CubeSat LED Transmitter for Optical Communication Downlink

The CubeSat radio frequency (RF) spectrum licensing step has always been long and therefore stressful to developers. With more satellites entering orbit, spectral congestion in the available RF frequency bands has become a problem. An optical-only solution eliminates this problem and provides a path for a lower power and higher throughput data link. The risks of weather outages drive a solution that requires enough geographically diverse optical ground locations that a clear line-of-sight is always available. The Aerospace Corporation, with funding from NASA, has developed a CubeSat light emitting diode (LED) transmitter and an inexpensive commercial portable optical telescope to demonstrate an efficient, low-cost solution for low-data rate space-to-ground links. An LED was chosen over a laser because it is simpler, has a lower cost, has fewer safety considerations, and is more robust. It has a significantly wider beamwidth than a laser, which makes the link extremely tolerant of satellite pointing errors, but it will still be providing data rates equivalent to S-band radios.

The LED transmitter weighs 86 grams, has a 70 cm diameter circular footprint, and a height of 2.3 cm. It is comprised of two independent LED channels, both at 617 nm. The first channel is a beacon with a modulation rate of 10 Kbps and a beamwidth of 120 degrees. The second channel is a comm beam with a data rate of 1 Mbps and a beamwidth of 2.5 degrees. The optical ground terminal (OGT) is based on a commercial 28 cm diameter F/2.2 telescope that is pointed by a commercial motorized mount on a portable tripod. A stellar alignment calibration is performed at the chosen location site and commanded remotely to track an object using a two-line element (TLE) input. The ground terminal’s ability to track a low-earth orbit (LEO) satellite was initially demonstrated with the International Space Station. The LED transmitter was environmentally tested to TRL 6 and is ready for a flight demonstration and may be funded by NASA’s R5 CubeSat program.