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LLCD is the first NASA mission to demonstrate two-way, high-rate laser communications from lunar orbit aboard the Lunar Atmosphere Dust Environment Explorer (LADEE).
The main goal of LLCD is to prove the fundamental concepts of laser communications and transfer data at a rate of 622 megabits per second (Mbps), which is about five times the current state-of-the-art from lunar distances. Engineers expect future space missions to benefit greatly from the use of laser communications technology. This new ability will provide increased data transmission for real-time communication and 3-D high-definition video, while taking advantage of its lower on-orbit mass and power requirements. For example, using S-band communications, the LADEE spacecraft would take 639 hours to download an average-length HD movie. Using LLCD technology, download times will be reduced to less than eight minutes.
"LLCD is the first step on our roadmap toward building the next generation of space communication capability," said Badri Younes, NASA's deputy associate administrator for space communications and navigation (SCaN) in Washington. "We are encouraged by the results of the demonstration to this point, and we are confident we are on the right path to introduce this new capability into operational service soon."
- NASA Press Release 13-309, October 22nd, 2013
LLCD is establishing the ability to encode data onto a beam of laser light and validating a new form of communications from space, "optical communications." The term "optical communications" refers to the use of light as the medium for data transmission. In the most basic definition optical communications can refer to the use of a flashlight to spell out S.O.S. in morse code, using your vehicle's "blinker" to signal that you are going to make a right turn, or even using your remote to change the TV channel. Each of the previous examples of optical communications is done using the visible and the near-visible (infrared, etc.) portions of the electromagnetic spectrum. LLCD is operating in the near-infrared portion of the electromagnetic spectrum - in the realm of light photons. Light photons are small packets of electromagnetic waves, and when many are transmitted together "in synch," they form what is commonly known as a LASER beam.
Why Laser Communications?
The Internet is no longer limited by the slow speed of dial-up connections, so why should our satellites be?
For decades NASA's space communications networks have relied on the use of radio waves to transmit critical Earth and space science data from spacecraft down to Earth. While radio-frequency (RF) communications technology has been the most reliable form of communications across space, it has struggled to meet the data rate demands of current and future science missions. These constraints on NASA's systems are expected to grow exponentially over the coming decades. Furthermore, the radio and microwave portions of the electromagnetic spectrum are getting close to capacity. Concern around this issue led NASA to search for more capable and effective solutions for the future of space communications. One solution NASA is exploring is the use of optical communications, or laser-based communications technologies. During the past several decades, the volume of data from NASA's missions has increased exponentially and is expected to continue at even greater rates. Laser communications will enable NASA to work within an unregulated, less crowded section of the electromagnetic spectrum. Additionally the narrow beam widths of laser communications allow the reuse of optical frequencies.
Another motivation for exploring laser communications is the development of more efficient, cost-effective space communications equipment. Because RF wavelengths are longer, the size of their transmission beam covers a wider area; therefore, receiving antennas for RF data transmissions must be very large. Laser wavelengths are 10,000 times shorter, allowing data to be transmitted across narrower, tighter beams. The smaller wavelengths of laser-based communications are more secure, delivering the same amount of signal power to much smaller collecting antennas. This reduction in antenna size applies for both ground and space receivers, which reduces satellite size and mass. Laser communication terminals can support higher data rates with lower mass, volume and power requirements, a cost savings for future missions.
The LLCD mission consists of space-based and ground-based components. The Lunar Laser Space Terminal (LLST) is an optical communications test payload flying aboard the LADEE Spacecraft. The LLST is demonstrating laser communications using a data-downlink rate that is five-times the current communication capabilities from lunar orbit. The ground segment consists of three ground terminals that will perform high-rate communication with the LLST aboard LADEE. The primary ground terminal, the Lunar Laser Ground Terminal (LLGT) is located in White Sands, NM and was developed by MIT/Lincoln Laboratory and NASA. The ground segment also includes two secondary terminals located at NASA/JPL's Table Mountain Facility in California and the European Space Agency's El Teide Observatory in Tenerife, Spain.
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