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Friday, January 25, 2013

Laser communication with satellites


Satellites are only useful to us if we can communicate with them. Until now, that communication has been via radio signals. However, the good folks at NASA have successfully used lasers to send an image from the Goddard Space Flight Center in Greenbelt, MD to the Lunar Reconnaissance Orbiter (LRO), which, as the name implies, is currently orbiting the moon. In an appealing mix of art and science, the chosen image was the Mona Lisa.

We already track satellites using lasers, but we hadn’t previously been able to send information to those satellites via lasers. To send the image, the picture of the Mona Lisa was first broken down into individual gray-tone pixels, each of which was represented by number from zero to 4,095. Each pixel was transmitted as a single laser pulse at a rate of about 300 bits per second, and reassembled by the Lunar Orbiter Laser Altimeter (the instrument aboard the LRO which received the image, aka the LOLA). Defects caused by the Earth’s atmosphere were corrected as shown below.

black-and-white images of the Mona Lisa

To clean up transmission errors introduced by Earth's atmosphere (left), Goddard scientists applied Reed-Solomon error correction (right), which is commonly used in CDs and DVDs. Typical errors include missing pixels (white) and false signals (black). The white stripe indicates a brief period when transmission was paused.
Image courtesy: Xiaoli Sun, NASA Goddard

As verification of the success of the test, the image was returned to Earth by the LRO’s radio telemetry system. 

To be clear, this system wouldn't work with currently existing satellites that have launched without the equipment aboard to receive laser communications. The LOLA was already set up to receive such information for tracking purposes. However, going forward, new satellites and spacecraft will undoubtedly have this capability. David Smith from the Massachusetts Institute of Technology and principal investigator of the LOLA said this about the mission:
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.

Below is a video describing the experiment.