Publications

@conference{Kim2014c,

title = {Observing Large Ionospheric Spatial Decorrelation for Ground-Based Augmentation System in the Brazilian Region},

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

url = {http://adsabs.harvard.edu/abs/2014AGUFMSA13C4014K},

year  = {2014},

date = {2014-12-15},

booktitle = {AGU Fall Meeting 2014},

address = {San Francisco, CA},

abstract = {Ground-Based Augmentation Systems (GBAS) support aircraft precision approach and landing by broadcasting differential Global Positioning System (GPS) corrections and integrity information to aviation users. Under anomalous ionospheric condition, unacceptably large residual errors can occur due to anomalously large ionospheric spatial decorrelation, and this can pose integrity threats to GBAS users. Thus, the development of an ionospheric anomaly threat model is required to simulate worst-case ionospheric errors and develop mitigation strategies. Ionosphere in low latitudes is known to be much more intense than that in mid latitudes due to active geomagnetic effect, and investigation of low latitude ionospheric anomalies must take precedence before operation of GBAS. In this paper, ionospheric spatial decorrelation is investigated for GBAS operation in the Brazilian region. Dual-frequency observation data are collected from Brazilian GPS reference stations. This analysis is performed using data sets collected on scintillating days, less-scintillating days, and storm days from 2012 to 2014. Precise ionospheric spatial gradient on the L1 signal is automatically estimated from dual-frequency observation data using simple truth method and station pair method. In the Brazilian region, however, intense ionospheric scintillations cause a large numbers of cycle slips in carrier-phase data. The simple truth process removes a considerably large number of those data through short-arc and outlier removals, and thus potential ionospheric gradients may not be detected. This motivates a data recovery process which skips short-arc and outlier removals if there appears a large ionospheric spatial gradient in the removed data. We also use a series of methods to validate anomalous ionospheric spatial gradients using manual validation with L1 single frequency measurement, station-wide check, satellite-wide check, and time-step check. In particular, the time-step check validates localized ionospheric anomalies in a scale of several tens of kilometers. This method is useful when the anomalies are not validated by station-wide and satellite-wide checks due to the sparse distribution of Brazilian GPS reference stations. Using the above methods, we observe and validate large ionospheric spatial gradients.},

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@conference{Lee2014d,

title = {Space Weather Prediction Error Bounding for Real-Time Ionospheric Threat Adaptation of GNSS Augmentation Systems},

author = {Jinsil Lee and Moonseok Yoon and Jiyun Lee},

year  = {2014},

date = {2014-12-15},

booktitle = {AGU Fall Meeting 2014},

address = {San Francisco, CA},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Yoon2014c,

title = {Ionospheric Signatures of North Korean Nuclear Test on 12 February 2013},

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

url = {http://adsabs.harvard.edu/abs/2014AGUFMNH31C3881Y},

year  = {2014},

date = {2014-12-15},

booktitle = {AGU Fall Meeting 2014},

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 2006 and 2009 North Korean underground nuclear tests were observed using GPS measurements. In this work, we processed GNSS stations in South Korea and Japan and analyzed traveling ionospheric disturbances that were coincident with the 2013 North Korean underground test. North Korea conducted the third underground nuclear test at 2:57 UTC on February 12, 2013. The magnitude of earthquake generated by this event was registered to be an Mw 5.1 event. After analyzing GPS measurements from nearby stations, strong ionospheric perturbations were observed 15-30 minutes after the reported event, and the disturbances were shown to have primarily two different wave trains. The maximum VTEC perturbations turned out to be between 0.4 to 0.7 TECU. Five stations located in the northwest-to-southeast direction were also scrutinized for the propagation direction and amplitude variation related to ionospheric wave structures. The results clearly showed that the maximum amplitude of the waves may be higher as the stations are closer to the epicenter indicating that the waveforms may propagate away from the epicenter. In this research, we will analyze the characteristics of the detected ionospheric perturbations associated with the underground nuclear test. These findings are expected to verify our modeling results. We hope to get a better understanding of the influence of man-made hazards on the temporal and spatial variability of the global ionosphere.},

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@conference{Yoon2014b,

title = {The Use of SBAS GIVE for Improving CAT-I GBAS Availability in the Korean Region},

author = {Moonseok Yoon and Eugene Bang and Jiyun Lee},

url = {

http://hdl.handle.net/10203/193388},

year  = {2014},

date = {2014-10-24},

booktitle = {ISGNSS 2014},

address = {Jeju, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Bang2014b,

title = {Ionospheric Delay Estimation using Kriging for Korean SBAS},

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

url = {

http://hdl.handle.net/10203/193393},

year  = {2014},

date = {2014-10-23},

booktitle = {ISGNSS 2014},

address = {Jeju, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2014c,

title = {Fast location survey of DGNSS reference station to support UAV navigation},

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

doi = {10.1109/ICCAS.2014.6987893},

year  = {2014},

date = {2014-10-22},

booktitle = {2014 14th International Conference on Control, Automation and Systems (ICCAS 2014)},

address = {Seoul, South Korea},

abstract = {Mobile Differential Global Navigation Satellite Systems (Mobile DGNSS) formed with multiple UAVs can support missions on a battlefield by providing navigation with high accuracy and integrity. For time efficiency and maneuverability of the system, the reference stations that are deployed and sited on the battlefield should be able to determine their precise position rapidly before generating the differential corrections. This paper presents a methodology for fast location surveying of the reference stations, which improves the GNSS-based position estimates of reference stations using precise measurements of relative distances between the stations. Simulation results showed that the proposed method enables users to achieve the desired accuracy in less time than the simple method which averages stand-alone GNSS solutions only. A theoretical model of DGNSS position uncertainty induced by inaccurate surveyed locations of the reference stations was also defined in this paper. The model was validated by comparing it to the results from experiments conducted with a DGNSS test-bed equipped with three GNSS receivers and a pseudo-user.

},

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tppubtype = {conference}

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@conference{Kim2014b,

title = {High Integrity GNSS Navigation and Safe Separation Distance to Support Local-Area UAV Networks},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=12424},

year  = {2014},

date = {2014-09-08},

booktitle = {Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014)},

pages = {869 - 878},

address = {Tampa, Florida},

abstract = {A local-area Unmanned Aerial Vehicle (UAVs) network can provide local-area differential corrections and optimized guidance, including the maintenance of safe separation distances, for precise and safe operation of the UAVs. In networked UAV operations, only the uncorrelated component of position error between two UAVs in the same network contributes to potential separation violations, because “in-network” UAVs share the same source of guidance and differential corrections. Therefore, models of uncorrelated errors need to be defined to establish safe separation distances between UAVs. This paper develops a methodology to estimate safe separation distance for UAVs which share the same source of guidance and local-area differential corrections. Using this methodology, ionospheric and tropospheric models for Navigation System Error (NSE) are developed theoretically. The airborne multipath error and Flight Technical Error (FTE) components which depend on hardware, environment, and operational conditions are also determined though UAV flight experiments. The standard deviation of FTE was estimated in both straight and curved flight segments. The results show that a lateral FTE error of 0.78 meters in curved segments is much greater than lateral FTE in straight segments and vertical FTE in both segments (all about 0.4 meters). This is due to the momentum of the UAV when it is taking turns and the limited controller response time of the UAV. The flight experiments also show that UAV multipath errors are reasonably well-bounded by the standard airborne multipath model developed for the Ground Based Augmentation System (GBAS). From the estimated error models, simulations of safe separation distances were conducted under the “in-network” UAVs scenario. Simulation results for a 24-satellite GPS constellation and the probabilities of safe separation suggested in prior work show that vertical separations between UAVs in the same network vary from about 3.0 – 6.5 meters, while horizontal separations vary between 2.2 and 3.5 meters. These values change with time according to the visible GPS satellite geometry and are known to the controller in real time.},

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pubstate = {published},

tppubtype = {conference}

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@conference{Lee2014b,

title = {VPL Parameter Determination for Improved Performance of Advanced RAIM},

author = {Jinsil Lee and Dongwoo Kim and Jiyun Lee and Todd Walter},

url = {https://www.ion.org/publications/abstract.cfm?articleID=12486},

year  = {2014},

date = {2014-09-08},

booktitle = {Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014)},

pages = {3566 - 3574},

address = {Tampa, Florida},

abstract = {Advanced Receiver Autonomous Integrity Monitoring (ARAIM) helps ensure the integrity of vertical guidance using multi-constellations and new civil signals [1]. Among Global Navigation Satellite System (GNSS) error sources, signal-in-space (SIS) range error, which includes satellite ephemeris and clock errors, is most significant because combinations of signals on multiple frequencies are used to remove ionospheric errors, which are dominant in single-frequency operations. In order to ensure the integrity of ARAIM, an integrity parameter ? is proposed to be broadcast by air navigation service providers. This ? parameter is an inflation factor applied to broadcast URAs to ensure that SIS range errors are bounded to the required level of integrity. Based on a comprehensive analysis of SIS error characteristics, this paper determines the ? parameter for subgroups of satellites that have similar error behavior and for individual satellites to lower the conservatism of the computed protection levels. Three different grouping scenarios (a single value of ? for all satellites, one value of ? for each satellite block type, and one value of ? for each satellite) were used to determine ?. The performance for each scenario was evaluated by performing ARAIM availability simulations. The simulation results demonstrated the performance improvements from grouping satellites into block types compared to the case of using a single value of ? for all satellites. In particular, applying one value of ? for each satellite gave significantly higher availability than other scenarios. This benefit gets more evident when the Gaussian tail behaviors of SIS error distributions differ across satellites.},

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@conference{Choi2014,

title = {Selection of Korea SBAS Reference Stations By Analyzing GPS Satellite Clock and Ephemeris Error Bound},

author = {Pilhoon Choi and Jiyun Lee},

year  = {2014},

date = {2014-08-01},

booktitle = {2014 APNN & MAPWiST},

address = {Seoul},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2014,

title = {Conceptual Study of Real-time Ionospheric Threat Adaptation Using Space Weather Forecasting for GNSS Augmentation Systems},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=11538},

year  = {2014},

date = {2014-01-27},

booktitle = {Proceedings of the 2014 International Technical Meeting of The Institute of Navigation},

pages = {666 - 676},

address = {San Diego, California},

abstract = {Current GNSS augmentation systems attempt to consider all possible ionospheric events in their computations of worst-case errors in user position solutions. The resulting error bounds are conservative because of this need to cover all possible anomalous conditions that might go undetected. This conservatism can be mitigated by subdividing anomalous conditions into several classes of severity and using different values of threat-model bounds for each class. This is possible if the level of ionospheric activity is classifiable and predictable (at least over periods of tens of minutes to hours) from measurable space weather conditions. This paper presents a new concept of real-time ionospheric threat adaptation that adjusts the ionospheric threat model in real time instead of always using the same ‘worst-case’ threat model. This is done by utilizing a relationship between ionospheric activity and space weather indices. The worst-case threat is defined as a function of the values of space weather indices. Predicted values of space weather indices are used for determining the corresponding threat model. Since space weather prediction itself is not reliable due to prediction errors, an uncertainty model is derived from 15 years of historical data, and the prediction errors are bounded to the required level of integrity of the system being supported. This concept was demonstrated by assessing the performance of real-time ionospheric threat adaptation when applied to the Local Area Augmentation System (LAAS) threat model used for Category I precision approach in the Conterminous United States (CONUS). The disturbance-storm time (Dst) index was selected as a measure of space weather intensity. The relationship between final Dst and the worst ionospheric gradients (or "slopes") identified in CONUS was defined. The ‘predicted Dst bound’ derived by taking into account prediction error statistics was then used to determine the worst slope bounds in real-time. As a result, the upper slope bound of the current threat model (425 mm/km for LAAS) could be lowered about 94% of the time during the 15 years of data (from 1995 to 2009) with an integrity of (1 - 10-7) using the bounded prediction value of Dst for real-time threat determination.},

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tppubtype = {conference}

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@conference{Lee2013,

title = {Analysis of Correlation between Ionospheric Spatial Gradients and Space Weather Intensity under Nominal Conditions for Ground-Based Augmentation Systems},

author = {Jinsil Lee and Jiyun Lee},

url = {http://adsabs.harvard.edu/abs/2013AGUFMSM53D2245L},

year  = {2013},

date = {2013-12-09},

booktitle = {AGU Fall Meeting 2013},

address = {San Francisco, CA},

abstract = {Ground-Based Augmentation Systems (GBAS) support aircraft precision approach and landing by providing differential GPS corrections to aviation users. For GBAS applications, most of ionospheric errors are removed by applying the differential corrections. However, ionospheric correction errors may exist due to ionosphere spatial decorrelation between GBAS ground facility and users. Thus, the standard deviation of ionosphere spatial decorrelation (σvig) is estimated and included in the computation of error bounds on user position solution. The σvig of 4mm/km, derived for the Conterminous United States (CONUS), bounds one-sigma ionospheric spatial gradients under nominal conditions (including active, but not stormy condition) with an adequate safety margin [1]. The conservatism residing in the current σvig by fixing it to a constant value for all non-stormy conditions could be mitigated by subdividing ionospheric conditions into several classes and using different σvig for each class. This new concept, real-time σvig adaptation, will be possible if the level of ionospheric activity can be well classified based on space weather intensity. This paper studies correlation between the statistics of nominal ionospheric spatial gradients and space weather indices. The analysis was carried out using two sets of data collected from Continuous Operating Reference Station (CORS) Network; 9 consecutive (nominal and ionospherically active) days in 2004 and 19 consecutive (relatively 'quiet') days in 2010. Precise ionospheric delay estimates are obtained using the simplified truth processing method and vertical ionospheric gradients are computed using the well-known 'station pair method' [2]. The remaining biases which include carrier-phase leveling errors and Inter-frequency Bias (IFB) calibration errors are reduced by applying linear slip detection thresholds. The σvig was inflated to overbound the distribution of vertical ionospheric gradients with the required confidence level. Using the daily maximum values of σvig, day-to-day variations of spatial gradients are compared to those of two space weather indices; Disturbance, Storm Time (Dst) index and Interplanetary Magnetic Field Bz (IMF Bz). The day-to-day variations of both space weather indices showed a good agreement with those of daily maximum σvig. The results demonstrate that ionospheric gradient statistics are highly correlated with space weather indices on nominal and off-nominal days. Further investigation on this relationship would facilitate prediction of upcoming ionospheric behavior based on space weather information and adjusting σvig in real time. Consequently it will improve GBAS availability by adding external information to operation. [1] Lee, J., S. Pullen, S. Datta-Barua, and P. Enge (2007), Assessment of ionosphere spatial decorrelation for GPS-based aircraft landing systems, J. Aircraft, 44(5), 1662-1669, doi:10.2514/1.28199. [2] Jung, S., and J. Lee (2012), Long-term ionospheric anomaly monitoring for ground based augmentation systems, Radio Sci., 47, RS4006, doi:10.1029/2012RS005016.},

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tppubtype = {conference}

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@conference{Yang2013,

title = {Investigating Natural Hazards Using GNSS Measurements: The Chelyabinsk Meteor Ionospheric Impact},

author = {Yu-ming Yang and Attila Komjathy and Mark D. Butala and Anthony J. Mannucci and Richard B. Langley and Jonathan Snively and Michael P. Hickey and David Galvan and Jiyun Lee},

url = {https://www.ion.org/publications/abstract.cfm?articleID=11447},

year  = {2013},

date = {2013-09-16},

booktitle = {Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013)},

pages = {3480 - 3488},

address = {Nashville, TN},

abstract = {Introduction. Natural hazards, including earthquakes, volcanic eruptions, and tsunamis, have been significant threats to humans throughout recorded history. The Global Positioning System satellites have become primary sensors to measure the effects of such natural hazards. Signatures in the GPS data include seismic deformation displacements, co-seismic vertical displacements, and real-time ocean buoy positioning estimates. Another way to use GPS observables is to measure and monitor ionospheric total electron content (TEC) variations generated by post-seismic atmospheric disturbances caused by earthquakes, volcanic eruptions, tsunamis, meteors and nuclear explosions [e.g., Artru et al., 2005]. Prior JPL Work. Advances in very high precision ionospheric GPS data processing at JPL have demonstrated that ground-based GPS receivers are capable of detecting TEC perturbations generated by atmospheric acoustic and gravity waves [Komjathy et al., 2012]. The 2011 Tohoku earthquake and tsunami data processing results have, for instance, demonstrated that the gravity-wave-derived TEC perturbations are visible 45 minutes after the earthquake [Galvan et al., 2012]. Applying JPL’s data processing techniques to multiple events, we have found that a 2004 volcanic eruption in Japan showed approximately 1-minute period waves in ionospheric TEC, whereas the September 2009 earthquake near Samoa produced signatures with an 8-minute period [Galvan et al., 2011]. There remains much to learn about the characteristics of these interactions between the Earth’s surface and ionosphere, including how and why they differ from one event to the next. Recent Natural Hazard Event of High Interest. The Chelyabinsk meteor provided a unique opportunity to observe TEC disturbances generated by a fireball in the atmosphere. The small asteroid entered the atmosphere at 3:20 UT on February 15, 2013 moving at a speed of about 20 km/s. The object, with an almost 20 meters in diameter, then burst into pieces at a height of 30-50 km above the ground. Large fragments moving at a high speed caused a powerful flash and a strong shockwave, with most of the meteor’s energy released at a height of 5 to 15 km above the earth. Technical Approach. We use JPL’s PyIono package (a bias-fixing algorithm) to generate high-precision calibrated TEC measurements. Calibrating TEC measurements serves multiple purposes for us including quality checking (QC) of processed data, leveling the phase measurements using pseudoranges and comparing modeled TEC perturbations with measured ones [Mannucci et al, 1998]. Obtaining absolute TEC values is useful to understand background conditions for the perturbations [e.g., Komjathy et al, 2005]. However, we are primarily interested in monitoring small-scale variations in ionospheric electron density, hence the changes in TEC are of interest, rather than absolute TEC values. Subsequently, JPL’s PyIono uses TEC observations to compute de-trended TEC data. A Butterworth band-pass filter (corresponding to waves with periods between 33 and 3.3 minutes ranging between 0.5 and 5 mHz) is applied to focus on acoustic and gravity wave generated TEC observations. This type of filtering allows us to more easily detect perturbations within an expected range of frequencies, which we can infer from previous observations of tsunami periods, for example [Galvan et al., 2012]. Summary and Anticipated Results. We have processed data from 23 GPS stations within a radius of 1500 km from the impact location in Russia for February 15 and the surrounding days. Initial results of monitoring TEC perturbations using the JPL technique suggest a strong impact of the individual explosions on the ionosphere. Furthermore, we observed a statistically significant correlation between the reported meteor trajectory and the ionospheric signatures measured by GPS. Preliminary modeling results of wave structures are presented and compared with the observed perturbations to investigate the possible generation and propagation of waves associated with this unique hazard event. Acknowledgements. The authors would like to thank NASA Headquarters and the Earth Science and Interior NASA ROSES Grant (NNH07ZDA001N-ESI). This research was performed at the Jet Propulsion Laboratory, California Institute of Technology under contract to the National Aeronautics and Space Administration. REFERENCES Artru, J., V. Ducic, H. Kanamori, P. Lognonné, and M. Murakami (2005), Ionospheric detection of gravity waves induced by tsunamis, Geophysical Journal International, 160, 840–848, 10.1111/j.1365-246X.2005.02552.x. Galvan, D. A., A. Komjathy, M. P. Hickey, P. Stephens, J. Snively, Y. Tony Song, M. D. Butala, and A. J. Mannucci (2012), Ionospheric Signatures of Tohoku-Oki Tsunami of March 11, 2011: Model Comparisons Near the Epicenter. Radio Science, 2012RS005023. Galvan, D. A., A. Komjathy, M. P. Hickey, and A. J. Mannucci (2011), The 2009 Samoa and 2010 Chile tsunamis as observed in the ionosphere using GPS total electron content, Journal of Geophysical Research (Space Physics), 116, A06, 318, 10.1029/2010JA016204. http://dx.doi.org/10.1029/2010JA016204 Komjathy, A., L. Sparks, B. D. Wilson, and A. J. Mannucci (2005), Automated daily processing of more than 1000 ground-based GPS receivers for studying intense ionospheric storms, Radio Sci., 40, RS6006, doi:10.1029/2005RS003279. Komjathy, A., D.A. Galvan, P. Stephens, M.D. Butala, V. Akopian, B.D. Wilson, O. Verkhoglyadova, A.J. Mannucci, and M. Hickey (2012). “Detecting Ionospheric TEC Perturbations Caused by Natural Hazards Using a Global Network of GPS Receivers: the Tohoku Case Study.” Earth Planets Space, Vol. 64, 1-8, 2012. Mannucci, A. J., B. D. Wilson, D. N. Yuan, C. H. Ho, U. J. Lindqwister, and T. F. Runge (1998), A global mapping technique for GPS-derived ionospheric total electron content measurements, Radio Science, 33, 565–582, 10.1029/97RS02707.},

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@conference{Yoon2013,

title = {Characteristics of Ionospheric Irregularities during Storm-Induced Traveling Ionospheric Disturbances over the Korean Region},

author = {Moonseok Yoon and Jiyun Lee},

year  = {2013},

date = {2013-09-01},

booktitle = {The 6th KAIST-Kyushu University joint workshop},

journal = {The 6th KAIST-Kyushu University joint workshop},

address = {Deajeon, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

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@conference{Pullen2013bb,

title = {Guidance, Navigation, and Separation Assurance for Local-Area UAV Networks: Putting the Pieces Together},

author = {Sam Pullen and Jiyun Lee},

url = {https://www.ion.org/publications/abstract.cfm?articleID=11051},

year  = {2013},

date = {2013-04-23},

booktitle = {Proceedings of the ION 2013 Pacific PNT Meeting},

pages = {902 - 914},

address = {Honolulu, Hawaii},

abstract = {This paper examines the guidance methodology needed to implement networks of autonomously-flown unmanned aerial vehicles (UAVs) controlled by centralized ground stations (GSs). The intended operations would take place within a local area with a diameter of less than 10 km for most applications but potentially as large as 50 - 100 km. UAVs in these networks are envisioned to be potentially quite small and inexpensive but capable of automated flight orientation and stability with guidance updates provided by the GS at 0.5 to 2 second intervals. The GS also provides GNSS differential corrections and integrity information to support sub-meter-level 95% navigation accuracy with 10-7 error bounds in the 3 - 10 meter range. Position and timing solutions for each UAV are relayed back to the GS and support both operational (route planning) and tactical (path updating) guidance. This guidance needs to insure safe separation between UAVs within the network and (depending on the airspace used) separation from "out-of-network" UAVs as well as manned aircraft. The proposed guidance approach centers around "zones of influence" surrounding each UAV that include allowances for navigation error, UAV guidance error, and ground-system guidance error. The amount of error allocated to each error source depends upon the degree of error correlation between each UAV and its neighbors as well as the required probabilities of safe separation that must be maintained. The ground system maintains and updates zones of influence for each UAV within its operational area and guides each UAV it controls to remain within a "zone of operations" to insure that all UAV movements it commands avoid collisions with other vehicles or the ground. This paper provides examples of how this is done and how adjustments are made to reflect changes in navigation performance and the influx of UAVs operating outside the network.},

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tppubtype = {conference}

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@conference{Bang2013c,

title = {Constructing Ionospheric Irregularity Threat Model for Korean SBAS},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=10982},

year  = {2013},

date = {2013-04-23},

booktitle = {Proceedings of the ION 2013 Pacific PNT Meeting},

pages = {296 - 306},

address = {Honolulu, Hawaii},

abstract = {Single-frequency based Satellite-Based Augmentation Systems (SBAS), the augmentation of the Global Navigation Satellite System (GNSS), broadcast estimates of vertical ionospheric delays and confidence bounds on the delay errors at Ionospheric Grid Points (IGPs). Using an ionospheric irregularity undersampled threat model, the integrity bounds, called Grid Ionospheric Vertical Errors (GIVEs), must be augmented to bound ionospheric irregularity threats which may exist between or beyond Ionospheric Pierce Points (IPPs) under ionospheric storm conditions. Since the ionospheric disturbed conditions can vary significantly from one region to another region, threat models need to be built for regions where SBAS will be operational. This paper presents a new method for constructing an undersampled threat model for SBAS in the Korean region, examines the influence of threat model to system availability, and demonstrates the performance of a newly developed threat model. The existing method tabulates undersampled threats in the threat model as a function of two metrics which measure the density and uniformity of IPP distribution in a region. Thus, the threat model metrics, which characterize threatening undersampled geometries including the density of IPP distribution accurately, play a critical role in improving system performance. The first threat metric, fit radius, is defined by an IPP search method used for a planar fit algorithm. This paper first determines a range of the fit radius optimized for the Korean region by considering the ionospheric observability and quality of the planar fit. Next this paper proposes a new second metric, the Relative Bin Number (RBN) metric, alternative to the Relative Centroid Metric (RCM) currently used in WAAS. RBN is more effective than the existing threat metric in capturing the sparseness of the IPP distribution by measuring the ratio of the number of partitions in which IPPs are absent to the total number of partitions. In addition, other essential parameters for the Korean SBAS threat model construction, including GEO MT28 (Message Type 28), IGP formations, and the number of reference stations, are determined. In a preliminary assessment, the undersampled ionospheric threat model based on the new methodology increased the coverage of 99.9% availability for APV-I service from 18.48% to 91.10%.},

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tppubtype = {conference}

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@conference{Bang2013b,

title = {Assessing the availability of a single-frequency satellite based augmentation system in the Korea region},

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

year  = {2013},

date = {2013-02-01},

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

address = {Tokyo, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Pullen2013b,

title = {High-Integrity Local-Area Differential GNSS Architectures Optimized to Support Unmanned Aerial Vehicles (UAVs)},

author = {Sam Pullen and Per Enge and Jiyun Lee},

url = {http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=C7A57031DC57820AD1B115FE13808599?doi=10.1.1.658.3106&rep=rep1&type=pdf},

year  = {2013},

date = {2013-01-28},

booktitle = {Proceedings of the 2013 International Technical Meeting of The Institute of Navigation},

address = {San Diego, California},

abstract = {As the applications of Unmanned Aerial Vehicles (UAVs) expand, UAVs will be combined into networks that cooperate to perform various missions within 10 to 200 km of a centralized controller. GNSS is the primary source of navigation for UAVs operating over large areas, and UAVs combined into local networks can easily make use of local-area differential corrections integrated into their guidance commands to improve their navigation accuracy and integrity. This paper develops a Local-Area Differential GNSS (LADGNSS) architecture around a concept of local-area UAV network operations that emphasizes low cost for commercial applications and high integrity to allow UAVs to operate in close proximity to each other and potential "targets " while minimizing collision risk. Using the well-established Ground-based Augmentation System (GBAS) as a starting point, a simplified LADGNSS architecture is identified that retains most of the performance of GBAS at a far lower cost. Because LADGNSS performance will be limited by the characteristics of UAV receivers and flight dynamics, future work will be focused on identifying and understanding UAV receiver performance through a series of flight tests at the Korea Advanced Institute of Science and Technology (KAIST).},

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@conference{[Kim]:[ION]:[2013],

title = {Optimized GNSS Network Station Selection to Support the Development of Ionospheric Threat Models for GBAS},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=10890},

year  = {2013},

date = {2013-01-28},

booktitle = {Proceedings of the 2013 International Technical Meeting of The Institute of Navigation},

pages = {559 - 570},

address = {San Diego, California},

abstract = {Extremely large ionospheric spatial gradients present potential integrity threats to the users of global navigation satellite systems (GNSS) augmentation systems. Thus, these ionospheric anomalies need to be monitored by ground reference stations and users must be alarmed within time-to-alerts. The long-term ionospheric anomaly monitoring (LTIAM) tool has been developed to monitor ionospheric behavior continuously over the life cycle of ground-based augmentation systems (GBAS) and to build ionosphere threat models for all regions where GBAS will be fielded in the future. However, the use of poor-quality GNSS data degrades the accuracy of ionospheric delay estimates, produces many faulty anomaly candidates and thus adds a great burden to LTIAM processing. This paper develops a methodology to select a set of welldistributed, high-quality stations from GNSS reference station networks. An optimized set of thresholds for data quality metrics, which maximize the elimination of spurious gradients while minimizing unnecessary station removals, are established by the high-quality station selection method. This method performs better that the previously developed method which determine thresholds independently for each quality metric without considering the interrelations between other quality metrics. The welldistributed sub-network selection method is also proposed to remove geographically redundant stations in dense regions. The number of CORS stations in the Conterminous U.S. (CONUS) is reduced to 46% of the total stations when a desired baseline constraint is 100 km. This paper also verifies the performance of the proposed method by processing data collected from other GNSS reference station networks.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Choi2012b,

title = {Extreme Ionospheric Gradients Observed in South Korea during the Last Solar Cycle},

author = {Yunjung Choi and Minchan Kim and Jiyun Lee},

url = {http://adsabs.harvard.edu/abs/2012AGUFM.G11B0929J},

year  = {2012},

date = {2012-12-03},

booktitle = {AGU Fall Meeting 2012},

address = {San Francisco, CA},

abstract = {Ground-Based Augmentation Systems (GBAS) support aircraft precision approach and landing by providing differential corrections for Global Navigation Satellite System (GNSS) pseudorange measurements and integrity information to aviation users within several tens of kilometers of GBAS-equipped airports. During the peak of the last solar cycle, extreme ionospheric gradients as large as 412 mm/km at high elevation and 360 mm/km at low elevation were observed in the United States. For a GBAS user at a 200-foot decision height (DH) for Category I precision approach, a spatial gradient of 412 mm/km could cause a residual range error of 8 meters. To predict the maximum position errors that GBAS users might suffer from these ionospheric threats, an ionospheric anomaly "threat model" for GBAS was developed in the Conterminous U.S (CONUS). The threat model issued to simulate worst-case ionospheric errors and develop mitigation strategies under ionospheric disturbances. Ionospheric conditions should be investigated for all regions where GBAS will be fielded in the future. We presents a method to identify ionospheric anomalies that can pose a potential integrity risk to GBAS users and details the study of extreme ionospheric gradients observed in South Korea during the last solar cycle. GPS dual-frequency code and carrier-phase measurements collected from a total of 74 GPS reference stations in South Korea were processed to observe ionospheric anomalies. Precise ionospheric delay estimates are obtained using the simplified truth processing method and ionospheric gradients are computed using the well-known "station pair method". Ionospheric threats can be modeled as a spatially linear semi-infinite wedge moving with constant speed in mid-latitude regions. A total of 22 dates during the last solar maximum period in 2000 - 2004 were investigated to identify ionospheric anomalies occurred in South Korea. Ten of the dates were the days previously chosen to construct the current CONUS threat model. The other 12 dates are newly selected based on two space weather indices, planetary K (Kp) and disturbance, storm time (Dst). An ionospheric gradient of 90.97 mm/km was discovered at 0414UT on November 6, 2001 between stations NAWW and SONC, when PRN 21 was at 20.1° elevation. Most of severe gradients were observed from satellites at low elevation and traveling in a southerly direction of the Korean Peninsula. To locate enhanced-delay regions, we investigate both global ionospheric delay maps generated using the IONosphere MAP Exchange format (IONEX) data provided by International GNSS Service (IGS) and regional delay maps produced using the Korean GPS network stations. To validate observed ionospheric anomaly events, we examine whether similarly large ionospheric gradients are discovered at other nearby station pairs and other satellites whose Ionospheric Pierce Point (IPP) tracks take similar paths. The results from a series of checks support that the equatorial ionospheric anomaly caused severe ionospheric gradients observed from southern stations in South Korea. This study provides a better understanding of ionospheric behavior within the Korean Peninsula under ionospheric storm conditions, and helps investigate the operation and performance of GBAS. Ionospheric threat model developed in each region could be combined into a future global threat model for GBAS.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Jeong2012c,

title = {Algorithm Analysis for GNSS Spoofing Detection System},

author = {Seongkyun Jeong and Sanguk Lee and Jiyun Lee},

year  = {2012},

date = {2012-11-01},

booktitle = {2012 APISAT},

address = {Jeju, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Jeong2012b,

title = {Design analysis of precision Navigation System},

author = {Seongkyun Jeong and Taehee Kim and Jae-Eun Lee and Sanguk Lee and Jiyun Lee},

url = {https://ieeexplore.ieee.org/document/6393228},

year  = {2012},

date = {2012-10-17},

booktitle = {2012 12th International Conference on Control, Automation and Systems},

address = {JeJu Island, South Korea},

abstract = {As GNSS(Global Navigation Satellite System) is becoming more common all over the world, the satellite navigation system is used in many fields such as positioning, navigation, timing, land survey, and so on. The satellite navigation covers all around world and can be used in everywhere if the satellite signal could be received. But the performance of positioning has the limitation due to the way of satellite navigation. The satellite navigation system has the error elements which are ionospheric delay, tropospheric delay, orbit ephemeris error, and other errors in calculation of positioning. These error elements degrade the positioning accuracy and the reliability of satellite navigation system. If these problems which make the usage of satellite navigation system be limited are resolved, the usage will be extended more than present. ETRI(Electronics and Telecommunications Research Institute) is developing precision navigation system for sub-centimeter positioning. This system is for providing precision navigation service and securing the application technology in satellite navigation system. This technology has important point in future navigation market and application. In this paper, we introduce the precision navigation system and design process. The navigation algorithm to be applied in the system is analyzed.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Kim2012,

title = {Data Quality Improvements and Applications of Long-Term Monitoring of Ionospheric Anomalies for GBAS},

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

url = {https://www.ion.org/publications/abstract.cfm?articleID=10410},

year  = {2012},

date = {2012-09-17},

booktitle = {Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012)},

pages = {2159 - 2174},

address = {Nashville, TN},

abstract = {The Long-Term Ionospheric Anomaly Monitoring (LTIAM) tool is an automated software package designed to analyze past data and support continuous ionospheric monitoring of both nominal and anomalous ionospheric spatial gradients. While automated measurement screening is included, large gradients observed by LTIAM require manual validation to confirm that they were caused by the ionosphere instead of faulty measurements or data recording. Ground stations with poor data quality thus add greatly to the burden of LTIAM processing. This paper develops an automated approach to data quality measurement for CORS and IGS ground stations. This method is used to identify stations that are poor according to multiple quality metrics. Thresholds are established for each quality metric, and stations violating one or more thresholds are removed from use by LTIAM unless their geographical position is sufficiently important. Use of this method with CORS stations in the Conterminous U.S. (CONUS) eliminates the almost 90% of spurious or false gradients while only excluding 16% of the over 1500 CORS stations in CONUS. This paper also investigates past CONUS ionospheric storm data to understand the distribution of anomalous spatial gradients. Examining LTIAM outputs on known storm days with gradients between 50 and 200 mm/km demonstrates that these smaller (but still anomalous) gradients are far more likely than extreme gradients above 200 mm/km. The continued use of LTIAM over the next solar peak should help us refine our knowledge of this distribution as well as the overall likelihood of large spatial gradients under anomalous ionospheric conditions.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Choi2012,

title = {Ionospheric Observations of the November 2004 Storm in South Korea},

author = {Yunjung Choi and Minchan Kim and Jiyun Lee},

year  = {2012},

date = {2012-09-01},

booktitle = {The 5th KAIST-Kyushu University joint workshop},

address = {Deajeon, Korea},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Bang2012,

title = {Automated Ionospheric Front Velocity Estimation Algorithm for Ground-Based Augmentation Systems},

author = {Eugene Bang and Jiyun Lee and Jiwon Seo and Sam Pullen and Sigrid Close},

url = {https://www.ion.org/publications/abstract.cfm?articleID=10035},

year  = {2012},

date = {2012-01-30},

booktitle = {Proceedings of the 2012 International Technical Meeting of The Institute of Navigation},

pages = {1570 - 1580},

address = {Newport Beach, CA},

abstract = {Ionospheric anomalies, which may occur during severe ionospheric storms, could pose integrity threats to Ground-based Augmentation System (GBAS) users [1], [2], [3]. The ionospheric threat for a Local Area Augmentation System (LAAS), a GBAS developed by the U.S. Federal Aviation Administration (FAA), was modeled as a spatially linear, semi-infinite “front” (like a weather front) with constant propagation speed. The model is parameterized by the slope (or gradient) of the front, its width, and its ground speed. Along with the magnitude of ionospheric gradients, the speed of the fronts in which these gradients are embedded is an important parameter for GBAS integrity analysis. This paper proposes an automated velocity estimation algorithm for anomalous ionospheric fronts. To examine the performance of this automated algorithm, we obtained estimation results for the points of the current Conterminous U.S (CONUS) threat space and compared these estimates to those manually computed previously. This new algorithm proposed in this paper is shown to be robust to faulty measurement and modeling errors. In addition, this algorithm is used to populate the current threat space with newly-generated threat points obtained from the Long-Term Ionospheric Anomaly Monitoring tool [4]. A larger number of velocity estimates helps to better understand the motion of ionospheric fronts under geomagnetic storm conditions.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2012,

title = {Results from Automated Ionospheric Data Analysis for Ground-Based Augmentation Systems (GBAS)},

author = {Jiyun Lee and Sungwook Jung and Minchan Kim and Jiwon Seo and Sam Pullen and Sigrid Close},

url = {https://www.ion.org/publications/abstract.cfm?articleID=10029},

year  = {2012},

date = {2012-01-30},

booktitle = {Proceedings of the 2012 International Technical Meeting of The Institute of Navigation},

pages = {1451 - 1461},

address = {Newport Beach, CA},

abstract = {Extremely large ionospheric spatial gradients could cause potential integrity threats to Ground-Based Augmentation System (GBAS) users. The importance of understanding ionosphere behavior is not limited to cases of extreme ionospheric events. Broader knowledge of both nominal and anomalous ionospheric behavior would help improve the design and operation of GBAS. We developed an automated tool for long-term ionosphere monitoring to continuously monitor ionospheric behavior during the life cycle of GBAS. This paper presents the results obtained from processing ionospheric data using the automated tool. Pre-existing ionospheric storm data are processed to populate the current threat space with newly discovered ionospheric anomalies. Durations of ionospheric anomalies exceeding a threshold within a continuous arc are also investigated in this research. This tool also supplies broader statistical estimates of ionospheric behavior under all conditions. In this paper, we analyze day-to-day variations of typical ionospheric statistics observed from a dense GPS network. The results demonstrate that some correlation between the statistics and a geomagnetic index exists even on nominal days. The automated tool not only identifies gradients large enough to threaten GBAS users but also provides reliable ionospheric statistics.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2011d,

title = {Automated Front Speed Computation of Ionospheric Anomalies for Ground-Based Augmentation Systems},

author = {Jiyun Lee and Eugene Bang and Sungwook Jung and Sam Pullen},

year  = {2011},

date = {2011-05-17},

booktitle = {The 13th International Ionospheric Effects Symposium (IES2011)},

address = {Alexandria, VA},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2011cc,

title = {Enhancements of Long Term Ionospheric Anomaly Monitoring for the Ground-Based Augmentation System},

author = {Jiyun Lee and Sungwook Jung and Sam Pullen},

url = {https://www.ion.org/publications/abstract.cfm?articleID=9539},

year  = {2011},

date = {2011-01-24},

booktitle = {Proceedings of the 2011 International Technical Meeting of The Institute of Navigation},

pages = {930 - 941},

address = {San Diego, CA},

abstract = {Extremely large ionospheric gradients can pose a potential integrity threat to the users of ground-based augmentation systems (GBAS). A better understanding of the ionospheric behavior (not limited to that during extreme ionospheric activity) is important in the design and operation of GBAS to meet its integrity and availability requirements. A tool for long-term ionosphere monitoring was developed to build an ionosphere threat model, evaluate its validity over the system operation, monitor ionospheric behavior continuously, and update it when necessary. This paper presents the enhanced algorithms of long-term ionospheric anomaly monitoring and evaluates its performance using data from a ionospheric storm day, 20 November 2003, and a nominal day, 9 November 2004. The automation of data processing enables us to more accurately categorize ionospheric behavior under both nominal and anomalous conditions. This paper also demonstrates that the automated procedure of enhanced long-term ionosphere monitoring not only identifies gradients large enough to threaten GBAS users but periodically generates reliable statistics of ionospheric gradients under all conditions.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2010b,

title = {Estimation of GPS Receiver Inter-Frequency Bias for Operational Ionosphere Monitoring in GBAS},

author = {Jiyun Lee and Sungwook Jung and Eugene Bang},

year  = {2010},

date = {2010-11-01},

booktitle = {ENRI International workshop on ATM/CNS},

address = {Tokyo, Japan},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Lee2010,

title = {Long Term Monitoring of Ionospheric Anomalies to Support the Local Area Augmentation System},

author = {Jiyun Lee and Sungwook Jung and Eugene Bang and Sam Pullen and Per Enge},

url = {https://www.ion.org/publications/abstract.cfm?articleID=9372},

year  = {2010},

date = {2010-09-21},

booktitle = {Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010)},

pages = {2651 - 2660},

address = {Portland, OR},

abstract = {Extremely large ionospheric gradients can pose a potential integrity threat to the users of Local Area Augmentation System (LAAS), and thus the development of an ionospheric anomaly threat model is essential for system design and operation. This paper presents a methodology for long-term ionosphere monitoring which will be used to build an ionosphere threat model, evaluate its validity over the life cycle of system, continuously monitor ionospheric anomalies, and update the threat model when necessary. The procedure automatically processes data collected from external sources and networks and estimates ionospheric gradients at regular intervals. If extremely large gradients hazardous to LAAS users are identified, manual validation is triggered. This paper also investigates a simplified truth processing method to create precise ionospheric delay estimates in near real-time, which is the core of long-term ionosphere monitoring. The performance of the method is examined using data from the 20 November 2003 storm and the 31 October 2003 storm. It demonstrates the effectiveness of simplified truth processing within long-term ionosphere monitoring. From the case studies, the automated procedure successfully identified the two worst ionospheric gradients observed and validated to date.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Gao2009,

title = {Methodology and Case Studies of Signal-in-Space Error Calculation Top-down Meets Bottom-up},

author = {Grace Xingxin Gao and Haochen Tang and Juan Blanch and Jiyun Lee and Todd Walter and Per Enge},

url = {https://www.ion.org/publications/abstract.cfm?articleID=8697},

year  = {2009},

date = {2009-09-22},

booktitle = {Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009)},

pages = {2824 - 2831},

address = {Savannah, GA},

abstract = {Signal in space (SIS) errors are a major error source for the Global Positioning System (GPS). They are defined as any errors related to satellite transmission, mainly satellite position and clock errors. A better understanding and characterization of the signal in space errors are essential for GPS integrity, because the SIS errors are a metric to determine satellite outages or failures. The statistics of the SIS errors are an important factor to monitor the system performance in terms of integrity. We present two methods to calculate SIS errors. One is called top-down, which is based on high data rate dual frequency measurements obtained from the Wide Area Augmentation System (WAAS) or the National Satellite Test Bed (NSTB) networks. The SIS errors of a satellite are obtained by stripping off all non-SIS errors from the total pseudo-range error. We apply our algorithm to L1/L2 measurements now as an intermediate step to migrate to L1/L5 measurements when L5 signals are available in the future. The other way of characterizing SIS errors is the bottom-up method, which builds up the SIS errors by summing the satellite position errors and satellite clock errors, etc. The satellite position and clock errors are calculated by differentiating broadcast and precise ephemerides obtained from the International GNSS Service (IGS) network and the National Geospatial Intelligence (NGA) network, respectively. The top-down and bottom-up methods well complement each other. In the second part of the paper, we apply the top-down and bottom-up methods to two actual satellite outages from 2007. The results show that the two methods match well no matter whether a satellite is faulted or not. The discrepancies of the two methods are currently within +/- 4 meters and are independent of the carrier smoothing filter length.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Ramakrishnan2008,

title = {Targeted Ephemeris Decorrelation Parameter Inflation for Improved LAAS Availability During Severe Ionosphere Anomalies},

author = {Shankararaman Ramakrishnan and Jiyun Lee and Sam Pullen and Per Enge},

url = {https://www.ion.org/publications/abstract.cfm?articleID=7694},

year  = {2008},

date = {2008-01-28},

booktitle = {Proceedings of the 2008 National Technical Meeting of The Institute of Navigation},

pages = {354 - 366},

address = {San Diego, CA},

abstract = {The Local Area Augmentation System (LAAS) is a ground-based differential GNSS system designed to provide precision approach for aircraft landing at a LAASequipped airport. While most anomalies affecting the system can be mitigated in the range domain, positiondomain geometry screening is essential to mitigate threats from anomalous ionosphere spatial gradients. These can potentially cause large range-domain errors before detection by the LAAS Ground Facility (LGF). Existing algorithms for position-domain screening inflate the sigma values (óvig and ópr_gnd) broadcast by the LAAS Ground Facility (LGF). This ensures that subset satellite geometries (i.e. subsets of a set of approved GPS satellites for which the LGF broadcasts valid corrections) for which unacceptable errors can result are made unavailable to the user. These unsafe subsets are found by comparing the resulting Maximum Ionosphere-Induced Error in Vertical (MIEV) with maximum “safe” navigation system error (NSE) values derived from Obstacle Clearance Surface (OCS) applicable to CAT I precision approaches. Recent analyses of past ionosphere spatial gradients observed over the Conterminous United States (CONUS) resulted in very high maximum gradients for both low and high-elevation satellites. The new ionosphere anomaly “threat model” for LAAS CAT I specifies a maximum spatial gradient of 375 mm/km for low-elevation satellites (below 15o) while high-elevation (above 65o) satellites can experience gradients as high as 425 mm/km. Uniform inflation of the broadcast sigmas for all approved satellites results in a significant drop in system availability under the new threat model. To minimize this decline, this paper proposes a new algorithm to implement position-domain screening by inflating satellite-specific, targeted ephemeris decorrelation parameters (called “P-values”) and ópr_gnd values. Availability is assessed for ten major airports in the USA. Under normal conditions, 100% availability is achieved for eight airports, while availability for the two remaining airports exceeds 99%. Targeted inflation consistently results in better system availability compared to strategies that inflate all satellites by the same amount, such as the óvig approach.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}

@conference{Park2007,

title = {Data-Replay Analysis of LAAS Safety during Ionosphere Storms},

author = {Young Shin Park and Godwin Zhang and Sam Pullen and Jiyun Lee and Per Enge},

url = {https://www.ion.org/publications/abstract.cfm?articleID=7455},

year  = {2007},

date = {2007-09-25},

booktitle = {Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007)},

pages = {404 - 414},

address = {Fort Worth, TX},

abstract = {	As reported in [2,4,5], previous Stanford research has identified the potential for severe ionosphere spatial gradients to affect Local Area Augmentation System (LAAS) integrity. In previous work [1], real-time position-domain geometry screening was used to maximize LAAS availability in the presence of ionosphere anomalies by broadcasting an inflated value of ó vig so that the maximum-ionosphere-induced-error-invertical (MIEV) for all viable airborne “subset” geometries (subsets of the set of satellites visible to and approved by the LGF) is below a pre-determined safe limit. The results of this work are based on the LAAS ionosphere spatial-gradient “threat model” established and validated with ionosphere storm data observed from WAAS and IGS since 2000 [2,4]. This previous approach leads to marginal availability of the required integrity (95 to 99 percent) and does not give the higher availability that is desired (99.9 percent or higher). In this paper, data from the Ohio cluster of CORS stations on November 20, 2003 and the North Carolina cluster of CORS stations on October 29, 2003 from [6] are used to perform “data-replay” analysis for several independent station pairs with separations from 23 to 75 km. These separations are significantly further than the effective LAAS user-to-LGF separation at the CAT I decision height. Comparisons of the result of data-replay analysis with the result of worst-case simulation in the manner of [1] are made. The conclusion derived from these comparisons is that CAT I ionosphere analysis performed by worst-case simulation is conservative but not unreasonable.},

keywords = {},

pubstate = {published},

tppubtype = {conference}

}