Photonics to Play Key Role in Spaceflight Communications

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Making space communications more efficient for both near-Earth and deep-space missions is a priority for NASA, and photonics may provide the solution. Laser communications could significantly improve data rates in all space regions, from low-Earth orbit to interplanetary.

After more than 50 years of relying solely on radio frequency (RF) to send and receive data, several centers across NASA are experimenting with laser communications, which has the potential to provide data rates at least 10 to 100 times better than RF. These higher speeds would support increasingly sophisticated instruments and the transmission of live video from anywhere in the solar system. They would also increase the bandwidth for communications from human exploration missions in deep space, such as those associated with Journey to Mars.

Artist concept of satellite relaying data from Mars to Earth via laser.

Artist concept of satellite relaying data from Mars to Earth via laser. Courtesy of NASA's Goddard Space Flight Center.

NASA began using lasers for satellite laser ranging, a technique to measure distances, in 1964. The laser ranging system at Goddard Space Flight Center uses laser stations worldwide to bounce short pulses of light off reflectors installed on satellites and on the moon during the Apollo and Soviet rover programs. By timing the bounce of the pulses, engineers can measure distances and orbits with an accuracy of within a few millimeters.

The first laser communications pathfinder mission, the Lunar Laser Communications Demonstration (LLCD), was launched by Goddard in 2013. LLCD demonstrated that a space-based laser communications system was viable and could survive both launch and the space environment. The Goddard team is now planning a follow-on mission called the Laser Communications Relay Demonstration (LCRD) to prove the proposed system's longevity and provide engineers with the opportunity to learn how to best operate the system for near-Earth missions.

Scheduled to launch in 2019, LCRD will simulate real communications support, practicing for two years with a test payload on the International Space Station and two dedicated ground stations in California and Hawaii. This mission could be the last hurdle to implementing a constellation of laser communications relay satellites similar to the Space Network's Tracking and Data Relay Satellites.

"We have been using RF since the beginning, 50 to 60 years, so we've learned a lot about how it works in different weather conditions and all the little things to allow us to make the most out of the technology, but we don't have that experience with laser comm," said Dave Israel, exploration and space communications architect at Goddard and principal investigator on LCRD. "LCRD will allow us to test the performance over all different weather conditions and times of day and learn how to make the most of laser comm."

NASA's Jet Propulsion Laboratory (JPL) and Glenn Research Center are also building on LLCD's success to discover how laser communications could be implemented in deep-space missions.

The team at Glenn is developing Integrated Radio and Optical Communications (iROC), an idea for putting a laser communications relay satellite in orbit around Mars that could receive data from distant spacecraft and relay the signal back to Earth. The iROC system would use both RF and laser communications and promote interoperability among all of NASA's assets in space. By integrating both RF and laser communications systems, iROC could provide services both for new spacecraft using laser communications systems and older spacecraft like Voyager 1 that use RF.

The data-rate benefits of laser communications for deep-space missions are clear; less recognized is that laser communications can also save mass, space and/or power requirements on missions. That could be monumental on missions like the James Webb Space Telescope, which is so large that, even folded, it will barely fit in the largest rocket currently available. Although Webb is an extreme example, many missions today face size constraints as they become more complex. The Lunar Reconnaissance Orbiter mission carried both types of communications systems, and the laser system was half the mass, required 25 percent less power and transferred data at six times the rate of the RF system.

Laser communications could also benefit a class of missions called CubeSats, miniature satellites about the size of a shoebox. These missions are becoming more popular and require miniaturized communications and power systems.

Power requirements can become a major challenge on missions to the outer solar system. As spacecraft move away from the sun, solar power becomes less viable, so the less power a payload requires, the better. A small spacecraft battery saves space and is easier to recharge.

From communications to altimetry and navigation, photonics' importance to NASA missions cannot be understated. As the technology continues to evolve, NASA will continue to invest in novel ways to use photonics to provide solutions to some of its most pressing challenges in future spaceflight.

The original article appeared on the website.

NASA is using photonics to solve some of the most pressing upcoming challenges in spaceflight, such as better data communications from space to Earth. Courtesy of NASA’s Goddard Space Flight Center/Amber Jacobson, producer.

Published: October 2016
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