Since 1998, almost 2,000 shoebox-sized satellites known as CubeSats have been launched into space. Due to their small frame and the fact that they can be made from off-the-shelf parts, CubeSats are significantly more affordable to build and launch than traditional satellites that cost hundreds of millions of dollars.
CubeSats can be sent up in flocks to cheaply monitor large swaths of the Earth’s surface. But as increasingly capable miniaturized instruments enable CubeSats to take highly detailed images, the tiny spacecraft struggle to efficiently transmit large amounts of data down to Earth, due to power and size constraints.
A new laser-pointing platform enables CubeSats to downlink data using fewer onboard resources at significantly higher rates than is currently possible. Rather than send down only a few images each time a CubeSat passes over a ground station, the satellites should be able to downlink thousands of high-resolution images with each flyby.
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.
In addition, 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, lasers are considered 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. But laser communication also presents a significant challenge. Because the beams are much narrower than the beams from radio waves, it takes far more precision to point the beams at a receiver on the ground.
The laser-pointing platform, slightly larger than a Rubik’s Cube, 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. But the MEMS mirrors don’t provide feedback about where they’re pointing.
As a solution, a calibration technique was developed 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 color, or wavelength, into the optical system. So instead of just the data beam going through, a second calibration beam of a different color 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 color — away from the main beam. As the rest of the laser light travels out toward a ground station, the diverted beam is directed back into an onboard camera. This camera can also receive an uplinked laser beam, or beacon, directly from the ground station; this is used to enable the satellite to point at the right ground target.
If the beacon beam and the calibration beam land at precisely the same spot on the onboard camera’s detector, the system is aligned and researchers can be sure that the laser is properly positioned for downlinking to the ground station. If, however, the beams land on different parts of the camera detector, an algorithm directs the onboard MEMS mirror to tip or tilt so that the calibration laser beam spot realigns with the ground station’s beacon spot.
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.