MIT’s Laser-Pointing Platform Could Help CubeSats Transmit Large Amounts of Data to Earth

MIT has developed a new laser-pointing platform that may help launch small satellites into the high-rate data game.

Since 1998, nearly 2,000 shoebox-sized satellites called CubeSats have been sent into space. Owing to their tiny frame and the fact that they can be produced using off-the-shelf parts, CubeSats are considerably more affordable to construct and launch than conventional behemoths that cost hundreds of millions of dollars.

CubeSats have revolutionized satellite technology, as they can be launched in droves to economically monitor large areas of the Earth’s surface. But as more and more capable miniaturized instruments enable CubeSats to capture very detailed images, the miniature spacecraft finds it hard efficiently convey large amounts of data down to Earth, because of size and power limitations.

The new laser-pointing platform for CubeSats, which is described in the journal Optical Engineering, allows CubeSats to downlink data using fewer onboard resources at considerably higher rates than is presently possible. Instead of sending down just a few images each time a CubeSat flies over a ground station, the satellites should be able to downlink numerous high-resolution images with each flyby.

To obtain valuable insights from Earth observations, hyperspectral images, which take images at many wavelengths and create terabytes of data, and which are really hard for CubeSats to get down, can be used. But with a high-rate lasercom system you’d be able to send these detailed images down quickly. And I think this capability will make the whole CubeSat approach, using a lot of satellites in orbit so you can get global and real-time coverage, more of a reality.

Kerri Cahoy, Study Co-Author and Associate Professor of Aeronautics and Astronautics, MIT

Cahoy, who is the Rockwell International Career Development Associate Professor at MIT, is a co-author on the paper, along with graduate student Ondrej Cierny, who is the lead author.

Beyond radio

Satellites usually downlink data via radio waves, which for higher rate-links are transmitted to large ground antennas. Every big satellite in space communicates within high-frequency radio bands that enable them to convey large amounts of data rapidly. But bigger satellites can house the larger antenna dishes or arrays required to support a high rate downlink. CubeSats are extremely small, and also have limited access to frequency bands that could handle high-rate links.

“Small satellites can’t use these bands, because it requires clearing a lot of regulatory hurdles, and allocation typically goes to big players like huge geostationary satellites,” Cahoy says.

Furthermore, the transmitters vital for high-rate data downlinks can use more power than small satellites can accommodate while still aiding a payload. Therefore, scientists have examined lasers as an alternative method of communication for CubeSats, as they are considerably more compact in size and are more power efficient, packing a lot more data in their closely focused beams.

But laser communications also pose a major challenge: As the beams are a lot narrower than the beams from radio waves, it takes greater precision to point the beams at a receiver on the ground.

“Imagine standing at the end of a long hallway and pointing a fat beam, like a flashlight, at a bullseye target at the other end,” Cahoy says. “I can wiggle my arm a bit, and the beam will still hit the bullseye. But if I use a laser pointer instead, the beam can easily move off the bullseye if I move just a little bit. The challenge is to keep the laser on the bullseye even if the satellite wiggles.”

Color, diverted

NASA’s Optical Communications and Sensor Demonstration uses a CubeSat laser communications system that fundamentally tips and tilts the whole satellite to align its laser beam with a ground station. But this steering system entails time and resources, and to realize a higher data rate, a more robust laser—which can use a large fraction of the satellite’s power and produce substantial amounts of heat onboard—is desirable.

Cahoy and her team aimed to design a precise laser-pointing system that would lessen the amount of energy and time needed for a downlink, and allow the use of lower-power, narrower lasers yet still realize higher data transmission rates.

The researchers built a laser-pointing platform, somewhat larger than a Rubik’s Cube, which integrates a small, off-the-shelf, steerable MEMS mirror. The mirror, which is smaller than a single key on a computer keyboard, faces a small laser and is positioned such that the laser can bounce off the mirror, into space, and down toward a ground receiver.

Even if the whole satellite is a bit misaligned, you can still correct for that with this mirror. But these MEMS mirrors don’t give you feedback about where they’re pointing. Say the mirror is misaligned in your system, which can happen after some vibrations during launch. How can we correct for this, and know exactly where we’re pointing?

Ondrej Cierny, Study Lead Author and Graduate Student, MIT

As a solution, Cierny developed a calibration method that establishes by how much a laser is askew from its ground station target, and automatically modifies the mirror’s angle to precisely point the laser at its receiver.

The method includes an extra laser color, or wavelength, into the optical system. Therefore, instead of only the data beam going through, a second calibration beam of a different color is passed through with it. Both beams rebound off the mirror, and the calibration beam passes through a “dichroic beam splitter,” a type of optical element that diverts a particular wavelength of light—in this case, the extra color—away from the main beam. As the rest of the laser light journeys out toward a ground station, the diverted beam is directed back into an onboard camera. This camera can also accept an uplinked laser beam, or beacon, straight from the ground station; this is used to allow the satellite to point at the correct ground target.

If the beacon beam and the calibration beam land at exactly the same spot on the onboard camera’s detector, the system is aligned, and scientists can be certain that the laser is correctly located for downlinking to the ground station. If, however, the beams land on other parts of the camera detector, an algorithm put together by Cierny directs the onboard MEMS mirror to tip or tilt so that the calibration laser beam spot readjusts to match with the ground station’s beacon spot.

“It’s like the cat and mouse of two spots coming into the camera, and you want to tip the mirror so that one spot is on top of the other,” Cahoy says.

To test the accuracy of the method, the scientists formed a lab bench arrangement that included the laser-pointing platform and a beacon-like laser signal. The arrangement was engineered to mimic a scenario in which a satellite flies at 400-kilometers altitude above a ground station and conveys data during a 10-minute overpass.

They fixed the minimum required pointing accuracy at 0.65 mrad—a measure that matches to the angular error that is acceptable for their design to have. In their experiments, they changed the incoming angle of the beacon laser a few times and noticed how the mirror tipped and tilted to align with the beacon. In the end, the calibration method realized an accuracy of 0.05 mrad—a lot more precise than what the mission needed.

Cahoy says that the result means the method can be easily modified so that it can precisely match even narrower laser beams than initially planned, which can, in turn, allow CubeSats to convey large volumes of data, such as images and videos of vegetation, ocean phytoplankton, wildfires, and atmospheric gases, at high data rates.

This shows that you can fit a low-power system that can make these narrow beams on this tiny platform that is a factor of 10 to 100 smaller than anything that’s ever been built to do something like this before. The only thing that would be more exciting than the lab result is to see this done from orbit. This really motivates building these systems and getting them up there.

Kerri Cahoy, Study Co-Author and Associate Professor of Aeronautics and Astronautics, MIT

With assistance from the NASA Space Technology Mission Directorate as part of the new CubeSat Lasercom Infrared CrosslinK (CLICK) mission in partnership with the University of Florida and NASA Ames Research Center, the researchers hope to achieve just that.

This study was supported, partly, by the MIT Deshpande Center for Technological Innovation and NASA.