Youngsun Yun

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.

Automated determination of fault detection thresholds for integrity monitoring algorithms of GNSS augmentation systems

The reference stations of GNSS augmentation systems including GBAS (Ground Based Augmentation System) and SBAS (Space Based Augmentation System) are designed to provide correction and integrity information for airborne users so that they can estimate the positions accurately and reliably using GNSS signal measurements. Therefore the integrity monitoring algorithms of the reference stations should detect and exclude GNSS signals with faults or anomalies which could cause user position errors. Normally, the detection thresholds are determined based on the nominal data collected during the site assessment and installation process assuming that there will be no critical environmental changes that impact on the test statistics of integrity monitoring algorithms. However, the characteristics of the test statistics could change in case of new constructions or RF interference nearby, which could induce higher continuity and integrity risk than allocated to the monitor. In order to prevent it and establish updated thresholds, this paper proposes an automated fault detection threshold determination procedure and environment change indicators. Examples with real GPS data show that the proposed procedure detects the environment change and determines new thresholds successfully.

GBAS Reference Receiver Clock Adjustment Effects on Continuity and Integrity Performance

Ground-based augmentation system (GBAS) reference stations provide pseudo-range corrections (PRCs) and range rate corrections (RRCs) for visible GPS satellites along with the standard deviations of the PRC errors (σpr_gnd) and B-values for the reference receivers. When generating PRCs, a GBAS should adjust the reference receiver clock bias to limit the PRC values within a certain range specified by international standards. Since σpr_gnd should reflect the PRC errors accurately for system integrity, it must consider the effect of the clock adjustment procedure. Nevertheless, this has hardly been researched, since it is well known that the clock adjustment does not impact on the user position solution. This paper notices that σpr_gnd is also used for GBAS integrity monitoring functions which are implemented in range domain, not position domain. Thus, the work presented here investigates continuity and integrity performance risk in range domain in case a GBAS estimates σpr_gnd ignoring the clock adjustment procedure and uses it for integrity monitors. The work presented here analyzes the risk by a simulation and proposes strategies for σpr_gnd establishment considering the clock adjustment effects to prevent system performance risk.

Statistical Test for Performance Evaluation of Code Carrier Divergence Detection at a GBAS Reference Station

In order to provide precision approach service based on GBAS, air navigation service providers should validate the GBAS system performance against international standard requirements and receive approval. The GBAS manufacturer should define integrity threat, analyze the integrity monitors and evaluate performance of the implemented monitors to verify integrity performance which is critical to the aircraft safety. This paper describes a statistical test result to evaluate performance of the code carrier divergence monitors implemented in KARI Integrity Monitor System software. The paper introduces analysis and test procedure which is developed for the performance evaluation and describes the analysis, the test scenario generation and the test results. The results show that the implemented monitors can detect the expected minimum detectable errors with satisfying the probability of missed detection requirements.

Verification Test for GBAS Correction Information of KARI IMT

Korea Aerospace Research Institute (KARI) has implemented an integrity monitor testbed (IMT) to provide archived GPS data and test results for integrity monitoring algorithm development. To verify that the system is implemented based on international standard requirements, this paper represents the basic functional verification test results of the implemented testbed as a GBAS reference station. It compares the IMT generated GBAS message fields with those of PEGASUS, which is a baseline toolset accepted by international GBAS experts, to show the validity of the correction information. It also verifies the integrity and availability of the system through analysis on GBAS user data in the range and position domain.