Page 407 - Satellite Communications, Fourth Edition

The Space Link 387

S 2

a
GSO
d
–
2
Δf
—
2

a GSO
Figure 12.13 Finding the distance d between
two GEO satellites.
S 1

to get an estimate of distance d for an ISL spanning CONUS.

(136 55 )
d 2 42164 sin 54767 km
2
Although this may seem large, the range from earth station to satel-
lite is in the order of 41000 km, (Prob. 3.11) so distance involved with
three uplinks and three downlinks is 246000 km!
GEO satellites are often arranged in clusters at some nominal longitude.
For example, there are a number of EchoStar satellites at longitude 119°W.
The separation between satellites is typically about 100 km, the corre-
sponding longitudinal separation for this distance being, from Eq. (12.63),
approximately 0.136°. Because the satellites are relatively close together
they are subject to the same perturbing and drift forces which simplifies
positional control. Also, all satellites in the cluster are within the main lobe
of the earth-station antenna.
Because LEO satellites are not continuously visible from a given earth
location, an intricate network of satellites is required to provide contin-
uous coverage of any region. A typical LEO satellite network will utilize
a number of orbits, with equispaced satellites in each orbit. For exam-
ple, the Iridium system uses 6 orbital planes with 11 equispaced satel-
lites in each plane, for a total of 66 satellites. Communication between
two earth stations via the network will appear seamless as message
handover occurs between satellites via intersatellite links.
Radio frequency ISLs make use of frequencies that are highly atten-
uated by the atmosphere, so that interference to and from terrestrial sys-
tems using the same frequencies is avoided. Figure 4.2 shows the
atmospheric absorption peaks at 22.3 GHz and 60 GHz. Table 12.3
shows the frequency bands in use: