In October of last year, a team from NASA and MIT's Lincoln Laboratory made space communications history by beaming data, via laser, at speeds reaching 622 megabits per second, to Earth from a spacecraft orbiting the moon. Radio-frequency systems used for space communications today are usually tens of times slower.
NASA and Lincoln Lab engineers tested this first-ever two-way laser link between the moon and the earth, dubbed the Lunar Laser Communication Demonstration (LLCD), for about a month. And, as it turns out, the test was underwhelming: no jaw-clenching, fingernail-biting, arm-clutching moments. In other words, an engineer's dream.
"It worked like gangbusters," says Don Boroson, who led the LLCD design team at Lincoln Lab, and presented the demo's results at the SPIE Photonics West conference on 3 February.
The laser link exceeded the NASA and MIT engineers' expectations. It achieved error-free communication when the moon was high in the sky, when it was low (albeit at a lower, 311-Mbps, data rate), when it was near the sun, and when the atmosphere was turbulent. It even worked through thin clouds, an unexpected bonus since clouds are the Achilles' heel of laser communications.
"The system did what it needed to do," Boroson says. "The concept is right, and the system is reliable. We think it's ready for prime time."
Space scientists want to be able to receive hundreds of times more photographs and measurement data, and even high-definition videos, from probes sent to other planets. Not only are radio links too slow for that, they also need antennas that can be bigger than the spacecraft themselves. Laser systems are much smaller and could be up to 100 times as fast. Deep-space laser communication could also someday enable high-bandwidth communications with astronauts who go beyond the moon.
"Suppose you wanted to make a Google maps image of Mars," Boroson says, "and not even as crisp as Google maps. It would take decades to send that much data back with radio systems we have now. If you had a laser communication system with a 50-times-higher data rate, it would take tens of weeks. Then you could send all the data for a Google map in one year."
The LLCD's ground terminal in White Sands, New Mexico turns on its laser beam and points it at the satellite. When the space module scans and finds it, it points its narrow, 0.5-watt laser beam right back, and the two terminals then continuously fine tune their pointing. "What was a surprise was that we barely needed to scan; they locked in immediately," Boroson says. "So the very first night we turned up the bit rate higher and higher and achieved the maximum data rates of both the uplink and downlink." The system was designed to send 20 Mbps from Earth to the lunar orbiter and receive 622 Mbps back from the moon.
Over subsequent weeks, the team beamed HD videos to the moon and back. The signal takes 1.3 seconds to travel one way between the Earth and moon. Together with the processing delay, they received the videos back on Earth with a 7-second lag. They sent videos of NASA space shuttle launches, astronauts in space, a message from NASA administrator Charlie Bolden, and a video of Bill Nye the Science Guy. Then they hooked up a live video camera from the Lincoln Lab operation center. "So people in the operations center could see real-time videos and then on the other screen could see a video of them that they knew had been to moon," Boroson says. "That was popular."
The engineers were able to download the lunar orbiter's entire 1-gigabyte science data cache in less than five minutes. The orbiter's onboard radio system would have taken three days to send down the same data.
Another exciting achievement was that the team was able to operate the LLCD without using radio commands at all. They programmed the spacecraft to awaken the LLCD space terminal and have it automatically point at the ground terminal at specific times, showing that a radio link wasn't needed.
NASA's follow-up laser communication mission, the Laser Communication Relay Demonstration (LCRD), which is scheduled for launch in 2017, will attempt to establish laser links at a rate of over 1 gigabit per second between Earth and a satellite in geosynchronous orbit (which is ten times closer than the moon). The LCRD will operate for five years in order to demonstrate the reliability of laser communication technology. The design of some its major parts will be based very closely on parts that Boroson's team designed for the LLCD, he says.
