Chapter 1

Introduction
 OVERVIEW OF SATELLITE COMMUNICATIONS

❑ The use of satellites in communications systems offers a number of features not readily available with
other means of communications. Because very large areas of the earth are visible from a
satellite, the satellite can form the star point of a communication net linking together many users
simultaneously.

❑ many users simultaneously, users who may be widely separated geographically

❑ The same feature enables satellites to provide communication links to remote communities in
sparsely populated areas which are difficult to access by other means.
 OVERVIEW OF SATELLITE COMMUNICATIONS

❖ History of Satellite Communication

Radio communication by satellites is the outcome of research in the area of communications whose objective
is to achieve ever-increasing ranges and capacities with the lowest possible costs

❖ The imagination

❑ The concept of a global telecommunication system using satellites was first put forward in an article for the
British magazine “Wireless World “in May 1945 by the science fiction author Arthur C. Clarke.

❑ In this paper, Dr. Clarke proposed that three communication stations be placed in synchronous 24-hour orbit
could form a global communication system and make worldwide communication possible. He suggested the
use of three artificial satellites at 120O spacing.

❑ The technology is actually a radio relay in the sky. Satellite stations with transmitting and receiving antenna
(transponders) are launched into orbit by rockets where they receive signals from transmitting stations on
earth and relay these signals back to earth stations.
 OVERVIEW OF SATELLITE COMMUNICATIONS
❖The pioneers
The idea of artificial unmanned satellite communication was only implemented after the advent of space-
age technology and solid-state electronics. The US and the former Soviet Union lead in development of the
technology.

❑ On 4 October 1957, the entire world received a shock: The Soviet launched its Sputnik-1 satellite.
The Americans suddenly lost their complacency over their presumed technological superiority.

❑ It was not until the 31 January 1958 that the US launched its first satellite, the Explorer-1.

❑ The USSR countered this by launching a “flying laboratory”; and thus, the space race had begun. Neither
of these rockets carried communications satellites.

❑ The US army is credited with the first communication satellite (SCORE), which was launched Dec. 18,
1958. In late 1958, the satellite broadcasts a Christmas message recorded by President Eisenhower.
 OVERVIEW OF SATELLITE COMMUNICATIONS

❑ The first actual two-way communications came into being in 1960 with the passive ECHO-I (launched in
1960) and ECHO-2 (launched in 1964) satellites. Unlike today's satellites, these earlier models merely
reflected the transmitted signals (up-link signal) back to the earth station (down-link signal) and therefore
were not commercially viable.

❑ This was achieved because the satellite was packed into a low orbit (no greater than 6,000 miles above the
earth) due to the limitations of the rocket technology of those days.

❑ The low orbit resulted in satellite moving faster than the earth's rotation and as the satellite moved across
the horizon, the earth station had to rotate its antenna. Eventually, the satellite would disappear, and
tracking would be passed to another earth station or satellite.

❑ The North Atlantic region would have required about 50 satellites for continuous coverage; a very expensive
arrangement.
 OVERVIEW OF SATELLITE COMMUNICATIONS

❖ The early days

❑ Two breakthroughs in technology led to the present communication technology:
1) The first involved improved rocket technology including booster engines that could launch satellite into
higher, non-decaying orbits.
2) The second was the development of efficient solar energy panels to supply power to these satellites.

❑ This development changed the satellites from passive reflectors to active transponders.
❑ The first transponder-type satellite, COURIER, was sent into orbit in late 1960 by the US Department of Defense.
❑ The first commercial communication satellite, TELSTAR, was on-line in July of 1962.
❑ From that time, many additional communication satellites have been placed into orbit. The International
Telecommunication Satellite Organization (INTELSAT) series began with the launch of EARLY BIRD satellite
from Cape Kennedy on April 6, 1965.
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❖ Commercialization

❑ Satellite systems became a tremendous competitive force among the common carriers when the
(Federal Communications Commission) FCC legislated the "OPEN SKIES POLICY" in 1972, which
encouraged private industry to enter the domestic communication satellite market.
❑ Western Union's WESTAR, begun in 1974, today incorporates many transponders re-transmitting voice
channels and color TV channels at high data rates. Two of the transponders are reserved as backup in
case of a failure of one of other ten.
❑ Radio Corporation of America (RCA) launched "SATCOM" satellites in 1975 and in 1976 AT & T joined
the satellite communication family with its COMSTAR.
❑ The first commercial satellites were launched by Canada starting in 1972 and were designated by the
name "ANIK" which is Eskimo for "little Brother".
 OVERVIEW OF SATELLITE COMMUNICATIONS

❑ The Soviet Union launched the first set of Domestic Satellites (DOMSATs), the Molniya (lightning) in
1966. These satellites beamed TV and radio coverage to remote areas within the vast Soviet territory.
Four of those satellites were originally spaced equidistant in non-stationary orbit. As one satellite lost
contact with earth stations, the next in line took over.
❑ The US COMmunication SATellite corporation (COMSAT), although owned through stockholder shares,
regulates the use as well as operations and sets tariffs for US satellites. It sells user time on satellites
to many diverse users.
❑ Presently, the US uses the largest share of satellite time (24%), followed by Great Britain (13%) and
France (6%). Other users include Germany, Italy, Canada, Iran and South American Countries.
❑ COMSAT has a counterpart in Russia called INTERSPUTNIK, which controls all aspects of satellite use
for Russia and its East European allies.
❑ COMSAT was credited with the first satellite communication system for ships at sea with the 1976
Marisat System.
 OVERVIEW OF SATELLITE COMMUNICATIONS
❖ The Basic Satellite System
The basic satellite system consists of a space segment and a ground segment, as shown in Fig. 1.1.

1) The ground segment

❑ The ground segment consists of all the earth stations.

The earth stations are most often connected to the end-
user’s equipment by a terrestrial network or directly
connected to the end-user’s equipment as in the case of
small stations (Very Small Aperture Terminal, VSAT).

❑ Stations are distinguished by their size which varies

according to the volume of traffic to be carried on the
space link and the type of traffic. The largest are equipped
with antennas of 30-m diameter. The smallest have 0.4-m
antennas (direct television receiving stations).
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2) The space segment

The space segment contains the satellite and all terrestrial facilities for the control and monitoring of
the satellite. This includes the tracking, telemetry and command (TT&C) stations together with the satellite control
center where all the operations associated with station-keeping and checking the vital functions of the satellite are
performed.

❑ The radio waves transmitted by the earth stations are received by the satellite; this is called the uplink.
❑ The satellite in turn transmits to the receiving earth stations; this is the downlink.
❑ The quality of a radio link is specified by its carrier-to-noise ratio. The important factor is the quality of the
total link from station to station.
❑ If the system contains several satellites, these satellites can be connected by radio or optical links.
❑ The satellite consists of a payload and a platform. The payload consists of the receiving and transmitting
antennas and all the electronic equipment which supports the transmission of carriers. The platform consists
of all the subsystems which permit the payload to operate.
 OVERVIEW OF SATELLITE COMMUNICATIONS

The satellite has a dual role:

1) To amplify the received carriers for retransmission on the downlink. The carrier power at the input of
the satellite receiver is of the order of 100 pW to 1 nW. The carrier power at the output of the transmission
amplifier is of the order of 10 to 100 W. The power gain is thus of the order of 100 to 130 dB.

2) To change the frequency of the carrier to avoid re-injection of a fraction of the transmitted power into
the receiver.

❑ In a typical satellite system, provision is generally made for an operational satellite, a backup satellite in
orbit and a backup satellite on the ground. The reliability of the system will involve not only the reliability of
each of the satellites but also the reliability of launching.
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❖ Unique Aspects of Satellite Communications
There are two methods for providing wireless communication services: terrestrially based systems and
satellites. Each concept has its specific merits. The Satellite communication has many unique aspects; some
of which are as follows:

1) Broadband bulk transmission

The satellite technology provides a large communication
capacity. Satellite communication systems can be
designed to transmit, in a very cost-effective manner,
long-haul bulk transmission over a wide bandwidth or, in
the case of digital transmission, at a very high bit rate
with a relatively bit error rate (BER).
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2) Economic consideration

The Transmission cost of satellite systems is independent of distance between the earth sites. It makes no
difference if two sites are 100 or 1000 km apart. The signals transmitted from the satellite can be received by
all stations regardless of their distances from each other. A call across the ocean costs no more to service than a
call across the street.

3) Significant delay

The stations experience a significant signal delay. Since geostationary satellites, for example, are positioned
about 35860 km above earth, the transmission has to travel to the satellite and return to the receiving earth
station. A round trip transmission requires a minimum of about 240 msec. This delay may affect certain
applications. In the case of voice communication, some people find this delay annoying as it takes about 0.5
sec between the end of talkspurt by a person and the point at which he begins to hear the response.
 OVERVIEW OF SATELLITE COMMUNICATIONS
4) Signal quality (low BER)

Most terrestrial systems require repeaters placed at relatively short distances from each other. Each repeater is
a source of noise that introduces some signal degrade. The total transmission is degraded by the product of all
the individual degradations. Satellites also have excellent error rates and can be deployed almost instantly, a
major consideration for military communication. The satellite system with only one repeater in space can
provide signal quality that is better by orders of magnitude than that provided by microwave systems. A
typical satellite BER in the case of digital transmission is 10-8 versus 10-5 for microwave systems.

5) Security

The broadcast aspect of satellite communications may present security problems; as all stations under the
satellite antenna can receive the broadcasts. Consequently, transmissions are often changed (encrypted) for
satellite channels. From a security and privacy point of view, satellites are a complete disaster. This is because
everybody can hear everything. Encryption is necessary when security is required.
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❖ The Satellite Link Analysis
The objective of a communication system link analysis
is to achieve a specific performance for a signal as it is
transmitted from one point to another. For a satellite link,
the performance is impaired in transmission capability by:
➢ satellite downlink power,
➢ atmospheric propagation effects,
➢ satellite and earth terminal noise.

The link power budget calculations basically relate two

quantities, the transmit power and the receive power,
and show in detail how the difference between these two
powers is accounted for. The Link budget calculations are
usually made using decibel quantities.
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❖ The antenna gain

The dish surface contour

y 2 = 4 fx
f is the focal length and x is the coordinate
along the axis of the paraboloid

d 2 d is the antenna diameter

A=
4
the effective aperture area Ae

Ae =  ( A) =  ( )
d 2
2
where 1 is the antenna efficiency
antenna power gain (relative to isotropic radiation) is given by

4Ae G (dBi) = 10 log + 20 log  + 20 log d − 20 log 

G=
 2
G (dBi) = 10 log + 20 log f + 20 log d + 20.4
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❖ Equivalent isotropic radiated power (EIRP)

A transmit antenna focuses the transmitted power toward the receiver in preference to radiation
in other directions.

The maximum power flux density at some distance r from a transmitting antenna of gain GT is

GT PT
=
4r 2
EIRP = GT.PT

EIRP(dBW) = 10 log PT + GT (dBi)

The EIRP must be accurately controlled because

❑ high EIRP will cause interference to adjacent and co-channel carriers, and
❑ low EIRP will result in poor quality performance of the service.
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