ABSTRACT: Spatial segment is one of three segments of each satellite navigation systems (SNS). Nowadays two SNSs, GPS and GLONASS, are fully operational, two next SNSs, BeiDou in China and Galileo in Europe, are in final phase of the construction. In the case of China system this segment will consist of 35 satellites with three types of orbits ? medium (MEO), geostationary (GEO) and inclined geosynchronous (IGSO). As GEO and IGSO satellites can be used in China and Asia-Pacific region only, BeiDou MEO constellation with 27 fully operational satellites will be taken into account in this paper. The orbital planes of the Galileo constellation will be divided in ?slots? that contains at least one operational satellite. The Galileo reference constellation has 24 nominal orbital positions or operational slots in MEO homogeneously distributed in 3 orbital planes; i.e. 8 slots equally spaced per plane. As the error of user?s position obtained from both systems depends on geometry factor DOP (Dilution Of Precision) among other things the knowledge of the number of satellites visible by the user above given masking elevation angle Hmin and the distributions of DOP coefficient values, GDOP in particular, is very important. The lowest and the greatest number of satellites visible in open area by the observer at different latitudes for different Hmin, the percentage of satellites visible above angle H, distributions (in per cent) of satellites azimuths and GDOP coefficient values for different Hmin for BeiDou and Galileo systems at different latitudes are presented in the paper.

KEYWORDS: Global Navigation Satellite System (GNSS), Global Positioning System (GPS), Galileo, BeiDou, Dilution of Precision (DOP), GNSS Construction, GNSS Geometry, GNSS Visibility

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