Laser communications, illustrated here with a conceptual mission, could increase bandwidth for both Earth orbiting and deep space missions by factors of 10 to 100. (credit: NASA/GSFC)
Spacecraft missions face all sorts of constraints based on financial and technological limitations, from their overall size to the quantity and quality of instruments they carry. Another has been communications, particularly for missions far from Earth, where limited transmission power and the tremendous distances involved can result in data rates reminiscent of old-fashioned modems.
But, as spacecraft and their instruments get increasingly sophisticated and powerful, even spacecraft in Earth orbit are facing communications constraints. A case in point is a synthetic aperture radar (SAR) mission being jointly developed by NASA and the Indian space agency ISRO, known as NISAR. The spacecraft will be capable of providing large volumes of SAR imagery of the Earth—provided there’s a way to get it all down.
“We’re starting to reach a bottleneck in terms of the amount of data we can bring down from satellites,” said Gerald Bawden, a program scientist in NASA’s Earth science division at NASA Headquarters, during a panel discussion at the Fall Meeting of the American Geophysical Union in New Orleans last month.
In the case of NISAR, the spacecraft will be able to collect more data than can be downlinked through the agency’s Near-Earth Network of Ka-band antennas. That system can collect 26 terabits a day of data from NISAR: a huge volume, but Bawden said the spacecraft is capable of generating even more.
“We are actually underutilizing NISAR,” he said. The mission, he said, is considering adding additional antennas to provide more downlink capacity.
But the mission would be more efficient, he said, if it didn’t have to rely on radiofrequency communications altogether. “If we had designed NISAR with laser communications from the beginning, all we would need is about six minutes” per day to downlink all the data the spacecraft could collect. “I firmly believe that for future large data missions, like I just described with NISAR, laser communications or optical communications—however you’d like to phrase it—is were NASA needs to go.”
NASA does have laser communications, for both big-data Earth science missions and more distant spacecraft, on its radar. “We’re moving to optical, but we are not giving up on RF [radiofrequency],” said Phillip Liebrecht, assistant deputy associate administrator for space communications and navigation at NASA, during that panel discussion. “For higher data rate capabilities, optical beats everything hands down.”
Laser communications has a number of advantages beyond high bandwidth, Liebrecht said. Optical systems require less mass, power, and size than RF systems on spacecraft. “It’s also more secure because the beams become extremely narrow and that reduces the chance of interference and/or somebody snooping on your signals,” he said.
Scientists see other applications for laser communications beyond, well, communications. “There’s really three dimensions of value,” said Jim Garvin, chief scientist at NASA’s Goddard Space Flight Center. One dimension is the higher data rates laser communications offers. A second, he said, is that laser communications can also offer very accurate ranging, much as laser rangefinder instruments already offer.
A third advantage, he said, is “data collection flexibility” on planetary missions, including being able to monitor dynamic phenomena thanks to the ability to return large volumes of data. “Optical communications on an Enceladus orbiter could map the small moon in a couple of weeks,” he said, referring to the icy moon of Saturn thought to have a subsurface ocean that regularly produces geysers. “Imagine taking the heartbeat of small object as it vents out from its oceans.”
Laser communications for planetary missions doesn’t provide the gigabit or even terabit data rates promised for Earth-orbiting spacecraft, but is still far better than current radiofrequency communications using the Deep Space Network. Missions to the Moon, Garvin said, could enjoy data rates of 500 megabits per second to one gigabit per second. Mars missions could operate at up to 100 megabits per second. Even spacecraft on the distant fringes of the solar system could still return data at 1 to 2 megabits per second.
Contrast that, Garvin said, with the data returned by the Juno mission orbiting Jupiter, including some of the stunning closeup images of the giant planet. Those images, he said, trickle back at less than 300 kilobits per second. An optical communications system, he said, could be 10 to 100 times faster. “The Juno mission with that capability, even part time, could produce a richer data set in both spatial and temporal space.”
NASA is experimenting with laser communications, slowly. A laser communications experiment flew on the Lunar Atmosphere and Dust Environment Explorer spacecraft orbiting the Moon in 2013. In one test, the experiment achieved a data rate of 622 megabits per second.
The Laser Communications Relay Demonstration (LCRD) experiment will test laser communications on a spacecraft in Earth orbit. The payload, currently undergoing integration and testing at Goddard, was once considered to fly as a hosted payload on a commercial communications satellite but is now scheduled to fly on the Air Force’s Space Test Program 3 mission in 2019. Other laser communications experiments include a terminal planned for the International Space Station and another planned for future Orion spacecraft.
For deep space missions, the next big test for laser communications will come with Psyche, one of two new Discovery missions selected by NASA for launch in the early 2020s. The mission will fly a laser communications payload as an experiment, along with traditional RF communications. Liebrecht said the Psyche payload will demonstrate downlink rates of up to 125 megabits per second as the spacecraft approaches its destination, the asteroid Psyche.
Part of Psyche’s laser communications system, known as the Deep Space Optical Communications (DSOC) project, will be provided by LGS Innovations, a Virginia company that specializes in similar technologies. “We are honored to support the NASA DSOC’s goal to increase communications performance and data transmission rates by many times over conventional means, in a very small form factor,” said Kevin Kelly, CEO of the company, in a statement last week announcing the company’s part in the project.
The roadmap for laser communications demonstrations, Liebrecht said, also includes a CubeSat mission flying in 2019 that will test 200 gigabits per second using just a 3U CubeSat and small groundbased telescopes as a receiver. He said there are plans to use laser communications to test the use of quantum encryption that can provide more secure communications.
While Liebrecht said there will continue to be a role for RF communciations for the indefinite future, NASA is laying the groundwork for increased use of laser communications. “The recent launch of NASA’s last Tracking and Data Relay Satellite closed a chapter in the history of space communications,” the agency said in a December statement about testing of the LCRD payload, mentioning the August 2017 launch of the TDRS-M satellite.
“Future generations of Space Network satellites will incorporate laser technologies developed in this decade,” the statement added. “The LCRD mission is an important milestone of that journey.”