Design of a Hardware Platform for GPS-Based Orientation Sensing [report]

Daniel Kirkpatrick
2000 unpublished
Unmanned aerial vehicles (UAV's) have recently gained popularity in military, civil service, agriculture, commercial, and hobby use. This is due in part to their affordability, which comes from advances in component technology. That technology includes microelectromechanical systems (MEMS) for inertial sensing, microprocessor technology for sequential algorithm processing, field programmable gate arrays (FPGA's) for parallel data processing, camera technology, global navigation satellite
more » ... (GNSS's) for navigation, and battery technology such as the high energy density of lithium polymer batteries. Despite the success of the technology to date, there remains development before UAV's should be flying alongside manned aircraft or over populated areas. One concern is that UAV electronics are not as safe, reliable or robust as manned-aircraft electronics because UAV's are not certified by the FAA. Another concern for UAV operation is with control algorithms and sensors, particularly in the estimation of the aircraft state, which is the position, velocity, and orientation of the aircraft. Some problems, such as numerical stability of a control algorithm or flight in windy and turbulent conditions have only been solved for certain conditions of wind, weather, or maneuvers. Outside those conditions, the actual orientation of a flying craft can mislead to the control system, and the control system may not be able to recover without a crash. When pilots fly manned aircraft in instrument meteorological conditions, or conditions of limited visibility of the ground, terrain, and obstacles, the pilot must fly in a manner which avoids abrupt maneuvers which could disturb accuracy of the aircraft's instruments. In a UAV without a pilot, there i is a need to estimate the position and orientation of a UAV in an absolute manner unambiguous relative to the Earth. The position and orientation estimate must not depend on carefully controlled flight paths, but instead the estimate must be robust in the presence of UAV flight dynamics. This thesis describes the design, implementation, and evaluation of a hardware platform for GPS based orientation sensing research. In this work, we considered a receiver with three or four RF sections, each connected to an antenna in a triangular or tetrahedral pyramid constellation. Specific requirements for the receiver hardware and functionality were created. Circuitry was designed to meet the requirements using commercial off-the-shelf (COTS) radio frequency (RF) modules, a mid-sized microcontroller, an FPGA, and other supporting components. A printed circuit board (PCB) was designed, fabricated, assembled, and tested. A GPS baseband processor was designed and coded in Verilog hardware description language. The design was synthesized and loaded to the FPGA, and the microcontroller was programmed to track satellites. With the hardware platform implemented, live satellite signals were found and tracked, and experiments were performed to explore the validity of GPS based orientation sensing using short antenna baselines. The platform successfully allows the user to develop correlator designs and explore carrier phase based orientation measurement using only software/Verilog modifications. Initial results of carrier phase based orientation sensing are promising, but the presence of multipath signal interference shows room for improvement to the baseband processing code. ii Dedication This thesis is dedicated to my father, who tirelessly answered every question asked about how the world works; to my mother, who supported and encouraged me on my lifelong journey through education; to my wife, who held the kids at bay while I write this; and to everyone that let me explain the elegance and genius of GPS. iii Acknowledgments The origin of this project was part of a Portland State University, Maseeh College of Engineering and Computer Science (MCECS) "Beta project" (formerly known as an innovation project), which was awarded funds in 2011. The goal of that project was to advance technology toward creating a UAV which could locate lost people in remote mountain or forest terrain by flying search patterns and using infrared sensitive cameras. I would like to thank the MCECS Beta project board for their financial and technical support of the UAV project which, in part, funded this thesis project. Further, I would like to thank Rob Gaskell and Jeremy Booth for their participation in the Beta project, for their review of my schematic and HDL work on this thesis, and for listening to my explanations of how GPS works. I likely learned as much by explaining how GPS works to them as I did reading the subject for myself. Finally, I want to thank my thesis adviser, Dr James McNames, and committee members, Professors Mark Faust and Roy Kravitz, for their encouragement and willingness to accept only my best work. iv
doi:10.15760/etd.2194 fatcat:74qklhpcdfbalgu2l4r6juuss4