20 Dec 2018
MIT-designed platform offers precision that mini-satellites require to transmit precisely large data packets from orbit to earth.
The novel laser-pointing platform, which is described SPIE journal Optical Engineering, enables CubeSats to downlink data using fewer onboard resources at significantly higher rates than is currently possible. Its developers say that, rather than sending down only a few images each time a CubeSat passes over a ground station, the satellites should in future be able to downlink thousands of 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,” commented Kerri Cahoy, associate professor of aeronautics and astronautics at MIT.
“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.” 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.
Satellites typically downlink data via radio waves, which for higher rate-links are sent to large ground antennas. Every major satellite in space communicates within high-frequency radio bands that enable them to transmit large amounts of data quickly. But bigger satellites can accommodate the larger antenna dishes or arrays needed to support a high rate downlink. CubeSats are too small, and also have limited access to frequency bands that could support 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,” said Cahoy, who also has an appointment in MIT’s Department of Earth, Atmospheric and Planetary Sciences.
Furthermore, the transmitters required for high-rate data downlinks can use more power than miniature satellites can accommodate while still supporting a payload. For these reasons, researchers have looked to lasers as an alternative form of communication for CubeSats, as they are significantly more compact in size and are more power efficient, packing much more data in their tightly focused beams.
Cahoy and her team looked to develop a precise laser-pointing system that would minimize the amount of energy and time required for a downlink, and enable the use of lower-power, narrower lasers yet still achieve higher data transmission rates.
The team developed a laser-pointing platform, slightly larger than a Rubik’s Cube that incorporates 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 angled so that the laser can bounce off the mirror, into space, and down toward a ground receiver. Cierny also developed a calibration technique that determines by how much a laser is misaligned from its ground station target, and automatically corrects the mirror’s angle to precisely point the laser at its receiver.
The technique incorporates an additional laser wavelength, into the optical system. So instead of just the data beam going through, a second calibration beam of a different wavelength is sent through with it. Both beams bounce off the mirror, and the calibration beam passes through a “dichroic beam splitter,” a type of optical element that diverts a specific wavelength of light — in this case, the additional wavelength — away from the main beam.
Cahoy says that this achievement result means the technique can be easily tweaked so that it can precisely align even narrower laser beams than originally planned, which can in turn enable CubeSats to transmit large volumes of data, such as images and videos of vegetation, wildfires, ocean phytoplankton, and atmospheric gases, at high data rates.
With support from the NASA Space Technology Mission Directorate as part of the new CubeSat Lasercom Infrared CrosslinK (CLICK) mission in collaboration with the University of Florida and NASA Ames Research Center, the team hopes to do just that. This research was supported, in part, by the MIT Deshpande Center for Technological Innovation and NASA.
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