Int. Conferences

@conference{nokey,

title = {A Stochastic Modeling Approach for Phase Transition-Induced Errors in GBAS under Ionospheric Scintillation},

author = {Andrew K. Sun and Jiyun Lee and Sam Pullen and Maria Caamano},

doi = {10.33012/2025.20391},

year  = {2025},

date = {2025-09-12},

booktitle = {Proceedings of the 38th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2025)},

address = {Baltimore, MD},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {	 Impact of Dual-Frequency Multi-Constellation GBAS Data Link Constraints on Integrity Monitoring},

author = {Maria Caamano and Daniel Gerbeth and Susumu Saito and Andrew K. Sun and Jiyun Lee and Sam Pullen},

doi = {10.33012/2025.20303},

year  = {2025},

date = {2025-09-12},

booktitle = {Proceedings of the 38th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2025)},

pages = {276-290},

abstract = {The future architecture for Dual-Frequency Multi-Constellation Ground-Based Augmentation Systems (DFMC GBAS), known as GBAS Approach Service Type (GAST) E, is designed to rely on measurements from two frequency bands, L1/E1 and L5/E5a. However, unintentional Radio Frequency Interference (RFI) could affect one of the frequencies, typically L1/E1, potentially leaving GBAS with only a single operating frequency. In such cases, GAST E cannot function as intended, making it necessary to define single-frequency fallback modes, GAST D1 and GAST D2, to maintain CAT II/III service. A key challenge for these new modes is the need to transmit additional measurements through the limited GBAS data link bandwidth, which makes data compression methods at the ground station and data reconstruction methods at the airborne side essential. This paper evaluates the impact of the Message Type (MT) 23 compression and reconstruction method being proposed at the International Civil Aviation Organization (ICAO) level, together with the lower ground data update rate (0.33 Hz instead of 2 Hz), on one of the key integrity monitors for these fallback modes: the Dual Solution Ionospheric Gradient Monitoring Algorithm (DSIGMA). We first derive DSIGMA thresholds for L1/E1 and L5/E5a using nominal flight test data and evaluate their consistency with existing GAST D standards. We then assess monitor performance with simulated ionospheric gradients to evaluate the impact of these new constraints under controlled anomalous ionospheric conditions, followed by an evaluation with real flight data collected under strong scintillation. The results with both simulated and real data demonstrate that the MT23 compression and reconstruction method, as well as the ground data lower update rate, can be used without visible effects on DSIGMA monitoring.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Enhancing Availability of SS-RAIM Based CDGNSS Using Integer Aperture Bootstrapping},

author = {Dongchan Min and Jiyun Lee},

url = {https://doi.org/10.33012/2025.20006},

doi = {https://doi.org/10.33012/2025.20006},

year  = {2025},

date = {2025-01-27},

booktitle = {Proceedings of the 2025 International Technical Meeting of The Institute of Navigation},

pages = {331 - 342},

address = {Long Beach, CA},

abstract = {Carrier-phase Differential Global Navigation Satellite System (CDGNSS) is gaining prominence in autonomous vehicles that require precise and reliable navigation. Our team proposed an integrity architecture based on Solution Separation Receiver Autonomous Integrity Monitoring (SS-RAIM) for CDGNSS. In this previous work, precise fixed position solutions are employed only when the incorrect fix probability, computed from filter covariance matrices, falls below a predefined level depending on integrity risk requirements. However, this approach may unnecessarily restrict users to float position solutions since covariance matrices do not reflect the quality of current ambiguity estimates calculated from actual measurements. This paper proposes a method to utilize the Integer Aperture Bootstrapping (IAB) test within the SS-RAIM architecture to determine fixed position solution usage. The IAB test directly evaluates measurement quality using estimated float and fixed ambiguity solutions. For ambiguities passing the IAB test, the incorrect fix probability can be controlled by adjusting the test parameter. The IAB test provides a closed-form analytical expression for this incorrect probability, enabling precise integrity risk evaluation and Protection Level (PL) computation. We set the IAB test parameter to ensure that ambiguities passing the test meet specified incorrect fix probability constraints. Furthermore, this paper examines how IAB test results affect the Probability Density Functions (PDFs) of float and fixed position solutions for exact PL computation. Since the IAB test directly assesses current measurement quality, the available measurement space depends on test results, leading to changes in position solution PDFs. We derive these changes in PDFs depending on IAB test results and incorporate them into PL computation. Simulation results demonstrate that PLs are reduced by using the IAB test when compared to previous approaches.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {A Method of Estimating Residual Bending Error in GNSS-RO Absolute TEC},

author = {Jaehee Chang and Andrew K Sun and Jihyeok Park and Jiyun Lee and Y. Jade Morton},

url = {https://doi.org/10.33012/2025.20005},

doi = {https://doi.org/10.33012/2025.20005},

year  = {2025},

date = {2025-01-27},

booktitle = {Proceedings of the 2025 International Technical Meeting of The Institute of Navigation},

pages = {151-160},

address = {Long Beach, CA},

abstract = {Ionospheric refraction and dispersion effects induce residual bending error in the Total Electron Content (TEC) derived from Global Navigation Satellite System (GNSS) radio occultation (RO) measurements. This study proposes a practical method for the direct evaluation of residual bending error in the conventional absolute TEC retrieval. We derive the expression for the residual bending error in absolute TEC, by accounting for refraction residuals in both the carrier phase and pseudorange measurements. Two error components are identified: the altitudinally varying residual bending error from carrier phase TEC, and a constant bias from phase-to-pseudorange leveling. These errors are quantitatively assessed and analyzed by utilizing real observational data from COSMIC-2. Our analysis reveals that the residual bending error is predominantly influenced by vertical TEC gradients and can exceed the level of 10 TEC units (TECU). The findings underscore the need for estimating and correcting residual bending errors to improve the accuracy of absolute TEC retrievals, particularly under high solar activity conditions.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Investigation of the Development of Traveling Ionospheric Disturbances during the Total Solar Eclipses on 21 August 2017 and 8 April 2024},

author = {Andrew K. Sun and Hyosub Kil and Hyeyeon Chang and Woo Kyoung Lee and Jiyun Lee},

year  = {2024},

date = {2024-12-13},

booktitle = {AGU Fall Meeting 2024},

address = {Washington D.C.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Phase Screen Model for Extreme Scintillations: Evaluating Theoretical Limits in Propagation Approximations},

author = {Andrew K. Sun and Charles L. Rino and Y. Jade Morton and Brian Breitsch and Jiyun Lee},

year  = {2024},

date = {2024-12-13},

booktitle = {AGU Fall Meeting 2024},

address = {Washington D.C.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Minimum Separation Boundary for UAM Tactical Deconfliction Based on Avoidance Risk Ratio Model},

author = {Junsoo Kim and Gihun Nam and Dongchan Min and Jiyun Lee and Sam Pullen},

url = {https://ieeexplore.ieee.org/document/10748735},

doi = {10.1109/DASC62030.2024.10748735},

year  = {2024},

date = {2024-09-29},

urldate = {2024-09-29},

booktitle = {2024 AIAA DATC/IEEE 43rd Digital Avionics Systems Conference (DASC)},

address = {San Diego, CA},

abstract = {Urban Air Mobility (UAM) is a promising approach to urban transportation, leveraging advanced technologies to utilize the airspace above cities. However, the maturation of UAM would lead to increasing traffic density in constrained airspace and thus increasing risk of Mid-Air Collisions (MAC), necessitating robust deconfliction strategies to maintain safe separation between aircraft. Conventional aviation relies on large separation boundaries, which are impractical for the efficient use of UAM airspace. Our prior research introduced a methodology for determining the Minimum Separation Boundary (MSB) for UAM, incorporating strategic and tactical deconfliction. This methodology defined the Avoidance Risk Ratio (ARR) as a key parameter for assessing the performance of tactical deconfliction systems. However, the initial model assumed a constant ARR, which does not accurately reflect the dynamic nature of UAM operations. In this study, we present an improved ARR model characterized by a decaying function that varies with the magnitude of separation boundary and avoidance uncertainty. This refined model accounts for increased avoidance failure rates as separation decreases, allowing for a more precise evaluation of collision probability for a given separation boundary. The MSB for UAM tactical deconfliction is then determined by applying this ARR model in our existing collision risk framework. We performed a sensitivity analysis using the improved methodology to evaluate the impact of tactical deconfliction on the reduction of the MSB. The analysis demonstrates that implementing tactical deconfliction could reduce the separation boundary by 18% compared to scenarios without it, which is a more refined result than the prior research.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Impact of Phase Transitions Due to Ionospheric Scintillation on GBAS Integrity},

author = {Andrew K. Sun and Jiyun Lee and Sam Pullen},

year  = {2024},

date = {2024-09-20},

booktitle = {Proceedings of the 37th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2024)},

address = {Baltimore, MD},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Quantifying the Impact of Ionospheric Bending on GNSS-RO Absolute TEC Retrieval During Solar Cycle 25},

author = {Jaehee Chang and Andrew K. Sun and Jihyeok Park and Y. Jade Morton and Jiyun Lee},

year  = {2024},

date = {2024-09-16},

urldate = {2024-09-16},

booktitle = {10th International Radio Occultation Working Group (IROWG-10) },

address = {Boulder, CO},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Assessment of Ionospheric Perturbations during Total Solar Eclipses},

author = {Andrew K. Sun and Hyosub Kil and Hyeyeon Chang and Woo Kyoung Lee and Jiyun Lee},

year  = {2024},

date = {2024-07-15},

booktitle = {45th COSPAR Scientific Assembly},

address = {Busan, Republic of Korea},

abstract = {Solar eclipses are caused by the obscuring of the Sun by the Moon when it passes between the Earth and the Sun. These phenomena affect the Earth's ionosphere in two ways. The direct effect of partial and total occlusion of the Sun appears as a decrease in ionospheric plasma density. The other effect is the generation of traveling ionospheric disturbances (TIDs) by atmospheric gravity waves. The former is manifest from many studies, whereas the latter is controversial because inconsistencies exist among different studies. This study aims to resolve this controversy by examining TID detection methods. Caution must be exercised when interpreting TID-like perturbations during solar eclipses, as the detection of small-amplitude perturbations is sensitive to their detection method. Moreover, the rapid change in plasma density during solar eclipses can be falsely detected as TIDs. The total solar eclipses over the United States on 21 August 2017, and 8 April 2024, provide a great opportunity to assess the generation of TIDs in association with solar eclipses. By applying various perturbation detection methods to the total electron content (TEC) data from the Global Navigation Satellite System (GNSS), we assess TIDs accompanying solar eclipses alongside the investigation of the occurrence of similar perturbations before and after the eclipse, as well as during non-eclipse days.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Estimation of the Residual Error due to Ionospheric Bending in Relative TEC using COSMIC-2 GPS-RO Measurements},

author = {Jaehee Chang and Andrew K. Sun and Jiyun Lee and Hyeyeon Chang and Yang Wang and Lei Liu and Y. Jade Morton and Douglas Hunt},

url = {https://doi.org/10.33012/2024.19607},

doi = {https://doi.org/10.33012/2024.19607},

year  = {2024},

date = {2024-04-15},

booktitle = {Proceedings of the ION 2024 Pacific PNT Meeting},

pages = {22-35},

abstract = {The Total Electron Content (TEC) derived from Global Positioning System (GPS) radio occultation (RO) measurements contains residual bending errors that arise from signal refraction and dispersion in the ionosphere. Motivated by the need to ensure the accuracy of TEC retrievals, particularly under high solar activity conditions, we investigate the impact of ionospheric bending on the conventional dual-frequency TEC. This work marks a new attempt in assessing the residual bending error in TEC estimates using real RO measurements, in contrast to prior research efforts that relied primarily on simulation studies. We present an estimation method for the residual bending error based solely on measurement data, avoiding reliance on a priori knowledge of the true ionospheric state. Utilizing real RO observations from COSMIC-2, our analysis reveals that the residual bending error is largely influenced by the vertical gradient of TEC, with additional dependencies on solar activity and local time. Large residual bending errors were observed under high solar activity conditions, reaching up to 17.7 TECU in a 15-day dataset from January 2023. Residual bending errors in dual-frequency TEC were often found to exceed the 3 TECU threshold from COSMIC-2 accuracy requirements, suggesting the use of single-frequency TEC or the application of residual bending error correction as a potential alternative under certain conditions inducing high TEC gradients.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Determining Minimum Separation Boundary for UAM Tactical Deconfliction},

author = {Junsoo Kim and Gihun Nam and Dongchan Min and Noah Minchan Kim and Jiyun Lee},

url = {https://doi.org/10.33012/2024.19601},

doi = {https://doi.org/10.33012/2024.19601},

year  = {2024},

date = {2024-04-15},

booktitle = {Proceedings of the ION 2024 Pacific PNT Meeting},

pages = {502 - 513},

abstract = {Urban air mobility (UAM) is an emerging paradigm in future mobility that utilizes the airspace above urban areas and incorporates innovative technologies to provide safe and efficient transportation. However, as UAM reaches maturity, the increased density in constrained airspace will raise safety concerns, particularly the risk of mid-air collisions (MAC). This emphasizes the importance of deconfliction in ensuring safe separations from other aircraft and static obstacles. Deconfliction includes strategic and tactical parts, which refer to pre-flight and in-flight deconfliction, respectively. Conventional aircraft perform deconfliction using sufficiently spaced-out separation in wide airspace. Applying these extensive separation boundaries to UAM, however, could significantly compromise airspace efficiency. Therefore, it is necessary to determine a tightened separation boundary that enhances airspace efficiency while still meeting the safety requirement. Building on our previous research, which introduced a concept for a minimum separation boundary focusing on the strategic deconfliction, this study extends the scope to include the tactical deconfliction. The tactical deconfliction utilizes the detect-and-avoid (DAA) system to mitigate collisions when the strategic deconfliction fails. This study proposes a methodology for determining the minimum separation boundary for the tactical deconfliction, focusing on two key parameters representing the performance of the DAA system. Additionally, a case study was conducted by applying the proposed methodology to an airborne collision avoidance system X (ACAS X), a promising DAA system suitable for UAM. The results demonstrate that implementing the tactical deconfliction could reduce the separation boundary by 12% compared to scenarios without it.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Ionospheric Scintillation Effects across Multiple Carrier Frequency Bands Transmitted from LEO Satellites},

author = {Andrew K. Sun and Y. Jade Morton and Jiyun Lee},

doi = {10.33012/2024.19525},

year  = {2024},

date = {2024-01-23},

urldate = {2024-01-23},

booktitle = {Proceedings of the 2024 International Technical Meeting of The Institute of Navigation},

pages = {109 - 125},

address = {Long Beach, CA},

abstract = {Prior simulation studies have indicated that signals from LEO satellites exhibit higher signal dynamics and more significant scintillation effects compared to those transmitted from MEO satellites. This study employs a physics-based simulation model initiated with real GPS scintillation data. It extends the analysis to assess scintillation effects transmitted from LEO satellites across various carrier frequencies in the L band (L1, L2, and L5) and other bands (VHF, UHF, and S). The results provide quantitative evidence that signals at lower frequencies experience more frequent fades and higher phase dynamics. Analyses across various frequency bands validate that distinct signal dynamics determine the time scale of scintillation, resulting in temporal characteristics of frequency-dependent signal fading. The simulation also demonstrates a consistent rate of selective fading among different L-band signals across different signal dynamics. This frequency-selective fading allows the adaptation of inter-frequency aiding techniques in receivers to improve the robustness of LEO signal processing.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Study of the Characteristics and Sources of Late-Night Equatorial Electron Density Irregularities},

author = {Andrew K. Sun and Hyosub Kil and Hyeyeon Chang and Jiyun Lee},

year  = {2023},

date = {2023-12-16},

urldate = {2023-12-16},

booktitle = {AGU Annual Meeting 2023},

address = {San Francisco, CA},

abstract = {A long-term, as-yet-unsolved question regarding equatorial irregularities is: why do late-night irregularities exhibit different characteristics compared to post-sunset irregularities? In the equatorial region, irregularities develop preferentially at post-sunset around equinoxes and during solar maximum, whereas they preferentially develop at late night around June solstices and during solar minimum. Post-sunset irregularities are understood by the generation of equatorial plasma bubbles (EPBs) in association with the evening prereversal enhancement (PRE) in the vertical ion velocity. Late-night irregularities are also understood as the signatures of EPBs, but the generation conditions of EPBs at late night have not yet been clearly understood. This study investigates the characteristics and sources of late-night irregularities by analyzing the ROCSAT-1 and ICON observations. These observations enable us to investigate the solar cycle dependence of the behavior of late-night irregularities. Our investigation focuses on the generation conditions of EPBs at late night, but our study extends to the investigation of other sources of late-night irregularities. From our preliminary investigation, we have identified the sinusoidal variation of the electron density in correlation with the vertical ion velocity. This property is explained better by traveling ionospheric disturbances (TIDs) than by EPBs. The roles of EPBs and TIDs in the generation of late-night irregularities will be assessed through the examination of various aspects in the irregularities.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Preliminary Assessment of Scintillation Data from the Spire Global and PlanetIQ Radio Occultation Constellation},

author = {Hyeyeon Chang and Lei Liu and Y. Jade Morton and Tibor Durgonics and Jun Wang and Dominic J. Fuller-Rowell and Douglas Hunt and John Braun and Jan-Peter Weiss and Jaehee Chang and Andrew K. Sun and Jiyun Lee},

year  = {2023},

date = {2023-12-14},

urldate = {2023-12-14},

booktitle = {AGU Annual Meeting 2023},

address = {San Francisco, CA},

abstract = {Ionospheric scintillation, characterized by rapid fluctuations in amplitude and phase of signals as they propagate through ionospheric irregularities, is a space weather phenomenon that impacts satellite-based communication and navigation systems. Accurate identification of scintillation events is important to understand their impact on Earth systems. Numerous efforts have been dedicated to monitoring ionospheric scintillation using ground-based GNSS receiver networks. Fluctuations in the signals received by Low Earth Orbit (LEO) satellites that perform GNSS radio occultation (RO) measurements have also been utilized to monitor ionospheric scintillation.

Recently, low-cost CubeSats launched by commercial small satellite companies such as Spire Global, PlanetIQ, and GeoOptics are performing GNSS-RO measurements. These CubeSats collect high-rate data for scintillation observations. Both Spire Global and PlanetIQ have been awarded contracts by the National Oceanic and Atmospheric Administration (NOAA) for a GNSS-RO data pilot study. Before utilizing this data for scientific research, it is essential to evaluate the scintillation observations provided by the data to assess their reliability and potential.



This study presents a preliminary assessment of scintillation data from the Spire Global and PlanetIQ CubeSats. The assessment process employs a methodology to distinguish real ionospheric scintillation events from radio frequency interference and other anomalies. Events identified as ionospheric scintillation are then further evaluated in terms of rate of detection, scintillation indices magnitude distributions, irregularity heights and geographic areas where scintillations are detected, time of occurrence, and solar-geomagnetic conditions associated with the observed scintillations. Furthermore, this study compares the scintillation results obtained from Spire, PlanetIQ, and COSMIC-2 data, as well as available ground-based high rate GNSS receiver measurements.



The assessment results of this study will contribute to understanding of the performance of scintillation data from low-cost commercial RO missions. Filtering satellite data in this way to extract real scintillation phenomena can offer valuable insights into ionospheric responses to space weather events.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Performance Assessment of Ionospheric Electron Density Profiles retrieved from KOMPSAT-5 by Comparing with Ground-based and Space-based Observations},

author = {Hyeyeon Chang and Jaehee Chang and Andrew K. Sun and Wookyoung Lee and Hyosub Kil and Jiyun Lee},

year  = {2023},

date = {2023-12-13},

urldate = {2023-12-13},

booktitle = {AGU Annual Meeting 2023},

address = {San Francisco, CA},

abstract = {Korea Multi-Purpose Satellite (KOMPSAT)-5, launched in August 2013, is the first radio occultation (RO) mission in the Korean space program and carries the atmosphere occultation precision orbit determination (AOPOD) payload to provide data for precise orbit determination (POD) and GPS radio occultation (RO) measurements.

GPS RO data from KOMPSAT-5 is being processed and provided through a collaborative effort between KASI and the University Corporation for Atmospheric Research (UCAR). This data is then utilized by the National Oceanic and Atmospheric Administration (NOAA) for weather forecasting purposes. While the neutral atmospheric profiles from KOMPSAT-5 have been made available to the public via the COSMIC Data Analysis and Archive Center (CDAAC), the ionospheric electron density profiles have not yet been provided and validated.



In this study, the performance assessment of the ionospheric electron density retrieved from KOMPSAT-5 RO has been done. This study includes estimation of electron density profiles from KOMPSAT-5 by combining data from both the occultation antenna and POD antenna. To validate the electron density measurements from KOMPSAT-5 with reliable truth observations, digital ionosonde (digisonde) data, incoherent scatter radar (ISR), and COSMIC-2 RO data were used as the reference dataset. Furthermore, we aim to analyze the impact of horizontal ionospheric variations on the electron density retrieved by the RO technique by conducting performance evaluations based on different regions and local time periods.



The comparisons of F2 layer peak (NmF2) obtained from KOMPSAT-5 data to digisonde measurements reveal a high degree of correlation of 0.90. The comparisons with electron density profiles from ISR and COSMIC-2 RO data show that the profiles retrieved from KOMPSAT-5 RO data have good agreement with those from both ISR and COSMIC-2 RO data. The comparison results also show that the errors were more widely distributed in the lower latitude area and near the dusk/dawn terminators. This is believed to be due to the significant variations in the ionosphere in the horizontal direction at low latitudes and around dawn/dusk terminators, making it challenging to meet the spherical symmetry assumption, resulting in relatively large errors.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Preliminary Assessment of the Residual Error due to Ionospheric Bending in the COSMIC-2 RO TEC by Comparing Results from Single- and Dual-frequency Methods},

author = {Jaehee Chang and Andrew K. Sun and Hyeyeon Chang and Yang Wang and Lei Liu and Douglas Hunt and Y. Jade Morton and Jiyun Lee },

year  = {2023},

date = {2023-12-12},

urldate = {2023-12-12},

booktitle = {AGU Annual Meeting 2023},

address = {San Francisco, CA},

abstract = {The total electron content (TEC) derived from Global Positioning System (GPS) radio occultation (RO) measurements has proven to be one of the most essential observables in ionospheric studies. There are two approaches for calculating the TEC, one of which is the single-frequency method that requires the ionospheric excess phase obtained through the removal of the orbit and clock errors in the L1 or L2 phase measurements. The other is the dual-frequency method that uses the combination of L1 and L2 phase measurements, which is more efficient as it automatically cancels out the orbit and clock errors. Because of this advantage, the dual-frequency method has been the more conventional approach for TEC estimation, and is currently being implemented in the TEC processing algorithm at COSMIC Data Analysis and Archive Center (CDAAC). However, when using the assumption of straight-line propagation, there exists a residual error in both TEC estimates relative to the true straight-line TEC which arises from the bending and dispersion of the GPS signal ray paths in the ionosphere. The dual-frequency TEC is known to have a larger residual error compared to the single-frequency TEC, which has been demonstrated in previous studies through theoretical derivations and simulations. However, this theoretical representation of residual errors due to ionospheric bending has not been thoroughly investigated and validated using real RO measurements.

In this research, real RO observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) mission were used to quantify the residual error in the relative TEC obtained by the dual-frequency method compared to the single-frequency method. The single-differencing with simultaneously tracked reference and occulting GPS satellite observations was used for the precise removal of the receiver clock bias in order to obtain the ionospheric excess phase for the single-frequency TEC. The results verified that the difference between the dual-frequency and single-frequency TEC at common straight-line tangent altitudes was larger at regions of high vertical TEC gradients causing larger ray separations. It was found that the residual error in the dual-frequency TEC derived from the COSMIC-2 precise orbit determination (POD) antenna observations on January 3rd, 2023 can be up to 5.3 TECU larger than that in the single-frequency TEC in the case of an occultation event with the vertical TEC gradient reaching 4.8 TECU/km. This presentation will discuss detailed approaches in the single-frequency excess phase-based TEC estimation and the dependence of dual-frequency residual error on background ionospheric TEC values.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Safety Risk Assessment Based Minimum Separation Boundary},

author = {Junsoo Kim and Gihun Nam and Dongchan Min and Noah Minchan Kim and Jiyun Lee},

doi = {10.1109/DASC58513.2023.10311141},

year  = {2023},

date = {2023-10-01},

urldate = {2023-10-01},

booktitle = {2023 IEEE/AIAA 42nd Digital Avionics Systems Conference (DASC)},

pages = {1-8},

address = {Barcelona, Spain},

abstract = {Urban Air Mobility (UAM) is a promising next-generation mobility solution, leveraging airspace and state-of-the-art technologies for efficient and prompt transportation in urban areas. In UAM operations, constrained urban airspace and a high safety requirement level have presented new challenges in determining the separation boundary between aircraft; increasing airspace efficiency while maintaining safety is crucial. In general aviation and Unmanned Aerial Systems (UAS), separation boundaries are set to large values to readily meet a target level of safety (TLS). These large boundaries, however, can severely lower the traffic capacity of UAM airspace if applied directly. To address this issue, in this paper we propose a methodology for determining the safety-assured minimum separation boundary for UAM operations. Our methodology identifies system safety requirements, conducts safety risk assessments (SRA), and analyzes system integrity for a comprehensive systemic approach, which reduces the reliance on extensive flight data without compromising conservatism. We applied the proposed methodology to a Global Navigation Satellite System (GNSS)/Inertial Navigation System (INS) integrated navigation system as a case study for analysis. The results indicate that a significant reduction in separation boundaries is possible while still satisfying the safety requirements compared to existing separation boundaries for general aviation and UAS. Furthermore, this paper applies the proposed method to various navigation technologies such as multi-GNSS constellation, GNSS augmentation systems, and INS sensor with different grades with the goal of further reducing the separation boundary to satisfy the stringent vertical requirements of future UAM airspace.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Mid-Latitude Ionospheric Scintillation Impact on Availability of Dual-frequency GNSS Augmentation Systems},

author = {Andrew K. Sun and Jaehee Chang and Jiyun Lee and Brian Breitsch and Y. Jade Morton and Sam Pullen},

year  = {2023},

date = {2023-09-13},

urldate = {2023-09-13},

booktitle = {Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023)},

address = {Denver, Colorado},

abstract = {Dual-frequency service of GNSS augmentation systems can effectively mitigate most ionospheric threats to system users. However, ionospheric scintillation remains a significant concern for GNSS augmentation systems, even with the benefits of dual-frequency operation. Ionospheric scintillation is characterized by rapid amplitude and phase fluctuations caused by electron density irregularities in the ionosphere. These fluctuations can lead to phase transitions and signal fading, resulting in the degradation of system performance. The occurrence of scintillation is most severe in low-latitude regions, driven by factors such as equatorial plasma bubbles (EPBs). While scintillation is relatively rare in mid-latitudes, EPBs can extend toward higher latitudes during extreme geomagnetic storms, potentially causing scintillations in mid-latitude regions. Our recent work assessed the impact of low-latitude scintillation on dual-frequency GNSS augmentation systems. However, this approach might not fully apply to mid-latitudes due to different ranges of scintillation properties and conditions. In response, our study investigates potential scintillation effects in mid-latitude regions using high-rate GNSS scintillation data to define the threat space based on physics-based modeling parameters. To characterize the scintillation effects, we conduct simulations for all combinations of parameters within the threat space. This allows us to identify the worst-case parameters corresponding to the most severe scintillation effects and incorporate them into the protection level calculations. Finally, we assess the availability of a dual-frequency SBAS-like system under extreme mid-latitude scintillation impact scenarios. Our results indicate that scintillation impacts on availability are much less severe in mid-latitudes compared to low-latitudes. However, potential scintillation impacts still exist in mid-latitudes under extreme scintillation conditions. Our approach can be applied to various GNSS augmentation systems to assess the impact of scintillation on system performance. This contribution can support the development of performance standards for operations under scintillation and mitigation strategies to address its impact on system performance.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {SS-RAIM based Integrity Architecture for CDGNSS Systems against Satellite Measurement Faults},

author = {Dongchan Min and Noah Minchan Kim and Junsoo Kim and Jiyun Lee and Sam Pullen

},

doi = {10.33012/2023.19219},

year  = {2023},

date = {2023-09-13},

urldate = {2023-09-13},

booktitle = {Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023)},

pages = {2550 - 2569},

address = {Denver, Colorado},

abstract = {In this paper, we propose an integrity architecture for the Carrier Phase-based Differential Global Navigation Satellite System (CDGNSS). The CDGNSS utilizes carrier-phase measurements and is reliant on the correct estimation (or resolution) of cycle ambiguities. However, due to the nonlinear nature of ambiguity resolution, protection level computation for CDGNSS presents greater challenges compared to that of traditional code-based systems. The method we propose addresses these difficulties by employing Solution Separation-based Receiver Autonomous Integrity Monitoring (SS-RAIM). This architecture considers the integrity risk associated with the incorrect ambiguity resolution by including correct/incorrect ambiguity fix probabilities into the protection level equation. In order to apply the SS-RAIM to CDGNSS, the main contributions of this work, which have not been explored in the previous works, are twofold. First, we revisit the variance computation property of the solution separations-based detection test statistic. Second, we verify the applicability of the measurement overbounding method. These considerations, which arise from the differences in navigation algorithms between code-based and CDGNSS systems, are mathematically demonstrated. Finally, we present performance analysis results using an illustrative example of a dual-constellation, dual-frequency navigation system.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Integrity Assurance of LIRTK Using SS-RAIM Against Sensor Faults for UAV Applications},

author = {Noah Minchan Kim and Dongchan Min and Jiyun Lee},

doi = {10.33012/2023.19457},

year  = {2023},

date = {2023-09-13},

urldate = {2023-09-13},

booktitle = {Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023)},

pages = {2242 - 2257},

address = {Denver, Colorado},

abstract = {This study proposes an integrity assurance architecture of Loosely-Coupled Kalman Filter-based Inertial Aiding for RTK (LIRTK) using Solution Separation based Receiver Autonomous Integrity Monitor (SS-RAIM). An integrity risk allocation tree of LIRTK is developed for each sensor fault hypothesis including the nominal hypothesis, single-satellite fault hypotheses, an IMU sensor fault hypothesis, and an incorrect fix fault hypothesis. Two P(CF) requirements for integrity risk and false alarm rate were defined in order to calculate the Vertical Protection Level (VPL) of the fixed solution. In order to improve the fixed rate by lowering the P(CF) requirement for false alarm rate, an ambiguity check process is newly proposed. In the ambiguity check process, the fixed ambiguities estimated under nominal and fault hypothesis are compared. After passing the ambiguity check process, this study found a new lower bound that relaxes the P(CF) requirement for false alarm rate and validated its mathematical proof. VPL simulations were performed for different Global Navigation Satellite System (GNSS) measurement noise levels and Inertial Measurement Unit (IMU) sensor grades. The simulation results demonstrated that the ambiguity check process significantly improved the fixed rate of LIRTK and exhibited a much higher value compared to single-epoch RTK.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Network-Based Augmentation System (NBAS) Architectures Optimized to Support Urban Air Mobility (UAM)},

author = {Jiyun Lee and Gihun Nam and Dongchan Min and Andrew K. Sun and Sam Pullen},

doi = {10.33012/2023.18603},

year  = {2023},

date = {2023-01-25},

booktitle = {2023 International Technical Meeting of The Institute of Navigation},

address = {Long Beach, CA},

abstract = {Integrity-assured navigation is essential to support highly automated operation of Urban Air Mobility (UAM) at lower altitudes in urban or suburban area. This paper develops a Network-based Augmentation System (NBAS) architecture which utilize GNSS pseudorange corrections and monitoring information of reference stations installed at multiple vertiports to improve navigation accuracy and integrity. A new monitoring concept of enhancing detection capability for spatially-decorrelated errors is proposed by combining information of vertiports located farther from users. Simulations are conducted to compare the performance of the NBAS system to that of the conventional Ground-based Augmentation Systems (GBAS) architecture (i.e., single reference station) assuming the use of a dual-frequency and dual-constellation (GPS and Galileo) GNSS and the dual-frequency divergence-free smoothing method. Simulation results show a significant reduction in both ionosphere-induced and ephemeris-failure threats by utilizing the multiple monitor capability of networked ground stations and UAM vehicles.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Availability Assessment of Dual-Frequency GNSS-Based Augmentation Systems Under Equatorial Ionospheric Scintillations},

author = {Andrew K. Sun and Jaehee Chang and Jiyun Lee and Brian Breitsch and Y. Jade Morton},

doi = {10.33012/2023.18617},

year  = {2023},

date = {2023-01-25},

booktitle = {2023 International Technical Meeting of The Institute of Navigation},

pages = {937-949},

address = {Long Beach, CA},

abstract = {This study investigates a wide range of equatorial scintillations to evaluate their impacts on the availability of dual-frequency Global Navigation Satellite System (GNSS)-based augmentation systems. A two-component power-law phase screen model is used to implement scintillation simulations for all possible sets of phase screen parameters within their empirical range. Two types of scintillation effects, phase transition and loss of lock, are conservatively quantified in terms of navigation availability. This study specifies equatorial scintillation conditions with the scintillation indicators, S4 and tau0, to search for the worst scintillation impact among all simulation results assigned to each grid of S4 and tau0. Aviation availability of a GPS/Galileo dual-frequency advanced receiver autonomous integrity monitoring (ARAIM) is assessed for the scintillation scenario selected from the real scintillation data. The simulation results show that the availability is highly dependent on the reacquisition time and the C/N0 threshold which are closely related to the receiver design specifications and user settings. The proposed approach allows availability assessment of GNSSbased augmentation systems for any scintillation conditions specified by indicators {S4, tau0} where the indicators can be obtained in near real-time.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Characteristics and Sources of Electron Density Irregularities in the Low Latitude F Region},

author = {Hyosub Kil and Andrew K. Sun and Hyeyeon Chang and Larry J. Paxton and Romina Nikoukar and Jiyun Lee},

year  = {2022},

date = {2022-12-17},

booktitle = {AGU Fall Meeting 2022},

address = {Chicago, IL},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Study of the drivers of the equatorial ionization anomaly using ICON and COSMIC2 data},

author = {Hyosub Kil and Hyeyeon Chang and Andrew K. Sun and Jiyun Lee},

year  = {2022},

date = {2022-12-17},

booktitle = {AGU Fall Meeting 2022},

address = {Chicago, IL},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Preliminary results of electron density profiles retrieved from KOMPSAT-5 Radio Occultation Data},

author = {Hyeyeon Chang and Jaehee Chang and Andrew K. Sun and Woo Kyoung Lee and Jiyun Lee},

year  = {2022},

date = {2022-12-01},

urldate = {2022-12-01},

booktitle = {AGU Fall Meeting 2022},

address = {Chicago, IL},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Cycle Slip Mitigation for Total Electron Content Retrieval using GeoOptics Radio Occultation Data},

author = {Jaehee Chang and Hyeyeon Chang and Jiyun Lee},

year  = {2022},

date = {2022-12-01},

urldate = {2022-12-01},

booktitle = {AGU Fall Meeting 2022},

address = {Chicago, IL},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Networked UAV Detection and Alerting of Ionospheric Anomalies within LADGNSS Navigation Framework},

author = {Gihun Nam and Andrew K. Sun and Jiyun Lee and Sam Pullen },

doi = {https://doi.org/10.33012/2022.18424},

year  = {2022},

date = {2022-09-01},

booktitle = {Proceedings of the 35th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2022)},

pages = {1529-1536},

address = {Denver, Colorado},

abstract = {Local Area Differential GNSS (LADGNSS) is a simplification of the Ground-based Augmentation System (GBAS) architecture to provide navigation and guidance for nearby UAVs. Recent research has evaluated the impacts of severe ionospheric anomalies on Dual-Frequency, Multiple-Constellation (DFDC) LADGNSS and its monitors. To protect against all possible ionospheric anomalies, the system assumes that the worst-case undetected ionosphere-induced differential errors might affect users. The hypothetical impact of ionospheric anomalies can be lowered by improving monitoring capability, and one possible improvement not studied previously is to utilize two-way transmissions from the multiple networked UAVs supported by LADGNSS at any given time. Two-way datalinks are already part of the LADGNSS architecture to support UAV status reports and cooperative guidance, and they can be exploited to allow all UAVs connected to LADGNSS ground stations to share the threat information collected by any of them. This paper examines the benefit of networked UAVs for LADGNSS under anomalous ionospheric conditions by proposing a monitor strategy that leverages observations from networked UAVs to enhance ionospheric monitor capability. The strategy utilizes UAVs within the network which have superior ionospheric gradient monitor (IGM) detection capability to reduce the maximum undetected gradient of a specific user UAV, leading to reduced maximum undetected ionosphere-induced differential errors (MIEV) at that user. Simulation results show a significant reduction in the worst ionosphere-induced differential error by utilizing multiple monitor capability of networked UAVs, where the benefit increases with a larger number of UAVs that are more widely distributed.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Optimal Smoothing and Monitor Strategies for DFDC Local-Area DGNSS under Anomalous Ionospheric Conditions},

author = {Gihun Nam and Dongchan Min and Noah Minchan Kim and Jiyun Lee and Sam Pullen},

doi = {10.33012/2022.18166},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

booktitle = {Proceedings of the 2022 International Technical Meeting of The Institute of Navigation},

pages = {1156 - 1174},

address = {Long Beach, California},

abstract = {Dual-Frequency, Dual-Constellation Local-Area DGNSS (DFDC LADGNSS) modernizes and expands upon L1-based LADGPS to support autonomous vehicles with improved system availability and robustness against ionospheric disturbances. Recent research has evaluated several different LADGNSS smoothing strategies using measurements on GPS L1/L5 and Galileo E1/E5a frequencies to determine the optimal approach under nominal conditions. However, under anomalous ionospheric conditions, the optimal smoothing procedure may be different from the one preferred under nominal conditions. In addition, and system performance greatly depends on the ionospheric monitoring algorithm when a Single Frequency (SF) or Divergence-Free (DF) smoothing process is used. To examine this problem, the performance of SF and DF smoothing processes is evaluated by simulation in terms of Maximum Ionosphere-Induced Vertical Errors (MIEVs), which quantify the anomalous ionospheric impact on DFDC LADGNSS. IonosphereFree (IF) smoothing, which removes almost all ionospheric delay impacts, is evaluated based on its nominal (H0) Vertical Protection Level (VPL). A dual frequency ionospheric gradient monitor is also introduced to fully take advantage of multiple-frequency measurements. MIEVs and VPLs are generated for 27-satellite GPS and Galileo constellations to determine which smoothing and monitor strategies give the best performance (lowest MIEVs or VPLs) under anomalous ionospheric conditions.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Correct Fix Probability Improvement Method via INS Aiding to Single Epoch RTK System},

author = {Noah Minchan Kim and Dongchan Min and Jiyun Lee},

doi = {10.33012/2022.18167},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

booktitle = {Proceedings of the 2022 International Technical Meeting of The Institute of Navigation},

pages = {1304 - 1319},

address = {Long Beach, California},

abstract = {This paper introduces a method that utilizes an Inertial Navigation System (INS) to improve the Probability of Correct Fix (PCF) in a single-epoch Real-Time Kinematic (RTK) system. In the proposed system, only single-epoch GNSS measurements are used to avoid problems caused by cycle slips, and a time-propagated relative vector in the INS is used as a pseudo measurement in addition to the GNSS measurements to obtain an RTK solution with an improved PCF. We mathematically prove that the covariance matrix of an ambiguity float solution is always improved by the addition of prior information of an INS-aided relative vector compared to that of the existing single-epoch RTK. However, the PCF simulation results obtained by using the improved covariance matrix after applying the proposed method show that counter examples with a worse PCF of INS-aided solution than that of a GNSS-only solution may occur due to the characteristics of Z-transformation in the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) method. This study also mathematically proves that the PCF is always improved without a counter example when the original Z matrix calculated from the existing single-epoch RTK is applied. Thus, we propose a new algorithm that does not generate any counter examples, while increasing the PCF significantly based on the proof. Availability simulations were performed for different GNSS measurement noise levels and Inertial Measurement Unit (IMU) sensor grades. The results show that the system availability can be improved after applying the proposed method to the existing single-epoch RTK.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Bubble-like Ionospheric Irregularities observed in the United States during the 26 August 2018 Geomagnetic Storm},

author = {Hyeyeon Chang and Hyosub Kil and Andrew K. Sun and Shunrong Zhang and Jiyun Lee},

url = {https://ui.adsabs.harvard.edu/abs/2021AGUFMSA45B2215C/abstract},

year  = {2021},

date = {2021-12-01},

urldate = {2021-12-01},

booktitle = {AGU Fall Meeting 2021},

address = {New Orleans, LA},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Enhanced Local-Area DGNSS for Autonomous Vehicle Navigation:  Optimal Smoothing Strategy},

author = {Gihun Nam and Dongwoo Kim and Noah Minchan Kim and Jiyun Lee and Sam Pullen },

doi = {10.33012/2021.18066},

year  = {2021},

date = {2021-09-01},

urldate = {2021-09-01},

booktitle = {Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri},

pages = {4080 - 4096},

abstract = {Local Area Differential GNSS (LADGNSS) is one means of providing navigation and guidance to autonomous vehicles with very high accuracy and integrity. In previous work, the authors have developed a low-cost, portable LADGNSS system prototype based on the single-frequency (L1-only) Ground-based Augmentation System (GBAS) architecture developed for civil aviation. This work expands that system to use multiple frequencies (L5/E5 in addition to L1) and Galileo satellites in addition to GPS, creating a Dual-Frequency Dual-Constellation (DFDC) system. This creates additional options for the carrier smoothing of code phase that is critical to reducing code-phase (pseudorange) errors and provides additional means to detect and exclude signals affected by anomalous ionospheric behavior. This paper develops new models of nominal errors for DFDC LADGNSS to represent error correlation across time and among ground reference receivers. These models support detailed comparisons of different smoothing algorithms and time constants in the presence of multiple nominal error sources. Vertical Protection Levels (VPLs) for a set of candidate smoothing processes and time constants are generated for 27-satellite GPS and Galileo constellations to determine which give the best performance (lowest VPLs, and thus highest availability) under different operational scenarios, different user distances from the LADGNSS reference station, and different levels of nominal ionospheric activity (in mid-latitudes and in equatorial regions).},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {GNSS Carrier Frequency Dependence of Ionospheric Scintillation Index in Equatorial Regions},

author = {Andrew K. Sun and Jiyun Lee and Xiaoqing Pi and Martin Kriegel and Jens Berdermann},

year  = {2021},

date = {2021-09-01},

urldate = {2021-09-01},

booktitle = {Proceedings of the XXXIVth URSI General Assembly and Scientific Symposium, Rome, Italy},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{nokey,

title = {Preliminary Assessment of GeoOptics and Spire CubeSat RO Performance by Comparing with COSMIC-2},

author = {Jaehee Chang and Hyeyeon Chang and Jiyun Lee},

year  = {2021},

date = {2021-08-01},

urldate = {2021-08-01},

booktitle = {BIEN 2021, Daejeon},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Chang2020b,

title = {Effects of CICERO Receiver Characteristics on the Quality of Radio Occultation Data},

author = {Hyeyeon Chang and Jiyun Lee and Yang Wang and Brian Breitsch and Y. Jade Morton},

year  = {2020},

date = {2020-12-01},

booktitle = {AGU Fall Meeting 2020},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Chang2020,

title = {Preliminary Assessment of CICERO Radio Occultation Performance by Comparing with COSMIC I Data},

author = {Hyeyeon Chang and Jiyun Lee and Yang Wang and Brian Breitsch and Y. Jade Morton},

doi = {10.33012/2020.17754},

year  = {2020},

date = {2020-09-21},

booktitle = {Proceedings of the 33rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2020)},

pages = {3888-3900},

abstract = {Community Initiative for Cellular Earth Remote Observation (CICERO) is a planned constellation of low-earth-orbiting 6U cubeSats for performing GNSS radio occultation (RO) of Earth’s atmosphere and surface. The goal of CICERO is to provide the first and high-quality commercial radio occultation data from space at low costs while enhancing weather and climate forecasting capabilities. Before CICERO data are used for reliable weather forecasting, the assessment of its performance is necessary. This study shows the performance of CICERO by comparing it with that of COSMIC I. The performance analysis was carried out with respect to geographic distribution, altitude distribution, and refractivity error. The results show that CICERO can acquire global coverage of data at low altitudes as well as COSMIC I, which is important for climate research. In addition, the analysis of refractivity error and its impact on temperature demonstrates that the miniature version of the GNSS RO receiver could satisfy certain accuracy requirements of the GNSS RO measurements. Thus, the cubeSat constellation CICERO can provide radio occultation measurements comparable to those of the COSMIC I mission. The study demonstrates the capabilities of nanosat-based LEO cubeSats to improve data obtainability and accuracy for weather forecasting.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Sun2020,

title = {Performance Benefit from Dual-Frequency GNSS-based Aviation Applications under Ionospheric Scintillation: A New Approach to Fading Process Modeling},

author = {Kiyoung Sun and Hyeyeon Chang and Jiyun Lee and Jiwon Seo and Y.T. Jade Morton and Sam Pullen},

doi = {10.33012/2020.17184},

year  = {2020},

date = {2020-02-14},

booktitle = {Proceedings of the 2020 International Technical Meeting of The Institute of Navigation},

pages = {889 - 899},

address = {San Diego, California},

abstract = {Deep signal fading due to ionospheric scintillation may cause loss-of-lock on one or more satellites in GNSS receiver tracking loops, which can degrade the navigation availability of GNSS-based aviation applications. Scintillation impact can be mitigated via the frequency diversity, which decreases the chance of satellite loss in the presence of deep and frequent signal fades. This study presents an improved fading process model that generates correlated fading processes of dual-frequency signals and simulates the resulting scintillation impact to evaluate the availability benefit of utilizing dual-frequency GNSS in aviation applications under scintillation. The correlated fading process was described by combining single-frequency only and dual-frequency concurrent fading processes under the assumption that each fading process can be modeled as a Poisson process. Times between deep fading onsets and fading durations observed from GPS L1/L5 dual-frequency measurements collected at Hong Kong in March 2nd, 2014 were used to model the GPS L1 only, L5 only, and L1/L5 concurrent fading processes. Availability simulations for SBAS service supporting the LPV200 phase of flight were conducted by considering the effects of satellite geometry degradation and shortened carrier smoothing time, which are caused by signal losses from deep fades generated by the newly proposed fading process model. A parametric analysis of availability resulting from variations in both the probability of loss-of-lock (under deep fading) and the receiver reacquisition time (following loss-of-lock) was conducted to provide receiver requirement standards for SBAS-based aviation under severe scintillation. The results show noticeable availability improvement from dual-frequency SBAS-based aviation applications over existing single-frequency SBAS.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2020,

title = {Optimal Continuity Allocation for a Tightly-coupled KF-based GNSS/IMU Navigation System with Redundant IMUs},

author = {Jinsil Lee and Dongwoo Kim and Dongchan Min and Gihun Nam and Jiyun Lee},

doi = {10.33012/2020.17201},

year  = {2020},

date = {2020-02-14},

booktitle = {Proceedings of the 2020 International Technical Meeting of The Institute of Navigation},

pages = {1101 - 1116},

address = {San Diego, California},

abstract = {This paper introduces an optimal continuity allocation algorithm for a tightly-coupled Kalman-filter (KF)-based Global Navigation Satellite System (GNSS) and multiple Inertial Measurement Units (IMUs) integrated navigation system. Beyond our recent work on the integrity and continuity algorithm of a KF-based GNSS and a single IMU navigation system, the method proposed in this paper enables IMU sensor exclusion while assuring total continuity and integrity requirements under redundant IMU sensor configuration. Two filtering scenarios in the presence of redundant IMU sensors are considered: 1) individual filters that utilize state estimates from a single set of filters at a time, and uses other filters as backup solutions, and 2) a decentralized filter where all the parallel local filters process each IMU sensor measurement and local estimates are subsequently fused in a master filter to achieve the global estimations. First, an analytical equation that can evaluate the continuity risk probability under each filter scenario is derived. Second, based on the fact that sub-filters within the decentralized filter share the same GNSS measurements with different IMU sensors, an inter-filter correlation between test statistics of the decentralized filter is considered to tightly allocate continuity to each monitor. Inter-filter correlation is quantified recursively by the KF, and the monitor thresholds are determined from the formulated joint distributions by the inter-filter correlation. Lastly, a minimum bounding protection level (PL) is determined by optimally allocating continuity risk to each monitor. Continuity allocation considering the correlation significantly improves the overall availability by lowering the continuity burden of each monitor. Simulation results show the benefits of utilizing redundant IMU in terms of system availability and the benefits of taking into account the exact inter-filter correlation for the case of a decentralized filter when allocating continuity requirements for each monitor.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2019c,

title = {Integrity Algorithm to Protect against Sensor Faults in Tightly-coupled KF State Prediction},

author = {Jinsil Lee and Minchan Kim and Dongchan Min and Jiyun Lee},

doi = {10.33012/2019.16867},

year  = {2019},

date = {2019-09-23},

booktitle = {Proceedings of the 32nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2019)},

pages = {594 - 627},

address = {Miami, Florida},

abstract = {Recently, autonomous vehicles including self-driving cars, or autonomous drones are getting a great deal of attention with the expectation that they can partially replace or augment human performance while improving operational efficiency and increase the range of applications. There are many technical issues to solve for realizing the mission including mechanical and electrical problem, control and path planning, and navigation. Among the required technologies, navigation is one of the essential components for the development of autonomous vehicles because vehicles conduct control and path following operations autonomously by trusting the given navigation information. In this response, multi-sensor navigation systems have been actively developed to improve navigation accuracy as well as continuity under various operational conditions. Currently, various sensors including Global Navigation Satellite System (GNSS), Inertial Measurement Unit (IMU), vision sensor, barometer, magnetometer, and LIDAR (for cars) are integrated to provide the navigation solution. Many studies developed the integration algorithms of these sensors, and the accuracy and continuity are significantly improved. As the many inputs from various sensors are integrated, navigation solutions are more often exposed to potential sensor faults. For the practical use of the autonomous vehicle in a ‘real world’ environment, navigation safety in addition to the navigation accuracy and continuity is critical to ensure the safe operation of the autonomous vehicles. This raises the most obvious and important question which is “How can we prove that the solution from the multi-sensor navigation system is safe to use?”. This study leverages the concept of navigation integrity which is carried out in civil aviation navigation. Integrity is a measure of trust in navigation solutions, and integrity algorithm detects navigation sensor faults causing unacceptably large positioning error and computes position error bound (protection level, PL) against undetected sensor faults. In civil aviation, integrity algorithms have been developed for different types of GNSS standalone and augmentation systems including Receiver Autonomous Integrity Monitoring (RAIM), and Satellite or Ground Based Augmentation Systems (SBAS or GBAS) to ensure navigation integrity to the extremely high level of integrity (of the order of from 10-7 to 10-9). In an integrity algorithm, the PL is computed by quantifying the impact of undetected sensor faults which propagates through a filter estimation algorithm. Thus, a PL derivation algorithm should be designed depending on estimation filters. The developed integrity algorithms for civil aviation (reviewed in the previous paragraph) is designed based on the weighted least square (WLS) filter algorithm which is a snapshot estimator in accordance with the positioning algorithm. In contrast, the multi-sensor navigation systems mostly use a Kalman filter (KF), which is a ‘recursive’ filter, for estimating navigation solutions. The recursive filter utilizes all previous sensor information in addition to the current time sensor input to determine the state estimates. This means sensor faults occurred in the previous measurements affect the position estimates as well as sensor faults in the current time measurements. This fault propagation characteristic in a recursive KF should be carefully considered when designing a KF based integrity algorithm. This raises the need for the new integrity algorithm for multi-sensor navigation systems which utilizes KF for sensor integration. The KF based integrity algorithms that quantify the fault impact on state estimates are actively being discussed in recent publications [1]-[3]. One assumption made in the previous studies is that faults occur in the GNSS measurement sequence which is used for the KF measurement update. Studies in [1] and [2] quantified the worst-case integrity risk of KF estimates by determining the worst-case fault vector in an analytical approach based on the relationship between state error and range residual based monitor test statistics. [2] specifically applied the KF based integrity algorithm to evaluate the integrity risk against a GNSS spoofing within a GNSS/IMU integrated system with an assumption that the IMU is a fault-free sensor. As another approach, [3] proposed to compute a real-time PL equation for KF estimates. To improve real-time capability, it applied an external snapshot based WLS RAIM monitor for fault monitoring and computed PL by simulating the impact of remaining fault on KF estimates through the KF algorithm that users are utilizing for their positioning. Based on the fact that there are two main filtering steps which are a state prediction and measurement update, the integrity algorithm for the sensor faults occurring in the state prediction step is also required in multi-sensor navigation systems to fully assure the system safety. In the state prediction step, an estimated state in a previous epoch is propagated to the next epoch based on a given static model or using sensor measurements which provide information for the state prediction. In multi-sensor navigation systems, sensors which provide the information for vehicle motions are used for the state prediction step. There are sensors generally applied in state prediction step including IMU, and vision sensor. Those sensors are also vulnerable to faults [4]-[5], and integrity against those sensors should be considered within the system integrity algorithms. In [6], we illustrated how an unexpected step type IMU faults (potentially due to fracture, stiction, or delamination) affect the KF estimates when it is used for the state prediction in a GNSS/IMU integrated navigation system. It was also shown that the impact of fault on position solutions due to the IMU sensor faults could be expressed using the KF innovation vector in real-time. A PL was determined for the IMU sensor fault hypothesis using the KF innovation vector and additional noise bounding terms. In this paper, we extend the concept of integrity assurance against faults occurring in sensors used for state prediction, proposed in [6], to KF-based multi-sensor navigation systems. We analyze the key difference in the impact of faults occurring in the prediction step and the measurement update step on state estimates. Under the assumption that no simultaneous fault occurs on two integrated sensors (one for the state prediction, and the other for the measurement update), the most fundamental parameter we can utilize from a KF is a KF innovation vector. If the innovation can fully capture the recursive impact of faults on state estimates in real-time, the user PL can be formulated utilizing the innovation vector with additional uncertainty terms for noise bounding. Based on the analytical formulation of the recursive fault impact on user position under each fault condition, it is shown that the innovation vector can express the fault impact on user position only when the fault occurs in sensors used for the state prediction. In case of a fault occurring in sensors used for the measurement update step, additional information on the fault vector is required to fully express the magnitude of the resulting position error caused by the fault. In other words, the innovation vector cannot be used to compute user PL in real-time when there is a fault on sensors used for the measurement update. Table 1 summarizes the content in this paragraph. Based on the finding, this study proposes a generalized integrity architecture which can be used for a KF based multi-sensor navigation system to assure navigation integrity. In the integrity architecture, three fault hypotheses are defined which are a nominal hypothesis (H0), a fault hypothesis occurred in a sensor for the measurement update step (H1), and a fault hypothesis occurred in a sensor for the prediction step (H2). For the H0 hypothesis, nominal covariance from the KF is used for PL computation. Under the H1 fault hypothesis, an additional fault monitoring algorithm is required to detect faults and quantify the undetected fault magnitude after fault monitoring in order to compute the real-time protection level. The algorithms which use RAIM proposed by the previous study [3] can be utilized for computing a real-time PL for this fault hypothesis. Lastly, the KF innovation vector is used to detect the fault and to compute a PL for the H2 fault hypothesis. For the H2 fault hypothesis, fault detection is conducted on the user position domain by comparing the fault impact determined using the KF innovation vector in real-time with a pre-defined threshold without employing additional fault monitoring algorithms. The proposed integrity algorithm can be applied to various types of multi-sensor navigation systems to compute PL against the sensor faults occurring in the state prediction step. In this study, we verified and showed the applicability of the developed integrity algorithm using two types of KF based multi-sensor systems which are a GNSS/IMU integrated navigation system and a GNSS/vision integrated navigation system. IMU and Vision sensors were used to predict the filter state, and GNSS was used to update the filter. Three PLs are computed for each fault hypothesis. We employed the algorithm proposed in [3] for computing a real-time PL against GNSS faults within the KF. New PLs both for the IMU and vision sensor were determined using the KF innovation sequence based on the proposed integrity algorithm. The computed PLs for IMU and vision sensors well bounded the KF estimates both under no-fault condition and fault-injected condition. The proposed integrity algorithm for KF-based multi-sensor navigation systems against sensor faults in the KF prediction step can be broadly used to fully assure the safety of autonomous vehicles. References [1] M. Joerger, and B. Pervan (2013). "Kalman Filter-Based Integrity Monitoring Against Sensor Faults." Journal of Guidance, Control, and Dynamics 36(2): 349-361. [2] C. Tanil et al. (2017). "An INS monitor to detect GNSS spoofers capable of tracking vehicle position." IEEE Transactions on Aerospace and Electronic Systems. [3] Bhattacharyya, S. and D. Gebre-Egziabher (2015). "Kalman filter based RAIM for GNSS receivers." IEEE Transactions on Aerospace and Electronic Systems 51(3): 2444-2459. [4] Bhatti, Umar Iqbal, and Washington Yotto Ochieng. "Failure modes and models for integrated GPS/INS systems." The Journal of Navigation 60.2 (2007): 327-348. [5] Fu, Li et al. “Vision-Aided RAIM: A New Method for GPS Integrity Monitoring in Approach and Landing Phase” Sensors (Basel, Switzerland) vol. 15,9 22854-73. 10 Sep. 2015, doi:10.3390/s150922854 [6] J. Lee, et al., “Integrity assurance of Kalman-filter based GNSS/IMU integrated systems against IMU faults for UAV applications,” ION GNSS 2018, Miami, Florida, pp. 2484-2500.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Kim2019,

title = {Use of Local-area Differential GNSS System for Polar Exploration},

author = {Dongwoo Kim and Minchan Kim and Jinsil Lee and Jiyun Lee},

year  = {2019},

date = {2019-07-26},

booktitle = {The 13th International Symposium on Antarctic Earth Sciences},

address = {Incheon, Republic of Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Min2019,

title = {Deep Neural Network Based Multipath Mitigation Method for Carrier Based Differential GNSS Systems},

author = {Dongchan Min and Minchan Kim and Jinsil Lee and Jiyun Lee},

doi = {10.33012/2019.16856},

year  = {2019},

date = {2019-04-15},

booktitle = {Proceedings of the ION 2019 Pacific PNT Meeting (2019 ION PNT)},

pages = {451-466},

address = {Honolulu, Hawaii},

abstract = {Carrier based Differential Global Navigation Satellite System (CD-GNSS) is getting lots of attention as a promising technology for drones since it can provide centimeter-level accuracy. Unlike widely used CD-GNSS applications such as geodetic survey, CD- GNSS systems to be used for drone applications should provide a certain level of integrity along with accuracy. However, there is a critical challenge in guaranteeing the integrity of CD-GNSS systems for drone applications due to a long filter duration which is the time necessary to resolve cycle ambiguity correctly. One of the most dominant factors that limits reducing the filter duration is the code multipath error. In this response, this paper proposes a code multipath mitigation method using Deep Neural Network (DNN) for drone systems. It is well known that DNN is a powerful for nonlinear regression problems toward which multipath estimation is applicable and can be trained using a large quantity of data. The target scenario of this study is chosen as a drone flying at a sufficiently high altitude (where no multipath reflections exist from surrounding obstacles) before lowering its altitude for other phases of operation. Therefore, the dominant factor of multipath errors of the drone is the signal reflection from its body frame. Considering the fact, the input parameters were chosen for developing the DNN model as elevation, azimuth and tilt angle of antenna which characterize signals reflected by the frame and Signal to Noise Ratio (SVR) which characterizes the slight change of signals. The validity of the proposed model was investigated and its multipath mitigation performance was evaluated under the target scenario using real data. The results show that the multipath error is mitigated by about 30% in terms of the standard deviation of multipath error and the filter duration is reduced by about 66%.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2019b,

title = {Assessment and Mitigation of Ionospheric Spatial Decorrelation on GBAS: Lessons Learned},

author = {Jiyun Lee},

doi = {10.33012/2019.16840},

year  = {2019},

date = {2019-04-15},

booktitle = {Proceedings of the ION 2019 Pacific PNT Meeting (2019 ION PNT)},

pages = {807-826},

address = {Honolulu, Hawaii},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Chang2019,

title = {Assessment of Ionospheric Spatial Decorrelation for Daytime Operations of GBAS in the Brazilian Region},

author = {Hyeyeon Chang and Moonseok Yoon and Jiyun Lee and Sam Pullen and Leonardo Marini Pereira},

doi = {10.33012/2019.16673},

year  = {2019},

date = {2019-02-04},

booktitle = {Proceedings of the 2019 International Technical Meeting of The Institute of Navigation (ION ITM 2019)},

pages = {618-631},

address = {Reston, Virginia},

abstract = {	Extensive ionospheric studies were conducted to support the initial phase of system design approval (SDA) for the existing SLS-4000 GBAS installed at Galeão Airport (GIG) in Rio de Janeiro, Brazil. This paper focuses on determining the broadcast value of the standard deviation of vertical ionospheric gradients (or sigma_vig) that is required to bound ionospheric spatial gradients at the Brazilian region under nominal conditions during daytime hours. The days for the analysis were selected by a combination of high levels of daytime or nighttime scintillation and/or severe values of the geomagnetic storm index. The time-step method, which is useful for gaining sufficient samples at distances less than the physical separation distance of ground stations, was utilized to estimate ionospheric spatial gradients. The results show that the bounding sigma_vig values are in the range of 8-13 mm/km, when daytime was defined to exist between 6 AM and 6 PM local time. Because the time-step method estimates gradients over a time interval instead of at a single time, the results include temporal as well as spatial gradients, meaning that they overestimate the spatial gradients that are the desired results. Thus, a new method called “geometric modeling” was proposed to estimate ionospheric temporal gradients and evaluate the temporal effect added to the bounding sigma_vig values. As a result, a sigma_vig of 13 mm/km including an approximately 2 mm/km temporal gradient contribution, is conservative enough to bound ionospheric spatial decorrelation for daytime GBAS operations in the Brazilian region.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Yoon2019c,

title = {Integrity Risk Evaluation of Impact of Ionospheric Anomalies on GAST D GBAS},

author = {Moonseok Yoon and Jiyun Lee and Sam Pullen},

doi = {10.33012/2019.16685},

year  = {2019},

date = {2019-02-04},

booktitle = {Proceedings of the 2019 International Technical Meeting of The Institute of Navigation (ION ITM 2019)},

pages = {100-112},

address = {Reston, Virginia},

abstract = {To insure safe approach and landing operations of GAST D GBAS, this study develops a three-step worst-case parameter search and integrity risk evaluation method to identify the worst-case sets of ionospheric threat parameters and evaluate the worstpossible integrity risk. Parameters of the ionospheric gradient impact simulation are first classified into two groups, “Worst-Case (WC)” and “average,” based on the underlying “specific risk” integrity requirements. “WC” parameters are those for which a clear basis for averaging could not be established because the probabilistic distribution of these parameters cannot be established with confidence due to the lack of sufficient independent observation data. These “WC” parameters must use the worst-case value of that parameter in calculating integrity risk, meaning the value of the parameter that maximizes the integrity risk. Parameters chosen to be “average” are those which are known to be essentially random from the point of view of a GBAS precision approach impacted by an ionospheric front. The parameters treated as “average” can apply values from a sampled distribution (usually from a Uniform distribution) between the minimum and maximum values of that parameter as the basis for calculating integrity risk. The first step of this method performs a randomized search over all of the parameters with a very large total number of samples to identify one or more regions of the ionospheric threat space where the worst-case result (maximum integrity risk) is likely to lie. In the second step, the search for the worst-case set of threat parameters is more refined so that a single worst-case set of parameters can be selected for each region. Once each identified region has a single set of worst-case threat-model parameters, separate “Step 3” simulations can be conducted for each region to estimate integrity risk values, and these final results are what is compared to the SARPs anomalous ionosphere integrity requirements. This paper also evaluates the maximum integrity risk probabilities based on this three-step method using the SARPs ionospheric anomaly threat model and geometric parameters. The resulting integrity risk values for all regions are well below 10-9 , showing that the GAST D SARPs integrity requirement is met based on this combined worst-case/average definition of the anomalous ionospheric threat.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Sun2018bb,

title = {Ionospheric Scintillation Effects on GBAS Ground Station Pseudorange Errors},

author = {Kiyoung Sun and Moonseok Yoon and Michael Felux and Jiyun Lee},

url = {http://adsabs.harvard.edu/abs/2018AGUFMSA13C2790S},

year  = {2018},

date = {2018-12-10},

booktitle = {AGU Fall Meeting 2018},

address = {Washington D.C},

abstract = {Ground-Based Augmentation Systems (GBAS) provide differential corrections and integrity information for aviation users to support aircraft precision approach and landing. However, the navigation performance of GBAS can be severely degraded by ionospheric scintillation. In extreme cases, deep fades in GPS signal amplitude lead to satellite signal loss which reduces the number of satellites available for use and consequently degrades navigation accuracy and availability.

Even if a receiver can tolerate the signal loss described above, a serious risk still remains on the navigation performance due to intensified ground and airborne pseudorange errors, resulting from the receiver tracking process in the presence of scintillation. In GBAS, ground and airborne error sources include thermal noise and multipath, which cannot be removed from differential corrections. These errors are modeled and used for airborne positioning algorithm and integrity protection level computations. Thus, if the ground and airborne errors increased by scintillation are not bounded by the existing model, they may pose an integrity threat to GBAS users.



In this paper, the effects of scintillation on GBAS ground station pseudorange errors were analyzed using multi-frequency GPS data collected from the reference station in Bahir Dar, Ethiopia located near the equatorial region. To quantify the ground station pseudorange errors, the standard deviations of receiver noise and multipath estimates are calculated from the linear combinations of dual-frequency GPS measurements under strong scintillation. The processed statistics were compared to the existing GBAS ground error model which was determined from receiver noise and multipath under nominal conditions. GBAS availability was also evaluated by computing Vertical Protection Levels (VPLs) with the broadcast integrity parameters newly estimated under scintillation and comparing those to the Vertical Alert Limit (VAL).



The results show that the GBAS ground error statistics under strong scintillation noticeably exceed those defined by the current GBAS ground error model. While a new GBAS ground error model can guarantee the navigation integrity in scintillation conditions, the system may suffer availability degradation depending on the severity of the scintillation.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Sun2018b,

title = {Neural Network Aided Reconstruction of Ionospheric Tomography for Limited GNSS Measurements},

author = {Kiyoung Sun and Pil Hun Choi and Sunjin Kim and Jiyun Lee},

year  = {2018},

date = {2018-12-06},

urldate = {2018-12-06},

booktitle = {2018 10th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Daejeon, Rep. of Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2018b,

title = {Performance Comparison of Neural Networks for Geomagnetic Field Based Indoor Positioning System},

author = {Hyojin Lee and Harhee Park and Dongchan Min and Jiyun Lee},

year  = {2018},

date = {2018-12-06},

booktitle = {2018 10th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Daejeon, Rep. of Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2018,

title = {Integrity Assurance of Kalman-Filter based GNSS/IMU Integrated Systems Against IMU Faults for UAV Applications},

author = {Jinsil Lee and Minchan Kim and Jiyun Lee and Sam Pullen},

doi = {10.33012/2018.15977},

year  = {2018},

date = {2018-09-24},

booktitle = {Proceedings of the 31st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2018)},

pages = {2484 - 2500},

address = {Miami, Florida},

abstract = {This study proposes an integrity architecture for an Extended Kalman filter (EKF) based Global Navigation Satellite System (GNSS)/Inertial Measurement Unit (IMU) integrated system to assure integrity of unmanned aerial vehicle (UAV) navigation systems. An integrity risk allocation tree is developed for each sensor fault hypothesis including the nominal hypothesis, a GNSS fault hypothesis, and an IMU sensor fault hypothesis. This paper then proposes a real-time EKF vertical protection level (VPL) against IMU sensor faultsto assure navigation integrity based on the relationship between EKF innovations and EKF state errors resulting from potential IMU faults and measurement noise. Simulations for the derived EKF VPLs are conducted under a specific IMU-fault condition and under no-IMU-fault condition to investigate typical performance. This study will be extended to assure the navigation integrity of different types of multi-sensor systems for UAV applications.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Chang2017b,

title = {Assessment of Ionospheric Gradient Statistics for Operating GBAS in Geomagnetic Equatorial Region},

author = {Hyeyeon Chang and Jiyun Lee},

year  = {2017},

date = {2017-12-11},

booktitle = {AGU Fall Meeting 2017},

address = {New Orleans, LA},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Sun2017,

title = {Ionospheric Scintillation Effects on GBAS Ground Multipath Error in Low-latitude Regions},

author = {Kiyoung Sun and Jiyun Lee},

year  = {2017},

date = {2017-12-07},

booktitle = {2017 9th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Fukuoka, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}