Lasers help keep CubeSats on target to handle large data downlinks
An MIT team is working on a new aiming system that will allow CubeSats to use lasers for high-bandwidth communications with Earth. The new laser-pointing platform uses a second directional beam to keep the primary data beam on focus, allowing the CubeSat to transmit large amounts of data without the need for heavy antennae or wasting propellant.
With their low cost and size of about that of a loaf of bread, CubeSats have a tremendous potential to revolutionize the scientific, commercial, and military exploration and exploitation of space. Instead of a single, large satellite that may not be where it's needed when it's needed, CubeSats can be sent up at a moment's notice or in huge constellations for global coverage of things like the weather or anti-missile defenses.
But one major drawback for CubeSats is that they aren't very good when it comes to sending back data. Lacking the powerful radio transmitters and large antennae of their bigger siblings, CubeSats can only send back data the equivalent of a few images at a time. However, to make them more practical, CubeSats need to be able to send back things like hyperspectral images in large numbers and quickly. That means being able to transmit data by the terabyte at a high rate.
Even conventional radio has trouble with this sort of data flow and CubeSats only have limited access to the necessary radio frequencies due to availability and regulation, so in recent years space engineers have looked at lasers as a faster means of communications, such as experiments like NASA's Optical Communications and Sensor Demonstration. According to MIT, lasers not only are better when it comes to bandwidth, but they are more compact than radio rigs and are more power efficient.
So far, so promising, but lasers also have their problems due to the CubeSats' small size. Laser experiments on conventional spacecraft or the International Space Station (ISS) have elaborate aiming mechanisms to keep them focused on the receiving Earth station, but CubeSat systems have to tilt the entire satellite to aim the beam. This costs time, energy, and propellant that the little probe cannot afford to waste.
Led by Kerri Cahoy, associate professor of aeronautics and astronautics, the MIT team is developing a new way to precisely point a laser and keep it on target without needing to move the CubeSat or equipping it with a high-powered laser. Measuring a couple of inches on a side, the laser-pointing platform uses a small laser that bounces off a small, off-the-shelf, steerable MEMS mirror to aim it at the ground receiver.
The clever bit is that not only does the system aim the laser, but it helps to keep it locked on target. This can be tricky because the mirror may have been knocked about a bit during the rocket launch, so the system has to recalibrate it in orbit. It does this by firing two lasers of two different colors. One is the data beam and the other is the calibration beam.
As the beams are reflected by the mirror, the calibration beam goes through a special optical element that splits the beams according to their color. When this happens, the calibration beam is sent to an onboard camera on the CubeSat while the data beam heads for the target. This camera also receives a reference beam sent from the ground station. By comparing these two, the system can adjust the transmitter's mirror, so the laser stays locked on.
So far, the system has been tested under laboratory conditions that simulate a satellite passing overhead for 10 minutes at an altitude of 400 km (250 mi). By varying the angle of the reference beam, the team were eventually able to get the calibration to focus within 0.05 milliradians.
"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," says Cahoy. "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."
The research was published in Optical Engineering.