Laser communications test breaks data-rate record
BY STEPHEN CLARK
Posted: October 23, 2013
High-definition 3D video postcards from Mars and lightning fast data downloads are a step closer to reality after a successful laser linkup with a communications testbed aboard NASA's LADEE spacecraft, which arrived in orbit around the moon earlier this month.
The White Sands ground station shot a laser toward LADEE's expected position in lunar orbit, and the spacecraft's laser package locked on to the signal and responded by emitting its 4-inch-diameter, 0.5-watt light beam back to a receiving telescope in the New Mexico desert.
Over the 239,000-mile distance between the Earth and the moon, the 4-inch-diameter laser column disperses to a width of 3.5 miles by the time it reaches the ground. But the light beam still covers an much smaller than traditional radio waves coming from space missions, and laser communications is less forgiving of tiny pointing errors between ground and space terminals.
NASA said the laser test achieved a record-breaking download speed of 622 megabits per second and an error-free upload rate of 20 megabits per second. Both values fell within preflight predictions.
The Lunar Atmosphere and Dust Environment Explorer, or LADEE, arrived in lunar orbit Oct. 6 after a one-month transit following its successful launch from Virginia on Sept. 5 aboard a Minotaur 5 rocket.
LADEE's primary mission is to study the lunar atmosphere, a nebulous collection of atoms and molecules that scientists hope to observe over a 100-day science mission. Researchers hope to find out what mechanisms drive the moon's ultra-thin atmosphere by measuring its response to phenomena such as solar storms, impacts on the lunar surface and sunlight.
The main science phase of LADEE's mission will begin in mid-November. LADEE's three science instruments have passed their initial activation tests and deployed their aperture covers, according to Butler Hine, LADEE project manager at NASA's Ames Research Center at Moffett Field, Calif.
The mission's laser payload, conceived as a technology demonstrator, is a pathfinder for future deep space probes, including NASA's next Mars rover set for launch in 2020.
The ground and space terminals for LADEE's laser test were built by MIT Lincoln Laboratory.
NASA does not plan to exchange real science data during LADEE's demonstration, but the laser beam will carry code and information packets during its 30-day test to prove future missions could use the system for heaps of science observations, images and high-definition 3D video.
Laser linkups foster much faster data transfers between ground controllers and spacecraft than possible with radio transmissions. Officials compare the difference to the speed gained by switching from a dial-up modem to a fiber-optic line.
To illustrate the difference, NASA says the LADEE spacecraft's conventional S-band radio would take 639 hours to downlink the average-length HD movie. A system using laser technology could do the job in less than eight minutes.
Light waves in the optical bandwidth are more than 10,000 times shorter than in radio bands, according to MIT scientists, meaning laser communications systems can be smaller and weigh less, making them that much more attractive for mission designers.
"I think there's no question that as we send humans farther out into the solar system, certainly to Mars, if we want to have high-definition 3D video, we want to have laser communications sending that information back," said John Grunsfeld, associate administrator of NASA's science mission directorate.
The Lunar Laser Communication Demonstration is the first test of optical communications beyond Earth orbit.
"LLCD is the first step on our roadmap toward building the next generation of space communication capability," said Badri Younes, NASA's deputy associate administrator for space communications and navigation in Washington. "We are encouraged by the results of the demonstration to this point, and we are confident we are on the right path to introduce this new capability into operational service soon."
A follow-on to LADEE's laser mission is scheduled for launch in 2017 aboard a commercial communications satellite built by Space Systems/Loral.
NASA will fund a hosted payload for the Loral-built satellite to relay data between the ground and other missions in low Earth orbit, including the International Space Station.