Detecting Ionospheric Threat for GBAS Using a Spatial-temporal Method

P. Zhao, Jun Zhang, R. Xue

Abstract:	Ground Based Augmentation System (GBAS) has been developed to augment the performance of Global Navigation Satellite System (GNSS) for the application of approaching and landing in civilian aviation to meet the stringent requirements of safety. In a GBAS prototype, navigation accuracy is improved by the local differential technology, and navigation integrity is secured by the integrity monitoring technology. GBAS broadcasts the differential corrections to the aircraft to reduce the space domain errors made by ephemeris, ionosphere and troposphere. The common assumption is, the aircraft and local ground facility (LGF) share the same propagation and ephemeris errors within several tens of kilometers distance. The ephemeris and troposphere errors always keep high correlation between aircraft and LGF. However, the ionosphere error does not follow the assumption when ionosphere anomalies occur, such as scintillation, disturbance and storm. Those anomalies affect navigation signal propagation in space with varying degrees, leading to a non-stationary, unpredictable and even immeasurable ionosphere delay, and finally degrading the performance of GBAS, becoming the most hazardous factor in precision approaching and landing. According to the empirical data that has been obtained by the Continuously Operating Reference Stations (CORS), gradients in slant ionospheric delay of as large as 425 mm/km (4mm/km normally) has been observed, which means more than 20m vertical navigation error could be made without detection [2]. Therefore, detecting ionosphere anomalies and estimating its impact becomes a necessary point for navigation integrity. In order to effectively detect ionospheric anomaly to ensure the integrity of GBAS with sustainable computation load, this paper will consider the attributes of the fault, analyze the existing methods and propose a novel spatial-temporal method for GBAS ionospheric anomaly detection. This paper includes six sections and the corresponding contents are as follows: In section 1, the background information about the conventional GBAS ionospheric anomaly detecting methods is given. Ionospheric anomaly threatens the integrity of GBAS during the aircraft approaching and landing. Developing an effective method for ionospheric anomaly detection without adding the computation load is very important to the application of civilian aviation. In section 2, the attributes of GBAS ionospheric anomaly are analyzed in detail, leading to build a model of moving wedge [1], of which the key parameters are width, slope and moving speed. The two attributes of GBAS ionospheric anomaly are spatial gradient and temporal gradient. The spatial gradient anomaly means the slant ionospheric delay over baselines of tens of kilometers has beyond the threshold. The temporal gradient anomaly means the change rate of the ionospheric delay on the ionospheric pierce point (IPP) has become abnormal. According to the data obtained from CORS and WAAS, researchers have built the wedge model so as to describe the GBAS ionospheric anomaly. Further, the fundamental theories and the mathematical model will also be introduced in this section. In section 3, the conventional detection methods are introduced and analyzed in detail. Many methods have been developed according to the spatial and temporal attributes. The representations is Delay-based method and Rate-based method. Delay-based method identifies the threat through detecting the differences of ionosphere delay between aircraft and LGF. This method suffers so large noise that it cannot work in most conditions. Rate-based method was proposed based on the ionospheric temporal gradient anomaly. The noise influence of this method can be ignorant. However, when the mobile receiver synchronizes with the ionosphere storm, the change rate of the ionospheric delay will be zero and cannot be detected. Advantages and disadvantages of the conventional methods according to the two attributes will be analyzed in detail so as to lead to the novel method. The integrity risk of these methods will also be analyzed in this section. In section 4, the novel method which combines the advantages of spatial detecting method and temporal detecting method will be presented, named spatial-temporal method. To avoid miss detection due to the defect of rate-based method, a spatial anomaly detecting method is implemented parallel. To reduce the impact of noise, Kalman filter is applied. The procedure of this algorithm will be presented in this section, and the simulation experiments will be carried out to compare our method and traditional method with aspects of detection threshold, accuracy and availability. In section 5, data that obtained from CORS is used to validate the usability of this method. CORS has hundreds of stations and therefore permits ionospheric measurement comparisons across baselines of tens of kilometers [3], thereby the data can be used to simulate the approaching process taking advantage of GBAS so as to examine the performance of the proposed method. We select data of the day that an ionosphere storm had happened, such as 20 Nov. 2003. Results relies on the empirical observation will show that this method can meet CAT I requirements even under the worst condition. Section 6 is the conclusion and the potential problems we will study. It is necessary for aviation application to prevent the risk brought by ionospheric anomaly for GBAS. This novel method proposed in this paper can detect the GBAS ionospheric threat and eliminate the impact effectively to meet the requirements of CAT I and ensure the availability of GBAS even in the worst case. [1] Hiroyuki Konno, Sam Pullen, Jason Rife, Per Enge: Ionosphere Monitoring Methodology for Hybrid Dual-Frequency LAAS, ION GNSS 19th International Technical Meeting of the Satellite Division, 2006. [2] Pullen, S., Y.S. Park, and P. Enge, Impact and mitigation of ionospheric anomalies on ground-based augmentation of GNSS. Radio Science, 2009. 44. [3] Datta-Barua, S., et al., Ionospheric Threat Parameterization for Local Area Global-Positioning-System-Based Aircraft Landing Systems. Journal of Aircraft, 2010. 47(4): p. 1141-1151.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1428 - 1434
Cite this article:	Zhao, P., Zhang, Jun, Xue, R., "Detecting Ionospheric Threat for GBAS Using a Spatial-temporal Method," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1428-1434.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In