Last week, NASA announced that the Lunar Laser Communication Demonstration (LLCD) on its Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft had made history by using a pulsed laser beam to transmit data over the 239,000 miles from the moon to Earth at a record-breaking data-download speed of 622 megabits per second (Mbps). This download speed is more than six times faster than the speed achieved by the best radio system ever flown to the moon. LLCD also demonstrated a data-upload speed of 20 Mbps on a laser beam transmitted from a ground station in New Mexico to the LADEE spacecraft in lunar orbit; this speed is 5,000 times faster than the upload speed of the best radio system sent to the moon. Finally, LLCD provided continuous measurements of the distance from Earth to the fast-moving LADEE spacecraft with an unprecedented accuracy of less than half an inch.
These tests were the first in a monthlong demonstration of the capabilities of the LLCD, which was developed by MIT Lincoln Laboratory researchers led by Don Boroson, a laboratory fellow in MIT LL’s Communication Systems Division. He describes below the highly improved communications capabilities that will enable NASA to significantly change the scope and design of future scientific space missions.
Q: What were the important results reported last week to the public?
A: This project has a number of “firsts,” as well as novel technologies. It has achieved the highest data rates ever transmitted from or to a lunar mission. It is NASA’s first space-based laser communications system. And it is by far the longest two-way laser communications link ever accomplished.
It includes signaling approaches that allow it to give errorfree performance through our turbulent atmosphere. The beam-stabilization system on the space terminal is based on inertial sensors, which can be scaled to work even at the most distant planets. And the ground receiver is based on arrays of small, inexpensive telescopes that are fiber-coupled to highly efficient superconducting nanowires, a photon-counting technology that was brought to its high state of maturity by joint MIT and Lincoln Lab teams.
Q: Why is this work important?
A: It is generally agreed that present-day science and exploration missions to deep space are constrained by the amount of data they can get back to Earth. Mars landers and orbiters gather much more information, in the form of images, for example, than they can send back over the huge distance — Mars is as much as 1,000 times farther from Earth than the moon — despite the incredible development of NASA’s radio-based Deep Space Network over the past 50 years.
It has been known for years that laser communications have the potential to deliver much higher data rates and use smaller space terminals than radio-based systems. But it has been an elusive goal to bring laser communications techniques and systems to the point where they can actually deliver on their promises.
With the success of LLCD, next-generation space mission designers can now feel more comfortable in including a laser communication system as part of their design. Although, for a number of reasons, some radio communications will likely always be needed, the designers will be able to make good trade-offs of mass and power on their spacecraft for the much higher data return they can get.
Q: What was MIT LL’s involvement?
A: The entire system design was based on technologies and concepts that Lincoln Laboratory has been developing for NASA for the past 10 years, and those grew out of our laser communications developments from the previous 20 years. When Lincoln Lab pointed out to the NASA sponsors that the pieces could add up to this demonstration, NASA made the mission happen.
Then, the Laboratory did the more detailed full-system design, the detailed design of the three modules that make up the space terminal, and the detailed design of the primary ground terminal. Lincoln Lab teams built, tested, and delivered these various parts to the spacecraft and to the ground site. Finally, we designed and built the Operations Center and have been staffing it round the clock, controlling and configuring the space and ground segments in a coordinated fashion.