Development of a European Ionosphere Threat Model in Support of GBAS Deployment

Emilien Robert, Petr Jonas, Jonathan Vuillaume, Daniel Salos, Louis Hecker, Philippe Yaya

Abstract: GBAS is a Ground Based Augmentation System for GNSS that provides precision approach operations to aircraft. The main vulnerability of this system is the ranging error caused by the ionospheric propagation delay, which is mitigated by the development of local ionosphere threat model. EUROCONTROL launched a project in 2012 to develop such a model for Europe in support of current and future GBAS implementations. First, the appropriate GNSS data have been selected and retrieved. Based on existing networks of ground receivers, several clusters of GNSS ground receivers have been selected. This selection process was a trade-off between a reasonable number of ground stations on one side, and the need to cover the European latitudes but also to have the smallest distances between ground stations on the other side. As a result, 14 clusters were selected, located in the Canary Island (Spain, 28°N), Madrid (Spain, 40°N), Corsica (France, 42°N), Toulouse (France, 43°N), Friuli region (Italy, 46°N), Paris (France, 48°N), Prague (Czech Republic, 50°N), London (United Kingdom, 51°N), Hamburg (Germany, 53°N), Malmö (Sweden, 55°N), Göteborg (Sweden, 57°N), Stockholm (Sweden, 59°N), Lulea (Sweden, 65°N), and Gällivare (Sweden, 67°N). Data were retrieved from a total of 215 ground receivers and from October 8th, 2012 to March, 31th, 2016. Stanford to define the CONUS threat model and provided to EUROCONTROL by the FAA has been used to process the GNSS data. This tool has been largely modified and complemented with additional functions to match the European specificities. The GNSS raw data processing has been divided into different steps. The first step processes GNSS raw measurement stored in RINEX files and performs various corrections to compute “true” ionosphere delay per line of sight. In order to improve the raw measurement processing, an innovative technique based on a statistical analysis has been developed to correct for the cycle slips. The second step estimates the ionosphere gradients for every pair of ground stations within a cluster and sorts them in “nominal” gradient if the gradient is lower than a threshold, “anomalous” otherwise. “Nominal” gradients are then plotted together in a complementary cumulative histogram. “Anomalous” gradients are scanned through various consistency checks and a third step computes the following additional characteristic for the consistent gradients: the ionosphere front speed, direction and width. The last step is the manual validation of the processing. Although various and efficient correction and consistency checks are applied to the raw measurement, the processing needs to be manually validated to separate the “anomalous” gradients coming from measurement artefact (“fake” gradients) with the “anomalous” true gradients coming from ionosphere spatial variation. The various raw measurement corrections and consistency checks have been gradually improved through the development of several versions of the tool in order to reduce the number of measurement artefact detected. Additionally, specific tools have been developed to reduce the required effort to manually validate a gradient. The combination of these developments allowed the manual validation of a very large number of ionosphere gradients. All data from October 8th, 2012 to October, 1st, 2017 have been processed, and a total of 15340 potential gradients have been manually validated out of which, 1561 were true ionosphere gradients. The largest measured ionosphere gradient has a slope of 403 mm/Km, which remains lower than the boundary of the threat model developed for the Contiguous United State. However, this event has been measured at very low latitude from the Canary Islands cluster, and GBAS users at mid-latitude may use a different boundary. The ionosphere gradient location analysis has shown that most of them are occurring either at low latitude (below 35°N) or high latitude (above 60°N). In addition, the analysis of the gradient time of occurrence has shown that most of the gradients occurred around the equinoxes and during local night hours. These characteristics are very consistent with typical ionosphere behaviour and reinforce the confidence in the overall data processing methodology.
Published in: 2018 IEEE/ION Position, Location and Navigation Symposium (PLANS)
April 23 - 26, 2018
Hyatt Regency Hotel
Monterey, CA
Pages: 1181 - 1190
Cite this article: Robert, Emilien, Jonas, Petr, Vuillaume, Jonathan, Salos, Daniel, Hecker, Louis, Yaya, Philippe, "Development of a European Ionosphere Threat Model in Support of GBAS Deployment," 2018 IEEE/ION Position, Location and Navigation Symposium (PLANS), Monterey, CA, April 2018, pp. 1181-1190.
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