© 2018, The Institute of Navigation, Inc. All rights reserved. Since anomalous ionospheric gradient can result in differential range errors for Ground-Based Augmentation System (GBAS) of several meters, it is one of the most important integrity risks. Because GBAS Approach Service Type D (GAST-D) that can support Category III precision approach and landing requires extremely high safety, its international standards specify ground and airborne integrity monitors to detect ionospheric anomalies in real time operations. An ionospheric threat model specifies upper bounds for ionospheric anomalies, and it is important for GBAS safety design. In the validation of Standards and Recommended Practices (SARPs) for GAST-D which was developed by International Civil Aviation Organization (ICAO), an ionospheric threat model is developed mainly based on knowledge of mid-latitude ionosphere, while at lower latitude region where plasma bubbles occur often which is the cause of steep gradient of the ionospheric delays. Therefore, it is important to investigate characteristics of such steep gradients in the low magnetic latitude region. This paper presents preliminary results of analyses of the spatial scale of ionospheric gradient in the low latitude region based on data of GNSS Earth Observation NETwork (GEONET), Japan for the statistical analyses using long-term data. An automatic system to pick up candidates of steep gradients has been developed. With this result, impact on GBAS ionospheric threat model will be assessed.