Design and Evaluation of Distributed Spacecraft Missions for Multi-Angular Earth Observation by Sreeja Nag B.S. Exploration Geophysics, Indian Institute of Technology, Kharagpur, 2009 M.S. Exploration Geophysics, Indian Institute of Technology, Kharagpur, 2009 S.M. Aeronautics and Astronautics, Massachusetts Institute of Technology, 2012 S.M. Technology and Policy, Massachusetts Institute of Technology, 2012 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 Engineering at the Massachusetts Institute of Technology June 2015 © 2015 Massachusetts Institute of Technology. All rights reserved. Signature of Author _________________________________________________________________________ Department of Aeronautics and Astronautics April 17, 2015 Certified by _______________________________________________________________________________ Olivier L. de Weck Professor of Aeronautics and Astronautics and Engineering Systems Thesis Supervisor Certified by _______________________________________________________________________________ David W. Miller Professor of Aeronautics and Astronautics Thesis Committee Member Certified by _______________________________________________________________________________ Kerri L. Cahoy Assistant Professor of Aeronautics and Astronautics Thesis Committee Member Certified by _______________________________________________________________________________ Charles K. Gatebe Senior Research Scientist at NASA Goddard Space Flight Center Thesis Committee Member Accepted by ______________________________________________________________________________ Paulo Lozano Associate Professor of Aeronautics and Astronautics Chair, Graduate Program Committee Page 2 of 243 Design and Evaluation of Distributed Spacecraft Missions for Multi-Angular Earth Observation by Sreeja Nag Submitted to the Department of Aeronautics and Astronautics On April 17, 2015 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Aeronautics and Astronautics Engineering ABSTRACT Distributed Spacecraft Missions (DSMs) are gaining momentum in their application to Earth science missions owing to their ability to increase observation sampling in spatial, spectral and temporal dimensions. This thesis identifies a gap in the angular sampling abilities of monolithic spacecraft in Earth observation missions and proposes to use distributed spacecraft to address this gap. The science performance metric is chosen to be Bidirectional Reflectance-Distribution Function (BRDF), which describes the directional and spectral variation of reflectance of an optically thick surface element at any time instant. Airborne instruments are the gold standard for BRDF estimation (e.g. Cloud Absorption Radiometer/CAR). They can collect thousands of reflectance measurements at the same ground spot, but are localized in space and time. Spaceborne platforms estimate BRDF by combining angular measurements over time, made along-track, cross- track or by autonomous pointing. However, their plane of data acquisition is very restricted with respect to the sun and the target itself might change over time of acquisition. Formations with spectrometer payloads can make multi-spectral reflectance measurements of a ground target, at many zenith and azimuthal angles simultaneously and estimate the angular signature of the surface. Constellations with overlapping ground spots can capture the angular components of global and temporally varying science products. This work demonstrates the performance impact and feasibility of a BRDF estimation mission using a systems engineering tool (driven by model based systems engineering or MBSE), intricately coupled with a science evaluation tool (driven by observing system simulation experiments or OSSEs). Formations and payload pointing strategies are optimized to maximize angular spread and minimize estimation errors. The effect of angular spread on spatial, spectral and radiometric sampling dimensions is quantified for available spectrometer payloads that fit within the 6U CubeSat standard. Technical feasibility within 6U is verified for attitude determination and control, propulsion, communication and onboard processing modules. DSM architectures are generated and compared to each other and monoliths, in terms of BRDF, albedo, gross primary productivity and total outgoing radiation. Performance is benchmarked with respect to data from previous airborne campaigns (NASA CAR), tower measurements (AMSPEC II) and ideal values from radiative transfer or climate models (UMGLO, COART); and accepted BRDF models. A formation of 6 small satellites produces lesser average error (21.82%) than larger monoliths (23.2%) over extended time periods, purely in terms of angular sampling benefits. The monolithic albedo error of 3.6% is shown to be outperformed by a formation of 3 satellites (1.86%), when arranged optimally and by a formation of 5 satellites (3.36%) when arranged in any way. An 8-satellite formation pushes albedo errors to 0.67% and reduces gross primary productivity errors from 89.77% (monolithic) to 78.69%. Thesis Supervisor: Olivier L. de Weck Title: Professor of Aeronautics and Astronautics and Engineering Systems Thesis Committee Member: David W. Miller Title: Professor of Aeronautics and Astronautics Thesis Committee Member: Kerri L. Cahoy Title: Assistant Professor of Aeronautics and Astronautics Page 3 of 243 Thesis Committee Member: Charles K. Gatebe Title: Senior Research Scientist at NASA Goddard Space Flight Center Page 4 of 243 Everything I am and ever hope to be, I owe to my mother. Page 5 of 243 Page 6 of 243 Acknowledgements First and foremost, I want to sincerely thank my advisor, Prof. Olivier de Weck, for consistent belief in my capabilities and for his guidance. I am particularly grateful to him for going out of his way on many occasions to provide unwavering support of my pursuit of unexpected opportunities and ever-evolving interests. His dedication to the topics of his students’ interests, irrespective of whether they align with his personal research goals, is very admirable and deeply appreciated. The quality of this dissertation would not have been what it currently is without the mentorship and invaluable advice from each of my PhD committee members, readers and advisors, Prof. David Miller, Prof. Kerri Cahoy, Prof. Jeffrey Hoffman and Prof. Dava Newman at MIT; and Dr. Charles Gatebe and Dr. Jacqueline LeMoigne at NASA Goddard Space Flight Center (GSFC). A special mention for Prof. Anantha Chandrakasan for being a very supportive mentor, especially during academically difficult times. I began my PhD research by showing up at GSFC for the summer of 2012. All I knew back then was that I wanted to make an impact in the Earth Sciences with large numbers of small satellites. I had the lucky privilege of meeting Dr. Warren Wiscombe, a senior scientist at GSFC and an MIT alum, who introduced to me to “BRDF” and asked me to go solve the “BRDF problem”. It has been nearly three years and every time I meet Warren, I learn something brand new in earth science. I am indebted to my Goddard family for the quality of my PhD: To Charles for giving me my GSFC break without ever having met me, for being a mentor whenever I needed and for a comprehensive introduction to Science@NASA; To Shana for sharing her home with me every time I visited for extended periods; To Jacqueline for supporting me at work ever so cherubically, for consistent mentorship and for taking me to my first launch (LADEE at Wallops); To Christine Chiu, Bert Pasquale, Tilak Hewagama, Shahid Aslam, Alexis Lyapustin, Georgi Georgiev, Crystal Schaaf and Miguel Roman for teaching me about algorithms and instruments in spite of their busy schedule; To Thomas Hilker and Forrest Hall for sharing their data and invaluable advice on photosynthetic measurements; To Jonathan Fentzke and Lars Dyrud for introducing me the world of dynamic and next-gen space entrepreneurship. Very deep thanks to Sangram Ganguly, Steve Hipskind and Chad Frost at NASA Ames Research Center (ARC) for giving me an opportunity to move to the wonderful San Francisco Bay Area. I have loved every bit of being here and hope the adventure continues. Starting life in a new place didn’t feel as difficult as they say out to be, thanks to Abhishek (owner of the Batmobile); Christina (the super mom); Fan, Jan, Jamie and Andres; Anirudh, Moumita, Archi, Annapoorna mashi and everyone else who smoothened my move. Generous funding for my work and study for my 3 years of PhD pursuit has come from the Schlumberger Faculty for the Future Fellowship, NASA Earth and Space Science Fellowship, MIT Zakhartchenko Fellowship, Zonta Amelia Earhart Fellowship, Draper Laboratory and the NASA InVEST Program, NASA GSFC’s Internal Research and Development (IRAD) Grant, USRA’s GESTAR program at NASA GSFC and BAERI’s ARC-CREST program at NASA ARC. Consistent funding from evaluators and mentors who believed in my projects and me enabled my PhD journey to be a smooth-sailing but intellectually challenging joyride. MIT’s International Students’ Office has been indispensable in my life of infinite paperwork as a green-horned, non-resident alien in the US of A. Marilyn Good and Julie Finn, you should take a bow for the years and years of very appreciated support you have provided to the SSL and SERG. Page 7 of 243 Work can never be satisfying through content alone. I got to travel so much