January 20th, 2013 by James Ayre
Next-generation long-distance laser communication will soon be a reality. Researchers at NASA recently completed the first demonstration of the technology by beaming an image of the Mona Lisa to the Lunar Reconnaissance Orbiter (LRO), currently orbiting the Moon.
The famous image was beamed about 240,000 miles from the Next Generation Satellite Laser Ranging (NGSLR) station at NASA’s Goddard Space Flight Center in Greenbelt, Md., to the Lunar Orbiter Laser Altimeter (LOLA) instrument on the spacecraft.
The researchers transmitted the data by “piggybacking” it on the laser pulses that are already regularly sent to the orbiter in order to track its position. The researchers were able to achieve the simultaneous transmission of the data while tracking the orbiter.
“This is the first time anyone has achieved one-way laser communication at planetary distances,” says LOLA’s principal investigator, David Smith of the Massachusetts Institute of Technology. “In the near future, this type of simple laser communication might serve as a backup for the radio communication that satellites use. In the more distant future, it may allow communication at higher data rates than present radio links can provide.”
Usually, satellites that venture further than Earth orbit rely on radio waves for all of the neccessary tracking and communication. Currently, LRO is the only satellite outside of Earth orbit that can be tracked by laser.
“Because LRO is already set up to receive laser signals through the LOLA instrument, we had a unique opportunity to demonstrate one-way laser communication with a distant satellite,” says Xiaoli Sun, a LOLA scientist at NASA Goddard and the lead author of the Optics Express paper, that details the work.
“Precise timing was the key to transmitting the image. Sun and colleagues divided the Mona Lisa image into an array of 152 pixels by 200 pixels. Every pixel was converted into a shade of gray, represented by a number between zero and 4,095. Each pixel was transmitted by a laser pulse, with the pulse being fired in one of 4,096 possible time slots during a brief time window allotted for laser tracking. The complete image was transmitted at a data rate of about 300 bits per second.”
“The laser pulses were received by LRO’s LOLA instrument, which reconstructed the image based on the arrival times of the laser pulses from Earth. This was accomplished without interfering with LOLA’s primary task of mapping the moon’s elevation and terrain and NGSLR’s primary task of tracking LRO.”
The laser transmission’s success was then verified by sending the image back to the Earth.
“Turbulence in Earth’s atmosphere introduced transmission errors even when the sky was clear. To overcome these effects, Sun and colleagues employed Reed-Solomon coding, which is the same type of error-correction code commonly used in CDs and DVDs. The experiments also provided statistics on the signal fluctuations due to Earth’s atmosphere.”
“This pathfinding achievement sets the stage for the Lunar Laser Communications Demonstration (LLCD), a high data rate laser-communication demonstrations that will be a central feature of NASA’s next moon mission, the Lunar Atmosphere and Dust Environment Explorer (LADEE),” says Goddard’s Richard Vondrak, the LRO deputy project scientist.
“The next step after LLCD is the Laser Communications Relay Demonstration (LCRD), NASA’s first long-duration optical communications mission. LCRD will help develop concepts and deliver technologies applicable to near-Earth and deep-space communication.”
A very interesting technology, especially for the purpose of deep-space communication, potentially allowing for much faster, and higher-quality, communication and control with/of very distant space probes.
Currently controlling very distant probes, such as the Voyager probes, involves incredible lag time, severely limiting the ability to control them. New technologies are certainly needed if anything beyond this solar system is ever going to be explored by humans.
Source: NASA/Goddard Space Flight Center
Image Credits: Xiaoli Sun, NASA Goddard; Mona Lisa via Wikimedia Commons
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