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Radiolocation Using AM Broadcast Signals

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Radiolocation Using AM Broadcast Signals Radiolocation Using AM Broadcast Signals by Timothy Douglas Hall B.S. Electrical Engineering, University of Missouri, 1993 M.S. Electrical Engineering, University of Missouri, 1994 Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology September 2002 Massachusetts Institute of Technology All Rights Reserved Signature of Author________________________________________________________ Department of Electrical Engineering and Computer Science 26 August 2002 Certified by ______________________________________________________________ Charles C. Counselman III Professor of Planetary Science Thesis supervisor Accepted by _____________________________________________________________ Arthur C. Smith Chairman, Department Committee on Graduate Student Radiolocation Using AM Broadcast Signals by Timothy Douglas Hall Abstract I have designed, built, and evaluated a passive radiolocation system that uses only signals of opportunity, that is, signals that exist for purposes other than radiolocation. The system estimates the relative position vector between a base station, which is a navigation receiver at a known location, and a rover, which is like the base station but free to move about. The relative position vector, called the baseline vector, is determined by multilateration from observations of the carrier phases of signals received from AM broadcast stations. This system determines the horizontal components of the baseline with about ten-meter uncertainties for baseline lengths up to about 35 kilometers. The navigation receivers are implemented as software radios on standard Intel®-based personal computers. The signals received by a one-meter vertical whip antenna are band- pass-filtered, amplified, and digitized. The entire AM band is digitized so simultaneous observation of all available signals is achieved. All further processing of the signals, including carrier-phase determination, is implemented in software run on the personal computer. The base station and rover record observed phase, frequency, and amplitude data on their local hard drives; and navigation algorithms are implemented in post-real- time. The interpretation of a carrier-phase observation in terms of position is ambiguous because one cycle of a carrier wave is virtually indistinguishable from the next. Previous attempts at signal-of-opportunity navigation using carrier phase sidestepped the ambiguity problem by requiring that the initial position of the rover be known and that phase variations be tracked without interruption. I designed and implemented an ambiguity-function method that enables the phase ambiguity to be resolved instantaneously without position initialization or signal-tracking continuity. I encountered several impediments to AM-broadcast-based radiolocation that, if not dealt with appropriately, reduce positioning accuracy, reduce ambiguity-resolution robustness, or both. AM transmitter position uncertainty directly causes receiver position- determination uncertainty. Since the error in published antenna positions sometimes exceeds 100 meters, I conducted sub-meter-accuracy geodetic surveys of 29 Boston-area 3 AM-broadcast antennas. The directional radiation patterns of the array antennas of many AM broadcast radio stations have phases that vary with azimuth angle. I developed and implemented a model for the phase of a directional antenna that nearly eliminated the errors caused by this effect. AM broadcast signals travel primarily as groundwaves, which propagate with phase velocities that depend on the electrical properties of the ground. Using simulations and empirical data, I designed and implemented a model for groundwave propagation that greatly reduced the errors caused by this effect over a broad geographic area. Proximate overhead and underground conductors, especially ones that are part of vast interconnected networks, can perturb phase locally by a radian or more, and in some cases can cause ambiguity-resolution failure. At night when the D-layer of the ionosphere recombines, signals in the AM band reflect off the ionosphere, which enables so-called skywave propagation. Since skywave can lead to interference with distant stations, regulations require many radio stations to significantly reduce power at night. Therefore, signals from far fewer AM radio stations are useful for nighttime navigation. Among signals that are still useful at night, skywave signals interfere with the desired groundwave signals and cause positioning performance accuracy to degrade by more than an order of magnitude. AM radiolocation positioning performance varies greatly with the local environment of the navigation receivers. Outdoors in the open, 95% of positioning errors are smaller than 15 meters for baselines up to 35 kilometers long. In wooded areas, where GPS positioning performance drops significantly, AM positioning performance is not affected. However, significant challenges remain to make AM positioning useful near tall buildings in urban areas, or inside structures. Thesis Supervisor: Charles C. Counselman III Title: Professor of Planetary Science 4 Acknowledgements I thank Prof. Charles C. Counselman III, my thesis advisor, for the countless hours he spent helping me with this work. Working with Chuck was not only educational but also enjoyable. I could not possibly have had a better advisor. I also thank the other members of my thesis committee: Prof. John Tsitsiklis and Prof. John Kassakian. Both took time out of their vacations to read my thesis for which I am very grateful. I am grateful to many people at MIT Lincoln Laboratory for their support. Specifically, I thank Dr. Pratap Misra for introducing me to the field of radiolocation and for being an outstanding mentor. Pratap’s unwavering encouragement while I was working on my thesis is greatly appreciated. I also thank Dr. Jay Sklar for providing me with a research assistantship after other funding at the lab dried up. Many thanks also go to Brian Adams for providing me with lab and office space and for helping me on countless occasions with experiment logistics. I also thank the National Science Foundation for supporting my research through a Graduate Research Fellowship for the first three years of my program at MIT. Finally, I thank my family and friends for their love and support. I especially thank my parents, Ron and Gayle Hall, for their unquestioning love and encouragement. I believe my success in life is due in no small part to their selfless commitment to family. My family is my anchor. Last, but certainly not least, I thank Kristin Little for her love and friendship. 5 Table of Contents Abstract..................................................................................................... 3 Acknowledgements .................................................................................. 5 Table of Contents..................................................................................... 7 List of Figures .......................................................................................... 11 List of Tables............................................................................................ 17 Chapter 1: Introduction.......................................................................... 19 1.1 Navigation Using GPS ....................................................................................... 19 1.1.1 Navigation Using Differential GPS............................................................ 20 1.1.2 Navigation Using Observations of GPS Carrier Phases............................. 20 1.2 Navigation Using Signals of Opportunity.......................................................... 21 1.3 Motivation.......................................................................................................... 21 Chapter 2: Navigation Receiver ............................................................. 23 2.1 Frequency Band ................................................................................................. 23 2.2 AM Navigation System Considerations............................................................. 23 2.3 AM Navigation Receiver Hardware................................................................... 24 2.3.1 Antenna ...................................................................................................... 24 2.3.2 Pre-Amp ..................................................................................................... 25 2.3.3 Low Pass Filter........................................................................................... 26 2.3.4 Amplifier .................................................................................................... 27 2.3.5 Power Supply, Gain Control, and Peak Detection ..................................... 28 2.3.6 Calibrator.................................................................................................... 29 2.3.7 A/D Converter............................................................................................ 30 2.3.8 Clock Circuitry........................................................................................... 31 2.3.9 Computer.................................................................................................... 31 2.3.10 Construction ............................................................................................. 32 7 2.4 AM Navigation Receiver Software...................................................................
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