As CubeSat capabilities continue to improve, many missions need high-speed communication to downlink data. Data rates using radio frequency (RF) communications are constrained by antenna size and power. Laser communications (lasercom) systems can use a much narrower beam width for a given aperture size due to having shorter wavelengths. Higher data rates can be achieved with optical communication than with RF assuming the same power level and similar efficiencies, but the primary challenge of lasercom systems is the precise pointing required for link closure.
Optical communication requires higher pointing accuracy, not only for the transmitter but also for the receiver, because of the directionality of the laser beam. This means that an optical ground station must be able to track a satellite with high accuracy. For an optical ground station such as the Optical Communications Telescope Laboratory (OCTL) from the Jet Propulsion Laboratory (JPL) or the Optical Ground Station (OGS) of the European Space Agency (ESA), the telescope is part of a fixed facility, and its pointing can be precisely calibrated using stars over a long period of time. However, these meter-class optical ground stations have costs and logistical complexities similar to those of the large aperture RF ground stations currently used for CubeSats requiring high data rates.
To address this challenge, the MIT STAR Lab is developing a portable ground station with an amateur telescope for the Nanosatellite Optical Downlink Experiment (NODE) project. State of the art amateur telescopes provide good control capability with gimbals, but the user must align the gimbals with respect to an inertial, Earth-fixed frame. Even for an experienced amateur astronomer, this is a non-trivial problem, and it can take hours to get the fine alignment within a few arcminutes accuracy.
We propose a novel approach to track a satellite with an amateur telescope. To resolve the alignment problem, we use a wide field of view star camera to determine its orientation with respect to an inertial frame. Star sensors are accurate to the arcsecond level, and they have the advantage of providing orientation with a single measurement. Using multiple star sensor measurements at different gimbal angles, it is possible to calculate the alignment of the gimbals in the Earth-fixed frame and the alignment of the star sensor in the gimbal frame. Once the alignment is obtained, satellite tracking can be achieved easily with a known orbit and precise Earth rotation model such as the International Earth Rotation and Reference System Service (IERS). We present the alignment calibration method and the preliminary tracking results using a Celestron CPC 1100 XLT to validate our approach
