An Investigation of Local-Scale Spatial Gradient of Ionospheric Delay Using the Nation-Wide GPS Network Data in Japan

This paper presents results from an investigation of ionospheric delay that might influence GBAS (GroundBased Augmentation System) especially with a focus to local-scale spatial gradient using the nation-wide dense GPS network in Japan (GEONET; GPS earth observation network), which currently consists of about 1,000 GPS stations with a typical separation of 20 km. GBAS is a system based on the differential GPS technique for aircraft precision approach and landing near and at the airport using C/A pseudorange. In general, ionospheric delay at user receivers will be removed simultaneously with the other error sources using differential correction dataset that is transmitted from a ground segment of GBAS. However, a large spatial gradient of ionospheric delay between ground GPS monitoring station and aircraft is now a major integrity risk for GBAS because it must protect any users within its service area anytime from positioning errors greater than a certain threshold. Therefore, we need to estimate localscale spatial and temporal variation of ionospheric delay exactly. To accomplish our purpose, we used ‘Total Electron Content (TEC) database’ provided by Kyoto University, Japan. We have already investigated local spatial gradient for both of S-N (South to North) and W-E (West to East) directions using TEC observations at co-located 4 stations within several 10 km along each of S-N and W-E direction extracted from one-year-data-set in 2001. The results in S-N gradient were consistent with ‘equatorial anomaly’ resulting in large TEC gradients toward South near Japan and W-E gradient represented almost daily variation [1]. In this paper, we further improved this local-scale analysis in time resolution and applied it to datasets of the other years and the other areas in Japan. First, we examined local spatial gradient with an original time interval of 30 seconds. Averaged over 10 minutes in order to reduce noise involved in the old analysis. Secondly, we applied this improved analysis to datasets of the densest area in 2001, 2002 and 2003 to examine relation of local spatial gradient to the ionospheric activity level, which became weaker. We also analyzed dependency of local spatial gradient on latitude by comparison of the northern