NASA is venturing into a new era of space communications using lasers, beginning with the Lunar Laser Communications Demonstration (LLCD).

Space communications technology has relied on the the use of radio waves to transmit critical Earth and space science data from space down to Earth. While radio frequency communications technology has been the most reliable form of space communication, it has struggled to meet the data rate demands of current and future science missions.  These constraints on NASA's systems are expecting to grow exponentially over the coming decades. This has led NASA to search for more capable and effective solutions for future space communications. One solution is for NASA to explore the use of optical communications or laser-based communications technologies. LLCD will demonstrate two-way, high-rate laser communications from lunar orbit aboard the Lunar Atmosphere Dust Environment Explorer (LADEE). It is scheduled to launch aboard a Minotaur-V Rocket from NASA's Wallops Flight Facility on Wallops Island, Va., later this year.  In order to make laser-based communications the norm, NASA needs to first prove that the concept is a viable option for missions. The way NASA proves the concept is through a real-life mission demonstration.

The main goal of LLCD is proving fundamental concepts of laser communications and transferring 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.

Laser Communications

LLCD will establish the ability to encode data onto a beam of laser light and, if successful, will validate 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 are done using the visible and the near-visible (infrared, etc.) portions of the electromagnetic spectrum.  LLCD will operate 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?

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. Although RF-based communications currently are the most reliable form of space communications, the radio and microwave portions of the electromagnetic spectrum are getting close to capacity. Laser communications will enable NASA to work within an unregulated, less crowded section of the electromagnetic spectrum. Furthermore, narrow beam widths 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 (about 100 miles); 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.

Mission Components

The LLCD mission consists of space-based and ground-based components. The Lunar Laser Space Terminal (LLST) is an optical communications test payload to fly aboard the LADEE Spacecraft and it will demonstrate laser communications 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.