Since 2009, NASA has tracked the position of its Lunar Reconnaissance Orbiter with a laser. But in an effort to push interplanetary communications technology forward, the space agency recently used the same device to beam a message to the spacecraft, namely a digitized image of Leonardo da Vinci's Mona Lisa. The successful test marked an important proof-of-concept that will drive the development of high data rate laser-communication.
To make it work, NASA had to transmit the data nearly 240,000 miles (386,243 km) in digital format, and it did so by using the Next Generation Satellite Laser Ranging (NGSLR) station at NASA's Goddard Space Flight Center in Greenbelt, Md. In turn, the spacecraft was able to receive the message by using its Lunar Orbiter Laser Altimeter (LOLA).
NASA sent the image in the form of discrete laser pulses that were decompiled by the Orbiter. Each pulse represented a different pixel of the digitized image.
"This is the first time anyone has achieved one-way laser communication at planetary distances," said LOLA's David Smith through NASA's official statement. "In the near future, this type of simple laser communication might serve as a backup for the radio communication that satellites use." Smith says that, in the more distant future, it may allow communication at higher data rates than present radio links can provide.
Precise timing was key. The image was divided into an array of 152 pixels by 200 pixels, and each dot was converted into a shade of gray, represented by a number between zero and 4,095. The pulse had to be fired in one of 4,096 possible timing slots during a brief time window allotted for laser tracking. In the end, the transmission rate reached a maximum 300 bits per second.
To clean up the image, NASA used the tried-and-true Reed-Solomon error correction, which is routinely used in CDs and DVDs.