Ground Based Augmentation System Research Articles

A comprehensive assessment of ionospheric gradients observed in Ecuador during 2013 and 2014 for ground based augmentation systems

Ground Based Augmentation Systems GBAS provide differential corrections to approaching and landing aircrafts in the vicinities of an airport. The ionosphere can introduce an error not accountable by those differential corrections, and a threat model for the Conterminous United States region CONUS was developed in order to consider the highest gradients measured. This study presents the first extensive analysis of ionospheric gradients for Ecuador, from data fully covering 2013 and 2014 collected by their national Global Navigation Satellite System GNSS monitoring network (REGME). In this work it is applied an automated methodology adapted for low latitudes for processing data from dual frequency receivers networks, by considering data from all available days in the date range of the study regardless the geomagnetic indices values. The events found above the CONUS threat model occurred during days of nominal geomagnetic indices, confirming: (1) the higher bounds required for an ionospheric threat model for Ecuador, and (2) that geomagnetic indices are not enough to indicate relevant ionospheric anomalies in low latitude regions, reinforcing the necessity of a continuous monitoring of ionosphere. As additional contribution, the events database is published online, making it available to other researchers.

Optimized GNSS Station Selection to Support Long-Term Monitoring of Ionospheric Anomalies for Aircraft Landing Systems

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

Ionospheric Gradient Threat Mitigation in Future Dual Frequency GBAS

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

GBAS Availability Assessment and Modeling of Ionospheric Scintillation Effects

The Japan Aerospace Exploration Agency (JAXA) is promoting a research and development program named DREAMS (Distributed and Revolutionary Efficient Air-safety Management System) to contribute to the realization of next-generation air transportation systems. One of the most important targets of this program is a reliable, high-precision navigation system. This paper describes an investigation into the effects of ionospheric plasma bubbles on the availability of Ground-based Augmentation Systems (GBAS). Although the effects of ionospheric anomalies on GBAS integrity have been well researched, their effects on GBAS availability have not received much attention. Since ionospheric plasma bubbles occur much more frequently than the Storm-enhanced Density (SED) phenomenon in regions at low magnetic latitude, their effects on availability as well as on integrity should be investigated. Copyright © 2016 Institute of Navigation

Code carrier divergence monitoring for dual-frequency GBAS

The ground-based augmentation system (GBAS) includes a ground monitor designed to protect against a code carrier divergence (CCD) fault originating from the satellite payload. The current single-fr...

GBAS Ionospheric Anomaly Monitoring Based on a Two-Step Approach.

As one significant component of space environmental weather, the ionosphere has to be monitored using Global Positioning System (GPS) receivers for the Ground-Based Augmentation System (GBAS). This is because an ionospheric anomaly can pose a potential threat for GBAS to support safety-critical services. The traditional code-carrier divergence (CCD) methods, which have been widely used to detect the variants of the ionospheric gradient for GBAS, adopt a linear time-invariant low-pass filter to suppress the effect of high frequency noise on the detection of the ionospheric anomaly. However, there is a counterbalance between response time and estimation accuracy due to the fixed time constants. In order to release the limitation, a two-step approach (TSA) is proposed by integrating the cascaded linear time-invariant low-pass filters with the adaptive Kalman filter to detect the ionospheric gradient anomaly. The performance of the proposed method is tested by using simulated and real-world data, respectively. The simulation results show that the TSA can detect ionospheric gradient anomalies quickly, even when the noise is severer. Compared to the traditional CCD methods, the experiments from real-world GPS data indicate that the average estimation accuracy of the ionospheric gradient improves by more than 31.3%, and the average response time to the ionospheric gradient at a rate of 0.018 m/s improves by more than 59.3%, which demonstrates the ability of TSA to detect a small ionospheric gradient more rapidly.

Estimation of overbound on ionospheric spatial decorrelation over low‐latitude region for ground‐based augmentation systems

In ground-based augmentation systems (GBAS), such as local area augmentation system (LAAS) of USA, variation of ionospheric delay between LAAS ground facility and the user leads to residual ionospheric error. Hence, the users need to compute the bounds on residual ionospheric error with the help of a parameter, σvertical_ionospheric_gradient (σvig). As σvig is a function of geographic region, the estimated value of σvig for mid-latitude regions (4 mm/km) is not appropriate for low-latitude regions where ionosphere is highly varying and is characterised by large spatial gradients. In this study, gradients are computed with processed total electron content data from 16 stations of global positioning system aided geo augmented navigation network and using the time-step method. The distribution of gradients is compared with one-sigma Gaussian distribution to obtain ‘inflation factor’. σvig overbound is computed for several days data comprising of quiet, moderate and storm conditions of ionosphere using the corresponding inflation factors. It is found that overbound values of 4.5 and 14.4 mm/km are required to bind the gradients under quiet and active conditions. These findings are useful for designing a safe and reliable GBAS for low-latitude stations.

3-D Ionospheric Plasma Bubble Model for Ground-Based Augmentation System

The ground-based augmentation system (GBAS) is now an important system for assisting the global navigation satellite system (GNSS) based aircraft navigation during landing phases. Since the ionospheric irregularity is one of the most serious problem in the high-precision GBAS, the impacts of ionospheric irregularities to GBAS in many countries need to be studied before actual installation and operation. However, most of previous studies are based on the rare ionospheric storm events observed in US in the mid-latitude region. For Thailand, which is located in equatorial and low-latitude region, the ionospheric irregularity known as plasma bubble is a common phenomenon after sunset, considered to have adverse impact to the integrity of GBAS operation. In this paper, we propose a simple 3-D ionospheric plasma bubble model for studying its impacts on GBAS operation in Thailand. The background electron density generated by the NeQuick2 model combined with the rectangular depletion are used in the near real-time simulation.

Analysis of Ionospheric Irregularity Observed near Suvarnabhumi International Airport in Thailand

The ionospheric irregularity is a major effect to the radio signals since the irregular electrons density can degrade the performance of the Ground-Based Augmentation System (GBAS). This work proposes the methods to calculate three parameters in the ionospheric threat model: front speed, front slope, and front width. The data of PRN9 satellite at three reference stations near Suvarnabhumi international airport, Thailand: KMITL, STFD and DPT stations, on 25th September 2012 are analyzed. The results show that the front speed is about 142.86 meters/second, the maximum front slope is-30 mm/km and the front width is higher than 25 kilometers.

Ionospheric bias detection in carrier phase–only ground-based augmentation systems

This work analyzes the design and performance of a planar ionospheric gradient monitor, based on differential carrier phase–only measurements between the elements of an antenna array. Small ionospheric disturbances are one of the potential threats to aerospace navigation and have to be monitored to prevent an excessive degradation of the position estimates. A carrier phase–based ionospheric gradient monitor enhances bias detection capabilities with respect to monitors that only exploit pseudorange measurements, provided that the ambiguous nature of the phase observations and the measurement error statistics are correctly handled. In this work we introduce a statistical test for detecting small ionospheric gradients, and we derive the analytical expression of the test probability distribution function for single- and two-baseline antenna arrays. A theoretical evaluation of the performance of the ionospheric gradient monitor is provided through inspection of the associated acceptance region, which defines the magnitudes of ionospheric gradient that cannot be detected with given integrity requirements. Due to the integer ambiguity of the carrier-phase measurements, multiple undesirable acceptance regions are produced.We provide a method to reduce the number and size of such regions.

Improvement of INS-Aided GPS Tracking Performance under Strong Ionospheric Scintillation

Ionospheric anomalies are major error sources that affect the performance of the GPS (Global Positioning System). In particular, ionospheric scintillation may cause GPS satellite signal loss, and multiple losses can degrade the availability of positioning/navigation services such as GBAS (Ground-Based Augmentation Systems). In order to improve the tracking performance of GPS receivers under scintillation conditions, a tracking loop aided by INS (Inertial Navigation System) has been developed, and flight tests to evaluate it were conducted around the island of Ishigaki where ionosphere scintillation frequently occurs. Phase tracking performance evaluation was carried out off-line using a software-defined GPS receiver which processes stored IF (Intermediate Frequency) data. The use of INS enabled a longer integration time of GPS signals, which increased the signal-to-noise ratio (SNR). As a result, the INS-aided tracking loop achieved continuous phase tracking even under strong scintillation conditions. Copyright © 2015 Institute of Navigation.

GBAS airport availability simulation tool

A global navigation satellite system augmentation system availability analysis tool has been developed to simulate a ground-based augmentation system (GBAS) prototype, an integrity monitor test bed, to evaluate its operational benefits at an airport of interest. The proposed availability simulation tool includes all GBAS ground facility algorithms as well as a graphical user interface that allows the user to modify simulation options and parameters. The output of the simulation tool is presented in a Stanford chart to help visualize the performance. The chart indicates the availability and integrity. The performance is evaluated primarily in the vertical position domain because of the weaker satellite geometry and more stringent required navigation performance as compared to those of the horizontal position domain. The simulation tool is implemented in Qt (http://www.qt.io/), an open-source cross-platform toolkit, allowing the tool to run on various devices. The computations are performed in the associated C++ code. The Newark Liberty International Airport (ICAO code: KEWR) is used as a simulation example to demonstrate the utility of the developed tool for investigating how reduced error models impact GBAS availability at the airport.

Flight test evaluation of ILS and GBAS performance at Gimpo International Airport

We address the differences in the ways the Instrument Landing System (ILS) and Ground-Based Augmentation System (GBAS) offer approach guidance, and their principles and methods. We perform comparative analysis of the accuracy of deviation between the GBAS Landing System (GLS) and ILS by means of flight tests, using the flight inspection aircraft at Gimpo International Airport. Our results show that the ILS deviation error increases as the distance between the threshold of runway and the aircraft increases; on the other hand, the GLS deviation error is stable, within the range of ±0.5 to ±2 m lateral and vertical deviation, respectively. We also analyze the alignment accuracy of the ILS and GLS based on the evaluation criteria of Category I of the Federal Aviation Administration, and our analysis results confirm that both systems satisfy the requirements (lateral: ±0.1°, vertical: ±0.05°).

김포국제공항의 GBAS 지상시험 및 성능 분석

위성항법지역보강시스템(GBAS)은 항공기 정밀접근을 지원하는 차세대 항행안전무선시설로, 최근 GBAS 설치 및 서비스를 제공하는 공항들이 전 세계적으로 증가하고 있다. 한국도 2013년 김포국제공항에 국내 최초로 미국 Honeywell사의 GBAS 지상장비인 SLS-4000을 설치하였으며, 지상시험을 통해 설치된 장비의 기능 및 성능을 점검하였다. 본 논문에서는 GBAS 지상시험에 대한 국내 GBAS CAT-I 시험평가 기준 및 방법을 소개하고, 김포국제공항에서 진행된 GBAS 시험평가 방법에 대해 기술하였다. 또한 GBAS 지상시험의 12개 시험항목 중 주요 5개 시험항목에 대한 상세한 시험평가 방법 및 분석 결과를 기술하였다. Ground based augmentation system (GBAS) is a next generation radio navigation aids to support precision approach of aircraft. Recently, airports installing GBAS and providing GBAS service are increasing all over the world. For the first time in Korea, SLS-4000 which is the GBAS ground equipment of Honeywell had been installed at Gimpo International Airport in 2013, and evaluated its functionality and performance of through the ground testing. This paper introduces a ground test and evaluation criteria on the CAT-I GBAS system, and describes testing methods for GBAS ground testing of Gimpo International Airport. In addition, detail testing methods and analysis results on major five of 12 ground test items are described.

비행검사용 항공기를 이용한 김포국제공항 GBAS 비행시험 및 성능평가

Ground Based Augmentation System(GBAS) is a system that offers an aircraft within 23 NM radius from the airport precision positioning service and precision approach service using the concept of Differential Global Positioning System(DGPS). After GBAS ground equipment installing at the airport, functionalities and performances of GBAS should be verified through the GBAS ground and flight testing. This paper describes the methods and results for GBAS flight test using the flight inspection aircraft at Gimpo International Airport. From the test results, we confirmed that the VDB data was received without misleading within the VDB coverage of Gimpo International Airport, and VDB field strength, protection level, and course alignment accuracy met the evaluation's criteria.

GBAS ionospheric anomaly monitoring strategy using Kullback-Leibler divergence metric

A significant anomaly of great importance for ground-based augmentation systems (GBAS) is the steep gradient in ionospheric delays caused by rare ionospheric storms. To avoid such hazardous errors, code-carrier divergence (CCD) monitors have been applied to support the use of GBAS in Category I (CAT I) operation. However, to meet the stricter requirements for CAT II/III operation, it may be necessary to greatly improve the speed with which existing CCD monitors detect ionospheric gradients or develop an innovative divergence monitor. One way of obtaining stricter integrity is to detect anomalous divergence behavior more rapidly than the CCD monitor does. Therefore, we attempted to devise a divergence monitor in which the detection time is reduced by using the Kullback-Leibler information. In simulations and experimental results, we demonstrate that the proposed divergence measure is quite capable of detecting ionospheric anomalies that are hazardous to GBAS and is a very promising alternative for the detection of deviations. A comparison with existing CCD monitors shows its increased fault detectability, which enables it to sense even small ionospheric gradients more rapidly than typical monitors.

GBAS 국내 운용을 위한 승인 절차 및 기준 연구

Ground-Based Augmentation System (GBAS) is providing precision approach and landing service with aircraft around airport. FAA granted System Design Approval (SDA) of SLS-4000 to Honeywell as the first GBAS category I system on September 2009. Since then, according to their own kind of approval process including System Design Approval, Facility Approval and Operational Approval, USA, Germany, Spain and Australia have approved GBAS category I system which are installed in some airports in order to provide commercial GBAS service. Recently, KARI has also installed GBAS category I system into Gimpo international airport to establish operational technology of GBAS domestically and to validate effectiveness of GBAS system in Korea. This paper introduces overseas trends and activities regarding approval process of GBAS system and presents approval process and criteria appropriate for future commercial operation of GBAS in Korea.

Distributed ionosphere monitoring by collaborating mobile receivers

This paper presents a novel ionospheric anomaly monitoring method for ground-based augmentation systems, based on a network of collaborative airborne dual-frequency receivers. The method allows aircraft to navigate safely using a single frequency or divergence-free position solution, methods that are more accurate than the ionosphere-free position solution. Each receiver measures ionospheric delays; measurements are shared among collaborators to estimate the local ionospheric gradient, which may be different for each satellite during a storm.

GBAS ionospheric threat model assessment for category I operation in the Korean region

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

GBAS ground monitoring requirements from an airworthiness perspective

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