Visibility and Geometry of Glonass Constellation

Nowadays there are two worldwide satellite navigation systems - American GPS, fully operational and Russian GLONASS, no fully operational. The number of GLONASS satellites is less than nominal 24; in June 2009 spatial segment consists of 20 satellites, 18 operational and 2 in maintenance. The number of GLONASS satellites visible in open and restricted area, the distributions (in per cent) of the Geometric Dilution of Precision (GDOP) coefficient values and No Fix (without 3D position) for different numbers of GLONASS satellites (interval [18, 24]), for different masking elevation angles (interval [0°, 25°]) at different observer's latitudes (9 zones, each 10° wide), latitude of Poland (zone 50°-60°), in particular, are demonstrated in the paper. Additionally the detailed distributions of satellite azimuths (8 intervals, each 45° wide) and the percentage of satellite visible in open area above given angle at different latitudes for different numbers of satellites are showed. The knowledge of all these distributions are very important, especially after the publication of the U. S. Government report in which we can read that in 2010 the overall GPS constellation will fall below the number of satellites required the actual level of GPS position fix accuracy.