Fig. 4From: Feasibility analysis of GNSS-based ionospheric observation on a fast-moving train platform (GIFT)Satellite distribution in sky view for different GNSS systems, i.e., GPS in (a), GLONASS in (b), Galileo in (c), and BDS in (d). Its time duration is from 15:08 to 16:28 HKT on 19 June 2017. The gray mask indicates that satellites in the mask are not visible for the moving stationBack to article page