May 23, 2014
Groundbreaking Earth-To-Moon Broadband Uplink To Be Detailed At CLEO: 2014
redOrbit Staff & Wire Reports - Your Universe Online
Most of us have gotten used to being connected no matter where we go, but if mankind ever starts living or working on the moon or some far-off asteroid, how would they be able to check their email or post killer selfies on Facebook? Thankfully, researchers from the Massachusetts Institute of Technology (MIT) are close to a solution.
Details about the technology will be unveiled Monday, June 9 at the annual Conference on Lasers and Electro-Optics (CLEO) in San Jose, California. During their presentation, the team will also be providing the first comprehensive overview of the on-orbit performance of the Lunar Laser Communication Demonstration (LLCD).
The LLCD is a laser-based communication uplink system between the Earth and the moon, and last year, the MIT team made history by surpassing the previous record transmission speed by a factor of 4,800. During a 30-day mission that concluded last December, NASA reported that LLCD reached data download and upload speeds to the moon at 622 megabits per second (Mbps) and 20 Mbps, respectively.
“For example, LLCD demonstrated error-free communications during broad daylight, including operating when the moon was to within three degrees of the sun as seen from Earth,” the US space agency said. “LLCD also demonstrated error-free communications when the moon was low on the horizon, less than 4 degrees, as seen from the ground station, which also demonstrated that wind and atmospheric turbulence did not significantly impact the system. LLCD was even able to communicate through thin clouds, an unexpected bonus.”
According to Mark Stevens of the MIT Lincoln Laboratory, the CLEO 2014 presentation will mark the first time that the team demonstrates both the implementation overview, as well as the actual performance of the network. He added that the on-orbit performance was close to predictions, making them confident they have a good grasp of the underlying physics behind the technology.
“Communicating at high data rates from Earth to the moon with laser beams is challenging because of the 400,000-kilometer distance spreading out the light beam,” he said in an Optical Society statement. “It's doubly difficult going through the atmosphere, because turbulence can bend light – causing rapid fading or dropouts of the signal at the receiver.”
The demonstration utilizes multiple techniques to overcome problems with signal fading over such a great distance, making it possible to achieve error-free performance over a vast array of optically challenge atmospheric conditions both in bright sunlight and darkness.
A New Mexico-based ground terminal uses four different telescopes, each of which are about six inches in diameter, in order to send the uplink signal to the moon. Each of the telescopes is fed by a laser transmitter, which sends data codes as invisible infrared light pulses. The total transmitter power is the sum of the four separate transmitters, resulting in a total of 40 watts of power.
According to Stevens, multiple telescopes are used because each one transmits light through a different column of air, which experiences different atmospheric bending effects. This method increases the chance that at least one of the laser beams will interact with a receiver, which is mounted on the Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite which is currently orbiting the moon.
The receiver collects the light using a slightly narrower telescope, then focuses it into an optical fiber similar to those used in Earth-based fiber optic networks. Afterwards, the signal is amplified about 30,000 times, and the pulses of light are converted into electrical pulses by a photodetector.
Those pulses are then converted a second time into data bit patterns that carry the actual message. Out of the 40-watt signals sent by the transmitter, less than one-billionth of one watt is actually received by the satellite. Even so, that’s approximately 10 times the signal necessary to achieve error-free communication, according to Stevens.
The CLEO: 2014 presentation will also detail how the large margins in received signal level will permit the system to operate through partly transparent thin clouds in the Earth's atmosphere. Stevens said that the team successfully “demonstrated tolerance to medium-size cloud attenuations, as well as large atmospheric-turbulence-induced signal power variations, or fading, allowing error-free performance even with very small signal margins.”