Types of Satellites

There are, in general, four types of satellite:
Geostationary satellite (GEO)
High elliptical orbiting satellite (HEO)
Middle-earth orbiting satellite (MEO)
Low-earth-orbiting satellite (LEO)
An HEO satellite is a specialized orbit in which a satellite continuously swings
very close to the earth, loops out into space, and then repeats its swing by the
earth. It is an elliptical orbit approximately 18,000 to 35,000 km above the
earth’ssurface, not necessarily above the equator. HEOs are designed to give
better coverage to countries with higher northern or southern latitudes.
Systems can be designed so that theapogeeis arranged to provide continuous
coverage in a particular area. By definition, anapogeeis the highest altitude-point of the orbit, that is, the point in the orbit where the satellite is farthest
from the earth. To clarify some of the terminology, we provide,Fig. 2.3,which
shows the geometric properties of an elliptical orbit. By geometry,

Sm¼a
ffiffiffiffiffiffiffiffiffiffiffiffiffi
1e
2
p
ð2:1Þ
Sp¼
Sm
a
¼að1e
2
Þð2:2Þ
where the eccentricity, or the amount by which the ellipse departs from a
circle, is
e¼
Sf
a
ð2:3Þ

The general equation of an ellipse can thus be written as
r¼
að1e
2
Þ
1þecosy
ð2:4Þ
It is apparent from (2.4) that if e¼0, the resulting locus is a circle.
An MEO is a circular orbit, orbiting approximately 8,000 to 18,000 km
above the earth’s surface, again not necessarily above the equator. An MEO
satellite is a compromise between the lower orbits and the geosynchronous
orbits. MEO system design involves more delays and higher power levels than
satellites in the lower orbits. However, it requires fewer satellites to achieve the
same coverage.
LEO satellites orbit the earth in grids that stretch approximately 160 to
1,600km above the earth’s surface. These satellites are small, are easy to
launch, and lend themselves to mass production techniques. A network of
LEO satellites typically has the capacity to carry vast amounts of facsimile,
electronic mail, batch file, and broadcast data at great speed and communicate
to end users through terrestrial links on ground-based stations. With advances
in technology, it will not be long until utility companies are accessing
residential meter readings through an LEO system or transport agencies and
police are accessing vehicle plates, monitoring traffic flow, and measuring
truck weights through an LEO system.
FIGURE2.3 Geometric properties of an elliptical orbit (Sf ¼semifocal length;
Sp¼semi parameter; Sm¼semiminor axis; r¼radius distance, focus to orbit path;
y¼position angle).
In the United States, the three satellite types (HEO, MEO, and LEO) are
collectively called LEOs; that is, low-earth orbiting satellite systems.By
frequency designation, the LEOs are grouped as big and little LEOs, as
described in Table 2.1.
LEOs are subject to aerodynamic drag caused by resistance of the earth’s
atmosphere to the satellite passage. The exact value of the force caused by the
drag depends on atmospheric density, the shape of the satellite, and the
satellite’s velocity. This force may be expressed in the form
Fd ¼0:5raCdAeq
v
2
kg-m=sec
2
ð2:5Þ
where
ra¼atmospheric density. This density is altitude-dependent, and its
variation is exponential.
Cd ¼coefficient of aerodynamic drag.
Aeq ¼equivalent surface area of the satellite that is perpendicular to the
velocity,v.
v¼velocity of the satellite with respect to the atmosphere. The
magnitude of this velocity is discussed in Sec. 2.2.
If the massms of the satellite is known, the accelerationad
due to aerodynamic
drag can be expressed as
ad ¼
Fd
ms
m=sec ð2:6Þ
The effect of the drag is a decrease of the orbit’s semimajor axis due to the
decrease in its energy. A circular orbit remains as such, but its altitude
decreases whereas its velocity increases. Due to drag, the apogee in the
elliptical orbit becomes lower and, as a consequence, the orbit gradually
becomes circular. The longer the influence on the orbit, the slower the satellite
becomes, and it eventually falls from orbit. Aerodynamic drag is more
significant at low altitudes (200 to 400 km) and negligible only about
3000 km because, in spite of the low value of atmospheric density encountered
TABLE2.1 Frequency Classification of LEOs [1]
Type Frequency Usage
Big LEO >10GHz Voice and data services. Require more spectrums.
Little LEO <10GHz Data services, in the lowest orbit. Often require
relatively small amounts of spectrum.

at the altitudes of satellites, their high orbital velocity implies that perturba-tions due to drag are very significant.
Ageostationaryorbit is a nonretrograde circular orbit in the equatorial
plane with zero eccentricity and zero inclination. The satellite remains fixed
(stationary) in an apparent position relative to the earth; about 35,784 km away
from the earth if its elevation angle is orthogonal (90

) to the equator. Its
period of revolution is synchronized with that of the earth in inertial space.
The geometric considerations for a geostationary satellite communication
system are discussed later in the text.
Commercial GEOs providefixed satellite service(FSS) in the C and Ku
bands of the radio spectrum. Some GEOs use the Ku band to provide certain
mobileservices. The International Telecommunication Union (ITU) (see Chap.
7) has allocated satellite bands in various parts of the radio spectrum from
VHF to 275GHz. Table 2.2 shows satellite communications frequency bands
and the services they perform, whileTable 2.3shows typical links frequency
bands.
Frequency bands in the UHF are suitable for communicating with small
or mobile terminals, for television broadcasting, and for military fleet
communication. The band of frequencies suitable for an earth–space–earth
radio link is between 450MHz and 20GHz. Frequencies between 20 and
50GHz can be used but would be subject to precipitation attenuation.
However, if an availability greater than 99.5% is required, a special provision
such as diversity reception and adaptive power control would need to be
TABLE2.2 Communication Satellite Frequency Bands Allocation
Band Frequency range (GHz) Services
VHF 0.03–0.3 Messaging
UHF 0.3–1.0 Military, navigation mobile
L 1–2 Mobile, audio broadcast radiolo-cation
S 2–4 Mobile navigation
C 4–8 Fixed
X 8–12 Military
Ku 12–18 Fixed video broadcast
K 18–27 Fixed
Ka 27–40 Fixed, audio broadcast, intersatel-lite
mm waves >40 Intersatellite

employed. Higher frequencies are more suitable forintersatellite links (ISL)and may become useable as the orbital congestion arises at the lower
frequencies. Another benefit of higher-frequency communication systems is
that system components generally become smaller. For satellites, this translates
to lighter weight, lower power, and reduced cost, and more importantly, it
means increased mobility and flexibility.