Publications

@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}

}

@conference{Chang2017,

title = {Case study for GBAS Operation in Equatorial Region on Nominal day at Daytime},

author = {Hyeyeon Chang 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}

}

@conference{Hong2017,

title = {Analysis on the Casualty risk from UAS Collision for Deriving Safe Separation Distance},

author = {Giseon Hong 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}

}

@conference{Lee2017e,

title = {Vertical Protection Level (VPL) derivation of multi-sensor integrated LAD-GNSS for UAV applications},

author = {Jinsil Lee and Jiyun Lee},

year  = {2017},

date = {2017-11-14},

booktitle = {ITSNT 2017},

address = {Toulouse, France},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Kim2017d,

title = {High-Integrity and Low-Cost Local-Area Differential GNSS Prototype for UAV Applications},

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

doi = {10.33012/2017.15110},

year  = {2017},

date = {2017-09-25},

booktitle = {Proceedings of the 30th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2017)},

pages = {2031 - 2054},

address = {Portland, Oregon},

abstract = {As civilian use of unmanned aerial vehicles (UAVs) increases, safe operation of UAVs while preventing collisions with either humans or ground structures has become a significant concern. To perform autonomous UAV missions Beyond Visual Line-Of-Sight (BVLOS) or in low-altitude airspace safely, achieving high accuracy and reliability of navigation solutions is required. This motivates the development of a cost-effective local-area UAV network that utilizes a Local-Area Differential Global Navigation Satellite System (LAD-GNSS) navigation solution [1, 2]. LAD-GNSS achieves a level of integrity comparable to that of Ground Based Augmentation System (GBAS) Category I operations by monitoring navigation faults at the ground station and by broadcasting integrity information to the UAV [3]. The architecture of this system involves space-conserving hardware configurations and several simplified GBAS integrity monitoring algorithms to reduce both the cost and the complexity of the system. LAD-GNSS is designed to support UAVs with a minimum operating altitude of either 50 ft plus obstacle height (within 5 km of the ground facility) or 200 ft (within 20 km of the ground facility) by providing an accurate position solution and a tight uncertainty bound on its position error. A prototype of LAD-GNSS has been developed and evaluated for both accuracy and integrity performance. The key ground and airborne functions of this system are shown in Figure 1 below. One notable characteristic of this prototype is that it utilizes a two-way datalink between the ground facility and the airborne user, which provides a major improvement in system flexibility. The two-way datalink enables the system not only to allocate integrity risk to each fault hypothesis dynamically to obtain the minimum safe protection level [4] but also to simplify the geometry screening needed to mitigate ionospheric anomalies by computing the maximum error in vertical position (MIEV) only for the satellites known to be tracked by each UAV. Specifically, each UAV continuously sends its GNSS measurements to the ground station, so that error corrections and integrity information can be generated by the ground station just for this known satellite geometry. This information is then broadcast back to the UAV to allow it compute its position solution. The integrity status of each UAV, including its current protection levels, is maintained by the ground facility and is used to guide each vehicle while maintaining safe separation from nearby obstacles and other UAVs. The LAD-GNSS prototype is composed of two parts: a ground module and an onboard module. The ground module operates in a manner similar to a GBAS ground facility. Most of the computations regarding integrity monitoring are performed in the ground module. The onboard module computes position solutions using the corrections broadcast from the ground module. The position solutions are then fed into a flight controller, which is the 3DR Pixhawk. The hardware components of the prototype for the ground module and the airborne module are described as follows. For the ground module, the equipment includes a single pair of NovAtel OEM-V3 receivers and a NovAtel GPS 703 GGG antenna (choke ring type) that can receive L1, L2 and L5 GNSS signals. An Intel NUC mini-PC is used to perform integrity monitoring calculations. For the onboard module, the same receiver model as one of the ground module is mounted on the octocopter UAV platforms. A NovAtel compact GNSS antenna is used for the onboard antenna. Due to the limited UAV payload capacity, a small single-board Raspberry Pi processor, which is capable of performing just the positioning calculations, is loaded on the UAV instead of a mini-PC. An Xbee Pro1 S1 modem, which can support communications over a 1-mile range, is provisionally used for the communication link. The software for both the ground and airborne modules runs in the Linux C language. Flight tests were conducted to evaluate the performance of this LAD-GNSS prototype in Yeongwol. Yeongwol has been designated by the government of South Korea as a permitted area of 95 km^2 for UAV flight tests. The flight paths chosen were designed to cover areas of interest considering practical applications of UAVs, such as agriculture, surveying, mapping, and reconnaissance. The distance of the designed path above the sparsely populated area is 1.5 km. The true trajectory of the UAV was derived by post-processing based on double differenced carrier-phase measurements. The Navigation System Error (NSE) of the system was then computed as the difference between the true UAV path (from post-processing) and the path estimated by LAD-GNSS. While LAD-GNSS can be the primary source of navigation for UAVs, other navigation sensors are required to provide reliable navigation in case of GNSS signal interference or blockage. In this study, the prototype has been provisionally extended to incorporate multiple sensors to assure complete UAV navigation system safety. With the inclusion of multiple sensors, a new fault hypothesis, which is a multi-sensor failure, is added to the existing LAD-GNSS integrity fault tree. The onboard module integrates Inertial Measurement Unit (IMU) sensor output with the LAD-GNSS solution using a Kalman filter (KF). A KF innovation-based fault detection algorithm is developed to protect the UAV from IMU sensor faults. In addition, optimal protection levels are computed by allocating the newly introduced multi-sensor integrity risk dynamically together with the LAD-GNSS integrity risk. This allocation to potential multi-sensor failures would change depending on sensor type and quality. Thus, optimal allocation is essential and would be effective for the proposed multi-sensor integrated system. The extended prototype was also tested at the Yeongwol test site, and its performance was compared with the flights that used LAD-GNSS only. References: [1] S. Pullen, P. Enge, and J. Lee, "High-Integrity Local-Area Differential GNSS Architectures Optimized to Support Unmanned Aerial Vehicles (UAVs)," Proceedings of ION ITM 2013, San Diego, CA, Jan. 28-30, 2013. [2] M. Kim, K. Kim, J. Lee, and S. Pullen, "High Integrity GNSS Navigation and Safe Separation Distance to Support Local-Area UAV Networks," Proceedings of the 27th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 869-878. [3] M. Kim, J. Lee, D. Kim, J. Lee, and S. Pullen, “Design of Local Area DGNSS Architecture to Support UAV Networks: Optimal Integrity/Continuity Allocations and Fault Monitoring,” Proceedings of ION PNT 2017, Hawaii, May 2017. [4] B. Pervan, S. Pullen, and J. Christie, “A Multiple Hypothesis Approach to Satellite Navigation Integrity,” Navigation: Journal of the Institute of Navigation, Vol. 45, No. 1, Spring 1998, pp. 61-84.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Kim2017b,

title = {Design of Local Area DGNSS Architecture to Support Unmanned Aerial Vehicle Networks: Concept of Operations and Safety Requirements Validation},

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

doi = {10.33012/2017.15091},

year  = {2017},

date = {2017-05-01},

booktitle = {Proceedings of the ION 2017 Pacific PNT Meeting},

pages = {992 - 1001},

address = {Honolulu, Hawaii},

abstract = {Local-area differential global navigation satellite systems (LAD-GNSS) support unmanned aerial vehicles (UAVs) with high integrity and accuracy. This study investigates three major issues in fully establishing LAD-GNSS and analyzes their performance. First, we define the concept and requirements of UAV operation, including segregation of UAV operation coverage in low-altitude airspaces, and the derivation of navigation requirements, such as alert limits (ALs), for each operational coverage. Second, we design a LAD-GNSS architecture by simplifying the hardware and monitoring algorithms of both the ground facility and onboard module, using the well-established ground-based augmentation system (GBAS) as a starting point. Lastly, we perform theoretical performance evaluations comparing position uncertainty bounds, which are represented by protection levels (PLs), to the corresponding navigation requirements for each coverage airspace. We derive and compare PLs and ALs under both nominal and malfunction cases. In addition, we describe a method for deriving PLs for excessive acceleration and code-carrier divergence fault scenarios, which are bounded by using a maximum-allowable error in range. Using these PLs, integrity/continuity allocations are ideally and dynamically assigned to each single-fault hypothesis to obtain optimized PLs that are identical for all fault scenarios. We find that vertical protection levels (VPLs) are reduced by approximately 16% when implementing the optimal allocation method.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Kim2017c,

title = {Monte Carlo Simulation for Impact of Anomalous Ionospheric Gradient on GAST-D GBAS},

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

doi = {10.33012/2017.15049},

year  = {2017},

date = {2017-05-01},

booktitle = {Proceedings of the ION 2017 Pacific PNT Meeting},

pages = {47-55},

address = {Honolulu, Hawaii},

abstract = {In this paper, a tool based on Monte Carlo simulation is developed for probabilistic analysis of the effects of anomalous ionospheric gradients on GAST-D GBAS approach and landing operations. While parameters associated with ionospheric gradients and satellite geometry are assumed to be at their worst-case values in the traditional certification approach, Monte Carlo simulation takes advantage of the random characteristics of these parameters. In the Monte-Carlo approach, simulation parameters are randomly chosen from the probabilistic distributions chosen for to represent the ionospheric threat model. More than ?10?^9 sampled events are combined into a probability of Hazardously Misleading Information (PHMI), defined as the sum of the probabilities of missed detections (P_mds) weighted by the probability of occurrence of the anomalous ionospheric event leading to each P_md value. The results are represented in terms of a cumulative distribution function showing the overall probability of exceeding a given differential range error size. The maximum allowable range error due to ionospheric gradients to support GAST-D is 2.75 m according to the reformulated requirements of the Standards and Recommended Practices (SARPs). The simulation results showed that the PHMI for differential range errors exceeding 2.75 m was well below ?10?^(-9). Furthermore, individual ionospheric events were investigated to ensure that integrity requirements were met in the traditional interpretation. The maximum differential range error in the simulation results was 2.74 m. Most of the largest differential range errors are generated from the specific geometry where the baseline direction (between the GBAS ground facility and the landing threshold point, or LTP) is closely aligned with the propagation direction of the ionospheric front.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Choi2016,

title = {Near Real-time GNSS-based Ionospheric Model using Expanded Kriging in the East Asia Region},

author = {Pil Hun Choi and Eugene Bang and Jiyun Lee},

url = {http://adsabs.harvard.edu/abs/2016AGUFM.G31B1064C},

year  = {2016},

date = {2016-12-12},

urldate = {2016-12-12},

booktitle = {AGU Fall Meeting 2015},

address = {San Francisco, CA},

abstract = {Many applications which utilize radio waves (e.g. navigation, communications, and radio sciences) are influenced by the ionosphere. The technology to provide global ionospheric maps (GIM) which show ionospheric Total Electron Content (TEC) has been progressed by processing GNSS data. However, the GIMs have limited spatial resolution (e.g. 2.5° in latitude and 5° in longitude), because they are generated using globally-distributed and thus relatively sparse GNSS reference station networks. This study presents a near real-time and high spatial resolution TEC model over East Asia by using ionospheric observables from both International GNSS Service (IGS) and local GNSS networks and the expanded kriging method. New signals from multi-constellation (e.g,, GPS L5, Galileo E5) were also used to generate high-precision TEC estimates. The newly proposed estimation method is based on the universal kriging interpolation technique, but integrates TEC data from previous epochs to those from the current epoch to improve the TEC estimation performance by increasing ionospheric observability. To propagate previous measurements to the current epoch, we implemented a Kalman filter whose dynamic model was derived by using the first-order Gauss-Markov process which characterizes temporal ionospheric changes under the nominal ionospheric conditions. Along with the TEC estimates at grids, the method generates the confidence bounds on the estimates using resulting estimation covariance. We also suggest to classify the confidence bounds into several categories to allow users to recognize the quality levels of TEC estimates according to the requirements for user's applications. This paper examines the performance of the proposed method by obtaining estimation results for both nominal and disturbed ionospheric conditions, and compares these results to those provided by GIM of the NASA Jet propulsion Laboratory. In addition, the estimation results based on the expanded kriging method are compared to the results from the universal kriging method for both nominal and disturbed ionospheric conditions.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Chang2016,

title = {Validation of Large Ionospheric Gradient Observed in India on 15th, February 2015},

author = {Hyeyeon Chang and Moonseok Yoon and Jiyun Lee},

year  = {2016},

date = {2016-12-09},

booktitle = {2016 8th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Daejeon, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Hyeon2016,

title = {Application of Weighted Least Square RAIM to Multi-Sensor Navigation Integrated with Space-Based Augmentation System},

author = {Eunjeong Hyeon and Jiyun Lee},

year  = {2016},

date = {2016-12-09},

booktitle = {2016 8th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Daejeon, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2016b,

title = {Integration of Onboard Sensors and Local Area DGNSS to Support High Integrity Unmanned Aerial Vehicles (UAV) Navigation},

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

doi = {10.33012/2016.14702},

year  = {2016},

date = {2016-09-12},

booktitle = {Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016)},

pages = {1477 - 1484},

address = {Portland, Oregon},

abstract = {A Local-Area Differential GNSS (LAD-GNSS) provides high integrity and accuracy for Unmanned Aerial Vehicles (UAVs). The integration of onboard sensors to the LAD-GNSS can support continuous and reliable navigation by overcoming GNSS signal vulnerability. This paper proposes the concept of a high integrity navigation architecture into which the LAD-GNSS and onboard sensors are integrated using an extended Kalman filter (EKF). Key components of the integrated system developed in this work, include its integrity/continuity requirement design, fault-monitoring strategies, and the corresponding protection level (PL) computation formulations. In this paper, the total integrity requirements are divided into mainly the following four hypotheses: a fault-free condition, a reference receiver failure, all other failures of LAD-GNSS, and an onboard sensor failure. According to the integrity allocation, fault monitoring strategies and PL computation formulations were proposed. The ground monitor of the LAD-GNSS and the onboard Receiver Autonomous Integrity Monitoring (RAIM) are utilized to monitor a single satellite failure sequentially, and an innovation-based fault-monitoring method is applied to monitor onboard sensor failures. To evaluate the performance of the integrated system, the VPLs under the four hypotheses were calculated using data collected from flight experiments which used an Inertial Navigation Sensor (INS) as an onboard sensor. The results show that the PL for the INS failure hypothesis has the largest vertical protection level (VPL) for the most of the operation time and it suitably bounds vertical position errors during a flight test.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Bang2016bb,

title = {Expanded Ionospheric Estimation and Threat Model Algorithms for SBAS},

author = {Eugene Bang},

doi = {10.33012/2016.14700},

year  = {2016},

date = {2016-09-12},

booktitle = {Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016)},

pages = {1338 - 1349},

address = {Portland, Oregon},

abstract = {The largest contribution to the Vertical Protection Levels (VPLs) of Satellite-Based Augmentation Systems (SBAS) comes from the Grid Ionospheric Vertical Errors (GIVEs), which are confidence bounds for the vertical ionospheric delay estimates of SBAS. Thus, reducing GIVEs is a critical issue in improving the performance of SBAS. This paper introduces two expanded methodologies to improve the performance of SBAS by reducing the magnitudes of GIVEs. We initially propose an expanded kriging method which integrates ionospheric observables from a previous epoch to a current fit domain to improve the kriging fit performance. Secondly, we propose a new threat metric, Norm-based Angular Metric (NAM), which effectively captures the uniformity of the Ionospheric Pierce Point (IPP) distribution by measuring the angular distribution of the IPPs. We also construct an undersampled ionospheric irregularity threat model with a three-dimensional set of threat metrics by combining the newly developed metric and the existing Rfit and Relative Centroid Metric (RCM). The performance of the proposed algorithms is investigated by conducting availability simulations for SBAS in the Korean region. First, with the proposed kriging method, the coverage of 99.9% availability for APV-I service is increased by approximately 12% within South Korea. Second, the newly developed threat model widened the 99% availability coverage for APV-I service by approximately 13% when SBAS monitor stations are expanded to overseas locations.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2016c,

title = {Vertical Position Error Bounding for Integrated GNSS/Onboard Sensors to Support Unmanned Aerial Vehicle (UAV)},

author = {Jinsil Lee and Eunjeong Hyeon and Minchan Kim and Jiyun Lee},

year  = {2016},

date = {2016-09-01},

booktitle = {ICCAS 2016},

address = {Daejeon, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2016,

title = {Equatorial Plasma Bubble Effects on GBAS and Its Mitigation Techniques},

author = {Jiyun Lee},

year  = {2016},

date = {2016-06-01},

booktitle = {International Beacon Satellite Symposium 2016},

address = {Trieste, Italy},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Yoon2016,

title = {Assessment of Equatorial Plasma Bubble Impacts on Ground-Based Augmentation Systems in the Brazilian Region},

author = {Moonseok Yoon and Dongwoo kim and Jiyun Lee and Sarawoot Rungruengwajiake and Sam Pullen},

doi = {10.33012/2016.13423},

year  = {2016},

date = {2016-01-25},

booktitle = {Proceedings of the 2016 International Technical Meeting of The Institute of Navigation},

pages = {368 - 379},

address = {Monterey, California},

abstract = {A preliminary Brazilian ionospheric anomaly threat model for Ground Based Augmentation Systems (GBASs) was developed from a comprehensive analysis using many days of ionospheric data from Brazil to reflect low-latitude conditions. Most anomalous ionospheric spatial gradients in this model were caused by nighttime Equatorial Plasma Bubbles (EPBs). In particular, the largest observed EPB-induced gradient (~860 mm/km) is two times larger than the upper bound (425 mm/km) on spatial gradients for the Conterminous U.S. (CONUS). The higher bound in the Brazilian ionospheric threat model has a significant effect on GBAS performance and availability. More detailed performance evaluation based on the Brazilian threat model is needed before GBAS can be made operational there. This paper first assesses the performance of GBAS with Position Domain Geometry Screening (PDGS), which has been used in CONUS. Using the parameters of the preliminary EPB threat model, sets of worst-case geometries among an EPB gradient front and satellite Ionospheric Pierce Points (IPP) are generated. To calculate undetected range errors for hypothetical GBAS users, we derive an analytical ionosphere-induced differential range error model. Based on these inputs, we analyze the performance of GBAS under worst-case ionospheric conditions. The results show that inflating the standard deviation of the vertical ionospheric gradient only is not sufficient to completely remove potentially unsafe satellite geometries. This leads to additional inflation of the standard deviation of pseudorange correction errors. If the maximum EPB-induced gradient is larger than about 600 mm/km, the resulting CATegory-I (CAT-I) GBAS availability is significantly degraded, making it difficult to meet the system requirement using this approach. For this reason, we have also employed Monte Carlo analysis. The key difference from the previous approach is that credit is taken for a prior probability of an extreme EPB event instead of assuming that worst-case storms occur with a probability of one in PDGS. This stochastic approach assesses the overall user impact by running many anomalous ionospheric trials based upon numerous combinations of threat model parameters. For each subset geometry, it is assumed that either one or two satellites are affected by the worst-case gradient while the remaining satellites are affected by anomalous but non-worst-gradients. Therefore, the worst-case impact should be approximated by inclusion in the distribution of simulation results. A real-time mitigation scheme based on the Monte Carlo method, which also requires sigma parameter inflation, is proposed that takes credit for the prior probability. System availability evaluation using the Monte Carlo approach with a single worst-case satellite impact shows that the availability penalty is significantly reduced compared to the previous (worst-case) method.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Choi2015,

title = {Ionospheric Signatures of the Earthquake in South Korea on 31 March 2014},

author = {Pilhoon Choi and Moonseok Yoon and Yu-ming Yang and Jiyun Lee and Attila Komjathy},

url = {http://adsabs.harvard.edu/abs/2015AGUFMNH21C1835C},

year  = {2015},

date = {2015-12-14},

booktitle = {AGU Fall Meeting 2015},

address = {San Francisco, CA},

abstract = {Previous studies on interactions between the atmospheric waves and ionospheric perturbations concluded that the acoustic-gravity waves triggered by solid earth events such as earthquakes, tsunamis and underground nuclear tests may be used in detecting the ionospheric perturbations. Ionospheric perturbations have been observed using sounding radars and GPS remote sensing techniques since 1970s. As primary examples, ionospheric disturbances associated with the Tohoku earthquake and tsunami on March 11, 2011 were observed and analyzed using GPS measurements. In this work, we processed GPS stations in South Korea and analyzed traveling ionospheric disturbances that were coincident with the 2014 South Korean earthquake. The 31 March 2014 earthquake occurred at 19:48 UTC and the magnitude of this event was registered to be 5.0 Mw. This earthquake is the fourth strongest in South Korea since records began. After analyzing GPS measurements from nearby stations, strong ionospheric perturbations were observed about 20 minutes after the reported event, and the disturbances were shown to have primarily a wave train with periods of 60-120 minutes. The maximum VTEC perturbations turned out to be between 0.6 to 1.3 TECU. In this research, we will analyze the characteristics of the detected ionospheric perturbations associated with the earthquake and compare these results with those from man-made earthquakes such as underground nuclear tests. These findings are expected to verify our modeling results. We also hope to get a better understanding of the influence of both natural hazards and man-made hazards on the temporal and spatial variability of the global ionosphere.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Sun2015b,

title = {Statistical Assessment of Ionospheric Perturbation Amplitudes Caused by Tohoku-Oki Earthquake in Japan},

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

year  = {2015},

date = {2015-12-11},

booktitle = {2015 7th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Fukuoka, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2015c,

title = {Validation of Code Carrier Divergence Monitor for GBAS Category II/III Operations},

author = {Dong-Kyeong Lee and Jiyun Lee},

year  = {2015},

date = {2015-12-11},

booktitle = {2015 7th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Fukuoka, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{offorOperations2015,

title = {Validation of DSIGMA Monitor for GBAS Category II/III Operations},

author = {Validation of DSIGMA Monitor for GBAS Category II/III Operations},

editor = {Eunjeong Hyeon and Jiyun Lee},

year  = {2015},

date = {2015-12-11},

booktitle = {2015 7th Kyushu University-KAIST Symposium on Aerospace Engineering},

address = {Fukuoka, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2015b,

title = {Vertical Position Error Bounding for Integrated Sensors to Support Unmanned Aerial Vehicles (UAV)},

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

url = {https://web.stanford.edu/group/scpnt/pnt/PNT15/2015_Presentation_Files/S02-Lee-UAV.pdf},

year  = {2015},

date = {2015-11-11},

booktitle = {SCPNT 2015},

address = {Stanford, CA},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Kim2015c,

title = {Characterization of GLONASS Clock and Ephemeris Error for Advanced Receiver Autonomous Integrity Monitor (ARAIM)},

author = {Dongwoo Kim and Kihun Lee and Jaekyu Jang and Jiyun Lee},

year  = {2015},

date = {2015-11-02},

booktitle = {ISGNSS 2015},

address = {Kyoto, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Pullen2015,

title = {Optimal Scheduling of Arrivals at Low-Latitude Airports under Severe Ionospheric Spatial Gradients},

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

year  = {2015},

date = {2015-11-01},

booktitle = {The 4th ENRI International Workshop on ATM/CNS},

journal = {ENRI International Workshop on ATM/CNS},

address = {Tokyo, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2015,

title = {Preliminary Results from Ionospheric Threat Model Development to Support GBAS Operations in the Brazilian Region},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=13040},

year  = {2015},

date = {2015-09-14},

booktitle = {Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015)},

pages = {1500 - 1506},

address = {Tampa, Florida},

abstract = {The Brazil ionospheric study project aims to develop a new Ground-based Augmentation System (GBAS) ionospheric threat model which reflect Brazilian low-latitude conditions. This study utilizes dual-frequency GNSS data collected on about 120 active ionosphere days to assess the Brazilian ionospheric behavior. Over 1000 threat points were generated from post-processed data, and most of these ionospheric gradients are caused by night-time Equatorial Plasma Bubbles (EPBs). A significant number of ionospheric gradients exceeded the upper bounds (375 – 425 mm/km) of the Conterminous U.S (CONUS) threat model. In particular, the largest gradient of about 850 mm/km is twice as large as the maximum gradient observed in CONUS. Since a larger bound would have a significant effect on system performance and availability, more-detailed study of the behavior of the most severe ionospheric gradients is desirable before finalizing the threat model. This paper also defines other threat model parameters to model the geometry of EPBs. The observations from Brazil were examined to quantify other existing parameters used to model ionospheric storms in mid-latitude regions and newly introduced parameters for EPBs. The maximum depletion of 35 meters and transition zone lengths of about 20 – 450 km have been estimated for the EPBs which produced the most extreme gradients. These EPBs appeared to be moving roughly eastward and parallel to the geomagnetic equator (within ±35 degrees of the magnetic north) with speeds of about 40 – 250 m/s. EPBs occurred mostly during post-subset hours between 1800 and 0500 local time with the peak in the late evening just before local midnight. Defining and modeling these characteristics of EPBs are key to the development of a GBAS ionospheric mitigation and safety case for operational approval in Brazil and the southern hemisphere.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Circiu2015,

title = {Evaluation of GPS L5, Galileo E1 and Galileo E5a Performance in Flight Trials for Multi Frequency Multi Constellation GBAS},

author = {Mihaela-Simona Circiu and Michael Felux and Boubeker Belabbas and Michael Meurer and Jiyun Lee and Minchan Kim and Sam Pullen},

url = {https://www.ion.org/publications/abstract.cfm?articleID=12967},

year  = {2015},

date = {2015-09-14},

booktitle = {Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015)},

pages = {897 - 906},

address = {Tampa, Florida},

abstract = {This paper presents a noise and multipath performance analysis of the new signals roadcast by the Block IIF GPS satellites on L5 and Galileo satellites on E1 and E5a using airborne flight data. Improved performance has been previously observed using ground measurement and is now validated using data from different flight tests. This is especially true for GPS L5 and Galileo E5a. However, on the airborne side, the performance of Galileo E1 signal is closer to that of GPS L1. The impact of different parameters on the airborne multipath, including receiver correlator spacing, airframe structure, ground influence, and smoothing time, are investigated and discussed.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Felux2015b,

title = {Total System Performance in GBAS-based Landings},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=12764},

year  = {2015},

date = {2015-04-20},

booktitle = {Proceedings of the ION 2015 Pacific PNT Meeting},

pages = {773 - 778},

address = {Honolulu, Hawaii},

abstract = {The requirements for a CAT-II/III capable GBAS (the so called GBAS Approach Service Type (GAST) D) are derived from the definition of a safe landing. The respective performance requirements are given in terms of touchdown performance of the aircraft which has two main influencing parameters: the flight technical error (FTE) and the navigation system error (NSE). In the process of deriving and standardizing the GBAS requirements a fixed value for the FTE is assumed. In this paper we show potential benefits from using the deviations from the nominal approach path to assess the FTE performance during an approach instead of using conservative assumptions. Depending on the prevailing wind conditions, the FTE performance is typically better than the value which is derived in the certification of an aircraft. This opens the potential to either improve availability of the GBAS service or optimize the landing with respect to runway capacity or risk minimization for a runway overrun.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Yoon2015,

title = {Multi-dimensional Verification Methodology of Ionospheric Gradient Observation during Plasma Bubble Events in the Brazilian Region},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=12762},

year  = {2015},

date = {2015-04-20},

booktitle = {Proceedings of the ION 2015 Pacific PNT Meeting},

pages = {748 - 762},

address = {Honolulu, Hawaii},

abstract = {Ionospheric activity in equatorial regions is known to be significantly more variable and intense than in mid-latitude regions. Prior to the initiation of Ground-Based Augmentation System (GBAS) service in the Brazilian region, it is necessary to make provisions for being sufficiently robust to all possible ionospheric anomalies through development of an ionospheric anomaly threat model. In the Brazilian region, ionospheric spatial gradients larger than the upper bounds of the Conterminous U.S (CONUS) threat model developed for the mid-latitude region are frequently observed in the presence of Equatorial Plasma Bubbles (EPBs). The higher bounds in the resulting ionospheric threat model have a significant effect on performance, availability, and the eventual approval of the system. Thus, verification of observed ionospheric spatial gradients is important since observations verified to be due to EPBs will be used to determine the upper bound of the ionospheric anomaly threat model for GBAS in Brazil. This paper applies a two-phase procedure of validating extremely large ionospheric gradients caused by low-latitude ionospheric anomaly events in the Brazilian region. This multi-dimensional approach rules out the possibility of receiver-instigated events, single satellite faults, and post-processing errors creating an apparent gradient that is nonexistent. However, the relatively smaller scale of EPBs and the sparse distribution of the Brazilian network stations makes it difficult to use the existing validation methods developed for mid-latitudes. Thus, this paper also proposes a new verification methodology based on the time-step method in order to augment the two-phase validation procedure. This methodology utilizes multiple satellites and stations that exhibit a similar trend of ionospheric delay to that of the threat candidate. The time-step method enables us to estimate ionospheric gradients over any short baseline distance and thus compensates for the lack of ionospheric observability caused by a sparse distribution of GPS reference stations. It visualizes the equatorial anomaly event in both the time domain and spatial domain by utilizing all available sources including regional ionospheric delay maps, IPP tracks, the motion of EPBs, the location of stations, and the occurrence time of large gradients. Using this procedure, we verified an extreme ionospheric gradient of 501.2 mm/km observed at reference stations SAVO and SSA1 viewing PRN 21 on 31 December 2013. Simultaneous ionospheric observation data from another satellite passing across the same EPB region was as high as about 360 mm/km. Six station-satellite pairs exhibited similar trend of ionospheric delays and observed severe ionospheric gradients above 300 mm/km using the time-step based techniques. Threat event visualization supported the fact that satellite-station pairs spread over several hundreds of kilometers in east-west direction were impacted by the same EPB moving eastward. These factors give us complete support to confirm that the extreme ionospheric gradient observed here is caused by a real ionospheric anomaly and not due to measurement errors or other faults. In addition to the particular case presented in this paper, we have also completed verification of other extreme EPB events whose gradients were larger than 500 mm/km. However, the full description of other events is beyond the scope of the paper.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Kim2015b,

title = {Conceptual Study of Mobile Differential GNSS Architecture Utilizing UAV Networks},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=12710},

year  = {2015},

date = {2015-04-20},

booktitle = {Proceedings of the ION 2015 Pacific PNT Meeting},

pages = {828 - 838},

address = {Honolulu, Hawaii},

abstract = {Differential Global Navigation Satellite Systems (DGNSS) which feature rapid deployment and mobility can provide required navigation accuracy and integrity to a wide range of users at arbitrary locations where no permanently installed systems exist. This paper proposes a concept of mobile DGNSS whose reference stations are composed of multiple Unmanned Aerial Vehicles (UAVs) to facilitate mobility of system. Key algorithms of Mobile DGNSS architecture, developed in this work, include a decision making technical for selecting reference station location, a fast location surveying method and integrity monitors. Using the decision making technical, landing positions of UAV reference stations are selected by taking into account the geographical characteristics of the landing vicinity, and the distances from DGNSS service demands. To validate that the locations satisfy the minimum siting requirements upon landing, two tests are carried out: territorial inclination test for assuring a sufficient number of satellites in view and satellite visibility test for avoiding a harsh environment with obstacles. The fast location surveying method is developed to reduce the time taken for the determination of the surveyed positions within specific positional accuracy. The simulation results show that the proposed method enables users to achieve the desired positional accuracy in less amount of time than the general method of simply averaging stand-alone GNSS solutions only. Integrity monitors in addition to the well-established Ground Based Augmentation System (GBAS) monitors are developed to detect faults specific to the mobile DGNSS. The code multipath monitor and the station displacement monitor are designed to detect large multipath errors on code measurements and unexpected movements of the reference stations, respectively under uncontrolled siting conditions. Rapid start-up time is a critical factor to assess whether the proposed concept is feasible. We derive surveying position error budgets for different multipath environments and system architectures. A new Vertical Protection Level (VPL), which includes the bound of surveying errors, is defined for the mobile DGNSS users. The effect of the resulting surveying errors on the system is evaluated by estimating deployment time for various conditions.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}