An image of the Mona Lisa has traveled nearly 240,000 miles digitally, going from the Next Generation Satellite Laser Ranging station at NASA’s Goddard Space Flight Center in Greenbelt, Md., to the Lunar Orbiter Laser Altimeter (LOLA) instrument on NASA’s Lunar Reconnaissance Orbiter. By transmitting the image on top of on laser pulses regularly sent in order to track position, simultaneous laser communication and tracking was achieved.
“This is the first time anyone has achieved one-way laser communication at planetary distances,” says 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.”
Satellites that go outside of Earth orbit characteristically use radio waves for tracking and communication. The Lunar Reconnaissance Orbiter is the only satellite in orbit around a body other than Earth to be tracked by laser also.
“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,”
explained Xiaoli Sun, scientist at NASA Goddard and lead author of a paper detailing the work.
Timing a Pixel Array
The key to transmitting the image was accurate timing. Sun and his team divided the Mona Lisa image into an array of 152 pixels by 200 pixels. Each pixel was converted into a shade of gray, represented by a number between zero and 4,095. (I guess 50 shades of grey wasn’t to their liking.)
Every pixel was then 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.
Received by LRO’s Lunar Orbiter Laser Altimeter instrument, the laser pulses reconstructed the image based on the arrival times of the pulses from Earth. This was accomplished with no interruption of LOLA’s primary task of mapping the moon’s elevation and terrain and the Goddard Space Flight Center’s main task of tracking LRO. The image was then returned to Earth using the spacecraft’s radio telemetry system in order to verify the success of the laser transmission.
Error Correction by Reed-Solomon Coding
Transmission errors were introduced by turbulence in Earth’s atmosphere, even when the sky was clear. To overcome these effects, Sun and colleagues used Reed-Solomon coding, which is the same type of error-correction code commonly used in CDs and DVDs.
The illustration at the top of the page shows the image before and after correction. The white stripe indicates a brief period when transmission was paused.
“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 next step following the Lunar Laser Communications Demonstration 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.
Reference: Free space laser communication experiments from Earth to the Lunar Reconnaissance Orbiter in lunar orbit Sun, Xiaoli; Skillman, David R; Hoffman, Evan D; Mao, Dandan; McGarry, Jan F; McIntire, Leva; Zellar, Ronald S; Davidson, Frederic M; Fong, Wai H; Krainak, Michael A; Neumann, Gregory A; Zuber, Maria T; Smith, David, Optics Express, Vol. 21, Issue 2, pp. 1865-1871 (2013) http://dx.doi.org/10.1364/OE.21.001865