Types of Orbits
The orbits of satellites that might be used for personal communications systems
can be grouped into four categories: low Earth orbit (LEO), medium Earth
orbit (MEO), geostationary orbit (GEO), and elliptical Earth orbit (EEO).
Each has advantages and disadvantages in PCS designs. Table 5.1 shows sample
orbital elements for each of the satellite orbit types in units suitable for use with
most of the general-purpose orbital-prediction computer programs.
The Iridium (Iridium is a trademark of Iridium, LLC) system [11, 12] is a
LEO constellation of 66 satellites (plus 6 in-orbit spares) approximately 778 km
above the Earth in six equally spaced orbital planes at nearly 86.4° inclination to
the equator. Every point on Earth is in view of this “infrastructure in the sky,”
and the system is planned as a complement, or adjunct, to the existing “islands”
of terrestrial communications infrastructures.
The MEO characteristics are typified by the Global Positioning Satellite
(GPS) system of 24 satellites occupying six orbital planes inclined about 54.8°
from the equator and at an altitude of about 20,000 km. The GPS satellites are
spaced so that at least four are simultaneously visible from each point on Earth.
Although the GPS system is not a communications system, GPS receivers are
increasingly being included in personal communications devices. The GPS
system is often selected for time keeping and site synchronization in multiple-
transmitter terrestrial systems.
Orbital Elements for Four Types of Satellite Orbits
Orbital Element LEO MEO GEO EEO
Satellite Iridium GPS INTELSAT Molniya
Epoch
(revolution of epoch)
2006
231.40822226
(rev 48,635)
2006
230.32290049
(rev 11,658)
2006
230.72851175
(rev 74,158)
2006
230.72851175
(rev 412)
Orbital inclination 86.4009° 54.76780° 0.01080° 62.08880°
RAAN 171.4706° 263.9384° 1.62870° 214.5329°
Argument of perigee 77.8669° 155.0343° 148.7212° 266.2404°
Eccentricity 0.0002548 0.0097258 0.00026580 0.7242485
Mean anomaly 282.2821° 205.50360° 129.1125° 16.2853°
Mean motion 14.34216969 2.00563941 1.00274158 2.00707324
Drag –0.00000e–000 –0.00000e–000 –0.00000e–000 0.00000e–000
Satellites that are in orbits having a period of one sidereal day are referred
to as having a geosynchronous orbit. If, additionally, the inclination of the orbit
with respect to the equator is nearly zero and the orbital path is highly circular,
the orbit appears stationary to an observer on the Earth. Such an orbit is called a
geostationary orbit, or GEO). lists the orbital parameters of the
INTELSAT satellite as an example of a GEO satellite.
Sometimes, highly elliptical orbits are chosen for communications because
of certain desirable geometric relationships between the orbit and the intended
coverage area. The EEO of the Molniya satellite is an example. The Molniya sat-
ellites were designed to provide television coverage to the high latitudes of Sibe-
ria in Russia since those high latitudes are not well covered by the equatorial
geostationary satellites. The Molniya orbits are highly eccentric, having approxi-
mately one-half sidereal-day period paths, which appear at apogee over Siberia
for a useful six-hour operational period. There is a second apogee over Hudson
Bay, Canada. When viewed from a fixed point on Earth, the orbital track
appears to make a somewhat distorted, open-ended “U” shape with the Earth
tucked in just above the curved part of the “U.” The two open ends are, respec-
tively, about 41,000 km above Siberia and Hudson Bay. Because at apogee the
orbit appears from Earth to be slow moving, very modest tracking capabilities
are required. Four satellites can provide essentially continuous coverage over the
high latitudes of one hemisphere of the Earth. The Molniya satellites have been
in service since 1963, but their future in the Russian television service is
uncertain.
Big LEO Systems
The three big LEO systems, Globalstar, Iridium, and Odyssey, are the first to be
licensed in the United States, and they have agreed to cooperate in an effort to
secure global authorizations for the portions of the radio frequency spectrum to
be used by their mobile phones. Globalstar and Odyssey, which employ
CDMA, or code division multiple access, share a segment of spectrum for their
mobile links. That spectrum segment can accommodate other global systems
employing compatible technologies. Iridium is a TDMA, or time division mul-
tiple access, system and uses a separate segment for its mobile links. The agree-
ment conforms with the International Telecommunication Union’s frequency
authorizations for global mobile systems.
Globalstar LP, based in San Jose, California, is a partnership of 12 interna-
tional telecommunications service providers and equipment manufacturers that
are building a global mobile satellite telephone system to be operational in 1998.
Globalstar’s dual-mode (cellular-satellite) handsets will be compatible with the
world’s existing cellular and wire-line networks. Globalstar will sell access to the
Globalstar system to a worldwide network of regional and local telecommunica-
tions service providers, including its strategic partners.
Iridium, LLC, is an international consortium of leading telecommunica-
tions and industrial companies funding the development of the Iridium system.
The Iridium system is a 66-satellite telecommunications network designed to
provide global wireless services to hand-held telephones and pagers virtually
anywhere in the world. The first five Iridium satellites were successfully
launched from Vandenburg Air Force Base on May 5, 1997, at 1855 UTC on
board a Delta II vehicle; this was followed by the launch of seven more satellites
on a Proton vehicle from Baikonur, Kazakhstan at 1402 UTC on June 18,
1997. Iridium, LLC, has since gone through a reorganization.
Odyssey Telecommunications International, Inc., (OTI), which has TRW
and Teleglobe as initial shareholders, was to develop a constellation of 12
Odyssey satellites orbiting approximately 10,354 km above the globe. The pro-
ject was cancelled, and the license has been returned.
The reorganization of Iridium, LLC, and the demise of Odyssey under-
score the difficulty of the economics model for communications satellites. This
is particularly true as terrestrial mobile phone technology becomes more and
more pervasive globally.
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