is also more suitable to the small-form satellites envisioned for future constellations that may include thousands of spacecraft.
“Once we can show that this works from space to ground, that will demonstrate that the technology can survive the harsh environment of space and allow us continue the development of the transceiver for commercial use,” Gunter added. “This has the potential to open up a range of new capabilities, including the ability to provide high-volume data services to anywhere in the world.”
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