Browsing by Subject "GNSS"
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Item Advanced techniques for centimeter-accurate GNSS positioning on low-cost mobile platforms(2015-12) Pesyna, Kenneth Mark Jr.; Humphreys, Todd Edwin; Heath, Robert W., Ph. D.; Vikalo, Haris; York, Johnathan; Sanghavi, SujayOver the past decade, GPS and other Global Navigation Satellite System (GNSS) chipsets have become smaller, cheaper, and more energy efficient, so much so that they now come standard in most smartphones and tablets. Under good multipath conditions, one can expect 2-to-3-meter-accurate positioning with these chipsets, under adverse multipath, accuracy degrades to 10 meters or worse. Outside the mainstream of consumer GNSS receivers, however, centimeter---even millimeter---accurate GNSS receivers are used routinely in geodesy, agriculture, and surveying. The key to their accuracy is a radically different approach to positioning in which the standard code-phase (or pseudorange) positioning technique is replaced by differential carrier-phase positioning. Adopting this high-precision carrier-phase-based technique for consumer-grade mobile devices is possible, but comes with significant challenges. This dissertation identifies and addresses the challenges to performing centimeter accurate carrier-phase differential GNSS (CDGNSS) positioning on low-cost mobile devices. To this end, this dissertation makes three primary contributions. First, this dissertation develops a carrier phase reconstruction technique to address the high power consumption of current CDGNSS algorithms. The reconstruction technique enables a continuous and unambiguous phase time history to be reconstructed from intermittent phase measurements, permitting aggressive duty cycling of the mobile device's internal GNSS chip, decreasing energy consumption. Second, this dissertation demonstrates that a centimeter-accurate positioning solution is possible based on GNSS data collected using a smartphone, a first in the open literature. It is identified that the primary impediment to performing CDGNSS on smartphones lies not in the commodity GNSS chipset within the phone, but instead in the antenna, whose chief failing is its poor multipath suppression, resulting in long initialization times. It is demonstrated that wavelength-scale random antenna motion can be used to decorrelate multipath errors and reduce the initialization period---the so-called time-to-ambiguity-resolution (TAR)---of smartphones employing CDGNSS to obtain centimeter-level positioning fix. Finally, this dissertation develops a framework that tightly fuses smartphone camera image measurements with GNSS carrier phase measurements to reduce CDGNSS initialization times beyond what is achievable using antenna motion alone. The framework augments the traditional bundle-adjustment- (BA-)-based structure from motion (SFM) algorithm with the carrier phase differential GNSS (CDGNSS) algorithm in a way that preserves the key features of both algorithms, namely the sparseness of the matrices in BA and the integer structure of the ambiguities in CDGNSS. The framework is shown to produce a faster, more robust, and more accurate positioning solution than achievable with existing techniques.Item Analysis and synthesis of collaborative opportunistic navigation systems(2014-05) Kassas, Zaher; Humphreys, Todd Edwin; Arapostathis, Ari, 1954-Navigation is an invisible utility that is often taken for granted with considerable societal and economic impacts. Not only is navigation essential to our modern life, but the more it advances, the more possibilities are created. Navigation is at the heart of three emerging fields: autonomous vehicles, location-based services, and intelligent transportation systems. Global navigation satellite systems (GNSS) are insufficient for reliable anytime, anywhere navigation, particularly indoors, in deep urban canyons, and in environments under malicious attacks (e.g., jamming and spoofing). The conventional approach to overcome the limitations of GNSS-based navigation is to couple GNSS receivers with dead reckoning sensors. A new paradigm, termed opportunistic navigation (OpNav), is emerging. OpNav is analogous to how living creatures naturally navigate: by learning their environment. OpNav aims to exploit the plenitude of ambient radio frequency signals of opportunity (SOPs) in the environment. OpNav radio receivers, which may be handheld or vehicle-mounted, continuously search for opportune signals from which to draw position and timing information, employing on-the-fly signal characterization as necessary. In collaborative opportunistic navigation (COpNav), multiple receivers share information to construct and continuously refine a global signal landscape. For the sake of motivation, consider the following problem. A number of receivers with no a priori knowledge about their own states are dropped in an environment comprising multiple unknown terrestrial SOPs. The receivers draw pseudorange observations from the SOPs. The receivers' objective is to build a high-fidelity signal landscape map of the environment within which they localize themselves in space and time. We then ask: (i) Under what conditions is the environment fully observable? (ii) In cases where the environment is not fully observable, what are the observable states? (iii) How would receiver-controlled maneuvers affect observability? (iv) What is the degree of observability of the various states in the environment? (v) What motion planning strategy should the receivers employ for optimal information gathering? (vi) How effective are receding horizon strategies over greedy for receiver trajectory optimization, and what are their limitations? (vii) What level of collaboration between the receivers achieves a minimal price of anarchy? This dissertation addresses these fundamental questions and validates the theoretical conclusions numerically and experimentally.Item GPS L2 C signal survey and the development of the emergent MATLAB L2 C (EMAL2) receiver(2012-12) Bright, Marlon Wayne; Humphreys, Todd Edwin; Schutz, BobThe United States Department of Defense has introduced two new GPS civilian signals on its “Link 2” (L2) and “Link 5” (L5) center frequencies. The first of these new civilian signals to reach full operational capability in the GPS constellation will be the L2 C signal. The L2 C signal boasts new signal structure features aimed at better tracking performance in comparison to the legacy L1 C/A signal. Amongst these are two new chip-by-chip interleaved spreading code sequences, Civilian Moderate (CM) and Civilian Long (CL), and a new, higher resolution navigation message, CNAV. The two new C codes are longer than the legacy C/A code and feature a data less pilot signal (CL) for improved tracking performance in weak signal environments. This work investigates L2 C acquisition and tracking considerations and implements algorithms for acquiring and tracking the signal in a software-defined receiver developed in MATLAB. The Emergent MATLAB L2 C (EMAL2) receiver was developed for the purpose of GPS signal simulator testing. This software-defined receiver differs from legacy receivers containing application specific integrated circuits (ASICs) in that all of EMAL2’s digital signal processing is done in software able to run on a general purpose processor. This approach offers greater flexibility and ease in configuration over ASICs for tracking a number of different types of signal structures in the receiver. The EMAL2 receiver’s design and implementation is described here-in. Initial testing of the EMAL2 receiver was conducted with live-sky signal data captured by antennas and front-ends at the University of Texas Radionavigation Laboratory (UT RNL). The data was processed by the GRID receiver (also at the UT RNL) to provide EMAL2 baseline received signal characteristics.Item Secure navigation and timing without local storage of secret keys(2014-05) Wesson, Kyle D.; Humphreys, Todd Edwin; Evans, Brian L. (Brian Lawrence), 1965-Civil Global Navigation Satellite System (GNSS) signals are broadcast unencrypted worldwide according to an open-access standard. The virtues of open-access and global availability have made GNSS a huge success. Yet the transparency and predictability of these signals renders them easy to counterfeit, or spoof. During a spoofing attack, a malefactor broadcasts counterfeit GNSS signals that deceive a victim receiver into reporting the spoofer-controlled position or time. Given the extensive integration of civil GNSS into critical national infrastructure and safety-of-life applications, a successful spoofing attack could have serious and significant consequences. Unlike civil GNSS signals, military GNSS signals employ symmetric-key encryption, which serves as a defense against spoofing attacks and as a barrier to unauthorized access. Despite the effectiveness of the symmetric-key approach, it has significant drawbacks and is impractical for civil applications. First, symmetric-key encryption requires tamper-resistant receivers to protect the secret keys from unauthorized discovery and dissemination. Manufacturing a tamper-resistant receiver increases cost and limits manufacturing to trusted foundries. Second, key management is problematic and burdensome despite the recent introduction of over-the-air keying. Third, even symmetric-key encryption remains somewhat vulnerable to specialized spoofing attacks. I propose an entirely new approach to navigation and timing security that avoids the shortcomings of the symmetric-key approach while maintaining a high resistance to spoofing. My first contribution is a probabilistic framework that develops necessary components of signal authentication. Based on the framework, I develop an asymmetric-key cryptographic signal authentication technique and a non-cryptographic spoofing detection technique, both of which operate without a secret key stored locally in a secure receiver. These anti-spoofing techniques constitute the remaining two contributions of this dissertation. They stand as viable spoofing defenses for civil users and could augment---or even replace---current and planned military anti-spoofing measures. Finally, I offer an in-depth case study of the security vulnerabilities and possible cryptographic enhancements of a modern GNSS-based aviation surveillance technology in the context of the technical and regulatory aviation environment.