Journal Articles

@article{nokey,

title = {Navigation Safety Assurance of a KF-Based GNSS/IMU System: Protection Levels Against IMU Failure},

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

doi = {10.33012/navi.612},

year  = {2023},

date = {2023-09-01},

urldate = {2023-09-01},

journal = {NAVIGATION: Journal of the Institute of Navigation},

volume = {70},

issue = {4},

abstract = {This study introduces a navigation integrity and continuity algorithm against an inertial measurement unit (IMU) sensor fault within a Kalman filter (KF) that ensures a high level of safety for IMU-integrated safety-critical navigation applications. A representative example of an IMU integrated navigation system is a global navigation satellite system (GNSS)/IMU system. Most previous studies have focused on GNSS faults when evaluating the integrity and continuity of a KF-based GNSS/IMU navigation system, leaving the IMU fault hypothesis unaddressed. Unlike GNSS, which is applied in the measurement update step within the KF, IMU measurements are applied in the state prediction step, which results in different fault propagation characteristics in the user state error compared with those in GNSS. This paper analytically derives the sequential IMU fault impacts on user state errors. Based on this investigation, a KF innovation-based fault detector and protection-level equations are developed, which can safely bound user state errors against sequential IMU fault impacts.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {DNN-Based Approach to Mitigate Multipath Errors of Differential GNSS Reference Stations},

author = {Dongchan Min and Minchan Kim and Jinsil Lee and Mihaela-Simona Circiu and Michael Meurer and Jiyun Lee},

doi = {10.1109/TITS.2022.3207281},

year  = {2022},

date = {2022-09-28},

journal = {IEEE Transactions on Intelligent Transportation Systems},

pages = {1-7},

abstract = {One of the major error components of differential global navigation satellite systems is a multipath error in a reference station. This paper introduces a deep neural network based multipath modeling method. A signal to noise ratio, as well as satellite geometry, is used as a feature parameter to capture the variation of the multipath error caused by unavoidable changes in the vicinity of the reference station. The performance of the proposed method is demonstrated for both normal and varying multipath cases using experimental data. The remaining multipath error after mitigation is well bounded by the standardized error model.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Undersampled Ionospheric Irregularity Threat Parameterization Using a Three-Dimensional Model for Satellite-Based Augmentation Systems},

author = {Eugene Bang and Jiyun Lee },

doi = {10.1109/TAES.2022.3199192},

year  = {2022},

date = {2022-08-17},

journal = {IEEE Transactions on Aerospace and Electronic Systems},

pages = {1-24},

abstract = {Single-frequency Satellite-based Augmentation Systems (SBASs) compute and broadcast estimates of vertical ionospheric delays and integrity bounds on the estimates called the Grid Ionospheric Vertical Errors (GIVEs) at Ionospheric Grid Points (IGPs). The dominant contribution of the GIVE comes from the undersampled ionospheric irregularity threat model. This paper presents a methodology for the undersampled ionospheric threat parameterization to reduce the magnitude of the GIVE. A threat model metric that measures the uniformity of angular separation of measurements was designed and incorporated into the current metric set, the fit radius and the relative centroid metric (RCM), such that threat geometries were parameterized rigorously based on the extended metric set. An undersampled threat model was constructed with the proposed three-dimensional metric set and historical ionospheric storm data from the global navigation satellite system stations in South Korea. We also simulated SBAS availability in the Korean region to demonstrate the benefit of the presented threat model methodology. In our preliminary assessment, the implementation of the proposed method was found to improve the 99.9% availability of the approach procedure with vertical guidance I by up to 12% when the approach was applied to the GIVE monitor algorithm.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Performance assessment of radio occultation data from GeoOptics by comparing with COSMIC data},

author = {Hyeyeon Chang and Jiyun Lee and Hyosang Yoon and Y. Jade Morton and Alex Saltman},

doi = {10.1186/s40623-022-01667-6},

year  = {2022},

date = {2022-07-11},

urldate = {2022-07-11},

journal = {Earth, Planets and Space},

volume = {74},

number = {108},

abstract = {Responding to the ever-growing demand for environmental information, the National Oceanic and Atmospheric Administration (NOAA) seeks to enter into contracts to purchase Global Navigation Satellite System (GNSS) radio occultation (RO) observations produced by commercial vendors at a low-cost. GeoOptics is one commercial vendor awarded a contract with NOAA. GeoOptics operates the Community Initiative for Cellular Earth Remote Observation (CICERO) constellation of low-earth-orbiting (LEO) 6U CubeSats. The 6U-sized CICERO will enable the deployment of GNSS array consisting of RO satellites in the Earth’s atmosphere to obtain many atmospheric observations which can improve weather forecasting. Applying GeoOptics RO data to reliable weather forecasting requires an assessment of its performance. This study analyzes the performance of GeoOptics CubeSats measurements by comparing it with the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) missions (COSMIC-1 and COSMIC-2). The performance analysis was carried on data coverage capabilities and measurement quality. The analysis of data coverage confirmed that GeoOptics can acquire global observational coverage with adequate low-altitude penetration capability, while there should be updated in local time coverage. The analysis of RO measurement quality showed that GeoOptics RO measurements are comparable to those of COSMIC-2, even though GeoOptics exhibited a lower signal-to-noise ratio (SNR). The potential of GeoOptics allows for the development of a GNSS array in the Earth’s atmosphere and a large amount of effective RO measurements to be obtained for reliable weather forecasting.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Error modelling method of extended Kalman filter-based terrain referenced navigation system for integrity assurance under nominal conditions},

author = {Haerhee Park and Jinsil Lee and Jiyun Lee},

doi = {10.1049/rsn2.12277},

year  = {2022},

date = {2022-05-17},

urldate = {2022-05-17},

journal = {IET Radar, Sonar & Navigation},

volume = {16},

number = {9},

pages = {1516-1529},

abstract = {Navigation safety is further emphasised for terrain referenced navigation (TRN) systems used in middle-range unmanned aerial vehicles (UAVs) than those in cruise missiles flying through mountainous areas, because middle-range UAVs may fly to regions and cities containing both combatants and civilians. For safety assurance of TRN systems, this paper proposes a method to compute protection levels (PLs) to assure the required level of integrity under nominal conditions. For obtaining PLs of TRN systems, all the components of the TRN measurement errors should be identified and overbounded. This study models every error component: the radar altimeter error, the vertical error in the terrain elevation database, the lateral offset induced error, and the interpolation error. Based on the characteristics of TRN systems using terrain information for obtaining navigation solutions, this study developed new models for the lateral offset induced error and the interpolation error as a function of terrain roughness by examining the effect of terrain roughness on the error sources. The simulation results demonstrate that the latitude and longitude PLs obtained by applying the proposed models can sufficiently overbound the navigation solution error for TRN systems under nominal conditions; especially the proposed method shows remarkable performance in regions with low terrain roughness.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {The Origin of Multitude Plasma Depletions Detected During the 12 February 2000 and 29 October 2003 Geomagnetic Storms},

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

doi = {10.1029/2021JA030169},

year  = {2022},

date = {2022-03-21},

urldate = {2022-03-21},

journal = {Journal of Geophysical Research: Space Physics},

volume = {127},

number = {3},

abstract = {Large amplitude plasma density irregularities have occasionally been detected at night in the midlatitude F region during geomagnetic storms. They are often interpreted in terms of equatorial plasma bubbles (EPBs) because midlatitude irregularities have the morphology of EPBs. This study assesses whether morphology can be a determining factor in ascribing the origin of such midlatitude ionospheric irregularities. We address this question by analyzing the observations of the First Republic of China satellite (ROCSAT-1) and Defense Meteorological Satellite Program (DMSP)-F14 and -F15 satellites during the geomagnetic storms on 12 February 2000 and 29 October 2003. On both days, ROCSAT-1 detects plasma depletions at midlatitudes in broad longitude regions and DMSP satellites detect isolated severe plasma depletions whose widths and depths are much wider and deeper than those of typical EPBs. The distinguishing characteristics during the storms are the detection of midlatitude depletions only in the Southern Hemisphere and the occurrence of some of these depletions before 19 hr local time and at the longitudes where EPBs are absent in the equatorial region. These characteristics are not explained satisfactorily by the characteristics of EPBs. Considering the detection of some of the midlatitude depletions at the equatorward edge of ionospheric perturbations in midlatitudes, midlatitude depletions are likely ionospheric perturbations that originated from higher latitudes. Because midlatitude depletions can originate from different sources, the morphology alone is not a determining factor of their origin.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Optimal Parameter Inflation to Enhance the Availability of Single-Frequency GBAS for Intelligent Air Transportation},

author = {Halim Lee and Sam Pullen and Jiyun Lee and Byungwoon Park and Moonseok Yoon and Jiwon Seo},

doi = {10.1109/TITS.2022.3157138},

year  = {2022},

date = {2022-03-17},

urldate = {2022-03-17},

journal = {IEEE Transactions on Intelligent Transportation Systems},

pages = {1-8},

abstract = {Ground-based Augmentation System (GBAS) augments Global Navigation Satellite Systems (GNSS) to support the precision approach and landing of aircraft. To guarantee integrity, existing single-frequency GBAS utilizes position-domain geometry screening to eliminate potentially unsafe satellite geometries by inflating one or more broadcast GBAS parameters. However, GBAS availability can be drastically impacted in low-latitude regions where severe ionospheric conditions have been observed. Thus, we developed a novel geometry-screening algorithm in this study to improve GBAS availability in low-latitude regions. Simulations demonstrate that the proposed method can provide 5-8 percentage point availability enhancement of GBAS at Galeão airport near Rio de Janeiro, Brazil, compared to existing methods.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Kalman-filter-based Integrity Evaluation Considering Fault Duration: Application to GNSS-based Attitude Determination},

author = {Dongwoo Kim and Jiyun Lee},

doi = {10.1007/s10291-022-01234-2},

year  = {2022},

date = {2022-02-23},

urldate = {2022-02-23},

journal = {GPS Solutions},

volume = {26},

number = {51},

abstract = {This study presents a new Kalman-filter-based integrity monitoring algorithm that considers fault duration length a variable. The existing integrity monitoring algorithms that assumed a single fault duration with a constant prior probability were extended to account for the multiple hypotheses of fault duration with different prior probabilities. The methods for integrity risk computations were presented for both single and multiple cumulative innovation monitors. Performance analysis of the proposed method was carried out by applying it to a GNSS/INS attitude heading reference system. In this process, the prior probabilities of the fault hypotheses were modeled through Monte Carlo simulations. The results show that the heading integrity risk of the single-hypothesis approach can be reduced by a factor of 12 when applying the multi-hypothesis approach, and the heading integrity risk of the multi-hypothesis approach can be further reduced by a factor of 5 by using multiple monitors. The sensitivity analyses demonstrate that the integrity risk strongly depends on the quality of carrier phase measurements and decreases as a false alarm probability or alert limit increases.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Ionospheric disturbances in low- and midlatitudes during the geomagnetic storm on 26 August 2018},

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

doi = {10.1029/2021JA029879},

year  = {2022},

date = {2022-01-24},

urldate = {2022-01-24},

journal = {Journal of Geophysical Research: Space Physics},

volume = {127},

number = {2},

abstract = {Plasma density depletions at midlatitudes during geomagnetic storms are often understood in terms of equatorial plasma bubbles (EPBs) due to their morphological similarity. However, our study reports the observations that reveal the generation of plasma depletions at midlatitudes by local sources. During the geomagnetic storm on 26 August 2018, the Defense Meteorological Satellite Program and Swarm satellites detected plasma depletions at midlatitudes in the Asian sector in the absence of EPBs in the equatorial region. This observation and the total electron content (TEC) maps over Japan demonstrate that traveling ionospheric disturbances (TIDs) are the sources of midlatitude plasma depletions in the Asian sector. Near the west coast of the United States, the development of a narrow TEC depletion band was identified from TEC maps. The TEC depletion band, which is elongated in the northwest–southeast direction, moves toward the west with a velocity of approximately 240 m/s. The TEC at the TEC depletion band is about 5 TEC units (1016 m−2) smaller than the ambient TEC. As this band is confined to the midlatitudes, this phenomenon is not associated with an EPB. The characteristics of the TEC depletion band are consistent with those of medium-scale TIDs. Observations in the Asian sector and the TEC depletion band over the United States demonstrate that plasma depletions can develop at midlatitudes by local sources. Therefore, the morphological similarity between midlatitude irregularities and EPBs or their coincident occurrence does not provide corroborating evidence of their connection.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kim2021,

title = {Closed-form analysis of undetected range errors due to ionospheric impacts for GBAS category I operations},

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

doi = {10.1002/navi.442},

year  = {2021},

date = {2021-08-27},

journal = {NAVIGATION},

volume = {68},

number = {3},

pages = {507-519},

abstract = {Ionospheric anomalies may cause large differential range errors in Ground-Based Augmentation System (GBAS) users. To mitigate those integrity threats, worst-case ionosphere-induced position errors for potentially usable satellite geometries must be bounded by the GBAS ground facility. This mitigation method requires us to compute the worst-case range error for each satellite affected by a hypothetical ionospheric front. This paper presents a simulation-based method for deriving a closed-form expression of undetected ionosphere-induced range errors. Two types of ionospheric impact scenarios are defined in terms of the motion of an ionospheric front. Explicit expressions for outputs of the code-carrier smoothing filter and the code-carrier divergence monitor are derived to reduce the computational load of ionospheric impact simulations. An exhaustive search algorithm is applied to generate the worst undetected range error among all possible ionospheric impact conditions. Finally, a closed-form expression that bounds the maximum ionospheric range errors is determined as a linear function of the magnitude of gradient and the relative speed of the ionospheric front.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sun2021,

title = {Markov Chain-based Stochastic Modeling of Deep Signal Fading: Availability Assessment of Dual-frequency GNSS-based Aviation under Ionospheric Scintillation},

author = {Andrew K. Sun and Hyeyeon Chang and Sam Pullen and Hyosub Kil and Jiwon Seo and Y. Jade Morton and Jiyun Lee},

doi = {10.1029/2020SW002655},

year  = {2021},

date = {2021-06-24},

journal = {Space Weather},

volume = {19},

number = {9},

pages = {e2020SW002655},

abstract = {Deep signal fading due to ionospheric scintillation severely impacts global navigation satellite system (GNSS)-based applications. GNSS receivers run the risk of signal loss under deep fading, which directly leads to a significant decrease in navigation availability. The impact of scintillation on GNSS-based applications can be mitigated via dual-frequency signals which provide a backup channel. However, the benefit of dual-frequency diversity highly depends on the correlation of fading processes between signals at different frequencies. This paper proposes a Markov chain-based model that simulates the actual behavior of correlated fading processes in dual-frequency channels. A set of recorded scintillation data was used to capture transitions among all fading states based on the fading and recovery of each signal frequency. A statistical study of deep fading characteristics in this data revealed that the Markov chain-based model accurately generates realistic correlated fading processes. Using the proposed model, aviation availability of localizer performance with vertical guidance down to a 200-foot decision height (“LPV-200”) under a strong scintillation scenario is analyzed by considering the effects of signal outages due to deep fading. A parametric analysis of the availability resulting from variations in mean time to loss of lock, mean time to reacquisition, and ionospheric delay uncertainty was conducted to investigate the performance standards on GNSS-based aviation under scintillation. The analysis results demonstrate a significant benefit of frequency diversity on aviation availability during scintillation. This model will further enable the assessment of GNSS-based availability for aviation and other applications under a full range of scintillation conditions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Chang2021,

title = {Ionospheric spatial decorrelation assessment for GBAS daytime operations in Brazil},

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

doi = {10.1002/navi.418},

year  = {2021},

date = {2021-05-06},

journal = {Navigation},

volume = {68},

number = {2},

pages = {391-404},

abstract = {Extensive ionospheric studies were conducted to support the initial phase of system design approval for the existing SLS-4000 GBAS installed at Antonio Carlos Jobim International Airport (formerly Galeão International Airport) (GIG) in Rio de Janeiro, Brazil. This paper focuses on determining the broadcast value of the standard deviation of vertical ionospheric gradients (or σ_vig) that is required to bound ionospheric spatial gradients in Brazil under nominal conditions during daytime hours. The time-step method is useful for gaining sufficient samples at distances less than the physical separation distance of ground stations and was utilized to estimate ionospheric spatial gradients. A new method called “geometric similarity” was developed to estimate ionospheric temporal gradients and evaluate the temporal effect added to the bounding σ_vig values. As a result, a σ_vig of 13 mm/km, including a temporal gradient contribution of approximately 2 mm/km, is conservative enough to bound ionospheric spatial decorrelation for daytime GBAS operations in Brazil.



},

keywords = {},

pubstate = {published},

tppubtype = {article}

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@article{Yoon2020,

title = {Extreme ionospheric spatial decorrelation observed during the March 1, 2014, equatorial plasma bubble event},

author = {Moonseok Yoon and Dongwoo Kim and Jiyun Lee},

doi = {10.1007/s10291-020-0960-x},

year  = {2020},

date = {2020-02-17},

journal = {GPS Solutions},

volume = {24},

number = {47},

abstract = {The ground-based augmentation system must make provisions to being sufficiently robustness to ionospheric anomalies through the development of an ionospheric anomaly threat model. For developing the threat model in Brazil, earlier work found that ionospheric spatial decorrelations larger than those in the midlatitude regions were frequently observed during the peak of Solar Cycle #24 (current cycle). We provide details of a study of the extreme ionospheric spatial decorrelation observed over Brazil during the March 1, 2014, equatorial plasma bubble (EPB) event. As viewed by two Brazilian GNSS reference stations in São José dos Campos, PRN 03 descended to an elevation angle of about 19° in the northern sky. A spatial decorrelation of 850.7 mm/km at the GPS L1 signal at 01:04:00 UT between the two stations SJCU (23.21° S, 45.96° W) and SSJC (23.20° S, 45.86° W) over a baseline of 9.72 km was discovered, when the line of sight of PRN 03 passed through the transition zone of the EPB. Since the EPB-induced ionospheric scintillation can corrupt the ionospheric gradient estimates, multiple gradient observations were made from multiple stations and satellites to verify the largest gradient observation. Severe gradients discovered at other station–satellite pairs support that the event of PRN 03 is a real anomaly as opposed to a receiver fault or the result of post-processing errors. Since the availability loss was estimated to be 41.7% with the Brazilian threat model, remedies to reduce over-estimated ionospheric impact when evaluating and mitigating ionospheric integrity risk are presented.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yoon2020b,

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

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

doi = {10.1002/navi.339},

year  = {2020},

date = {2020-01-03},

journal = {Navigation},

volume = {1},

number = {12},

abstract = {This study develops a three‐step Monte Carlo method to evaluate the worst possible integrity risk of ionospheric spatial gradients for GAST D GBAS. Impact simulation parameters are classified into two groups, “worst‐case” and “average,” based on the underlying integrity requirements. Unlike “average” parameters, “worst‐case” parameters are those for which a clear basis for averaging could not be established because the probabilistic distribution of these parameters cannot be developed with sufficient confidence due to the lack of observation data. In calculating integrity risk, these “worst‐case” parameters use the worst‐case value that maximizes the integrity risk. Each step of the randomized search narrows down the parameter ranges in sequence and identifies the two worst‐case parameter sets (based on the largest position error and the largest missed‐detection probability) for each worst‐case region identified in the initial step. The resulting integrity risk values are well below 10−9, showing that the GAST D SARPs integrity requirement is met.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Jeong2019b,

title = {GNSS Spoofing Detection for Moving Receiver using GNSS Augmentation System},

author = {Seongkyun Jeong and Minchan Kim and Jiyun Lee},

year  = {2019},

date = {2019-10-01},

journal = {International Journal of Aeronautical and Space Sciences},

volume = {Accepted},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yoon2019,

title = {Assessment and Mitigation of EPB Impacts on Category-I GBAS operations in the Brazilian Region},

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

doi = {10.1002/navi.328},

year  = {2019},

date = {2019-08-22},

journal = {Navigation},

volume = {66},

number = {3},

pages = {643-659},

abstract = {Prior to initiating GBAS service in equatorial regions, it is vital to evaluate potential integrity threats posed by equatorial plasma bubble (EPB)‐induced ionospheric gradients and assess availability when implementing ionospheric threat mitigation methods. Earlier work developed a preliminary EPB model with a gradient bound larger than twice that for mid‐latitude ionospheric storms. Position‐domain geometry screening (PDGS) with this higher gradient bound decreases availability to 58.3% at the Galeão International Airport, Brazil, during nighttime. A new mitigation method using Monte Carlo simulation randomizes ionospheric scenarios using randomly generated parameter combinations within the threat model and assesses the ensemble impacts. By taking credit for a prior probability of an extreme EPB, this algorithm determines the inflated integrity parameters to meet the safety requirement in the probabilistic definition. This paper shows that with this method, the system availability for category I precision approaches dramatically improved to 89.6% when a data‐driven prior probability of 10‐5 was applied.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Jeong2019,

title = {Synthesis Algorithm for Effective Detection of GNSS Spoofing Attacks},

author = {Seongkyun Jeong and Jiyun Lee},

doi = {10.1007/s42405-019-00197-y},

year  = {2019},

date = {2019-07-26},

journal = {International Journal of Aeronautical and Space Sciences},

abstract = {As the strength of global navigation satellite system (GNSS) signals is very low, they are vulnerable to interference and susceptible to attacks motivated by economic, military, and security reasons. These threats are gradually increasing. The most common attack is jamming, in which a strong signal is used to make a receiver miss the GNSS signal. As attacks become more sophisticated, they are expected to evolve to spoofing interference, in which the receiver is deceived. Spoofing interference is a larger threat because the receiver cannot recognize that they are being targeted by an attacker. For this reason, it is becoming more important to monitor GNSS signals so that they can be evaluated in terms of reliability. In this paper, we analyze spoofing detection methods with respect to the navigation solution, measurements, and navigation messages obtained by a receiver. We propose an advanced detection method for overcoming the limitations of each individual detection algorithm. The proposed methods enhance the performance of spoofing detection and reduce the false alarm rate. This research can be applied directly to GNSS signal monitoring systems and will be helpful for enhancing the stability of satellite navigation systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2019,

title = {Correlation between Ionospheric Spatial Decorrelation and Space Weather Intensity for Safety-Critical Differential GNSS Systems},

author = {Jinsil Lee and Jiyun Lee},

doi = {10.3390/s19092127},

year  = {2019},

date = {2019-05-08},

journal = {Sensors},

volume = {19},

number = {9},

pages = {2127},

abstract = {An ionospheric spatial decorrelation is one of the most dominant error factors that affects the availability of safety-critical differential global navigation satellite systems (DGNSS). This is because systems apply significant conservatism on the error source when ensuring navigation safety due to its unpredictable error characteristic. This paper investigates a correlation between GNSS-derived ionospheric spatial decorrelation and space weather intensity. The understanding of the correlation has significant advantages when modeling residual ionospheric errors without being overly pessimistic by exploiting external sources of space weather information. An ionospheric spatial decorrelation is quantified with a parameter of spatial gradient, which is an ionosphere total electron content (TEC) difference per unit distance of ionospheric pierce point (IPP). We used all pairs of stations from dense GNSS networks in the conterminous United States (CONUS) that provide an IPP separation distance of less than 100 km to obtain spatial gradient measurements under both ionospherically quiet and active conditions. Since the correlation results would be applied to safety-critical navigation applications, special attention was paid by taking into consideration all non-Gaussian tails of a spatial gradient distribution when determining spatial gradient statistics. The statistics were compared with space weather indices which are disturbance storm time (Dst) index and interplanetary magnetic field (IMF) Bz index. As a result, the ionospheric spatial decorrelation showed a significant positive correlation with both indices, especially under active ionospheric conditions. Under quiet conditions, it showed positive correlation slightly weaker than those under active conditions, and the IMF Bz showed preceding response to the spatial gradient statistics revealing the potential applicability for predicting the spatial decorrelation conditions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2018c,

title = {Effects on the Positioning Accuracy of a GPS Receiver with Array Antenna and Time Delay Compensation for Precise Anti-Jamming},

author = {Kihoon Lee and Junpyo Park and Jiyun Lee},

doi = {10.2322/tjsass.61.171},

year  = {2018},

date = {2018-07-04},

journal = {Transactions of the Japan Society for Aeronautical and Space Sciences},

volume = {61},

number = {4},

pages = {171-178},

abstract = {As more GPS receivers are used in navigation systems to obtain precise position information, concerns about GPS jamming vulnerability are growing. The most effective way to overcome this jamming weakness is to use an array antenna that consists of many antenna elements and RF channels. However, an array antenna causes two side effects: a nulling pattern and a time delay error in positioning performance. We analyze the effects on the positioning accuracy of a GPS receiver equipped with a precise time-delay-compensated array antenna to overcome jamming situations. We present an analysis of the theoretical gain pattern of a 4-array antenna and experimental verification of GPS signal attenuation by obtaining a satellite's CN0 value in the near jamming direction. We show the results of the time delay measurement in the array antenna system using two independent methods and present a new baseband linear interpolation algorithm that is evaluated as having a 0.95 ns RMS error after compensating for the invariant RF time delays. Finally, using the realistic gain pattern and time-delay-compensation results, we assess the position error of the GPS receiver and show the possibility of attaining a precise navigation system in jamming environments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Choi2018,

title = {Conceptual Satellite Orbit Design for Korean Navigation Satellite System},

author = {Moonseok Choi and Dae Hee Won and Jongsun Ahn and Sangkyoung Sung and Jiyun Lee and Jeongrae Kim and Jae-Gyu Jang and Young Jae Lee},

doi = {10.2322/tjsass.61.12},

year  = {2018},

date = {2018-01-01},

journal = {Transactions of the Japan Society for Aeronautical and Space Sciences},

volume = {61},

number = {1},

pages = {12-20},

abstract = {A regional navigation satellite system is a prospective candidate for use in the Korean navigation satellite system (KNSS), which will have South Korea and the remainder of East Asia as its service area. However, orbit design is a prerequisite for any navigation satellite system. This paper implements a conceptual design process prior to orbit design for an indigenous KNSS. Orbits are examined in terms of suitability, and an orbit combination based on the dilution-of-precision (DOP) performance is presented. Through simulation, an orbit combination capable of providing a stable DOP for the Korean Peninsula is proposed. Moreover, the orbit combination proposed incorporates design constraints such as satellite unavailability or potential position errors in the north-south direction, with the Korean Peninsula as a reference position. The simulation results suggest that the KNSS requires an orbit combination involving geostationary orbit (GEO) and elliptically inclined geosynchronous orbit (EIGSO), along with backup satellites in EIGSO; thus, the proposed system consists of 11 satellites in total.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Saito2017,

title = {Ionospheric delay gradient model for GBAS in the Asia-Pacific region},

author = {Susumu Saito and Surendra Sunda and Jiyun Lee and Sam Pullen and Slamet Supriadi and Takayuki Yoshihara and Michael Terkildsen and Frédéric Lecat and ICAO APANPIRG Ionospheric Studies Task Force},

doi = {10.1007/s10291-017-0662-1},

year  = {2017},

date = {2017-10-01},

journal = {GPS Solutions},

volume = {21},

number = {4},

pages = {1937-1947},

abstract = {We investigated characteristics of anomalous spatial gradients in ionospheric delay on GNSS signals in the Asia-Pacific (APAC) low-magnetic latitude region in the context of the ground-based augmentation system (GBAS). The ionospheric studies task force established under the Communications, Navigation, and Surveillance subgroup of International Civil Aviation Organization (ICAO) Asia-Pacific Air Navigation Planning and Implementation Regional Group, analyzed GNSS observation data from the Asia-Pacific region to establish a regionally specified ionospheric threat model for GBAS. The largest ionospheric delay gradient value in the analyzed data was 518 mm/km at the L1 frequency (1.57542 GHz), observed at Ishigaki, Japan in April 2008. The upper bound on the ionospheric delay gradient for a common ionospheric threat model for GBAS in the ICAO APAC region was determined to be 600 mm/km, irrespective of satellite elevation angle.



},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yoon2017b,

title = {Equatorial Plasma Bubble Threat Parameterization to Support GBAS Operations in the Brazilian Region},

author = {Moonseok Yoon and Jiyun Lee and Sam Pullen and Joseph Gillespie and Navin Mather and Rich Cole and Jonas Rodrigues de Souza and Patricia Doherty and Rezy Pradipta},

doi = {10.1002/navi.203},

year  = {2017},

date = {2017-09-17},

journal = {Navigation},

volume = {64},

number = {3},

pages = {309-321},

abstract = {The Brazil ionospheric study project aims to develop a new ground‐based augmentation system (GBAS) ionospheric threat model to better reflect Brazil's low‐latitude conditions. Data processing from the global navigation satellite system for 123 active ionospheric days identified 1017 anomalous ionospheric gradients caused by nighttime equatorial plasma bubbles (EPBs). A significant number of gradients, including the largest verified gradient of 850.7 mm/km, exceed the upper bound (375–425 mm/km) of the conterminous United States (CONUS) threat model. This paper defines a series of parameters to model the geometry of EPBs. A maximum ionospheric delay drop of 35 m and a transition zone between 20 and 450 km are estimated for EPBs that move roughly eastward and parallel to the geomagnetic equator with speeds between 40 and 250 m/s. These parameters are key to the development of a GBAS ionospheric mitigation and safety case for operational approval in Brazil and other low‐latitude locations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2017c,

title = {Monitoring and Mitigation of Ionospheric Anomalies for GNSS-Based Safety Critical Systems: A review of up-to-date signal processing techniques},

author = {Jiyun Lee and Y.T. Jade Morton and Jinsil Lee and Hee-Seung Moon and Jiwon Seo},

doi = {10.1109/MSP.2017.2716406},

year  = {2017},

date = {2017-09-06},

journal = {IEEE Signal Processing Magazine},

volume = {34},

number = {5},

pages = {96-110},

abstract = {The ionosphere has been the most challenging source of error to mitigate within the community of global navigation satellite system (GNSS)-based safety-critical systems. Users of those systems should be assured that the difference between an unknown true position and a system-derived position estimate is bounded with an extremely high degree of confidence. One of the major concerns for meeting this requirement, known as integrity, is ionosphere-induced error or discontinuity of GNSS signals significant enough to threaten the safety of users. The potentially hazardous ionospheric anomalies of interest in this article are ionospheric spatial decorrelation and ionospheric scintillation under disturbed conditions. As the demand of safety-critical navigation applications increases with the rapid growth of the autonomous vehicle sector, ionospheric monitoring and mitigation techniques become more important to support such systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Closas2017,

title = {Advances in Signal Processing for GNSSs},

author = {Pau Closas and Marco Luise and Jose-Angel Avila-Rodriguez and Christopher Hegarty and Jiyun Lee},

doi = {10.1109/MSP.2017.2716318},

year  = {2017},

date = {2017-09-06},

journal = {IEEE Signal Processing Magazine},

volume = {34},

number = {5},

pages = {12-15},

abstract = {Examines global navigation satellite systems (GNSS). The papers in this special issue address the design of special GNSS signals, a topic of particular interest in the past and still of great relevance today. It continues with the discussion of effective techniques for receiver performance enhancement and finishes with the analysis of some vulnerabilities. GNSS technology today is ubiquitous in many transversal infrastructures and has become the backbone of all applications where precise position, navigation, and timing (PNT) of user equipment is required. Moreover, GNSS is the pervasive PNT technology in outdoor environments, where its performance, coverage, and reliability exceeds that of other technical solutions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2017d,

title = {Design and evaluation of symmetric space–time adaptive processing of an array antenna for precise global navigation satellite system receivers},

author = {Kihoon Lee and Jiyun Lee},

doi = {10.1049/iet-spr.2016.0277},

year  = {2017},

date = {2017-08-07},

journal = {IET Signal Processing},

volume = {11},

number = {6},

pages = {758-764},

abstract = {The most effective method for overcoming the interference vulnerability of global navigation satellite system (GNSS) receivers is to use an adaptive array antenna which has the capability of nulling or beamforming to a certain direction. The space-time adaptive processing (STAP) algorithm, which is very effective in signal processing of the array antenna for antiinterference, is studied. The phenomenon of pseudorange error in STAP is analysed with a new superposition method of linear line correlation functions. From this analysis, a new symmetric STAP algorithm, which uses an appropriate constraint condition for GNSS signals, is proposed to completely prevent this pseudorange error. The new STAP algorithm performance is verified and confirmed by simulations and experiments with a four-element array antenna. According to the simulation results, the new STAP algorithm has no pseudorange error, whereas the already existing STAP algorithm has an error of 1.6 m. Also experiments with four RF channels analogue-to-digital converter data and a real-time receiver confirm the effectiveness of their precise STAP algorithm which is easy to implement.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2017b,

title = {Real-Time Ionospheric Threat Adaptation Using a Space Weather Prediction for GNSS-Based Aircraft Landing Systems},

author = {Jinsil Lee and Sam Pullen and Seebany Datta-Barua and Jiyun Lee},

doi = {10.1109/TITS.2016.2627600},

year  = {2017},

date = {2017-07-01},

journal = {IEEE Transactions on Intelligent Transportation Systems},

volume = {18},

number = {7},

pages = {1752-1761},

abstract = {The use of ground-based augmentation systems (GBASs) is increasing in the national airspace system and also in many nations to support aircraft precision approaches and landing. An anomalous ionospheric event if undetected can cause a potential threat to users of single-frequency-based global navigation satellite system augmentation systems. Current GBAS utilize the pre-defined “worst case” ionospheric threat model in their computation of user position errors to consider all possible ionospheric conditions. This could lead to an excessive availability penalty by adding conservatism on the resulting error bounds. This paper proposes a methodology of real-time ionospheric threat adaptation that adjusts the ionospheric threat model in real time instead of always using the same threat model. This is done by using predicted values of space weather indices for determining the corresponding threat model based on an established relationship between space weather indices and ionospheric threats. Since space weather prediction itself is not reliable due to prediction errors, an uncertainty model was derived from 17 years of historical data. When applied to Category I GBAS in the Conterminous United States, this method lowered the upper bound of the current threat model about 95% of the time during the 17 years (from 1995 to 2011) using the bounded prediction value of the disturbance-storm time index.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kim2017,

title = {Feasibility Studies for Applications of Long-Term Ionospheric Anomaly Monitor},

author = {Minchan Kim and Eugene Bang and Sam pullen and Yong Jae Lee and Jiyun Lee},

doi = {10.1109/TAES.2017.2671782},

year  = {2017},

date = {2017-06-01},

journal = {IEEE Transactions on Aerospace and Electronic Systems},

volume = {53},

number = {3},

pages = {1581-1588},

abstract = {The results of processing preexisting ionospheric storm data are presented to demonstrate the use of the long-term ionospheric anomaly monitoring (LTIAM) tool for developing a ground-based augmentation system (GBAS) ionospheric anomaly threat model. The LTIAM not only detect the worst ionospheric gradients successfully, but also populate the current GBAS threat space with newly discovered ionospheric anomalies. As an application of LTIAM, the paper also proposes a method of utilizing its outputs to understand the distribution of anomalous spatial gradients. Examining the gradients above 50 mm/km on known storm days demonstrates that smaller (but still anomalous) gradients are far more likely than extreme gradients above 200 mm/km. The continued use of LTIAM over the current and upcoming solar peaks is discussed to estimate the likelihood of large spatial gradients.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Felux2017,

title = {Ionospheric Gradient Threat Mitigation in Future Dual Frequency GBAS},

author = {Michael Felux and Mihaela-Simona Circiu and Jiyun Lee and Florian Holzapfel},

doi = {10.1155/2017/4326018},

year  = {2017},

date = {2017-03-20},

journal = {International Journal of Aerospace Engineering},

volume = {2017},

number = {4326018},

pages = {10},

abstract = {The Ground Based Augmentation System (GBAS) is a landing system for aircraft based on differential corrections for the signals of Global Navigation Satellite Systems (GNSS), such as GPS or Galileo. The main impact on the availability of current single frequency systems results from the necessary protection against ionospheric gradients. With the introduction of Galileo and the latest generation of GPS satellites, a second frequency is available for aeronautical navigation. Dual frequency methods allow forming of ionospheric free combinations of the signals, eliminating a large part of the ionospheric threats to GBAS. However, the combination of several signals increases the noise in the position solution and in the calculation of error bounds. We, therefore, developed a method to base positioning algorithms on single frequency measurements and use the second frequency only for monitoring purposes. In this paper, we describe a detailed derivation of the monitoring scheme and discuss its implications for the use in an aviation context.



},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2017bb,

title = {Optimized GNSS Station Selection to Support Long-Term Monitoring of Ionospheric Anomalies for Aircraft Landing Systems},

author = {Jiyun Lee and Minchan Kim},

doi = {10.1109/TAES.2017.2650038},

year  = {2017},

date = {2017-02-01},

journal = {IEEE Transactions on Aerospace and Electronic Systems},

volume = {53},

number = {1},

pages = {236-246},

abstract = {Differential global navigation satellite systems (GNSS)-based aircraft precision approach and landing systems require the development of ionospheric threat models to insure that users are sufficiently protected against ionospheric anomalies. The long-term ionospheric anomaly monitor (LTIAM) is being used to build ionospheric threat models for ground-based augmentation systems (GBAS) and to continuously monitor ionospheric behavior over the life cycle of GBAS. While LTAIM exhaustively detects all potential anomalies, the use of poor-quality GNSS data degrades the accuracy of ionospheric delay estimates and produces many faulty anomaly candidates, thus adding a great burden to LTIAM processing. To select GNSS reference stations with high-quality data, an optimized set of thresholds for data quality metrics are established. The high-quality station selection method maximizes the elimination of spurious gradients while minimizing unnecessary station removals. When applied to the continuously operating reference stations (CORS) network in the Conterminous U.S. (CONUS), this method discards 90% of faulty candidates while only excluding 14% of the over 1600 CORS stations. The well-distributed subnetwork selection method is also proposed to remove geographically redundant stations in dense regions. The number of CORS stations in CONUS is reduced to 48% of total stations when a desired baseline constraint is 100 km. The results demonstrate that the optimal GNSS station section methods are applicable to a wide range of GNSS station networks that will be used for ionospheric monitoring.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yoon2017,

title = {Validation of Ionospheric Spatial Decorrelation Observed During Equatorial Plasma Bubble Events},

author = {Moonseok Yoon and Dongwoo Kim and Jiyun Lee},

doi = {10.1109/TGRS.2016.2604861},

year  = {2017},

date = {2017-01-01},

journal = {IEEE Transactions on Geoscience and Remote Sensing},

volume = {55},

number = {1},

pages = {261-271},

abstract = {A postprocessing methodology is developed to validate abnormal ionospheric spatial decorrelation observed during the equatorial plasma bubble (EPB) events. While the Global Navigation Satellite System (GNSS) remote sensing techniques have been used to measure the ionospheric gradients, the measurements can be easily corrupted during the ionospheric disturbances. Extremely large ionospheric gradients are required to be validated before being declared real ionospheric events as opposed to the artifacts of erroneous measurements. The use of existing methods is however limited due to the small size of EPBs compared with the baseline distances between GNSS network stations in equatorial regions. This paper proposes a new validation procedure which utilizes a time-step method to estimate gradients over any short distance. Equatorial anomaly events are visualized in multiple time series by combining all available sources, including severe gradients observed from the multiple widely spread stations, the estimated EPB and known satellite motions, and the known station locations. A similar ionospheric pattern over multiple station-satellite pairs supports the fact that they are impacted by the same EPB at different times and locations. An extreme ionospheric gradient of 3.09 TECU/km observed in the Brazilian region during the December 31, 2013 EPB event is validated to be real using this methodology. The results demonstrate the effectiveness of the methodology for validating EPB-induced ionospheric spatial decorrelation.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bang2016b,

title = {Preliminary availability assessment to support single-frequency SBAS development in the Korean region},

author = {Eugene Bang and Jinsil Lee and Todd Walter and Jiyun Lee},

doi = {10.1007/s10291-016-0522-4},

year  = {2016},

date = {2016-02-23},

journal = {GPS Solutions},

volume = {20},

number = {3},

pages = {299-312},

abstract = {Satellite-Based Augmentation Systems (SBASs) enhance the global navigation satellite system (GNSS) to support all phases of flight by providing required accuracy, integrity, continuity, and availability. The Korean SBAS program was recently initiated to develop a single-frequency SBAS aiming to provide Approach Procedure with Vertical guidance (APV)-I Safety-of-Life (SoL) service to aviation users by 2022 within the Korean region. We assess the preliminary availability of the single-frequency SBAS which will be deployed in the Korean peninsula. The resulting system performance shall be used as a baseline to design system components and specifications. The fundamental components of SBAS architecture, SBAS monitor network, geostationary earth orbiting satellite parameters, and ionospheric grid point mask, are defined and their effects on system performance are investigated. Ionospheric correction and integrity algorithm parameters including an ionospheric irregularity threat model are determined using data collected from the Korean GNSS network. The coverage of 99.9 % availability for APV-I service increases from approximately 70 % for the baseline case to 100 % when SBAS monitor stations are expanded to overseas. Even with the expanded monitor network, however, 90 % and less than 95 % availability for LPV-200 service can be achieved only in a very limited region.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{김기완2015,

title = {LADGNSS 항법지원을 받는 무인항공기의 비행 기술 오차 모델링 기법},

author = {김기완 and 김민찬 and 이동경 and 이지윤},

doi = {10.5139/JKSAS.2015.43.12.1054},

year  = {2015},

date = {2015-12-01},

journal = {한국항공우주학회지},

volume = {43},

number = {12},

pages = {1054-1061},

abstract = {민수용 무인항공기의 활용이 확대될 것으로 기대되면서 무인항공기의 항법 정확도와 항법무결성의 보장에 대한 문제가 중요해지고 있다. 최근 민수용 무인항공기를 대상으로 항법 정확도와 항법 무결성을 보장하는 지역보강항법시스템(Local-Area Differential Global Navigation Satellite System, LADGNSS)의 개념이 제시된 바 있다. LADGNSS는 무인항공기 간의 충돌을 방지하기 위한 최소분리거리 정보를 제공하여 무인항공기의 안전을 보장한다. 최소분리거리를 산출하기 위해서는 무인항공기의 비행기술오차(Flight Technical Error)에 대한 정보가 필요한데, 이 오차는 기존 유인항공기 분야에서 평균이 0인 정규분포로 모델링 되어 왔다. 하지만 무인항공기의 경우 유인항공기와 다르게 제어/항법장비나 비행경로 등에 대한 표준이 다변화 될 것으로 예상되며 비행기술오차에 대해서 일괄적으로 평균이 0인 정규분포를 가정하는 것은 무결성 정보 산출 시 과도한 보수성을 야기할 수 있다. 본 연구에서는 비행실험을 통해 무인항공기의 비행기술오차를 수집하고, 해당 오차의 특성을 잘 묘사할 수 있는 Johnson 분포 모델을 이용해 오차를 모델링 하였다. 오차모델에 대한 적합성을 평가하기 위해서 Kolmogorov-Smirnov Test와 Anderson-Darling Test를 수행하였다.



Navigation accuracy, integrity, and safety of commercial Unmanned Aerial Vehicle(UAV) is becoming crucial as utilization of UAV in commercial applications is expected to increase. Recently, the concept of Local-Area Differential GNSS (LADGNSS) which can provide navigation accuracy and integrity of UAV was proposed. LADGNSS can provide differential corrections and separation distances for precise and safe operation of the UAV. In order to derive separation distances between UAVs, modeling of Flight Technical Error (FTE) is required. In most cases, FTE for civil aircraft has been assumed to be zero-mean normal distribution. However, this assumption can cause overconservatism especially for UAV, because UAV may use control and navigation equipments in wider performance range and follow more diverse path than standard airway for civil aircraft. In this research, flight experiments were carried out to understand the characteristics of FTE distribution. Also, this paper proposes to use Johnson distribution which can better describe heavy-tailed and skewed FTE data. Futhermore, Kolmogorov-Smirnov and Anderson-Darling tests were conducted to evaluate the goodness of fit of Johnson model.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yun2015,

title = {RAIM algorithm considering simultaneous multiple ramp failures},

author = {Ho Yun and Deokhwa Han and Changdon Kee and Jiyun Lee and Moonbeom Heo},

doi = {10.1108/AEAT-07-2013-0126},

year  = {2015},

date = {2015-07-06},

journal = {Aircraft Engineering and Aerospace Technology},

volume = {87},

number = {4},

pages = {357-367},

abstract = {Purpose

The purpose of this paper is to develop and analyze a new multiple hypothesis receiver autonomous integrity monitoring (RAIM) algorithm, which can handle simultaneous multiple ramp failures.



Design/methodology/approach

The proposed algorithm uses measurement residuals and satellite observation matrices of several consecutive epochs for failure detection and exclusion. It detects failures by monitoring the error vector rather than a projection of the error vector. The algorithm assumes that magnitude of range errors can vary with time, while the conventional sequential multiple hypothesis RAIM algorithm assumes that range errors are constant biases.



Findings

The algorithm can detect any instance of multiple failures, including failures that cannot be detected by the conventional RAIM algorithm. It can detect multiple failures with magnitudes of several tens of meters, even though the algorithm must solve an ill-conditioned problem. And it can also deal with ramp failures which cannot be detected by conventional sequential multiple hypothesis RAIM algorithm. The detection capability of the proposed algorithm is not dependent on satellite geometry or types of errors.



Practical implications

Implications for the development of the RAIM algorithm for aviation users are included. In particular, it can be a candidate for a future standard architecture in multiple constellations, multiple frequency and satellite-based augmentation system users.



Originality/value

A new multiple hypothesis RAIM algorithm with a relative RAIM concept is proposed. Also presented is a detailed explanation of the algorithms, including rigorous mathematical expressions, and an analysis of differences in detection capability between the conventional multiple hypothesis RAIM algorithm and proposed algorithm.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kim2015,

title = {GBAS ionospheric threat model assessment for category I operation in the Korean region},

author = {Minchan Kim and Yunjung Choi and Hyang-Sig Jun and Jiyun Lee},

doi = {10.1007/s10291-014-0404-6},

year  = {2015},

date = {2015-07-01},

journal = {GPS Solutions},

volume = {19},

number = {3},

pages = {443-456},

abstract = {During extreme ionospheric storms, anomalous ionospheric gradients can become high enough to affect Global Navigation Satellite Systems (GNSS) Ground-Based Augmentation Systems (GBAS) and to threaten the safety of GBAS users. An ionospheric anomaly threat model for the Conterminous United States (CONUS) was developed based on extreme ionospheric gradients observed in CONUS during the last solar maximum period (2000–2004). However, in order to understand and mitigate ionosphere threats occurring in different geographical regions, ionospheric anomaly threat models have to be established for the relevant regions. To allow the certification of a GBAS ground facility in South Korea, a Korean ionospheric anomaly threat model must be determined. We describe the method of data analysis that was used to estimate ionospheric spatial gradients. Estimates of anomalous gradients in the Korean region were used to define and build an ionospheric anomaly threat model for this region. All gradient estimates obtained using Korean GNSS reference network data for potential ionospheric storm dates from 2000 to 2004 were included in this threat space. The maximum spatial gradient within this threat space is 160 mm of delay per km of user separation, which falls well within the bounds of the current ionospheric threat model for CONUS. We also provide a detailed examination of the two largest ionospheric spatial gradient events observed in this study, which occurred on November 10, 2004, and November 6, 2001, respectively.



},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Felux2015,

title = {GBAS ground monitoring requirements from an airworthiness perspective},

author = {Michael Felux and Jiyun Lee and Florian Holzapfel},

doi = {10.1007/s10291-014-0398-0},

year  = {2015},

date = {2015-07-01},

journal = {GPS Solutions},

volume = {19},

number = {3},

pages = {393-401},

abstract = {The ground-based augmentation system (GBAS) provides corrections for satellite navigation signals together with integrity parameters to aircraft and enables precision approach guidance. It will eventually replace the currently used instrument landing system. GBAS Approach Service Type C stations supporting CAT-I operations have been fully developed and certified, and first stations are operational. For the service type D, which is intended to support CAT-III operations including automatic approaches and landings, requirements have been drafted and are currently undergoing validation. One remaining issue is the requirement for monitoring of ionospheric anomalies in the ground subsystem. Large gradients in the concentration of free electrons in the ionosphere can lead to significant positioning errors when navigation is based on differential methods. We give a review of the derivation of currently proposed performance requirements for such a monitor. Next, we show that the required level of safety from an airworthiness perspective can be achieved even with relaxed monitoring requirements compared to the currently drafted standards. These relaxations result from satellite geometry assessments on the ground and actual approach characteristics toward a runway. We show that with this method, it is sufficient to monitor for gradients in the range of about 450–550 mm/km while current standards require detection already from 300 mm/km. A remote monitoring receiver near the touchdown point can monitor the post-correction differential range error and use it as test statistic for GBAS performance monitoring and protection against ionospheric disturbances.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ratna2015,

title = {Automatic ionospheric scintillation detector for global navigation satellite system receivers},

author = {Devanaboyina Venkata Ratna and Gampala Sivavaraprasad and Jiyun Lee},

doi = {10.1049/iet-rsn.2014.0232},

year  = {2015},

date = {2015-06-18},

journal = {IET Radar, Sonar & Navigation},

volume = {9},

number = {6},

pages = {702-711},

abstract = {Severe ionosphere scintillations have been known to affect the performance and measurement accuracy of Global Navigation Satellite System (GNSS) receivers. The scintillation in signal amplitude and phase reduces the number of available GNSS satellites by causing the loss of lock in GNSS receivers. Hence, the investigation of ionospheric scintillations is imperative for monitoring the activities of the atmosphere, ionosphere and space weather. Scintillations can be modelled as a function of scintillation indices like amplitude scintillation index (S4), phase scintillation index ( σ Ø), C/N and elevation angle with respect to the time. In this study, the GNSS Ionospheric Scintillation and TEC monitor receiver located at the K L University, Vaddeswaram, India, sited in low latitudes, provided the data for the real-time analysis of ionospheric scintillations. This paper describes an ionospheric scintillation model (RTISM), which determines the automatic threshold for different scintillation signals using the Neyman Pearson detector. The results of the RTISM model include estimation, detection and mitigation of ionospheric scintillations using wavelet analysis, Hilbert-Huang transform and binary hypothesis test. The RTISM model has been tested for major scintillation events observed during the geomagnetic storms that occurred in the maximum solar activity periods of the 24th solar cycle (2013-2014).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {GNSS Airborne Multipath Error Modeling Under UAV Platform and Operating Environment},

author = {Minchan Kim and Kiwan Kim and Dong-Kyeong Lee and Jiyun Lee},

doi = {10.11003/JPNT.2015.4.1.001},

year  = {2015},

date = {2015-03-15},

urldate = {2015-03-15},

journal = {Journal of Positioning, Navigation, and Timing},

volume = {4},

number = {1},

pages = {1-7},

abstract = {In the case of an unmanned aerial vehicle (UAV) equipped with a GNSS sensor, a boundary line where the vehicle can actually exist can be calculated using a navigation error model, and safe navigation (e.g., precise landing and collision prevention) can be supported based on this boundary line. Therefore, for the safe operation of UAV, a model for the position error of UAV needs to be established in advance. In this study, the multipath error of a GNSS sensor installed at UAV was modeled through a flight test, and this was analyzed and compared with the error model of an existing manned aircraft. The flight test was conducted based on a scenario in which UAV performs hovering at an altitude of 40 m, and it was found that the multipath error value was well bound by the error model of an existing manned aircraft. This result indicates that the error model of an existing manned aircraft can be used in operation environments similar to the scenario for the flight test. Also, in this study, a scenario for the operation of multiple UAVs was considered, and the correlation between the multipath errors of the UAVs was analyzed. The result of the analysis showed that the correlation between the multipath errors of the UAVs was not large, indicating that the multipath errors of the UAVs cannot be canceled out.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kim2014,

title = {A Comprehensive Method for GNSS Data Quality Determination to Improve Ionospheric Data Analysis},

author = {Minchan Kim and Jiwon Seo and Jiyun Lee},

doi = {10.3390/s140814971},

year  = {2014},

date = {2014-08-14},

journal = {Sensors},

volume = {14},

number = {8},

pages = {14971-14993},

abstract = {Global Navigation Satellite Systems (GNSS) are now recognized as cost-effective tools for ionospheric studies by providing the global coverage through worldwide networks of GNSS stations. While GNSS networks continue to expand to improve the observability of the ionosphere, the amount of poor quality GNSS observation data is also increasing and the use of poor-quality GNSS data degrades the accuracy of ionospheric measurements. This paper develops a comprehensive method to determine the quality of GNSS observations for the purpose of ionospheric studies. The algorithms are designed especially to compute key GNSS data quality parameters which affect the quality of ionospheric product. The quality of data collected from the Continuously Operating Reference Stations (CORS) network in the conterminous United States (CONUS) is analyzed. The resulting quality varies widely, depending on each station and the data quality of individual stations persists for an extended time period. When compared to conventional methods, the quality parameters obtained from the proposed method have a stronger correlation with the quality of ionospheric data. The results suggest that a set of data quality parameters when used in combination can effectively select stations with high-quality GNSS data and improve the performance of ionospheric data analysis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Won2014,

title = {GNSS integration with vision-based navigation for low GNSS visibility conditions},

author = {Dae Hee Won and Eunsung Lee and Moonbeom Heo and Sangkyung Sung and Jiyun Lee and Young Jae Lee},

year  = {2014},

date = {2014-04-01},

journal = {GPS Solutions},

volume = {18},

number = {2},

pages = {177-187},

abstract = {In urban canyons, buildings and other structures often block the line of sight of visible Global Navigation Satellite System (GNSS) satellites, which makes it difficult to obtain four or more satellites to provide a three-dimensional navigation solution. Previous studies on this operational environment have been conducted based on the assumption that GNSS is not available. However, a limited number of satellites can be used with other sensor measurements, although the number is insufficient to derive a navigation solution. The limited number of GNSS measurements can be integrated with vision-based navigation to correct navigation errors. We propose an integrated navigation system that improves the performance of vision-based navigation by integrating the limited GNSS measurements. An integrated model was designed to apply the GNSS range and range rate to vision-based navigation. The possibility of improved navigation performance was evaluated during an observability analysis based on available satellites. According to the observability analysis, each additional satellite decreased the number of unobservable states by one, while vision-based navigation always has three unobservable states. A computer simulation was conducted to verify the improvement in the navigation performance by analyzing the estimated position, which depended on the number of available satellites; additionally, an experimental test was conducted. The results showed that limited GNSS measurements can improve the positioning performance. Thus, our proposed method is expected to improve the positioning performance in urban canyons.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yoon2014,

title = {Medium-scale Travelling Ionospheric Disturbances in the Korean Region on 10 November 2004: Potential impact on GPS-Based navigation Systems},

author = {Moonseok Yoon and Jiyun Lee},

doi = {10.1002/2013SW001002},

year  = {2014},

date = {2014-03-07},

journal = {Space Weather},

volume = {12},

number = {4},

pages = {173-186},

abstract = {Extreme medium‐scale traveling ionospheric disturbances (MSTIDs) occurred at midlatitudes in East Asia during a geomagnetically active time on 10 November 2004. Using the Global Positioning System (GPS) observation data from Korean GPS reference stations, the characteristics of the MSTIDs on 10 November 2004 and their potential impact on GPS‐based navigation systems in the Korean region are analyzed. The MSTIDs were first observed in the northeast part of South Korea at about 10:00 UT and propagated southwestward with successive wavefronts which extended from northwest to southeast. The peak‐to‐peak amplitudes of vertical total electron content (TEC) disturbances decreased from about 29 to 10 total electron content unit (1 TECU = 1016 el m−2), and the wavelengths lengthened from about 360 to 580 km from 12:53 to 14:38 UT. The propagation velocity of MSTID wavefronts was estimated using three nearby reference stations showing that velocity gradually decreased from about 254 m/s at 11:46 UT to 76 m/s at 21:26 UT. The ionospheric irregularities in small‐scale regions accompanied by the MSTIDs were spatially and temporally varied from about 10:00 to 22:00 UT in response to the movement and intensity change of the MSTIDs. This event also generated anomalously large ionospheric spatial gradients which could cause unacceptable residual pseudorange errors for users of GPS augmentation systems. Frequent loss of the GPS signals, which occurred due to the intense ionospheric irregularities, could also degrade the continuity and availability of GPS‐based navigation systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Won2014b,

title = {Selective Integration of GNSS, Vision Sensor, and INS Using Weighted DOP Under GNSS-Challenged Environments},

author = {Dae Hee Won and Eunsung Lee and Moonbeom Heo and Seung-Woo Lee and Jiyun Lee and Jeongrae Kim and Sangkyung Sung and Young Jae Lee},

doi = {10.1109/TIM.2014.2304365},

year  = {2014},

date = {2014-03-04},

journal = {IEEE Transactions on Instrumentation and Measurement},

volume = {63},

number = {9},

pages = {2288-2298},

abstract = {Accurate and precise navigation solution can be obtained by integrating multiple sensors such as global navigation satellite system (GNSS), vision sensor, and inertial navigation system (INS). However, accuracy of position solutions under GNSS-challenged environment occasionally degrades due to poor distributions of GNSS satellites and feature points from vision sensors. This paper proposes a selective integration method, which improves positioning accuracy under GNSS-challenged environments when applied to the multiple navigation sensors such as GNSS, a vision sensor, and INS. A performance index is introduced to recognize poor environments where navigation errors increase when measurements are added. The weighted least squares method was applied to derive the performance index, which measures the goodness of geometrical distributions of the satellites and feature points. It was also used to predict the position errors and the effects of the integration, and as a criterion to select the navigation sensors to be integrated. The feasibility of the proposed method was verified through a simulation and an experimental test. The performance index was examined by checking its correlation with the positional error covariance, and the performance of the selective navigation was verified by comparing its solution with the reference position. The results show that the selective integration of multiple sensors improves the positioning accuracy compared with nonselective integration when applied under GNSS-challenged environments. It is especially effective when satellites and feature points are posed in certain directions and have poor geometry.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kumar2014,

title = {Equatorial Ionospheric Anomaly (EIA) and comparison with IRI model during descending phase of solar activity (2005- 2009)},

author = {Sanjay Kumar and A. K. Singh and Jiyun Lee},

doi = {10.1016/j.asr.2013.12.019},

year  = {2014},

date = {2014-03-01},

journal = {Advances in Space Research},

volume = {53},

number = {5},

pages = {724-733},

abstract = {The ionospheric variability at equatorial and low latitude region is known to be extreme as compared to mid latitude region. In this study the ionospheric total electron content (TEC), is derived by analyzing dual frequency Global Positioning System (GPS) data recorded at two stations separated by 325 km near the Indian equatorial anomaly region, Varanasi (Geog latitude 25°, 16/ N, longitude 82°, 59/ E, Geomagnetic latitude 16°, 08/ N) and Kanpur (Geog latitude 26°, 18/ N, longitude 80°, 12/ E, Geomagnetic latitude 17°, 18/ N). Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the descending phase of solar activity from 2005 to 2009. It is found that the maximum TEC (EIA) near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semi-annual variation is seen with two maxima occurring in both spring and autumn. Results also showed the presence of winter anomaly or seasonal anomaly in the EIA crest throughout the period 2005–2009 only except during the deep solar minimum year 2007–2008. The correlation analysis indicate that the variation of EIA crest is more affected by solar activity compared to geomagnetic activity with maximum dependence on the solar EUV flux, which is attributed to direct link of EUV flux on the formation of ionosphere and main agent of the ionization. The statistical mean occurrence of EIA crest in TEC during the year from 2005 to 2009 is found to around 12:54 LT hour and at 21.12° N geographic latitude. The crest of EIA shifts towards lower latitudes and the rate of shift of the crest latitude during this period is found to be 0.87° N/per year. The comparison between IRI models with observation during this period has been made and comparison is poor with increasing solar activity with maximum difference during the year 2005.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Won2013,

title = {Geometrical distortion integrated performance index for vision-based navigation system},

author = {Dae Hee Won and Sebum Chun and Seung-Woo Lee and Sangkyung Sung and Jiyun Lee and Jeongrae Kim and Young Jae Lee},

doi = {10.1007/s12555-012-0194-y},

year  = {2013},

date = {2013-12-01},

journal = {International Journal of Control, Automation and Systems},

volume = {11},

number = {6},

pages = {1196-1203},

abstract = {This paper proposes weighted dilution of precision (WDOP) as an indicator of the accuracy of position and attitude in vision-based navigation. WDOP accurately represents the tendencies of navigational errors. It is obtained by weighted least squares. The weight is determined by the deployment of feature points and the geometrical distortion of the vision sensor. The performance of WDOP was verified by simulation. The values of the dilution of precision (DOP) and WDOP were computed and analyzed by comparison with the navigational errors. Additionally, a correlation test was used to determine how well they reflect the trends of the navigational errors. Simulation results showed that WDOP was strongly correlated with navigational errors, which makes it a parameter that can be used to determine the quality of a vision-based navigation system. The proposed WDOP can be used as a practical indicator of navigation performance.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bang2013,

title = {Methodology of automated ionosphere front velocity estimation for ground-based augmentation of GNSS},

author = {Eugene Bang and Jiyun Lee},

doi = {10.1002/rds.20066},

year  = {2013},

date = {2013-10-03},

journal = {Radio Science},

volume = {48},

number = {6},

pages = {659-670},

abstract = {Extreme ionospheric anomalies occurring during severe ionospheric storms can pose integrity threats to Global Navigation Satellite System (GNSS) Ground‐Based Augmentation Systems (GBAS). Ionospheric anomaly threat models for each region of operation need to be developed to analyze the potential impact of these anomalies on GBAS users and develop mitigation strategies. Along with the magnitude of ionospheric gradients, the speed of the ionosphere “fronts” in which these gradients are embedded is an important parameter for simulation‐based GBAS integrity analysis. This paper presents a methodology for automated ionosphere front velocity estimation which will be used to analyze a vast amount of ionospheric data, build ionospheric anomaly threat models for different regions, and monitor ionospheric anomalies continuously going forward. This procedure automatically selects stations that show a similar trend of ionospheric delays, computes the orientation of detected fronts using a three‐station‐based trigonometric method, and estimates speeds for the front using a two‐station‐based method. It also includes fine‐tuning methods to improve the estimation to be robust against faulty measurements and modeling errors. It demonstrates the performance of the algorithm by comparing the results of automated speed estimation to those manually computed previously. All speed estimates from the automated algorithm fall within error bars of ± 30% of the manually computed speeds. In addition, this algorithm is used to populate the current threat space with newly generated threat points. A larger number of velocity estimates helps us to better understand the behavior of ionospheric gradients under geomagnetic storm conditions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{김민찬2013b,

title = {종합적 품질평가 기법을 이용한 국내 GPS 상시관측소의 데이터 품질 분석},

author = {Minchan Kim and Jiyun Lee},

doi = {10.5139/JKSAS.2013.41.9.689},

year  = {2013},

date = {2013-09-01},

urldate = {2013-09-01},

journal = {한국항공우주학회지},

volume = {41},

number = {9},

pages = {689-699},

abstract = {전리층 폭풍 시 발생할 수 있는 극심한 전리층 이상현상은 GNSS 보강시스템 사용자의 안전을 위협하는 대표적인 요인이므로 전리층 위협모델을 기반으로 한 지상 모니터링을 통해 적시에 감지 및 경보가 이루어 져야한다. GNSS 관측 데이터를 기반으로 전리층 분석을 수행하고 그 결과로 위협모델을 개발하기 때문에 각 관측소의 데이터 품질은 시스템 성능에 큰 영향을 미칠 수 있다. 전 세계적으로 GNSS 상시관측소 수가 많이 증가함에 따라 품질이 떨어지는 데이터를 산출하는 관측소 또한 증가하였다. 본 연구에서는 GNSS 데이터 품질평가 기법 이용하여 국내 GPS 상시관측소 데이터의 품질을 비교하고 품질이 떨어지는 데이터가 전리층 지연오차 및 기울기 추정치에 미치는 영향을 분석하였다. 품질평가 결과 국내 상시관측소간 데이터 품질에 큰 차이를 보였고 이 품질은 일정기간 유지된다는 것을 확인하였다. 본 연구에서 분석한 결과를 바탕으로 전리층 위협모델 개발을 위한 GNSS 데이터 품질 기준을 제시할 수 있다.



During extreme ionospheric storms, anomalous ionospheric delays and gradients could cause potential integrity threats to users of GNSS (Global Navigation Satellite System) augmentation systems. GNSS augmentation ground facilities must monitor these ionospheric anomalies defined by a threat model and alarm the users of safely-of-life applications within time-to-alerts. Because the ionospheric anomaly threat model is developed using data collected from GNSS reference stations, the use of poor-quality data can degrade the performance of the threat model. As the total number of stations increases, the number of station with poor GNSS data quality also increases. This paper analyzes the quality of data collected from Korean GPS reference stations using comprehensive GNSS data quality check algorithms. The results show that the range of good and poor qualities varies noticeably for each quality parameter. Especially erroneous ionospheric delay and gradients estimates are produced due to poor quality data. The results obtained in this study should be a basis for determining GPS data quality criteria in the development of ionospheric threat models.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Jeong2012,

title = {Long Term Ionospheric Anomaly Monitoring for Ground Based Augmentation Systems},

author = {Sungwook Jeong and Jiyun Lee},

doi = {10.1029/2012RS005016},

year  = {2012},

date = {2012-07-25},

journal = {Radio Science},

volume = {47},

number = {RS4006},

abstract = {Extreme ionospheric anomalies can pose a potential integrity threat to ground‐based augmentation of the Global Positioning System (GPS), and thus the development of ionospheric anomaly threat models for each region of operation is essential for system design and operation. This paper presents a methodology for automated long‐term ionospheric anomaly monitoring, which will be used to build an ionospheric anomaly threat model, evaluate its validity over the life cycle of the system, continuously monitor ionospheric anomalies, and update the threat model if necessary. This procedure automatically processes GPS data collected from external networks and estimates ionospheric gradients at regular intervals. If ionospheric gradients large enough to be potentially hazardous to users are identified, manual data examination is triggered. This paper also develops a simplified truth processing method to create precise ionospheric delay estimates in near real‐time, which is the key to automating the ionospheric monitoring procedure. The performance of the method is examined using data from the 20 November 2003 and 9 November 2004 ionospheric storms. These results demonstrate the effectiveness of simplified truth processing within long‐term ionosphere monitoring. From the case studies, the automated procedure successfully identified extreme ionospheric anomalies, including the two worst ionospheric gradients observed and validated previously based on manual analysis. The automation of data processing enables us to analyze ionospheric data continuously going forward and to more accurately categorize ionospheric behavior under both nominal and anomalous conditions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Won2012,

title = {Weighted DOP with Consideration on Elevation-Dependent Range Errors of GNSS Satellites},

author = {Dae Hee Won and Jongsun Ahn and Seung-Woo Lee and Jiyun Lee and Sankyung Sung and Heung-Won Park and Jun-Pyo Park and Young Jae Lee},

doi = {10.1109/TIM.2012.2205512},

year  = {2012},

date = {2012-07-11},

journal = {IEEE Transactions on Instrumentation and Measurement},

volume = {61},

number = {12},

pages = {3241-3250},

abstract = {This paper proposes the weighted dilution of precision (WDOP) with consideration of the satellite elevation angle in order to improve the performance of dilution of precision (DOP), which is a standard tool to quantify the positional precision of the Global Navigation Satellite System (GNSS). The WDOP is calculated by assigning different weights to visible GNSS satellites depending on their elevation angles. In order to demonstrate the effectiveness of WDOP, the conventional DOP and WDOP were mathematically analyzed and a comparative analysis was conducted using actual Global Positioning System data. Results showed that WDOP represents the position error trends more accurate than the conventional DOP, particularly when low-elevation measurements were used for positioning calculation. Therefore, the WDOP could be a promising replacement of DOP as a tool for representing and quantifying errors in GNSS positioning.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Seo2012b,

title = {Targeted Parameter Inflation within Ground-Based Augmentation Systems to Minimize Anomalous Ionospheric Impact},

author = {Jiwon Seo and Jiyun Lee and Sam Pullen and Per Enge and Sigrid Close},

doi = {10.2514/1.C031601},

year  = {2012},

date = {2012-03-01},

journal = {Journal of Aircraft},

volume = {49},

number = {2},

pages = {587-599},

abstract = {Anomalous ionospheric conditions can cause large variations in propagation delays of transionospheric radio waves, such as global navigation satellite system (GNSS) signals. Although very rare, extremely large spatial variations pose potential threats to ground-based augmentation system (GBAS) users. Because GBAS provide safety-of-life services, namely precision approach and landing aircraft guidance, system safety must be guaranteed under these unusual conditions. Position-domain geometry-screening algorithms have been previously developed to mitigate anomalous ionospheric threats. These algorithms prevent aircraft from using potentially unsafe GNSS geometries if anomalous ionospheric conditions are present. The simplest ground-based geometry-screening algorithm inflates the broadcast sig_vig parameter in GBAS to signal whose geometries should not be used. However, the sig_vig parameter is not satellite-specific, and its inflation affects all satellites in view. Hence, it causes a higher than necessary availability penalty. A new targeted parameter inflation algorithm is proposed that minimizes the availability penalty by inflating the satellite-specific broadcast parameters: sig_prgnd and P values. In this new algorithm, sig_prgnd and P values are inflated by solving optimization problems. The broadcast parameters obtained from this algorithm provide significantly higher availability than optimal sig_vig inflation at Newark Liberty International Airport and Memphis International Airport without compromising system safety. It is also demonstrated that the computational burden of this algorithm is low enough for real-time GBAS operations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lee2011c,

title = {Observations of low‐elevation ionospheric anomalies for ground‐based augmentation of GNSS},

author = {Jiyun Lee and Seebany Datta-Barua and Godwin Zhang and Sam Pullen and Per Enge},

doi = {10.1029/2011RS004776},

year  = {2011},

date = {2011-11-22},

journal = {Radio Science},

volume = {46},

number = {RS6005},

abstract = {Extreme ionospheric anomalies occurring during severe ionospheric activity can pose an integrity threat to users of Global Navigation Satellite System (GNSS) Ground Based Augmentation Systems (GBAS). While most very large spatial gradients in slant ionospheric delay were observed on high‐elevation satellites, several extreme gradients were also observed on satellites below 15 degrees elevation. This paper details the study of anomalous ionospheric spatial gradients for low‐elevation satellites observed from the 20 November 2003 geomagnetic storm in the Conterminous United States (CONUS). As viewed by a cluster of Continuously Operating Reference Stations (CORS) receivers in northern Ohio, SVN 26 came into view around 20:30 Universal Time (UT) on this day, rose to an elevation angle of about 15 degrees, and set around 22:00 UT. A spatial gradient of 360 mm/km was discovered at 21:20 UT between CORS stations GARF and WOOS, when SVN 26 was at 11 degrees elevation. Ionospheric delay measurements are vulnerable to semi‐codeless L2 tracking errors and data post‐processing errors, especially when satellites are at low elevation. This paper presents a series of methods to validate observed ionospheric anomaly events using station‐wide checks, satellite‐wide checks, and manual verification with single‐frequency measurements. Spatial gradients discovered at other station pairs and another low elevation satellite with a similar azimuth angle, SVN 29, support that the event of SVN 26 is an ionospheric anomaly as opposed to a receiver fault.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Seo2011,

title = {A Real-Time Capable Software-Defined Receiver Using GPU for Adaptive Anti-Jam GPS Sensors},

author = {Jiwon Seo and Yu-Hsuan Chen and David S. De Lorenzo and Sherman Lo and Per Enge and Dennis Akos and Jiyun Lee},

doi = {10.3390/s110908966},

year  = {2011},

date = {2011-09-19},

journal = {Sensors},

volume = {11},

number = {9},

pages = {8966-8991},

abstract = {Due to their weak received signal power, Global Positioning System (GPS) signals are vulnerable to radio frequency interference. Adaptive beam and null steering of the gain pattern of a GPS antenna array can significantly increase the resistance of GPS sensors to signal interference and jamming. Since adaptive array processing requires intensive computational power, beamsteering GPS receivers were usually implemented using hardware such as field-programmable gate arrays (FPGAs). However, a software implementation using general-purpose processors is much more desirable because of its flexibility and cost effectiveness. This paper presents a GPS software-defined radio (SDR) with adaptive beamsteering capability for anti-jam applications. The GPS SDR design is based on an optimized desktop parallel processing architecture using a quad-core Central Processing Unit (CPU) coupled with a new generation Graphics Processing Unit (GPU) having massively parallel processors. This GPS SDR demonstrates sufficient computational capability to support a four-element antenna array and future GPS L5 signal processing in real time. After providing the details of our design and optimization schemes for future GPU-based GPS SDR developments, the jamming resistance of our GPS SDR under synthetic wideband jamming is presented. Since the GPS SDR uses commercial-off-the-shelf hardware and processors, it can be easily adopted in civil GPS applications requiring anti-jam capabilities.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}