Figure 74.1
depicts several kinds of satellite links and orbits. The geostationary earth
orbit (GEO) is in the equatorial plane at an altitude of 35,786 km with a
period of one sidereal day (23h 56m 4.09s). This orbit is sometimes called the
Clarke orbit in honor of Arthur C. Clarke who first described its usefulness
for communications in 1945.
The hub retransmits
through the satellite to another small terminal. Such links require two hops with
attendant time delays. With high gain satellite antennas and relatively
narrow-band digital signals, direct single-hop mesh interconnections of VSATs may
be used.
GEO
satellites appear to be almost stationary from the ground (subject to small
perturbations) and the earth antennas pointing to these satellites may need
only limited or no tracking capability.
An orbit for
which the highest altitude (apogee) is greater than GEO is sometimes referred
to as high earth orbit (HEO). Low earth orbits (LEO) typically range from a few
hundred km to about 2000 km.
Medium earth
orbits (MEO) are at intermediate altitudes. Circular MEO orbits, also called
Intermediate Circular Orbits (ICO) have been proposed at an altitude of about
10,400 km for global personal communications at frequencies designated for
Mobile Satellite Services (MSS) [Johannsen, 1995].
LEO systems
for voice communications are called Big LEOs. Constellations of so-called
Little LEOs operating below 1 GHz and having only limited capacity have been
proposed for low data rate non-voice services, such as paging and store and
forward data for remote location and monitoring, for example, for freight
containers and remote vehicles and personnel [Kiesling, 1996].
Initially,
satellites were used primarily for point-to-point traffic in the GEO fixed
satellite service (FSS), e.g., for telephony across the oceans and for
point-to-multipoint TV distribution to cable head end stations. Large earth
station antennas with high-gain narrow beams and high uplink powers were needed
to compensate for limited satellite power.
This type of
system, exemplified by the early global network of the International Telecommunications
Satellite Organization (INTELSAT) used Standard-A earth antennas with 30-m
diameters. Since then, many other satellite organizations have been formed
around the world to provide international, regional, and domestic services.
As
satellites have grown in power and sophistication, the average size of the
earth terminals has been reduced. High gain satellite antennas and relatively
high power satellite transmitters have led to very small aperture earth terminals
(VSAT) with diameters of less than 2 m, modest powers of less than 10 W [Gagliardi,
1991] and even Smaller ultra-small aperture terminals (USAT) diameters
typically less than 1 m.
Terminals
may be placed atop urban office buildings, permitting private networks of
hundreds or thousands of terminals, which bypass terrestrial lines. VSATs are
usually incorporated into star networks where the small terminals communicate
through the satellite with a larger Hub terminal.
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