Global Positioning System Reference
In-Depth Information
h (Orbital altitude)
α
User
R e (Earth's radius)
θ
O (Earth's center)
Figure 2.13
Relationship between elevation angle and Earth central angle ( θ ).
, the
corresponding Earth central angle, , can be computed. Rider then defines a
so-called half street width parameter, c , which is related to the Earth central angle,
From (2.13), given an orbital altitude, h ¸ and a minimum elevation angle,
α
,
and the number of satellites per orbital plane, S , as follows:
π
(
)
(2.14)
cos
θ
=
cos
c
cos
S
Finally, Rider's analysis produces a number of tables that relate optimal combi-
nations of orbital inclination, i , half street width, c , and number of orbital planes, P ,
for various desired Earth coverage areas (global versus mid-latitude versus equato-
rial versus polar) and various levels of coverage (minimum number of satellites in
view).
Practical applications of the theoretical work [16-18] have included the
IRIDIUM LEO mobile satellite communications constellation, which was originally
planned to be an Adams/Rider 77-satellite polar constellation and ended up as a
66-satellite polar constellation, the ICO MEO mobile satellite communications con-
stellation, which was originally planned to be a Rider 10-satellite inclined circular
constellation, and the Globalstar LEO mobile satellite communications constella-
tion, which was originally planned to be a Walker 48-satellite inclined circular
constellation of 8 planes.
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