Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 598

Figure 11.1 GLONASS satellite constellation.

sion. A 21-satellite constellation provides continuous 4-satellite visibility over 97%
of the Earth's surface, whereas a 24-satellite constellation provides continuous
observation of no fewer than 5 satellites simultaneously from more than 99% of
the Earth's surface. Under the 21-satellite concept, the performance of all 24 satel-
lites will be determined by GLONASS controllers and the “best” 21 will be acti-
vated. The remaining three will be held for backup or in reserve. Periodically, the
mix will be evaluated and, if necessary, a new best set of 21 will be defined [1, 2, 13,
14].

Once the permanent system of 21 operational and 3 spare satellites is estab-
lished, a single satellite failure would not lower the system design probability of suc-
cessful fix determination below 94.7%. (Available reference material did not specify
whether the term “successful fix determination” is referring to a two-dimensional
and/or a three-dimensional fix.) When necessary to maintain system accuracy, new
satellites will be launched and used either to replace malfunctioning satellites or held
in reserve for future use [1].
Each GLONASS satellite is in a 19,100-km circular orbit referenced to the
Earth's surface with an inclination of 64.8 . The orbital period is 11 hours and 15

minutes. The current orbital configuration and overall system design (including sat-
ellite nominal L-band antenna beamwidths of 35

°

° to 40 ° ) provide navigation service

to users up to 2,000 km above the Earth's surface [1].