Portable Optical Ground Stations for Satellite Communication by Kathleen Michelle Riesing B.S.E., Princeton University (2013) S.M., Massachusetts Institute of Technology (2015) Submitted to the Department of Aeronautics and Astronautics in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Aeronautics and Astronautics at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2018 ○c Massachusetts Institute of Technology 2018. All rights reserved. Author............................................................................ Department of Aeronautics and Astronautics May 24, 2018 Certified by. Kerri L. Cahoy Associate Professor of Aeronautics and Astronautics Thesis Supervisor Certified by. David W. Miller Jerome Hunsaker Professor of Aeronautics and Astronautics Certified by. Sertac Karaman Associate Professor of Aeronautics and Astronautics Certified by. Jamie W. Burnside Senior Staff Member, MIT Lincoln Laboratory Accepted by....................................................................... Hamsa Balakrishnan Associate Professor of Aeronautics and Astronautics Chair, Graduate Program Committee 2 Portable Optical Ground Stations for Satellite Communication by Kathleen Michelle Riesing Submitted to the Department of Aeronautics and Astronautics on May 24, 2018, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Aeronautics and Astronautics Abstract Small satellite technical capabilities continue to grow and launch opportunities are rapidly expanding. Several commercial constellations of small satellites for Earth observation and communications are making their way onto orbit, increasing the need for high bandwidth data downlink. Laser communications (lasercom) has the potential to achieve high data rates with a reduction in power and size compared to radio frequency (RF) communications, while simultaneously avoiding the significant regulatory burden of RF spectrum allocation. Lasercom benefits from high carrier frequencies and narrow beamwidths, butthe resulting challenge is to precisely point these beams between transmit and receive terminals. Arcsecond to sub-arcsecond pointing is required from both the space ter- minal and the ground station. While existing lasercom ground stations have primarily utilized professional telescopes at observatory-class facilities, making optical ground stations more affordable and transportable is a key enabler for expanding lasercom to small satellites and new applications, as well as establishing networks to mitigate the effects of weather. This thesis focuses on the development of a portable optical ground station utiliz- ing a commercial off-the-shelf (COTS), low-cost telescope. The Portable Telescope for Lasercom (PorTeL) reduces mass and cost by at least 10× compared with existing op- tical ground stations. To enable the use of a low-cost telescope, several contributions are made to the state-of-the-art approach to optical ground station design, point- ing, and tracking. A system architecture is proposed that enables rapid deployment in approximately 30 minutes and that is capable of tracking satellites in low-Earth orbit (LEO) to within arcseconds of accuracy. A novel telescope calibration algo- rithm is developed that is agnostic to initial mount/telescope orientation and utilizes a star tracker for rapid, automated alignment. This eliminates the need for manual pre-alignment (e.g., leveling of the mount) and provides a generic framework that ex- tends to different mount types. An approach to tracking LEO objects to betterthan 5 arcseconds RMS using only two-line element sets (TLEs) and low-cost hardware is presented. Pointing and tracking algorithms and performance are validated by using a physical ground station setup to track LEO objects, including the International Space Station (ISS). 3 Thesis Supervisor: Kerri L. Cahoy Title: Associate Professor of Aeronautics and Astronautics Thesis Committee Member: David W. Miller Title: Jerome Hunsaker Professor of Aeronautics and Astronautics Thesis Committee Member: Sertac Karaman Title: Associate Professor of Aeronautics and Astronautics Thesis Committee Member: Jamie W. Burnside Title: Senior Staff Member, MIT Lincoln Laboratory 4 Acknowledgments This research has been supported by NASA Space Technology Research Fellow- ship Grant #NNX14AL61H. Thank you to NASA, my NSTRF mentor Dr. Michael Krainak, and the entire NSTRF team for facilitating my graduate research, providing generous funding to attend conferences and travel for work, and for the opportunity to do internships at NASA JPL and NASA GSFC. My graduate school experience would not have been nearly as rewarding without this fellowship. Thank you also to the MIT Deshpande Center for Technological Innovation for providing an Innovation Grant that in part supported my research. A special thank you to my committee members, Prof. David Miller, Prof. Sertac Karaman, Jamie Burnside, and in particular my thesis advisor Prof. Kerri Cahoy, who tirelessly advocates for me and all of her students. Thank you also to Anthony Zolnik, whose support made testing from the roof of MIT Building 37 possible. To all my friends in STAR Lab and SSL – Kit, Tam, Charlotte, Pratik, and many more – thank you for making grad school fun. Thank you to Hyosang Yoon for being both a great friend and mentor – apart from classes, you have taught me the most in my time at MIT and have helped me whenever I’ve asked. Also, special thanks to David Sternberg and Chris Jewison for being one year older and wiser and guiding me through the process. You guys have been great coaches, friends, and therapists. Thank you to Emily Clements – it has been so helpful having an officemate who can review my work and remind me of important deadlines. Thank you to Britta Kelley for being a continual source of support and laughter. Finally, this thesis would not have been possible without my family. Many people say this, but few mean it quite so literally. When January in Boston brought a week of single-digit temperatures and subsequent panic about collecting enough data for my thesis, I packed up my equipment and drove south. My dad and grandpa’s Florida vacation turned into a long attempt to track satellites. Thanks Dad and Grandpa for letting me crash your vacation and helping me drag my telescope out into a field at 4 AM. The next stop was my grandparent’s house in North Carolina, where my grandpa, mom, and aunt provided the same level of support while my brother in Atlanta waited on-call in case of clouds. Those three weeks reflected a lifetime’s worth of unwavering support and self-sacrifice. Let it never be said that the telescope wasn’t portable. 5 6 Contents 1 Introduction 17 1.1 Motivation . 18 1.1.1 Laser Communication for Small Satellites . 18 1.1.2 Optical Ground Networks . 20 1.1.3 Communication for Rapid Response . 21 1.1.4 Space Situational Awareness . 22 1.2 Ground Station Overview . 22 1.2.1 Atmospheric Effects . 22 1.2.2 Components . 23 1.3 Ground Station Challenges . 26 1.3.1 Pointing, Acquisition, and Tracking . 26 1.3.2 Cost and Transportability . 28 1.4 Thesis Contributions and Roadmap . 30 2 Ground Station Design 33 2.1 Literature Review . 33 2.1.1 Fixed Optical Ground Stations . 33 2.1.2 Transportable Optical Ground Stations . 39 2.1.3 Summary and Research Gap . 42 2.2 Portable Telescope for Lasercom . 43 2.2.1 Architecture . 43 2.2.2 Hardware . 45 2.2.3 Pointing and Tracking Requirements . 50 3 Pointing Model 53 3.1 Problem Description . 53 3.2 Literature Review . 55 3.2.1 Professional Telescopes . 55 7 3.2.2 Amateur Telescopes . 57 3.2.3 Research Gap . 59 3.3 Formulation . 60 3.4 Calibration . 64 3.4.1 Coarse Calibration . 64 3.4.2 Fine Calibration . 64 3.5 Gimbal Angle and Rate Commands . 67 4 Satellite Tracking 71 4.1 Problem Description . 71 4.2 Literature Review . 72 4.2.1 Optical Ground Stations . 72 4.2.2 Amateur Telescopes . 73 4.2.3 Research Gap . 74 4.3 Approach . 75 4.3.1 Coarse Stage . 76 4.3.2 Fine Stage . 80 5 Operation and Results 83 5.1 Ground Station Deployment . 83 5.1.1 Physical Setup . 83 5.1.2 Test Locations . 84 5.2 Calibration of Pointing Model . 85 5.3 Star Pointing Results . 90 5.4 Satellite Tracking Results . 92 5.4.1 Coarse Tracking . 92 5.4.2 Fine Tracking . 94 6 Conclusion 103 6.1 Summary . 103 6.2 Contributions . 104 6.3 Future Work . 105 A Vector Notation 109 B Quaternion Conventions 111 8 Nomenclature Acronyms AFRL Air Force Research Laboratory AO Adaptive optics APD Avalanche photodiode BPSK Binary phase shift keying CMOS Complementary metal–oxide–semiconductor COTS Commercial off-the-shelf DLR German Aerospace Center (German: Deutsches Zentrum für Luft- und Raumfahrt) DOF Degree(s) of freedom EKF Extended Kalman filter ERS European Remote Sensing Satellite ESA European Space Agency FCC Federal Communications Commission FOV Field of view FSM Fast-steering mirror Gbps Gigabit per second GEO Geostationary orbit 9 INNOVA IN-orbit and Networked Optical Ground Stations Experimental Veri- fication Advanced Testbed IR Infrared ISS International Space Station JAXA Japan Aerospace Exploration Agency JPL Jet Propulsion Laboratory JSpOC Joint Space Operations Center LCRD Laser Communications Relay Demonstration LEO Low-Earth orbit LLCD Lunar Laser Communication Demonstration LLGT Lunar Lasercom Ground Terminal LOS Line-of-sight LUCE Laser Utilizing Communications Equipment Mbps Megabit per second MITLL Massachusetts Institute of Technology Lincoln Laboratory NASA National Aeronautics and Space Administration NICT National