NASA Glenn developing enhanced communications for future Mars missions
CLEVELAND, Ohio — NASA’s recent announcement of briny water flowing on the surface of Mars makes the prospect of humans going to our neighboring planet in the near future even more compelling. But before humans go there, it is essential for their success and survival that we greatly enhance our ability to communicate data over interplanetary distances. Achieving that crucial enhancement is the task of the IROC program at the NASA Glenn Research Center in Cleveland, Ohio.
IROC stands for Integrated Radio and Optical Communication. Leading the program is Dr. Dan Raible.
“We’ve kind of outgrown the radio frequency,” Raible said. “Radio frequency is great. It’s very robust. But it’s very slow when you get far away.” Raible explained there are inherent limitations with the RF signal.
“The limitations are really speed,” he told SpaceFlight Insider.
Just like with a cell phone, if one goes out into the country and gets too far from a tower, it takes longer to communicate or download information because the power of the RF signal goes down the farther one gets from the tower. In space, the same is true, but over vastly greater distances.
“The further you get from Earth, the slower you have to talk to send the message across,” Raible said. “Going interplanetary, to Mars, for example, it can take days, sometimes even weeks to send a picture, a jpeg. So we’re very limited on the types of information we can send over these communications systems the farther out in space we go.”
These limitations do not hinder only our Mars spacecraft but limit the communications capability of our spacecraft throughout the Solar System.
“NASA right now has about 100 spacecraft across the Solar System,” Raible said. “We’re leaving about 90-95 percent of the data on those spacecraft, never to be recovered. So the communications with those spacecraft truly are the bottleneck in getting our science data returned from our investments on board.”
To eliminate this bottleneck and vastly improve our communications capability, Raible and his team at NASA Glenn are working to develop an optical system for space communication using a laser.
“We can use a laser beam,” Raible said, “and we can impart the data on the spacecraft onto the laser and transmit it 100 to 1000 times faster than we can with an RF beam.”
The laser the team is using is in the infrared, and so would not be visible to human vision. It is also very low power. Even out at Mars, the system will require only a 10-watt laser, yet the beam will be able to be detected from Earth.
“So it is very low power,” Raible said. “But we can go very fast with that.”
His team is running a prototype of the system through simulated exercises in the lab, with the effects of interplanetary distance and delay programmed into the simulation.
“The data rates that we are sending in a prototype here in this lab are 267 megabits per second,” Raible explained. “To put a context on that number, the fastest we’ve ever communicated to Mars was 6 megabits per second. MRO (Mars Reconnaissance Obiter) has a camera, a wonderful instrument, aboard, but it still takes a couple of days to transmit an image back to Earth at 6 megabits per second. And it can only do that when Mars and Earth are very close. When the Earth and Mars are close, at 267 megabits per second, we could stream multiple channels of high-definition video back at the same time.”
No optical communication hardware has gone to Mars yet. However, the optical communication capability has already been proven in space, aboard the Lunar Atmospheric Dust Environmental Explorer (LADEE), which was launched in 2013 and ended its mission in April 2014. Aboard LADEE was a secondary experimental payload, the Lunar Laser Communications Demonstration (LLCD). Like the prototype in the IROC lab, the LLCD aboard LADEE used a small telescope to send the laser communication beam.
“It was a very small telescope, 10 centimeters, that did send optical comm back to the Earth,” Raible said. “From the Moon we did a rate of 622 megabits per second, which just blew the doors off of anything we’ve ever done from the Moon. So with that speed, you could do 30 channels of high-definition video all together at the same time. That was our first experiment. Now our next step is to take it further, out to Mars, and show that same capability from there.”
Their success with the LLCD aboard LADEE was just a first step toward achieving the communications capability they hope to perform at Mars.
“Our goal,” Raible said, “is to get up to about a gigabit per second of data before humans go out to Mars.”
The IROC lab is set up as an experimental proving ground for all the elements that will make up the system for successful optical communication with spacecraft at Mars. Jennifer Napier is lead investigator for developing the system’s modulation hardware. Her hardware successfully receives data from an RF signal and loads it onto the laser beam.
In the lab simulations the team runs, the RF signal comes from a mockup rover on a simulated Mars background or set. The rover’s RF signal is received and loaded onto the laser and sent back to ‘Earth’ through a small modified telescope with a special gold-finish to the optics mirror, since the laser is in the infrared range. The telescope creates a finely columnated laser to send through space back to Earth, or, in the lab’s case, across the room. The laser beam is then received by a telescope of moderate size.
The laser signal from the LLCD aboard LADEE at the Moon was received by pointing the receiving telescope at the position in the sky where the spacecraft was supposed to be, and receiving the signal.
“This is not like the acres and acres of giant ground-based radio frequency antennas used to receive the data from space,” Raible said. “For optical, all it takes is a telescope of 2 meters or so in diameter to receive the signal.”
The laser signal is detected in the telescope by tremendously cooled-down photomultiplier tubes. The tubes are cooled down to about 4 kelvin, in order to detect and measure the laser signal.
“The important thing to know is that when we measure the signal coming back from the laser, what we’re actually looking for are single photons of energy,” Raible said. “Which is an incredibly small delicate unit. And when we receive one of those single photons of energy, we’ve actually received multiple bits of information. From one photon from that laser, we can detect upwards of 12 to 14 bits per single photon. So it is a very efficient way of communicating – when considering the tradeoff of slow speed and high power required to send an RF signal.”
As a comparison to the 10-watt laser needed for optical communication at Mars, the camera aboard MRO uses more than 200 watts of power to send the RF signal. Additionally, due to the physics of how the RF signal ‘spreads’ as it goes out into space, most of that signal’s energy misses Earth. The laser signal is much more power-efficient, and much faster because the laser beam does not spread as much as the RF signal, allowing more of its energy to reach the detector in the telescope on Earth.
“The hardware is smaller. The detector is smaller. It’s much better for scalability,” Raible said.
However, despite the tremendous enhancement to our communications capability that optical/laser communication presents, transitioning to it will not happen overnight. That is why Raible’s program is called Integrated Radio and Optical Communication (IROC). This need for integration has resulted in the development of a new piece of communication hardware for future Mars missions.
“You put a telescope and antenna together and you get teletenna,” Raible said with a smile. “You also get antescope. But that doesn’t sound as good. So we made up this thing and called it teletenna.”
The teletenna prototype in the IROC lab utilizes a parabolic dish or annulus for the RF signal. Out in the front of the dish is a mounted coax that radiates the RF signal toward the parabolic reflector, which focuses it, and the beam goes out toward Earth.
“On the inside of that we [tested] the optical capability,” Raible said. “On the parabolic reflector, we have an optical quality surface just in the center portion.” The laser comes in from the rear at the center onto a small mirror on the front mount, which reflects it back to the optical finished center of the parabolic dish, which then focuses and sends the signal on toward Earth.
“The full-size teletenna will be 3 meters in diameter,” Raible said. “It will not be heavy. It will not be made out of machined aluminum like the smaller prototype.”
The annulus for the flight model will be a mesh made from a material called molybdenum. It will be gold-plated and can be seen through like a screen door. It is being used for communication dishes in space now but has never been flown out to deep space. The Mars mission with the first teletenna will be its first use in deep space.
“This is really a hybridization of the RF and the optical,” Raible said, “putting them together in one piece of hardware. We’re taking the heritage and legacy of radio communication and marrying that with the optical capability, in one unit, and having it be light-weight, compact, and low power, so that people are going to be willing to accept that. Because if we sold them two independent systems, no one would buy it. They would say I already have the RF system and I can’t afford the mass, power and cost of a whole other system aboard.”
Raible says the hybrid system is needed as a transitional phase between the experimental and operational stages of using optical as the dominant system. Just like the network of RF antennas around the globe, an analogous network of optical receivers will have to be established around the Earth. Because of the geographic and political issues involved in establishing such a network, Raible expects it will take a couple of decades to do so.
In the meantime, the newly developed teletenna will almost surely be aboard NASA’s next Mars spacecraft.
“After the next Mars rover which we are for now calling Mars 2020, the plan is to send another orbiter,” Raible said. “About 2022. We don’t know when it will happen exactly, but we are trying to get our capability established by 2030. So between 2022 to 2026 we want to be on those orbiter mission[s] for deployment. We want to make sure that it’s out there when the humans come out, and that our capability is in place, that it’s proven, and that they can really lean on it and make sure [it’s] a part of their everyday functionality for mission success.”
Raible expects that by the time humans arrive at Mars, there may at first be one or two orbiters equipped with optical communications and perhaps three or four others with RF, depending on how long the existing orbiters at Mars (Odyssey, Express, MRO, and MAVEN) remain functioning. Communications operations will have to be scheduled through the RF and optical capabilities as they are available to the astronauts and other communicating hardware. There will be optical communications aboard the astronaut’s spacecraft, and when they are on the far side of Mars, their signal can be relayed through one of the optical-equipped orbiters.
“We want to eventually be able to stream video from Mars, especially when we send humans to Mars,” Raible said. “This is going to be very important, not only for public dissemination of what we’re doing, but [also] for safety and the mental health of the astronauts, to be able to live and see their families. These are missions of two-year duration typically. So they’re going to need connectivity back home.”
The kind of connectivity Raible speaks of will be essential to the astronauts’ survival so far from home, but will also connect everyone on Earth to the experience of exploring the neighboring planet in a way that would be impossible through RF communication alone.
The developments of the IROC program will clearly be an essential part of our future explorations of Mars, both for those involved in the missions to the distant planet, and for those who want to share in that exciting experience from Earth.
Michael Cole is a life-long space flight enthusiast and author of some 36 educational books on space flight and astronomy for Enslow Publishers. He lives in Findlay, Ohio, not far from Neil Armstrong’s birthplace of Wapakoneta. His interest in space, and his background in journalism and public relations suit him for his focus on research and development activities at NASA Glenn Research Center, and its Plum Brook Station testing facility, both in northeastern Ohio. Cole reached out to SpaceFlight Insider and asked to join SFI as the first member of the organization’s “Team Glenn.”