Advanced techniques for centimeter-accurate GNSS positioning on low-cost mobile platforms
Over 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.