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Optical Methods for Assessing Fade Margins 10-13
Joint Probability (%)
60
50
40
30
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10
0
ALL
CCCC
CCCS
CCCB
CCSS
CCSB
CCBB
Mixed State
CSSS
CSSB
CSBB
CBBB
SSSS
SSSB
SSBB
SBBB
BBBB
=One
=Two
>=Four
=Three
Satellites
Visible
Figure 10-9: The joint probability of state mixtures for two, three, and four potentially
visible satellites assuming the Globalstar constellation for observations made in
Singapore.
10.8 Satellite Diversity
The geometry of the satellites in their constellation and the nature of the associated states
corresponding to each satellite position at any instant of time suggest the employment of
satellite diversity. Although satellite diversity using the Globalstar constellation has been
previously described in Chapter 6 (Section 6.6), for purposes of completeness and chapter
continuity, we again review the results here.
10.8.1 Single and Joint Fade Distributions
In Figure 10-10, Figure 10-11 and Figure 10-12 are given a series of cumulative fade
distributions for Tokyo, London, and Singapore assuming the Globalstar constellation
relative to each geographic location. The distributions in each of these figures
correspond to different types of satellite diversity communications systems. The
distribution for the “Highest Satellite” is derived from a satellite diversity scenario where
communications are switched to the satellite that assumes the highest elevation angle in
the respective constellation, regardless of the path-state. The distribution labeled “Best
Satellite” is also derived from multiple satellites where switching takes place relative to
the satellite giving the smallest fade. In calculating this distribution, a decision for “best
satellite” was made every 18 seconds before “hand over” was executed. At the 10%
probability, the fades exceeded are 15 dB, 13 dB, and 19 dB for Tokyo, London, and
Singapore, respectively. The average elevation angles for the constellation of satellites
for Tokyo and Singapore are similar (40° and 38°, respectively). Nevertheless, Singapore
tends to show larger fades for the “Best” and multiple satellite scenarios because of the