This paper presents a basic investigation of ionospheric delay variation effect on GBAS (Ground-Based Augmentation System) especially with a focus to plasma bubble, which often occurs at geomagnetic low latitude regions after sunset to midnight in spring and fall seasons. Plasma bubble is an ionospheric phenomenon with a low electron density area in ionosphere, which horizontal scales in the east-west and north-south directions are about 100 km and several 1,000 km, respectively. It is also known that it propagates eastward with a speed of about 100 m/s. As a result, it produces a large temporal and spatial gradient of ionospheric propagation delay on GPS measurements. Although GBAS is a system based on a differential GPS technique for aircraft precision approach near airport, a large spatial gradient in ionospheric delay often produces a significant positioning error on GBAS through a carrier smoothing method with a time constant of 100 seconds. In order to investigate ionospheric delay variations due to plasma bubble, Electronic Navigation Research Institute (ENRI) performed preliminary GPS observations with a high sampling rate GPS receiver at Naha of Okinawa island (26.2N, 127.7E) in Japan during about one month of each spring in 2004 and 2005. We extracted a typical event from the observational data and examined its abrupt variation of ionospheric delay. As a result, a rapid slant delay change with a rate of about 8mm/s was observed in satellite signal with an elevation angle of 50 degrees. The same event was also observed at co-located GEONET (GPS Earth Observation Network) sites within an area in horizontal scale of several 10 km. Comparing their results, a large spatial gradient was found as 95.2 mm/km in vertical delay with an averaging time of 100 seconds, which was the same period of a time constant in GBAS carrier smoothing. Because such a large spatial gradient due to plasma bubble seems to affect on GBAS CAT-II/III integrity, it is needed to model plasma bubble effects on GBAS with parameters of ionospheric spatial gradient, velocity of ionospheric front and so on. For this purpose, we aimed to investigate a statistical characteristic of plasma bubble especially in temporal and spatial variations of ionospheric delay using a dense GPS observational system, which was installed at Ishigaki island (24.9N, 124.2E) in Japan and has been operated by ENRI since February, 2005. This system is designed to observe ionospheric disturbances including its propagation velocity with co-located five GPS scintillation receivers (GSV4004B). Using this system, it is expected to estimate a spatial gradient along propagation direction of plasma bubble by combining a temporal change rate of ionospheric delay (temporal gradient) and a propagation velocity with an assumption of a constant velocity. As first step for the ulterior analysis of a spatial gradient, we mainly examined characteristics of temporal gradient in this paper.