Characteristics of Ionospheric Gradients in the Transition Region from Magnetic Low to Mid-latitudes for GBAS Implementation

Abstract

Ionospheric delay gradients are one of the most important integrity risks in Ground-Based Augmentation System (GBAS) because it is based on the differential GNSS (DGNSS) technique with L1 single frequency signals. Because the ionospheric threats have different primary sources especially for the magnetic latitude, the ionospheric threat model needs to be established by reflecting the local ionospheric conditions. The ICAO Asia-Pacific (APAC) region has defined the ionospheric threat model that can be used in the low magnetic latitude part of the region. However, the data used for construction of the model are restricted below 25 degree in north latitude, more detailed data analysis are needed. In lower magnetic latitude include Japan, plasma bubbles which occurs around magnetic equator region are considered as main cause of ionospheric threat. It is important to understand the behavior of ionospheric gradients which is induced by plasma bubbles in lower magnetic region to establish the optimization of ionospheric threat model for lower magnetic latitude region. Especially, to clarify the poleward limit of the “magnetic low latitude” region is very important in order to refine the availability of GBAS safety design. In addition, the characteristics of ionospheric gradients of transition region around magnetic low latitude to mid latitude is also important for the optimization of the ionospheric model. The ionospheric threat model for GBAS define the parameters of the ionospheric gradient (slope, width, depth and speed). Among those parameters, the slope of the ionospheric gradients has been studied relatively well, while there are few studies focusing on the other parameters. Especially, the slope and the width are very important parameters, because steep ionospheric gradients with small spatial scales may not be detected neither by GBAS ground stations nor by an airborne receiver when the gradient stands between the GBAS ground station and airplanes. In this study, we have analyzed the data from 2014 to 2017 and identified more than 10 cases of steep gradients for estimation of ionospheric gradients parameters in lower magnetic latitudes. The steep gradients were found mostly at 20 degree or less in magnetic latitude. The occurrences of the steep gradients decreased as the magnetic latitude increases. The speeds of steep gradients were around 100 m/s. The directions of the ionospheric gradients were mostly in the east-west direction. The characteristics of the gradients indicate that these steep gradients are mostly associated with plasma bubbles. By integrating these results, the ionospheric threat model can be improved to enhance safety and availability of GBAS especially in terms of the width and speed of the gradient. It will also be possible to establish the ionospheric threat models at the transition region from magnetic low to mid-latitudes.