Precision Closed-Loop Laser Pointing System for the Nanosatellite Optical Downlink Experiment
Abstract: The use of advanced small-satellite platforms has become increasingly more popular in the recent years. Several private companies are investing enormous capital into constellations of small satellites that are designed to provide highly data-intensive global services, such as rapid Earth imaging or fast worldwide Internet access. The scientific community is also interested in the development of miniature and high throughput platforms, for instance in the area of microwave radiometry or hyperspectral imaging. The current state of the art nanosatellite radio frequency (RF) communications systems struggle to keep up with the increasing downlink demand and satellite data processing capabilities. Laser communications (lasercom) offers various advantages: increased bandwidth, smaller size, weight, power consumption, and a license-free spectrum. While the narrow beamwidths allow lasercom to achieve higher data rates than RF, they, however, also result in higher pointing requirements for the spacecraft. Precision laser pointing systems have been successfully demonstrated on bigger satellites, but not on a nanosatellite scale, where the size and weight constraints are so severe. The Nanosatellite Optical Downlink Experiment (NODE) developed at MIT is a lasercom terminal designed to demonstrate the technologies required for a high-speed optical downlink using commercial off-the-shelf components within the constraints of a typical 3U CubeSat. NODE augments the bus attitude control system with a compact fine laser pointing stage to compensate for the spacecraft body pointing error. This thesis focuses on the development and laboratory verification of the laser pointing system for NODE. A control scheme utilizing a miniature fast steering mirror (FSM) used to track a beacon uplink signal from the ground station is presented. An on-orbit FSM calibration algorithm is developed to improve the control robustness and precision. A novel sampling approach that enables closed-loop FSM control is proposed and implemented. The method focuses on simultaneous sampling of the beacon and an internal feedback signal on a single detector. Finally, a hardware-in-the-loop testbed is built in the laboratory with components that were selected for NODE, and the system is functionally verified and analyzed with regards to pointing accuracy. Experimental results show that the pointing requirements given by the mission link budget are met, and that the system performs reliably under various laboratory-simulated conditions.
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