Ground Based Augmentation System Research Articles

Overbounding Ifree errors based on bayesian Gaussian mixture model for ground-based augmentation system

A dual-frequency measurement is employed in conjunction with an innovative Ifree filtering technique for mitigating the primary sources of Ifree influence on ground-based augmentation systems (GBAS) to safeguard the reliability of GBAS. The protective level achieved through the conventional Gaussian overbounding approach that are considered as much conventional technique. This adherence to tradition results in decreased reliability and a higher likelihood of false alarms. In contrast, the utilization of the Ifree algorithm contributes to reducing errors associated with dual-frequency measurements. This paper proposes the overbounding process according to Bayesian Gaussian mixture model (GMM) for maintaining Ifree-based GBAS range error. The Bayesian GMM is utilized for single-frequency model errors to examine the ambiguity estimations. The Monte Carlo (MC) simulation is established for defining estimated GMM assurance level accuracy which is attained through the general estimation method. Then, the last Bayesian GMM which is utilized for overbounding Ifree error distribution is investigated. According to the property of convolution invariance, the vertical protection in position field is determined without presenting difficult numerical calculations.

Improved Time-Step Method for Bounding Nominal Spatial and Temporal Ionospheric Gradients for Ground-Based Augmentation Systems in Hong Kong

Improved Time-Step Method for Bounding Nominal Spatial and Temporal Ionospheric Gradients for Ground-Based Augmentation Systems in Hong Kong

ASSESSMENT OF STEC ESTIMATION QUALITY USING GNSS PPP FIXED

Abstract. GNSS (Global Navigation Satellite System) data can be used for geodetic remote sensing, particularly for monitoring the ionosphere in the context of Space Weather. One of the important parameters derived from GNSS measurements for ionospheric analysis is the Slant Total Electron Content (STEC). By utilizing GNSS data from multiple frequencies or even a single frequency, the STEC can be computed using an appropriated linear combination, like geometry free. However, when computing an ionospheric gradient between two IPP (Ionospheric Pierce Point) from the same satellite, the precision of the STEC estimate can become a limiting factor. In some cases, the uncertainty in the estimate may be greater than the actual gradient value itself. This poses challenges, especially for augmentation systems like GBAS (Ground Based Augmentation System), where accurate ionospheric gradients are crucial. An alternative approach to improve these limitations is to estimate the STEC using a different approach, like Precise Point Positioning (PPP). For such case, the coordinates of the GNSS stations are constrained to known values (PPP-Fixed), while other parameters such as clock biases, tropospheric delays, and ionospheric delays (including STEC) can be estimated. The results of an experiment carried out to assess the quality of STEC for such application are presented and have shown good results. Ionospheric gradients are agreed in the mm level.

Assessment of Number of Critical Satellites for Ground-Based Augmentation System Continuity Allocation to Support Category II/III Precision Approaches.

The ground-based augmentation system (GBAS) is a regional system supporting navigation and ensuring the integrity of aircraft near airports during precision approaches. Standardized at the international level, GBAS Approach Service Types (GASTs) C and D, which are defined for the GPS L1 signal, support CAT I and II/III precision approaches with decision heights of 200 and 50 ft, respectively. However, the future GBAS, GAST E, which utilizes dual-frequency and multi-constellation signals, and the GAST D1, defined for both GPS L1 and Galileo E1 signals, require the establishment of standards. To define the continuity requirement, the number of critical satellites must be considered. Currently, there is a lack of analysis on the number of critical satellites for various GBAS service types available to the public. This paper aims to evaluate the number of critical satellites for future GBAS service types, employing optimized GPS and Galileo constellations and assessing all potential protection levels worldwide. The methodology to model the difference of position solutions using the 30 s and 100 s smoothing filters is presented in detail to compute the protection level for GASTs D and D1. The resulting number of critical satellites can be used to define the continuity allocation of future GBAS.

Optimization of Protection Level of GBAS with Gaussian Mixture Model

The Gaussian mixture model (GMM) is commonly used to model the heavy tail of the ground-based augmentation system (GBAS) range error distribution. In practice, Gaussian over-bounding based on a GMM is used to over-bound the heavy tail of the ranging errors, but the GBAS protection levels (PLs) based on the Gaussian over-bounding tend to be overestimated. Based on the idea of solution separation and overcoming the shortcoming of its direct reference to GBAS, this paper analyses the constraint conditions and objective functions of the optimal protection level based on solution separation under a GMM distribution, and proposes that multi-hypothesis solution set classification can effectively reduce the computational complexity. At the same time, least squares optimization and dynamic allocation of integrity risk are used to further reduce the protection level. This paper verifies the validity of the parameters of the GMM based on actual airport GBAS data, performs simulation verification of the typical scenarios of CAT I and CAT II/IIIa global GBAS under the Beidou 3 constellation, and analyses the performance improvement effect under different solution set traversal depths. The results show that when the traversal depths of CAT I and CAT II/IIIa are 4 and 6, the vertical protection level component of the ground ranging error is reduced by 14% and the total vertical protection level is reduced by 10%.

An Efficient Fault Detection and Exclusion Method for Ephemeris Monitoring

The ephemeris fault needs to be detected and mitigated in the ground-based augmentation system to provide precision approach for the aircraft. In the current fault detection and exclusion (FDE) method, the double-differenced carrier phase (DDCP) observation is used as a test statistic to detect a faulty satellite caused by an ephemeris fault, taking advantage of the residual spatial gradient. However, the current FDE method cannot distinguish whether the fault comes from a reference satellite (RS) or a non-reference satellite (NRS) in DDCP. One way to address this issue is to pre-validate the RS before it can be used to form a DDCP test statistic for detecting ephemeris fault on the NRS. The RS is pre-validated using the previous ephemeris for any newly acquired and re-acquired satellite. This method is developed in detail to present the shortcomings. A more efficient FDE method using multiple hypothesis testing to detect ephemeris fault on both the RS and NRS simultaneously in real time is proposed. Moreover, to facilitate the application in integrity monitoring, the test risks and minimum detectable error are analyzed. The numerical results of the proposed FDE method show an improved performance in detecting ephemeris fault on the RS and a comparable performance on the NRS compared with the current FDE method.

Variability of Vertical Total Electron Content Gradients in the Brazilian Sector

Ground Based Augmentation Systems are designed to meet demanding requirements during airport approach and landing sections of aircraft routes. It computes corrected and smoothed differential corrections and other information. These messages are transmitted to aircraft, which may use the corrections to improve their position estimation. However, residual correction errors remain, due to ionospheric spatial and temporal gradients between Ground Based Augmentation System reference receivers and approaching aircrafts. Large vertical ionospheric delay gradients detected during combinations of extreme geophysical environments indicate that there are problematic conditions for Ground Based Augmentation System operation over the Brazilian region. Motivated by the above conditions, data from Rede Brasileira de Monitoramento Contínuo, the Brazilian public network of dual-frequency Global Navigation Satellite System receivers operated by Instituto Brasileiro de Geografia e Estatística were analyzed to estimate ionospheric vertical Total Electron Content gradients. The effects from different locations (represented by the dip latitude) and geophysical conditions (represented by solar activity, season, and local time) on variations of ionospheric vertical Total Electron Content gradients have been statistically quantified. The results from the present work provide information to more detailed performance assessments of Ground Based Augmentation System procedures under equatorial and low-latitude ionospheric regions, aimed at flexibly delimiting the conditions for feasible operation in such regions.

Application Scheme Design of BeiDou Ground-based Augmentation System for Guided Rockets

China’s BeiDou Ground-based Augmentation System (GBAS) improves the accuracy of BeiDou satellites navigation system to the sub-meter level. GBAS is widely used in the navigation fields, such as civil vehicles, ships, drones, which produces significant economic benefits and lays the foundation for application of high dynamic missile. According to the application requirements of guided rocket, this paper designed a high dynamic real time differential (RTD) based on GBAS, and proposed the composition, principle, working flow, missile equipment devices and ground terminal, etc. Through the design analysis, it is hoped that this paper can provide reference for high dynamic application of GBAS.

Flight Testing GLS Approaches Enabled by Wide Area Corrections in Kerkyra, Greece

Many airports with a high value to commercial air traffic have spatial or budgetary constraints which prevent the installation of a precision approach system. We previously designed a low-cost precision approach system which combines the advantages of both ground-based and satellite-based augmentation systems by using a converter between them in order to allow GAST-A approach types. We installed, operated, and flight-tested such a system at Kerkyra Airport using an A320 aircraft. During these, we recorded data from a commercial multimode receiver as well as GPS raw data in order to prove the feasibility of the system. Data were analyzed using the Pegasus toolset as well as a highly precise reference trajectory computed from postprocessed carrier phase data. The data recorded show excellent performance for approach guidance that is no different from that of the more expensive GPS landing system GLS and provides guidance in accordance with the localizer performance with vertical guidance standards. Our low-cost precision approach system can provide precision approach-like guidance to appropriately equipped transport aircraft. Kerkyra Airport is extremely limited in availability of usable surface area, such that conventional precision landing aids cannot be placed on airport property. The system provides the ground-based augmentation system approach service type A, a category defined in Annex 10 to the convention of Chicago. This category has not seen any operational use until now but offers an opportunity to provide precision approaches based on GLS where guidance down to a certain altitude will be sufficient.

Advanced Warning of Threatening Equatorial Plasma Bubbles to Support GBAS in Low Latitudes

In low-latitude regions such as Brazil, the single-frequency Ground-Based Augmentation System (GBAS) is not yet fully operational due to the influence of ionospheric variability. The most critical issue for GBAS is the occurrence of ionospheric gradients large enough to compromise the system's integrity. This can result in unsafe operation because a single GBAS ground station cannot identify all threatening gradients in real time. This paper develops and validates a strategy to detect and alert in advance the presence and effects of equatorial plasma bubbles (EPBs), which create the largest threat to GBAS operations in low latitudes. This alerting system uses surrounding stations in the vicinity of the GBAS facilities being supported to monitor the ionosphere state in real-time and send alerts to the served GBAS facility when threatening conditions are detected on one or more tracked satellites. Time-step measurements of ionospheric gradients and data availability at these surrounding stations are used to generate these alerts when needed. An architecture is proposed for such a system along with a complete validation of the method. Validation uses a reliable dataset from the peak of ionospheric Solar Cycle 24 containing post-processed station-pair gradients in time, amplitude scintillation indices, and ionospheric maps. All gradients from the dataset classified as threatening to GBAS operation were encompassed by the alerts issued by the real-time methodology within the adopted success criteria. Based on these results, an EPB alerting approach based on this method presented and validated in this paper can significantly enhance single-frequency GBAS integrity and availability in low latitudes.

Detection of GPS C/A Code Self-Interference: Monitor Overview and Applicability

<h3>Abstract</h3> Self-interference can cause large errors of up to tens of meters on the GPS C/A code pseudorange measurement. Although the probability of an occurrence of large self-interference errors is small, to enable the use of C/A code phase measurements in high accuracy or safety-of-life applications, a detection or mitigation method is needed. A stressful case of self-interference is modeled on a GPS hardware simulator to investigate monitor performance using real signal results. A summary of contributions in this paper follows. Self-interference pseudorange error characteristics within and mitigation requirements for the Ground Based Augmentation System (GBAS) Approach Service Type D (GAST-D) environment are identified. Existing monitors are evaluated for their response to self-interference and their ability to detect the error based on the GAST-D environment. This includes the potential for misidentification of the error. A novel Frequency Domain Cross-Correlation (FDCC) detector is proposed that can uniquely identify self-interference error with no siting constraints.

Multilayer Ionospheric Model Constrained by Physical Prior Based on GNSS Stations

The need for accurate modeling of the ionosphere plays an important role in the global navigation satellite system (GNSS) positioning. The traditional multi-layer VTEC model without prior has been used for modeling the ionospheric delay error. However, it is assumed that the electron density of the ionosphere is compressed into multiple thin layers at fixed heights in the lack of capturing ionospheric physics. In this paper, the data enhancement method by virtual observations is proposed to build the constrained multi-layer VTEC model to capture physical features from empirical ionospheric models. The extraction methods of physical knowledge have been developed by prior VTEC based on principal component analysis and model coefficients based on emulated basis function. The constrained multi-layer modeling has been verified based on simulation and real measurement of GNSS data in Yunnan, China collected from Qianxun Ground-based Augmentation System on November 3, 2021. The receiver DCB error estimated by the multi-layer model with prior constraint is significantly lower than that of the single-layer model and the traditional multi-layer model. The experimental test shows that the constrained multi-layer model achieves the accuracy of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0.5 \,\rm {TECU}$</tex-math></inline-formula> for the independent reference station. The dSTEC of the proposed two multi-layer models are significantly lower than those of the single-layer model for low elevation angles, and the RMSE of dSTEC is reduced by 63 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> with the cutoff elevation angle of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{\circ }$</tex-math></inline-formula> . The spatial distribution of the multi-layer VTEC model shows consistency with the tomography model to verify vertical feature capturing capability. Compared with the Un-differenced and Un-combined Precise Point Positioning (UCPPP) without ionospheric constraint, the multi-layer constrained model based on the test data improves the convergence time approximately by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$36.55\%$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$18.78\%$</tex-math></inline-formula> in the horizontal (H), up (U) directions, respectively. These results demonstrate that the proposed multi-layer models not only improve ionospheric delay estimation precision but also can obtain the VTEC distribution capturing the physical characteristics of the ionosphere. The proposed multi-layer models may be valuable for the ionospheric delay modeling of satellite navigation systems in harsh variable ionospheric conditions.

Characterization of the GNSS RFI Threat to DFMC GBAS Signal Bands.

This article presents analysis results from a long-term multi-site Global Navigation Satellite System (GNSS) Radio Frequency Interference (RFI) monitoring campaign in the context of Ground Based Augmentation System (GBAS) Dual Frequency Multi Constellation (DFMC) concept operation. GBAS resilience against unintentional RFI is an important area for investigation as the ground station receivers often must operate adjacent to high-traffic roads where chances of being affected by RFI are high. To be able to develop algorithms and reaction strategies necessary to ensure continuity and availability of service, knowledge of interference signal characteristics and frequency band/bands affected, as well as relative occurrence rates between the considered frequencies and frequency combinations, is necessary. The analysis presented in the article covers the prevalence and properties of the RFI events observed on the GPSs L1 and L5 and the Galileo E1 and E5a frequency bands that are considered by the on-going DFMC GBAS concept development initiatives. Due to being spectrally adjacent, the observed event analysis is also carried out for the Galileo E5b and GLONASS G1 frequency bands. The article also addresses the issue of spectral occupancy distribution of the observed events and presents new interesting RFI event types captured during the considered monitoring period.

Optimal Parameter Inflation to Enhance the Availability of Single-Frequency GBAS for Intelligent Air Transportation

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

A Novel Design of Flat-Top Beamforming Anti-Multipath Antenna

This article discusses a novel design method of wide-beam circular polarization array antenna for satellite navigation. For special applications such as satellite navigation guiding aircraft landing, high-performance ground reference station antennas are required. In the system of satellite navigation guiding aircraft landing, the antenna of satellite navigation receiver needs to have the following radiation characteristics: the shape of antenna pattern is close to an ideal hemisphere, the upper half of space has uniform right-hand circular polarization gain, and the beam has the characteristic of sharp cutoff below 5 degrees of elevation. Whether the GNSS reference station antenna can have a wider beam circular polarization characteristic to receive more navigation satellites signal; whether it can suppress the multipath effect to a greater extent to reduce the multipath error; and whether it can work in a wider frequency band, so that several major satellite navigation systems work together, the above factors directly affect the satellite navigation and positioning accuracy of the entire system. The beam of this antenna has the following feature: sharp cutoff near the horizon for multipath mitigation at low elevation angles. Comprehensive design thereby accelerates system design. It owns wide beam, good circular polarization characteristics, low elevation angle sharp cutoff, and ultralow sidelobe radiation characteristics that achieve the purpose of receiving satellite navigation signal and multipath. The low sidelobe characteristic of the antenna is beneficial to suppress multipath effect. According to the theoretical knowledge of the symmetrical dipole and the four-arm helical antenna, the simple structure of the symmetrical element is fully utilized, and the printed balun easily realizes the dual-frequency operation of the antenna. Combined with the wide-beam circular polarization characteristic of the four-arm helical antenna structure, the shaped beam array antenna is designed. The wide-beam circularly polarized antenna unit not only covers the frequency band of the satellite navigation system but also satisfies the system performance and key feature requirements of the array antenna for the unit; that is, it satisfies the wide-beam circular polarization characteristics of the upper half space. According to the linear array basic theory and array antenna synthesis theory, Woodward Lawson sampling synthesis algorithm is used to synthesize the array amplitude and phase weights corresponding to the specific antenna pattern required for the Ground-Based Augmentation System (GBAS) engineering. The shaped beam array antenna consists of 21 units, of which 11 antenna units are directly fed, and the feeding ports of the rest are dummy ports. It is an equally spaced array antenna. The spacing between antenna units is close to half wavelength. They are fed according to the value corresponding to the excitation function. The array antenna is simulated by HFSS (High-Frequency Structure Simulator). Compared with traditional satellite navigation antennas, the antenna in this paper has excellent front-to-rear ratio and flat-top beam pattern. The results show that the wide-beam circularly polarized array antenna meets the engineering requirements of GBAS applications and lays the solid foundation for GBAS engineering applications.

Research on accuracy performance evaluation technology of Beidou GBAS reference receiver

The ground based augmentation system (GBAS) uses ground subsystems including reference receivers to provide differentially correction Global Navigation Satellite System (GNSS) data to improve the overall accuracy and integrity of the system. The accuracy of the GBAS ground subsystem is measured in the pseudo-range domain, so it is necessary to perform pseudo-range domain accuracy testing of the GBAS ground equipment. Based on the B-value method, this paper presents the evaluation process of GBAS pseudo-range domain accuracy based on the BDS-3 new frequency points of B1C and B2a. The B-value methodology is based on broadcast pseudo-range correction data, which covers the differential correction data of all reference stations and considers ground system monitoring, and can reflect the impact of the multipath synthesis effect of different receiver combinations on the accuracy of the pseudo-range domain. The real-collected data shows that the built test system can reach a GAD-B3 accuracy level, and the B-value method can effectively evaluate the pseudo-range domain accuracy of BDS-3 GBAS reference receivers.

GBAS flight test integrity simulation evaluation

In order to improve the navigation performance of the conventional ground-based augmentation system (GBAS) Category I precision approach phase, the performance ver ification of the integrity technology of the GBAS based on BeiDou satellite navigation system is studied. The integrity and system pseudo-range error models of the CAT I precision approach based on BeiDou are investigated, combined with the provisions in relevant standards and the calculation method of integrity. The experimental results show that the integrity data calculated by the system meets the CAT I precision approach integrity requirements of the Radio Technical Commission for Aeronautics specification and provides a timely warning when anomalies occur in the pseudo-rang error model through changes in the protection level.

Construction of nominal ionospheric gradient using satellite pair based on GNSS CORS observation in Indonesia

Ground-Based Augmentation System (GBAS) is a GNSS augmentation system that meets International Civil Aviation Organization (ICAO) requirements to support precision approach and landing. GBAS is based on local differential GNSS technique with reference stations located around an airport to provide necessary integrity and accuracy. The performance of the GBAS system can be affected by gradient in the ionospheric delay between aircraft and reference stations. A nominal ionospheric gradient, which is bounded by a conservative error bound, is represented by a parameter σvig. The parameter σvig is commonly determined using station pair to GNSS Continuous Operating Reference Station (CORS) data. The station-pair method is susceptible to doubling of the estimation error of receiver inter-frequency bias (IFB) and is not suitable with the CORS conditions in Indonesia. We propose a satellite-pair method that is found to be more suitable for the CORS network over Indonesia which is centered in Java and Sumatra islands. An overall value of σvig (5.21 mm/km) was obtained using this method along with preliminary results of a comparison of σvig from Java and Sumatra islands.Graphical

Assessing Availability of GNSS-GBAS Landing Systems in GAST-D/F Performance

The Ground-Based Augmentation System (GBAS) has been recently approved as CAT II performance (GBAS Approach Type D (GAST-D)) for the precision approach operations, and by using the GPS signals only. But the requirements of CAT III performance (GAST-F) are tended to be approved using dual Constellation by adding the European Galileo system. In this research, the availability of CAT III was assessed using Galileo system. A simulation tool was used to estimate which level of integrity and accuracy is needed for CAT II and CAT III performances, considering the new innovated Binary Offset Carrier (BOC) modulation and the increased power of +6 dB in Galileo signals. The results showed a promising performance of Galileo over Europe space.

An Error Overbounding Method Based on a Gaussian Mixture Model with Uncertainty Estimation for a Dual-Frequency Ground-Based Augmentation System

To ensure the integrity of a ground-based augmentation system (GBAS), an ionosphere-free (Ifree) filtering algorithm with dual-frequency measurements is employed to make the GBAS free of the first-order ionospheric influence. However, the Ifree algorithm outputs the errors of two frequencies. The protection level obtained via the traditional Gaussian overbound is overconservative. This conservatism may cause false alarms and diminish availability. An overbounding framework based on a Gaussian mixture model (GMM) is proposed to handle samples drawn from Ifree-based GBAS range errors. The GMM is employed to model the single-frequency errors that concern the uncertainty estimation. A Monte Carlo simulation is performed to determine the accuracy of the estimated GMM confidence level obtained by using the general estimation approach. Then, the final GMM used to overbound the Ifree error distribution is analyzed. Based on the convolution invariance property, vertical protection levels in the position domain are explicitly derived without introducing complex numerical calculations. A performance evaluation based on a real-world road test shows that the Ifree-based vertical protection levels are tightened with a small computational cost.