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Session B2: Trends in GNSS Augmentation Systems

Impact of Phase Transitions due to Ionospheric Scintillation on GBAS Integrity
Andrew K. Sun, Jiyun Lee, Korea Advanced Institute of Science and Technology; Sam Pullen, Stanford University
Date/Time: Wednesday, Sep. 18, 4:00 p.m.

Best Presentation

The Ground-Based Augmentation System (GBAS) provides differential corrections and integrity monitoring of Global Navigation Satellite Systems (GNSS) through different levels of service known as GBAS Approach Service Types (GAST). GAST D, an upgrade to GAST C, incorporates additional integrity monitors to detect anomalous ionospheric delays and alert users when significant discrepancies between estimated and actual ionospheric delays are detected. GAST F takes a more direct approach by measuring the differential ionospheric delay using dual-frequency (DF) Ionospheric Gradient Monitoring (IGM) between the reference station and airborne users. For dual-frequency multi-constellation (DFMC) GBAS, GAST F/X is proposed for future service type that uses a DF combination of measurements to estimate and remove ionospheric delays. This approach significantly improves the system’s accuracy and integrity under ionospheric anomalies, especially in equatorial regions. The choice of service type depends on the specific requirements of the operation and the prevailing ionospheric conditions.

However, ionospheric scintillation remains a significant concern for GBAS operations. Ionospheric scintillation, often accompanied by irregular structures such as equatorial plasma bubbles, involves rapid fluctuations in the amplitude and phase of GNSS signals. This can lead to deep signal fading, resulting in the loss of receiver tracking lock. This may require reverting to single-frequency (SF) mode as a backup for DF service or even cause the complete loss of satellite signals for SF service. Previous studies by Saito et al. (2023) and Silva et al. (2023) have investigated signal loss to assess the availability of the service during scintillation utilizing real GNSS data collected from GBAS experimental testbeds in equatorial regions.

This paper focuses on a lesser-known but significant effect of ionospheric scintillation: phase transition. Unlike abrupt cycle slips that occur after loss of phase lock, phase transition is characterized by gradual full-cycle phase changes during phase unwrapping. These diffractive effects stem from the propagation environment while maintaining phase lock loop tracking, which is distinct from typical cycle slips. The gradual nature of phase transitions, coupled with other phase contributions, complicates their identification using conventional cycle slip detection methods. Consequently, they can persist undetected and lead to bias-like errors in phase measurements.

Phase transitions can introduce additional errors in smoothed pseudoranges, significantly increasing these errors during severe scintillation, where they can occur frequently. Since the diffractive effects are independent of the carrier frequency, they cannot be removed by DF combinations. This leads to residual errors in DF combinations that are even larger than those in SF measurements, cancelling the benefit of DF combinations which are the primary modes used in DFMC GBAS. Therefore, it is essential to account for scintillation-induced errors in the DF bounding error sigmas. Moreover, the impact of these additional errors on integrity monitor test statistics can degrade monitoring performance, potentially leading to false alerts or missed detections.

Previous studies, including Saito et al. (2018), have observed increased GAST D integrity monitor test statistics during scintillation that are correlated with increasing amplitude scintillation (S4) indices. However, the precise cause of this increased noise in test statistics remained unclear. In general, the performance of integrity monitors under scintillation has not been fully characterized. In our recent work (Sun et al., 2023), we addressed this gap by investigating the impact of scintillation-induced noise, particularly those due to phase transitions, on the system availability of DF Advanced Receiver Autonomous Integrity Monitoring (ARAIM). This research confirmed that phase transitions introduce additional errors that must be considered in formulating error bounds for protection level calculations. These additional errors complicate GBAS system performance because multiple integrity monitors are affected, and these effects are correlated with each other. Therefore, it is imperative to conduct a comprehensive analysis of the impact of phase transitions on smoothed pseudoranges across different modes and monitor test statistics to assess scintillation impacts on GBAS system performance.

In response, this study analyzes and characterizes the effects of phase transitions during scintillation on smoothed pseudoranges in both SF and DF modes as well as GBAS integrity monitor test statistics. We utilize 50-Hz raw GNSS data collected from Brazil to investigate additional errors in smoothed pseudoranges during scintillation. Various GBAS integrity monitors, including the code carrier divergence (CCD) monitor, dual-solution pseudorange ionospheric gradient monitor (DSIGMA), and IGM, are evaluated. To isolate the effects of phase transitions in the smoothed pseudoranges and monitor test statistics, we employ detrended GNSS measurements as inputs for the smoothing filters and integrity monitors. The detrended measurements are obtained by removing common error terms, including geometric range, satellite and receiver clock errors, and tropospheric delay, from GNSS measurements. The results to date identify increasing variations (noise) in smoothed pseudoranges and monitor test statistics as the S4 index increases.

We also obtain error bounds for smoothed pseudoranges and GBAS integrity monitor test statistics as a function of scintillation indices (amplitude scintillation index, S4, and scintillation signal decorrelation time, tau0) through scintillation simulations. We employ a scintillation simulator developed by Xu et al. (2020), which utilizes S4 and tau0 as inputs and provides statistically consistent phase screens for scintillation simulations. To analyze the full range of scintillation behaviors, we select pairs of S4 and tau0 values ranging from 0.3 to 1.0 and 0.2 to 1.3 sec, respectively, corresponding to a set of 12 scintillation scenarios. The simulation results enable the calculation of error bounds for scintillation-induced errors in smoothed pseudoranges and test statistics under consistent scintillation scenarios, facilitating threshold determination for all scintillation conditions specified by S4 and tau0 pairs.

In summary, this study identifies phase transitions during scintillation and evaluates their impacts on smoothed pseudoranges and integrity monitor test statistics in GBAS. Incorporating phase transitions into error models and integrity monitor test statistics is crucial for accurately calculating protection levels and assessing GBAS availability under scintillation. These findings also contribute to assessing different augmentation systems under scintillation, including ARAIM and Satellite-Based Augmentation System (SBAS), guiding their future development for operations in equatorial regions.

References
S. Saito et al., “Impact Assessment of Ionospheric Scintillation Associated with Plasma Bubbles on GAST-D Ground Integrity Monitors," Proceedings of the 31st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2018), Miami, FL, 2018, pp. 2186-2194.
S. Saito et al., “Evaluating Performance of Ionospheric Anomaly Monitor for DFMC GBAS with Flight Data in Ionospheric Disturbed Conditions," Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023), Denver, CO, 2023, pp. 1144-1155.
C. M. Silva et al., “Assessing the Performance of Dual-Frequency Multi-Constellation GBAS Architectures during Periods of Ionospheric Scintillation in Brazil," Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023), Denver, CO, 2023, pp. 1855-1878
K. A. Sun et al., Availability Assessment of Dual-Frequency GNSS-Based Augmentation Systems Under Equatorial Ionospheric Scintillations," Proceedings of the 2023 International Technical Meeting of The Institute of Navigation, Long Beach, CA, 2023, pp. 937-949
D. Xu et al., A two-parameter multifrequency GPS signal simulator for strong equatorial ionospheric scintillation: modeling and parameter characterization,” NAVIGATION. vol. 67, no. 1, pp. 181-195, 2020.

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