Along with reducing power consumption and weight, laser-communications research has focused on correcting for the effects of atmospheric turbulence. Cloud cover was an issue during the October demonstration. During 50 test passes, clouds sometimes obscured the primary ground station at White Sands, N.M. Mission managers were able to shift operations to a Jet Propulsion Laboratory facility at Table Mountain, Calif. Another option in Spain at 14,000 feet was well above the cloud deck.
NASA managers have already launched an effort to make future laser-based space networks "delay- and disruption-tolerant." The reason, acknowledged David Israel, NASA's principle investigator for the laser relay demonstration, is simple: "Clouds happen."
"The real trick was pointing a very narrow laser beam from 239,000 miles away to a four-mile area," at the White Sands ground station, added LLCD mission manager Don Cornwell. The initial challenge in pointing a laser in lunar orbit was accounting for the 1.5-second time difference it takes for a beam of light to reach Earth from the moon. Hence, controllers at NASA's Goddard Space Flight Center in Maryland had to point the laser slightly ahead of the ground stations, much as a quarterback leads a receiver on a long pass.
The second challenge was overcoming spacecraft vibration that could affect the accuracy of the laser pointer. Slight vibrations were measured and cancelled out, but Cornwell says the slight oscillations were not as bad as expected.
"Pointing and tracking was our biggest concern," but locking the laser beam onto its ground station target turned out to be one of the easiest parts of the demonstration, Cornwell says in an interview with EE Times. Since the optical system operated error-free while downloading and uploading data on each of 50 passes, program managers said very little troubleshooting was required. Hence, they were able to focus on different operational profiles, such as transmitting data when the moon was setting.
LLCD hitched a ride aboard NASA's Lunar Atmosphere and Dust Environment Explorer, which scanned the lunar horizon for signs of a faint atmosphere. That part of the mission was less successful, and the low-flying LADEE crashed into the moon on April 17, a day or so earlier than expected.
The upcoming laser relay demonstration will be carried to geosynchronous orbit in 2017 or 2018 as a hosted payload aboard a commercial satellite being built by Space Systems/Loral. SSL will host the laser relay demonstration under a $3 million NASA contract. The space agency said the deal marks the first time a NASA payload will be flown on a US-made commercial communications satellite.
The planned two-year laser project will demonstrate optical relay techniques that would bring an operational laser-based communications network closer to reality. If the technology works, said Israel, the next step is "can we make it operational?" Added Israel, "It's really about the scalability" of space communications networks.
The relay experiment also will attempt to demonstrate that data from one optical link can be relayed to another link. Two ground stations would "work on the whole hand-over aspect" of the demonstration, Israel said. Other planned experiments include multiplexing data for use by multiple users at different locations and demonstrating a DVR-like "store/forward" capability that would allow video, for example, to be stored and relayed later at lower data rates.
Current rover operations on Mars require that orbiting RF communication satellites be in the proper line-of-site position to relay commands back and forth between mission managers on Earth and the Curiosity and Opportunity rovers. An operational laser relay network could someday provide a "24-hour view" of surface operations by relaying data from the far side of Mars or the moon, Israel said.
He also is working the Interplanetary Networking Special Interest Group to make a future Internet-in-space a reality. Along with developing "opportunistic networks" that can adjust, for example, to a ground station socked in by clouds, the group is also trying to use emerging software-defined network technology that is currently all the rage in the terrestrial communications sector.
— George Leopold is a freelance writer based in Washington, where he covers NASA and space policy. He is the former executive news director at EE Times.