Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Gregory Myer
M.S. Final
Jun 23, 2017, 9:00 am - 10:00 am
Ionospheric Scintillation Effects On GPS Pseudorange and Carrier Phase Measurements and Positioning Algorithms To Improve Positioning Solution Accuracy During Scintillation
Abstract: The ionosphere is an important cause of disturbances on GNSS signals, especially in high latitudes and equatorial areas. Previous studies indicate that while ionospheric scintillation may cause abrupt, random fluctuations in carrier phase measurements, its impact on pseudorange is less serious. Since modern GNSS receivers, especially those for high precision applications, use carrier phase corrected pseudoranges to improve accuracy of position solutions, there exists the need to have a better understanding of the scintillation effects on carrier phase measurements and developing means to mitigate scintillation induced errors in navigation solutions.

In this thesis, scintillation impacts are demonstrated on carrier phase and pseudorange measurements using real scintillation data collected at low and high latitudes, and the effect on positioning is investigated and mitigated. To obtain a more insightful and quantitative understanding of the impact, the data was used to generate position solutions using several custom navigation processing algorithms. The results clearly indicate that sudden carrier phase discontinuities during strong scintillation lead to the degradation of carrier-smoothed pseudorange accuracy and consequently, results in large position errors. During strong scintillation with no carrier phase discontinuities, comparatively smaller position errors are found due to phase fluctuations that cause to small changes in the range measurements.

Based on this analysis, we present several adaptive positioning techniques to mitigate scintillation induced position errors. One algorithm simply replaces the carrier-smoothed pseudorange with the unsmoothed pseudorange for satellites that are affected by cycle slips or experiencing outages on the carrier phase measurements. Another adaptive algorithm uses the GDOP to determine if a scintillating satellite can be completely removed from the navigation processing to improve positioning accuracy.

Results show that during segments of data affected by cycle slips, substitution of the unsmoothed pseudorange is able to reduce position errors by at least 60%, depending on the severity of the cycle slip. The algorithms that substitute the unsmoothed pseudorange were found to increase errors as compared to a conventional technique that repairs cycle slips, which indicates that it is still best to use the carrier-smoothed pseudoranges as long as there are no discontinuities. After repairing cycle slips, results from this thesis show that the adaptive technique based on the analysis of the GDOP was able to reduce maximum errors on average by 13% on all of the data sets when comparing to a conventional algorithm. It was also found that a new carrier-smoothing technique can reduce maximum errors by 7.9%. Alternative approaches for future improvements are also discussed.
Adviser: Jade Morton
Co-Adviser: N/A
Non-ECE Member: Clayton Shonkwiler
Member 3: Jesse Wilson
Addional Members: N/A
G. Myer, Y. Morton, and B. Schipper, “Ionospheric scintillation effects on GPS pseudorange and carrier phase measurements and an adaptive algorithm to limit position errors during scintillation,” in Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, pp. 971 – 988, 2017.
Program of Study: