Pointing System Performance Analysis for Optical Inter-satellite Communication on CubeSats by Hyosang Yoon B.S., Korea Advanced Institute of Science and Technology (2008) S.M., Korea Advanced Institute of Science and Technology (2010) Submitted to the Department of Aeronautics and Astronautics in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2017 @ Massachusetts Institute of Technology 2017. All rights reserved. Author ....................... Signature redacted Department of Aeronautics and Astronautics May 24, 2017 Certified by.....Signature redacted ............... Kerri L. Cahoy Associate Professor of Aeronautics and Astronautics Thesij Supervisor Certified by ....... Signature redacted ..... Steven R. Hall Professor of eronVbics 4nd Asironautics Certified by............. Signature redacted ....... David W. Miller Professo.-ro r cs and Astronautics Accepted by ............ Signature redacted ('* Youssef M. Marzouk Associate Professor of Aeronautics and Astronautics MASSACHUSETTS INSTITUT E Chair, Graduate Program Committee OF TECHNOLOGY SEP 272017 LIBRARIES ARCHIVES Pointing System Performance Analysis for Optical Inter-satellite Communication on CubeSats by Hyosang Yoon Submitted to the Department of Aeronautics and Astronautics on May 24, 2017, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract Free-space optical communication using lasers (lasercom) is a leading contender for future space-based communication systems with potential advantages over radio fre- quency (RF) communication systems in size, weight, and power consumption (SWaP). Key benefits are due to the shorter wavelength: additional bandwidth and narrow beam width. The narrower beam supports higher energy density for a given aperture size, so that lasercom can transmit data at the same rate with smaller SWaP as well as improve link security since the beam footprint is smaller. Lasercom is an attractive option for improving inter-satellite links (ISL) for resource- constrained CubeSats, which have emerged as a standard form of a small satellite since 1999. However, lasercom requires much more accurate pointing because of its nar- rower beam width. Accurate pointing is not trivial for most CubeSat platforms due to their resource constraints. A typical 3U CubeSat is 34 cm x 10 cm x 10 cm with less than 5 kg mass and about 10 W of available orbit-average power. This thesis presents pointing and tracking technologies to support lasercom on CubeSats. It covers three critical issues: (1) attitude determination and control of CubeSats, (2) relative orbit determination, and (3) development of a miniaturized fine beam pointing module. New attitude determination and control algorithms are de- veloped, simulated, and validated with hardware in the loop demonstrations; results indicate that lasercom at data rates competitive with or better than RF is feasible on CubeSats. For attitude determination and control (ADC), this thesis develops a new attitude estimation algorithm, which is called Attitude and Parameter estimation Kalman fil- ter (APKF). Attitude determination (AD) is thought to be more challenging than attitude control (AC) for CubeSats because of the limited capabilities of sensors that are compatible with the small form factor and resource constraints of CubeSats. The largest difference between a CubeSat and a larger satellite is the gyroscopes that measure rotation rates. Since a CubeSat is normally not able to accommodate high quality gyroscopes, the APKF is used to improve estimation without relying on gy- roscope measurements. The APKF estimates CubeSat attitude and body rates as 3 well as other unknown parameters such as the moment of inertia (MOI), actuator alignment, and the residual dipole moments. For relative orbit determination, this thesis describes an estimation algorithm that fuses different types of orbital measurements using the Kalman filter. There are three measurements that can be used in the relative orbit estimation for low earth orbiting (LEO) lasercom crosslink CubeSats: Global Navigation Satellite System (GNSS) nav- igation solutions for an individual satellite (e.g. Satellite A or "SatA"), beacon beam measurements at SatA, and GNSS navigation solutions of the other satellite (SatB) transferred through ground station networks. The GNSS and beacon are measured at SatA, so these can be assumed to have negligible time delay, but the arrival time of the SatB navigation solutions will be an out-of-sequence measurement (OOSM) whose arrival time will be delayed due to the ground station relay. To fuse the sensor data with different measurement times, a new algorithm called the Augment Fixed- Lag Smoother (AFLS) is developed. To update the Kalman filter with an OOSM, the AFLS generates the estimates at the measurement time of the OOSM by interpo- lation. The AFLS is applied to a nonlinear system as the extended AFLS (EAFLS). The Satellite Tracking Kalman Filter (STKF) is developed using the EAFLS. The fine pointing system (FPS) is necessary because while the CubeSat attitude determination and control and the orbit determination developments cover the Cube- Sat's body pointing capability, due to the extremely narrow beam desired for high-rate laser communications, body pointing alone cannot satisfy the beam pointing require- ments. The example case used in this thesis is a CubeSat design concept mission with an inter-satellite laser communication link. To reduce the pointing error, a FPS needs to be implemented as the final stage for beam pointing. This thesis demon- strates the feedback control loop of the FPS using a hardware-in-the-loop test. A key component of the FPS is the miniaturized micro-electro-mechanical systems (MEMS) fast steering mirror (FSM) which is the actuator used to point the laser beam. Using a commercial-off-the-shelf (COTS) MEMS FSM that is also planned for use on the flight module, the fine pointing control loop has been demonstrated with results that show that it is feasible to meet the pointing requirement for a 3U CubeSat mission whose goal is 20 Mbps link at 25 km to 1000 km crosslink range. By developing and demonstrating the critical technologies for both spacecraft body pointing and the fine beam pointing, this thesis has demonstrated the feasibility of a CubeSat lasercom crosslink at a data rate and form factor that can outperform RF, leading to a high-speed and secure ISL for CubeSats. Thesis Supervisor: Kerri L. Cahoy Title: Associate Professor of Aeronautics and Astronautics Thesis member: Steven R. Hall Title: Professor of Aeronautics and Astronautics Thesis member: David W. Miller Title: Professor of Aeronautics and Astronautics 4 Acknowledgments First of all, I would like to thank my thesis advisor, Prof. Kerri Cahoy. She is the one who selected me in the application process, and who supported me through the whole doctoral program. I am impressed with her enthusiasm for space missions and projects as well as her devotion to her students: it was not out of the ordinary to receive her reply emails at 3 in the morning. I would not have been able to do the research and projects without her support. Next, I am grateful to the rest of my thesis committee, Prof. Steven Hall and Prof. David Miller. I took a class on Kalman filtering (16.322) from Prof. Hall in my first semester at MIT and it plays a key role in this thesis. He gave me very constructive advice when I was stuck on control problems in my thesis research as well. Also, Prof. Miller's comments on pragmatic approaches for space missions were helpful to set up the structure of the lasercom pointing systems. I thank all my friends in STAR Lab and SSL. KitKat, TimTam, Emily, Christian, Roman, Rachel, Michael, Greg, Derek, Max, Caleb, David, Chris, Pratik, and every- one! It was really lucky to work with you guys. Especially, Kathleen Riesing, thank you for being my best friend and English teacher for three years. Also, a special thank you to my Korean friends at MIT. My appreciation to Sangyeon Cho, who went through all the challenges of studying abroad with me from the beginning, and to Jinwook Lee, who has been a great consultant for me on various topics. I am going to miss you guys and all the time we spent together. I would like to acknowledge the support from Samsung Scholarship. Not only their financial support, but also their effort to enable networking among the recipients who are some of the most brilliant people in Korea. Finally I thank my family: my mom, dad, brother, Eunji Bang, and Geumchae Yoon. Thank you so much for everything. I love you. 5 6 Contents 1 Introduction 17 1.1 Challenges in Laser Beam Pointing ..... ...... ...... .. 18 1.2 Problem Statement ..... .... ..... ..... ..... .... 21 1.3 Thesis Scope and Contributions .. ...... ....... ...... 21 1.4 Thesis Organization ......... ........ ........ ... 23 2 Gaussian Beam Optics and Pointing Requirement 25 2.1 Gaussian Beam Optics ..... ........ .. ..... 26 2.2 Aperture Diameter and Gaussian Beam Width ........ ..... 28 2.3 Crosslink Lasercom Reference Mission Overview ... ...... ... 29 2.3.1 CubeSat Crosslink Lasercom Reference Mission ...... .. 29 2.3.2 CubeSat Intersatellite Lasercom Optics .... ..... .... 31 2.3.3 Pointing Requirement for the CubeSat Lasercom Intersatellite Crosslink M ission .. ....... ....... ....... .. 32 3 The Kalman Filter Overview 33 3.1 The discrete-time Kalman filter ...... ............... 33 3.2 The Extended Kalman Filter ....