Quasi-Optical Discrete Lens Arrays for Synthetic Aperture Radar by GARY LOUIS RAIT B.S.E.E., Ohio Northern University, 1978 M.S.E.E., University of Southern California, 1980 M.B.A, Claremont Graduate School, 1988 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Electrical and Computer Engineering 2002 This thesis entitled: Quasi-Optical Discrete Lens Arrays for Synthetic Aperture Radar written by Gary Louis Rait has been approved for the Department of Electrical and Computer Engineering ________________________ Prof. Zoya B. Popović ________________________ Prof. William J. Emery Date ___________________________ The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. Rait, Gary Louis (Ph.D., Electrical Engineering) Quasi-Optical Discrete Lens Arrays for Synthetic Aperture Radar Thesis directed by Professor Zoya B. Popović Over the last twenty years, electronically-scanned arrays (ESAs) have benefited from significant research and development to become the state-of-the-art in both spaceborne and airborne synthetic aperture radar (SAR) remote sensing systems, but the resources required to provide the necessary RF performance are often substantial. While some alternative antenna technologies have been investigated, the use of Quasi-Optical (QO) antennas in SAR applications has not received much attention. This thesis focuses on the ability of QO antenna technology to provide the RF performance likely to be required by future SAR remote sensing systems, both spaceborne and airborne. Based on the types of SAR missions thus far conducted and the evolution of the corresponding mission-level requirements, the requirements for future SAR missions are extrapolated. SAR performance equations for curved-earth geometry are derived from the standard relationships described in the literature and are used to allocate mission-level requirements to the antenna level. This thesis concludes that QO antenna technology is applicable to spaceborne and airborne SAR systems and offers advantages over traditional ESA technology in certain cases. Antenna subsystem design work is done using both antenna technologies for each of three future SAR missions to determine the prime power, mass, and cost resources required to achieve the necessary performance. The fundamental efficiency with which the QO beamforming network performs and is implemented provides the potential for mass and cost savings. The ability of the QO beamforming network to easily accommodate multiple, simultaneous beams enables a new SAR operational mode that can provide either prime power reduction or wider-swath/finer-resolution coverage not achievable with any single- beam SAR antenna. iii DEDICATION I dedicate this to my wife Maureen who has always supported me and encouraged me to do better. She is the reason this is complete. I owe her everything. ACKNOWLEDGMENTS First and foremost, I wish to thank my thesis advisor, Professor Zoya Popović, for giving me this chance. She did not have the time to take on another student, let alone a non- traditional student without the sense to take a leave of absence from work to complete his research. She could have justifiably declined, but instead she took me into her student group and provided wise technical guidance and support. I would also like to thank the members of my committee, Professor William Emery, Professor Susan Avery, Professor K.C. Gupta, and Professor Penina Axelrad, for their review of this work. The classes I took from these professors were years ago, yet they did not hesitate to be on my committee. I am indebted to a few generations of Professor Popović’s students for the development of Quasi-Optical antenna technology as well as bearing with me over the years. Stein Hollung’s and Darko Popović’s work is especially applicable and is referenced herein. Ball Corporation financially enabled this effort and continues to provide me with challenging work opportunities in the antenna and SAR fields. In my 14 years with Ball I have had the privilege of working with the Jet Propulsion Laboratory on two spaceborne SAR missions and have learned much as a result. Continuing to work at Ball while completing this research was usually stressful and at times seemed hopeless. I wish to thank Janeil Walter for covering for me at work and bailing me out of conflicts on many occasions. Lastly, a friend for 14 years who worked with us as a subcontractor on the SIR-C mission, Ron Petersen, proofread all of the text and provided valuable feedback and moral support. I would not be here were it not for my family. My mother and father always emphasized higher-level education and gave me the opportunity to pursue it, an opportunity that they were not fortunate enough to have. Along with my brother, they also gave me the confidence necessary to pursue it. It was my wife’s daily indulgence and positive support, though, that pulled me through the last few laps of the race. v CONTENTS CHAPTER 1 INTRODUCTION………………………………………………………………….1 2 SAR MISSIONS AND ANTENNA TECHNOLOGY…………………………….7 3 SAR MISSION REQUIREMENT FLOWDOWN RESULTS…………………...45 4 QUASI-OPTICAL ANTENNA TECHNOLOGY………………………………..54 5 REPRESENTATIVE SAR MISSIONS FOR QO EVALUATION………………71 6 QO EVALUATION FOR AIRBORNE SAR…………………………………….81 7 QO EVALUATION FOR SPACEBORNE SAR………………………………..108 8 MULTIPLE-BEAM SAR OPERATION………………………………………..126 9 CONCLUSION AND FUTURE WORK………………………………………..135 BIBLIOGRAPHY…………………………………………………………………………..138 APPENDIX A SAR MISSION REQUIREMENTS FLOWDOWN……………………………146 B SAR PERFORMANCE MODEL……………………………………………….196 C QO EVALUATION FOR HIGH-RESOLUTION SPACEBORNE SAR………205 D ACRONYMS…………………………………………………………………...218 vi FIGURES FIGURE 2-1 The NASA multi-frequency, polarimetric AirSAR imaging system resides on the NASA DC-8 aircraft…………………………………………………………………10 2-2 The CCRS C/X-SAR imaging system resides on the Convair 580 aircraft…………11 2-3 The PHARUS imaging system resides on the Cessna Citation II aircraft…………..12 2-4 The DCRS EMISAR imaging system resides on a Gulfstream G3 aircraft…………13 2-5 Joint STARS is an operational military ground-target surveillance and command/control sensor resident on a Boeing 707-300 aircraft…………………….14 2-6 The TESAR high-resolution SAR utilizes the Predator UAV platform and features a gimbaled Ku-Band active phased array……………………………………………...15 2-7 NASA’s Seasat was the first spaceborne SAR for scientific remote sensing……….17 2-8 The SIR-A imaging system was the first Shuttle operational payload………………18 2-9 The Venera spacecraft was one of two launched by the Soviet Union in 1983 to study the planet Venus……………………………………………………………………..19 2-10 The Magellan system utilized a large reflector for SAR imaging of Venus as well as mission telecommunications………………………………………………………...20 2-11 The Almaz-1 satellite was distinguished by two large SAR antennas on either side of the spacecraft………………………………………………………………………...21 2-12 The ERS-1 imaging system was the first spaceborne SAR to generate commercially- available remote sensing data………………………………………………………..22 2-13 The SIR-C/X-SAR imaging system demonstrated the value of multi-parameter imagery………………………………………………………………………………23 2-14 CSA’s Radarsat-1 system provides multi-mode commercial remote sensing data today…………………………………………………………………………………24 2-15 SRTM used interferometric SAR processing to generate data from which the first contiguous topographic map of the Earth’s surface will be generated………………25 2-16 The most obvious difference between the ESA (a) and the AESA (b) is the presence of the element-level T/R modules…………………………………………………...33 2-17 ESA (a) and AESA (b) block diagrams highlighting ESA amplifiers and AESA transmit driver amplifiers and receive post-amplifiers………………………………40 vii 2-18 SAR antenna technology preference with electronic beam steering………………...44 3-1 SAR mission-level performance requirement flowdown to antenna performance requirements…………………………………………………………………………53 4-1 Quasi-optical antenna functionality………………………………………………….55 4-2 Traditional ESA or AESA antennas use constrained beamforming networks………55 4-3 The offset-fed reflector is a geometric analog to the QO antenna…………………..56 4-4 The planar QO antenna transforms a spherical wave to a plane wave electrically….57 4-5 Traditional ESA vs QO equivalent………………………………………………….58 4-6 Traditional AESA vs QO equivalent………………………………………………...58 4-7 Active, transmit/receive, X-Band lens………………………………………………61 4-8 Active X-Band lens unit cell layout…………………………………………………61 4-9 Passive, dual-polarized, X-Band, cylindrical lens…………………………………...62 4-10 Cylindrical lens steered beams………………………………………………………62 4-11 Parameters involved in calculation of gain of QO spatial beamforming network…..64 4-12 Generic QO block diagram showing noise temperature components and reference points………………………………………………………………………………...67 5-1 Approximate (flat-earth) relationships between swath (SW), slant range interval (R), and look angle interval (R)………………………………………………...76 s 6-1 AESA antenna block diagram for airborne high-resolution stripmap reference SAR mission……………………………………………………………………………….83 6-2 AESA antenna RF performance for airborne high-resolution stripmap reference SAR mission……………………………………………………………………………….84 6-3 AESA antenna mass for airborne high-resolution stripmap reference SAR mission..85 6-4 AESA antenna cost for airborne high-resolution stripmap reference SAR mission...86 6-5 Fully-distributed QO antenna block diagram for airborne high-resolution stripmap reference SAR mission………………………………………………………………88 6-6 Fully-distributed QO antenna RF performance for airborne high-resolution stripmap reference SAR mission………………………………………………………………89 6-7 Generic beamforming network efficiency calculations for the AESA………………92 viii 6-8 Generic beamforming network efficiency calculations for the passive ESA………..94 6-9 Generic beamforming network efficiency calculations for the fully-distributed QO antenna……………………………………………………………………………….96 6-10 Fully-distributed QO antenna mass for airborne high-resolution stripmap reference SAR mission…………………………………………………………………………97 6-11 Fully-distributed QO antenna cost for airborne high-resolution stripmap reference SAR mission…………………………………………………………………………98 6-12 Partially-distributed QO antenna block diagram for airborne reference SAR mission……………………………………………………………………………….99 6-13 Partially-distributed QO antenna RF performance for airborne reference SAR mission……………………………………………………………………………...100 6-14 Partially-distributed QO antenna mass for airborne reference SAR mission………101 6-15 Partially-distributed QO antenna cost for airborne reference SAR mission……….102 6-16 Centralized QO antenna block diagram for airborne reference SAR mission……..103 6-17 Centralized QO antenna RF performance for airborne reference SAR mission…...104 6-18 Centralized QO antenna mass for airborne reference SAR mission……………….105 6-19 Centralized QO antenna cost for airborne reference SAR mission………………...106 7-1 AESA antenna block diagram for spaceborne wide-swath reference SAR mission.109 7-2 AESA antenna RF performance for spaceborne wide-swath reference SAR mission……………………………………………………………………………...111 7-3 AESA antenna mass for spaceborne wide-swath reference SAR mission…………112 7-4 AESA antenna cost for spaceborne wide-swath reference SAR mission………….113 7-5 Fully-distributed QO antenna block diagram for spaceborne wide-swath reference SAR mission………………………………………………………………………..114 7-6 Fully-distributed QO antenna RF performance for spaceborne wide-swath reference SAR mission………………………………………………………………………..115 7-7 Fully-distributed QO antenna mass for spaceborne wide-swath reference SAR mission……………………………………………………………………………...116 7-8 Fully-distributed QO antenna cost for spaceborne wide-swath reference SAR mission……………………………………………………………………………...117 ix 7-9 Partially-distributed QO antenna block diagram for spaceborne wide-swath reference SAR mission……………………………………………………………………….……..118 7-10 Partially-distributed QO antenna RF performance for spaceborne wide-swath reference SAR mission……………………………………………………………..119 7-11 Partially-distributed QO antenna mass for spaceborne wide-swath reference SAR mission……………………………………………………………………………...120 7-12 Partially-distributed QO antenna cost for spaceborne wide-swath reference SAR mission……………………………………………………………………………...121 7-13 Centralized QO antenna block diagram for spaceborne wide-swath reference SAR mission……………………………………………………………………………...122 7-14 Centralized QO antenna RF performance for spaceborne wide-swath reference SAR mission……………………………………………………………………………...123 8-1 The AESA antennas of the SRTM interferometric SAR each utilized two simultaneous beams at different polarizations to increase swath and area coverage rate………………………………………………………………………………….127 8-2 The use of multiple, simultaneous beams in the along-track dimension…………...128 8-3 The azimuth-ambiguity-to-signal ratio is the ratio of the sum of the power received from the ambiguous doppler intervals to the power received from the unambiguous doppler interval……………………………………………………………………..130 8-4 Example two-beam composite transmit (one-way) pattern for one beam at zero doppler and the other steered 11 degrees away in the along-track dimension……..131 8-5 Example two-way pattern for zero-doppler beam showing the pattern degradation caused by the second beam steered 11 degrees away in the along-track dimension.131 A-1 Idealized flat-earth imaging geometry……………………………………………..148 A-2 Practical curved-earth imaging geometry…………………………………………..149 A-3 Real-aperture along-track resolution……………………………………………….151 A-4 Top view of SAR geometry………………………………………………………...152 A-5 Along-track target separation………………………………………………………153 A-6 Azimuth beamsteering in “spotlight” imaging mode………………………………159 A-7 SAR swath width…………………………………………………………………...160 A-8 Flat-earth approximation to swath width…………………………………………...161 x