1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
2
Chapter 17
Satellite Communication
© 2008 The McGraw-Hill Companies
3
Topics Covered in Chapter 17
 17-1: Satellite Orbits
 17-2: Satellite Communication Systems
 17-3: Satellite Subsystems
 17-4: Ground Stations
 17-5: Satellite Applications
 17-6: Global Positioning System
© 2008 The McGraw-Hill Companies
4
17-1: Satellite Orbits
 Satellites are launched and orbited for a variety of
purposes. The most common application is
communication in which the satellite is used as a
repeater.
© 2008 The McGraw-Hill Companies
5
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning
 The ability to launch a satellite and keep it in orbit
depends upon following well-known physical and
mathematical laws called orbital dynamics.
 In order for a satellite to go into orbit around the
earth, it must have some forward motion.
 When a satellite is launched, it is given both vertical
and forward motion.
© 2008 The McGraw-Hill Companies
6
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning
 Forward motion produces inertia, which tends to keep
the satellite moving in a straight line
 Gravity tends to pull the satellite toward the earth.
 The inertia of the satellite is equalized by the earth’s
gravitational pull.
 The satellite constantly changes its direction from a
straight line to a curved line to rotate about the earth.
© 2008 The McGraw-Hill Companies
7
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning
 The goal is to give the satellite acceleration and speed




that will exactly balance the gravitational pull.
Communication satellites are typically about 22,300
miles from the earth.
A satellite needs to travel about 6800 mi/hr in order to
stay in orbit at that distance.
A satellite rotates around the earth in either a circular or
elliptical path.
A satellite rotates in an orbit that forms a plane that
passes through the center of gravity of the earth called
geocenter.
© 2008 The McGraw-Hill Companies
8
17-1: Satellite Orbits
Figure 17-1: Satellite orbits. (a) Circular orbit. (b) Elliptical orbit.
© 2008 The McGraw-Hill Companies
9
17-1: Satellite Orbits
Figure 17-2: The orbital plane passes through the geocenter.
© 2008 The McGraw-Hill Companies
10
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning: Satellite
Height
 In a circular orbit, the height is the distance of the
satellite from the earth.
 In geometric calculations, the height is the distance
between the center of the earth and the satellite.
 When the satellite is an elliptical orbit, the center of the
earth is one of the focal points of the ellipse.
 The two points of greatest interest are the highest point
above the earth (the apogee) and the lowest point (the
perigee).
© 2008 The McGraw-Hill Companies
11
17-1: Satellite Orbits
Figure 17-4: Elliptical orbit showing apogee and perigee.
© 2008 The McGraw-Hill Companies
12
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning: Satellite
Speed
 Satellite speed varies depending upon the distance of
the satellite from the earth.
 For a circular orbit the speed is constant, but for an
elliptical orbit the speed varies depending upon the
height.
 Low earth satellites of about 100 mi in height have a
speed of about 17,500 mi/hr.
 Very high satellites such as communication satellites
typically travel at speeds of about 6800 mi/hr.
© 2008 The McGraw-Hill Companies
13
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning: Satellite
Period
 The period is the time it takes for a satellite to complete
one orbit.
 This time is also called the sidereal period.
 One revolution is the period of time that elapses
between the successive passes of the satellite over a
given meridian of earth longitude.
 Typical rotational periods range from about 1 ½ h for a
100-mi height to 24 h for a 22,300-mi height.
© 2008 The McGraw-Hill Companies
14
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning: Angle of
Inclination
 The angle of inclination of a satellite orbit is the angle
formed between the line that passes through the center
of the earth and the north pole, and a line that passes
through the center of the earth but that is also
perpendicular to the orbital plane.
 It is also defined as the angle between the equatorial
plane and the satellite orbital plane as the satellite
enters the northern hemisphere.
 When the satellite has an angle of inclination, the orbit
is said to be ascending or descending.
© 2008 The McGraw-Hill Companies
15
17-1: Satellite Orbits
Figure 17-5: (a) Angle of inclination. (b) Ascending and descending orbits.
© 2008 The McGraw-Hill Companies
16
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning: Angle of
Elevation
 The angle of elevation of a satellite is the angle that
appears between the line from the earth station’s
antenna to the satellite and the line between the earth
station’s antenna and the earth’s horizon.
 Noise in the atmosphere contributes to poor
performance.
 The minimum practical angle of elevation for good
satellite performance is 5°.
 The higher the angle of elevation, the better.
© 2008 The McGraw-Hill Companies
17
17-1: Satellite Orbits
Figure 17-6: Angle of elevation.
© 2008 The McGraw-Hill Companies
18
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning:
Geosynchronous Orbits
 To use a satellite for communication relay or repeater
purposes, the ground station antenna must be able to
follow or track the satellite as it passes overhead.
 Depending upon the height and speed of the satellite,
the earth station is able to use it only for communication
purposes for that short period when it is visible.
 The best solution to this problem is to launch a
synchronous or geostationary satellite.
© 2008 The McGraw-Hill Companies
19
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning:
Geosynchronous Orbits
 In a geosynchronous earth orbit (GEO), the satellite
rotates about the earth in exactly 24 h.
 It appears to be fixed or stationary. The antenna is
pointed at the satellite and remains in a fixed position,
making continuous communication possible.
 Most communication satellites in use today are of the
geosynchronous variety.
© 2008 The McGraw-Hill Companies
20
17-1: Satellite Orbits
Principles of Satellite Orbits and Positioning: Position
Coordinates in Latitude and Longitude
 The satellite location is specified by a point on the earth
directly below the satellite known as the subsatellite
point (SSP).
 Latitude is defined as the angle between the line drawn
from a given point on the surface of the earth to the
point at the center of the earth called the geocenter
and the line between the geocenter and the equator.
 The prime meridian is used as a reference point for
measuring longitude.
© 2008 The McGraw-Hill Companies
17-2: Satellite Communication
Systems
21
 Communication satellites are not originators of
information to be transmitted.
 Satellites are relay stations for earth sources.
 The transmitting station sends the information to the
satellite, which in turn retransmits it to the receiving
station.
 The satellite in this application is what is generally
known as a repeater.
© 2008 The McGraw-Hill Companies
17-2: Satellite Communication
Systems
22
 Repeaters and Transponders
 An earth station transmits information to the satellite.
 The satellite contains a receiver that picks up the
transmitted signal, amplifies it, and translates it on
another frequency.
 The signal on the new frequency is then retransmitted
to the receiving stations back on earth.
© 2008 The McGraw-Hill Companies
17-2: Satellite Communication
Systems
23
 Repeaters and Transponders
 The original signal being transmitted from the earth
station to the satellite is called the uplink.
 The retransmitted signal from the satellite to the
receiving stations is called the downlink.
 The transmitter-receiver combination in the satellite is
known as a transponder.
© 2008 The McGraw-Hill Companies
17-2: Satellite Communication
Systems
24
Figure 17-10: Using a satellite as a microwave relay link.
© 2008 The McGraw-Hill Companies
17-2: Satellite Communication
Systems
25
 Repeaters and Transponders: Frequency Allocations
 Most communication satellites operate in the microwave
frequency spectrum.
 The microwave spectrum is divided up into frequency
bands that have been allocated to satellites as well as
other communication services such as radar.
 The most widely used satellite communication band is
the C band.
 The C band uplink frequencies are in the 5.925- to
6.425-GHz range and the downlink frequencies are in
the 3.7- to 4.2-GHz range.
© 2008 The McGraw-Hill Companies
17-2: Satellite Communication
Systems
26
 Frequency bands used in satellite communication:
BAND
FREQUENCY
P
225–390 MHz
J
350–530 MHz
L
1530–2700 MHz
S
2500–2700 MHz
C
3400–6425 MHz
X
7250–8400 MHz
Ku
10.95–14.5 GHz
Ka
17.7–31 GHz
Q
36–46 GHz
V
46–56 GHz
W
56–100 GHz
© 2008 The McGraw-Hill Companies
27
17-3: Satellite Subsystems
 All satellite communication systems consist of two
basic parts, the satellite or spacecraft and two or more
earth stations.
 The satellite performs the function of a radio repeater
or relay station.
 Two or more earth stations may communicate with
one another through the satellite rather than directly
point-to-point on the earth.
© 2008 The McGraw-Hill Companies
28
17-3: Satellite Subsystems
 The heart of a communication satellite is the
communication subsystem.
 This subsystem is a set of transponders that receive
the uplink signals and retransmit them to earth.
 A transponder is a repeater that implements a
wideband communication channel capable of carrying
many simultaneous communication transmissions.
© 2008 The McGraw-Hill Companies
29
17-3: Satellite Subsystems
Figure 17-14: General block diagram of a communication satellite.
© 2008 The McGraw-Hill Companies
30
17-3: Satellite Subsystems
Communication Subsystems
 The main payload on a communication satellite is the
communication subsystem that performs the function of
a repeater or relay station.
 An earth station takes the signals to be transmitted,
known as baseband signals, and modulates a
microwave carrier.
 The three most common baseband signals are voice,
video, or computer data.
 Most modern satellites contain at least 12 transponders.
© 2008 The McGraw-Hill Companies
31
17-3: Satellite Subsystems
Communication Subsystems: Multichannel
Configurations
 Virtually all modern communication satellites contain
multiple transponders.
 This permits many more signals to be received and
transmitted.
 Each transponder operates on a separate frequency,
but its bandwidth is wide enough to carry multiple
channels of voice, video, and digital information.
 The two multichannel architectures used with
communication satellites are broadband and fully
channelized.
© 2008 The McGraw-Hill Companies
32
17-3: Satellite Subsystems
Power Subsystem
 Today virtually every satellite uses solar panels for its
basic power source.
 Solar panels are large arrays of photocells connected in
various series and parallel circuits to create a powerful
source of direct current.
 A key requirement is that the solar panels always be
pointed toward the sun.
 Solar panels generate a direct current that is used to
operate the various components of the satellite and to
charge secondary batteries that act as a buffer.
© 2008 The McGraw-Hill Companies
33
17-3: Satellite Subsystems
Telemetry, Command, and Control Subsystems
 All satellites have a telemetry, command, and control
(TC&C) subsystem that allows a ground station to
monitor and control conditions in the satellite.
 The telemetry system is used to report the status of the
onboard subsystems to the ground station.
 A command and control system permits the ground
station to control the satellite.
 Most satellites contain a small digital computer that acts
as a central control unit for the entire satellite.
© 2008 The McGraw-Hill Companies
34
17-3: Satellite Subsystems
Applications Subsystems
 The applications subsystem is made up of the special
components that enable the satellite to fulfill its intended
purpose.
 For a communication satellite, this subsystem is made
up of the transponders.
 An observation satellite may use TV cameras or
infrared sensors to pick up various conditions on earth
and in the atmosphere. This information is then
transmitted back to earth by a special transmitter
designed for this purpose.
© 2008 The McGraw-Hill Companies
35
17-4: Ground Stations



The ground station, or earth station, is the terrestrial
base of the system.
The ground station communicates with the satellite
to carry out the designated mission.
The earth station consists of five major subsystems:
1. The antenna subsystem
2. The receive subsystem
3. The transmit subsystem
4. The ground control equipment (GCE) subsystem
5. Power subsystem
© 2008 The McGraw-Hill Companies
36
17-4: Ground Stations
Antenna Subsystems
 All earth stations have a relatively large parabolic dish
antenna that is used for sending and receiving signals
to and from the satellite.
 Earth station dishes were 80 to 100 ft or more in
diameter, however, with higher power transmission,
antennas as small as 18 in in diameter are used.
 The antenna in an earth station must be steerable. That
is, it must be possible to adjust its azimuth and
elevation so that the antenna can be properly aligned
with the satellite.
© 2008 The McGraw-Hill Companies
37
17-4: Ground Stations
Receive Subsystems
 The downlink is the receive subsystem of the earth
station.
 It usually consists of very low noise preamplifiers that
take the small signal received from the satellite and
amplify it to a level suitable for further processing.
 The signal is then demodulated and sent on to other
parts of the communication system.
© 2008 The McGraw-Hill Companies
38
17-4: Ground Stations
Receive Subsystems: Receiver Circuits
 The receive subsystem consists of the LNA, down
converters, and related components.
 The purpose of the receive subsystem is to amplify the
downlink satellite signal and translate it to a suitable
intermediate frequency.
 The IF signal is then demodulated and demultiplexed as
necessary to generate the original baseband signals.
© 2008 The McGraw-Hill Companies
39
17-4: Ground Stations
Figure 17-18: Dual-conversion down converters. (a) RF tuning. (b) IF tuning.
© 2008 The McGraw-Hill Companies
40
17-4: Ground Stations
Receiver Ground Control Equipment
 The receiver ground control equipment (GCE)
consists of one or more racks of equipment used for
demodulating and demultiplexing the received signals.
 The down converters provide initial channelization by
transponder, and the demodulators and demultiplexing
equipment process the 70-MHz IF signal into the
original baseband signals.
 Other intermediate signals may be developed as
required by the application.
© 2008 The McGraw-Hill Companies
41
17-4: Ground Stations
Transmitter Subsystems
 The uplink is the transmitting subsystem of the earth
station.
 It consists of all the electronic equipment that takes the
signal to be transmitted, amplifies it, and sends it to the
antenna.
 In a communication system, the signals to be sent to
the satellite might be TV programs, multiple telephone
calls, or digital data from a computer.
 Signals modulate a carrier, are amplified, and sent to an
antenna via waveguides, combiners, and diplexers.
© 2008 The McGraw-Hill Companies
42
17-4: Ground Stations
Transmit Ground Control Equipment
 The transmit subsystem begins with the baseband
signals, which are first fed to a multiplexer, if multiple
signals are to be carried by a single transponder.
 The multiplexer output is then fed to a modulator.
 In analog systems, a wideband frequency modulator is
normally used.
 In digital systems, analog signals are first digitized with
PCM converters. The resulting serial digital output is
then used to modulate a QPSK modulator.
© 2008 The McGraw-Hill Companies
43
17-4: Ground Stations
Power Subsystems
 Most earth stations receive their power from the normal
ac mains. Standard power supplies convert the ac
power to the dc voltages required to operate all
subsystems.
 Most earth stations have backup power systems that
take over if an ac power failure occurs.
 The backup power system may consist of a diesel
engine driving an ac generator, which automatically
starts when ac power fails.
 Smaller systems may use uninterruptible power
supplies (UPS), which derive their main power from
batteries.
© 2008 The McGraw-Hill Companies
44
17-4: Ground Stations
Telemetry and Control Subsystems
 The telemetry equipment consists of a receiver and the
recorders and indicators that display the telemetry
signals.
 The signal may be received by the main antenna or a
separate telemetry antenna.
 A separate receiver on a frequency different from that of
the communication channels is used for telemetry
purposes.
© 2008 The McGraw-Hill Companies
45
17-4: Ground Stations
Telemetry and Control Subsystems
 In some satellite systems where communication is not
the main function, some instrumentation may be a part
of the ground station.
 Instrumentation is a general term for all the electronic
equipment used to deal with the information transmitted
back to the earth station.
© 2008 The McGraw-Hill Companies
46
17-4: Ground Stations
Very Small-Aperture Terminal
 A very small-aperture terminal (VSAT) is a miniature
low-cost satellite ground station.
 These units are extremely small and mount on the top
or side of a building and in some versions even fit into a
suitcase.
 Costs range from a few thousand dollars to no more
than about $6000 today.
 They can be installed very quickly by plugging them in
and pointing the antenna.
© 2008 The McGraw-Hill Companies
47
17-4: Ground Stations
Very Small-Aperture Terminal
 The most common application of VSATs today is in
connecting remote company or organization sites to a
main computer system.
 Gas stations and retail stores use VSATs as point-of-sale
(PoS) terminals to transmit sales transaction information
to the home office, check customer credit cards, and relay
inventory data.
 Tollbooths using SpeedPass and other radio-frequency
identification (RFID) of vehicles for tolls use VSATs.
 The settop box receiver used by consumers for Direct
Broadcast Satellite (DBS) TV reception is a receive-only
(RO) VSAT.
© 2008 The McGraw-Hill Companies
48
17-5: Satellite Applications
 Communication: The main application for satellites
today is in communication. Communication satellites
act as relay stations in the sky and permit reliable
long-distance communication worldwide.
 Direct Broadcast Satellite (DBS) service: This is a TV
signal distribution system designed to distribute
signals directly to consumers.
© 2008 The McGraw-Hill Companies
49
17-5: Satellite Applications
 Satellite Cell Phones. Satellite-based cellular
telephone service is under development. The
proposed new systems use low-earth-orbit satellites to
perform the relay services to the main telephone
system or to make connection directly between any
two cellular telephones using the system.
© 2008 The McGraw-Hill Companies
50
17-5: Satellite Applications
 Digital Satellite Radio: One of the newest satellite
applications is in digital satellite radio or the digital
audio radio service (DARS).
 This service provides hundreds of channels of music,
news, sports, and talk radio to car portable and home
radios.
 It provides full continuous coverage of the station you
select wherever you are in the United States.
 Its digital transmission techniques ensure high-quality
stereo sound that is immune to noise.
 The satellites transmit other information such as song
title and artist, type of music, and other data, which are
displayed on a LCD screen.
© 2008 The McGraw-Hill Companies
51
17-5: Satellite Applications
 Surveillance satellites can look at the earth and
transmit what they see back to ground stations for a
wide variety of purposes, including military
intelligence, meteorological applications, and
mapping.
 Satellite navigation systems can provide global
coverage unavailable with land-based systems
satellites.
© 2008 The McGraw-Hill Companies
52
17-6: Global Positioning System
 The Global Positioning System (GPS), also known
as Navstar, is a satellite-based navigation system that
can be used by anyone with an appropriate receiver to
pinpoint his or her location on earth.
 GPS was developed by the US Air Force for the
Department of Defense as a continuous global radio
navigation system.
 The GPS system consists of three major segments:
the space segment, the control segment, and the user
segment.
© 2008 The McGraw-Hill Companies
53
17-6: Global Positioning System
Space Segment
 The space segment is the constellation of satellites
orbiting above the earth that contain transmitters which
send highly accurate timing information to GPS
receivers on earth.
 The GPS consists of 24 main operational satellites and 3
active spare satellites arranged in six orbits of 3 or 4
satellites each.
© 2008 The McGraw-Hill Companies
54
17-6: Global Positioning System
Space Segment
 Each of the satellites contains four highly accurate atomic
clocks.
 These clocks are used to generate a unique
pseudorandom code identifying the specific satellite that
is transmitted to earth.
 The satellite also transmits a set of digitally coded
ephemeris data that completely defines its precise orbit.
© 2008 The McGraw-Hill Companies
55
17-6: Global Positioning System
Figure 17-22: The GPS space segment.
© 2008 The McGraw-Hill Companies
56
17-6: Global Positioning System
Control Segment
 The control segment of the GPS system refers to the
various ground stations that monitor the satellites and
provide control and update information.
 The master control station is operated by the U.S. Air
Force in Colorado Springs.
 Four additional monitoring and control stations
constantly monitor the satellites and collect range
information from each.
© 2008 The McGraw-Hill Companies
57
17-6: Global Positioning System
Control Segment
 The information is sent back to the master control
station in Colorado, where all the information is
collected and position data on each satellite
calculated.
 The master control station then transmits new
ephemeris and clock data to each satellite on the Sband uplink once per day.
© 2008 The McGraw-Hill Companies
58
17-6: Global Positioning System
GPS Receivers
 A GPS receiver is a complex superheterodyne
microwave receiver designed to pick up the GPS
signals, decode them, and then compute the location of
the receiver.
 The output is usually an LCD display giving latitude,
longitude, and altitude information and/or a map of the
area.
 The most widely used GPS receiver is the popular
handheld portable type, not much larger than an
oversized handheld calculator.
© 2008 The McGraw-Hill Companies
59
17-6: Global Positioning System
GPS Receivers
 The receiver performs a time multiplexing operation on
the four satellites within view of the receiver.
 The data is extracted from each of the four satellites
and stored in the receiver’s memory.
 Data from three satellites is needed to fix the receiver’s
position.
 If data from a fourth satellite is available, altitude can be
calculated.
© 2008 The McGraw-Hill Companies
60
17-6: Global Positioning System
Figure 17-23: A GPS receiver.
© 2008 The McGraw-Hill Companies
61
17-6: Global Positioning System
Figure 17-25: How triangulation works to locate a GPS receiver.
© 2008 The McGraw-Hill Companies
62
17-6: Global Positioning System
GPS Applications
 The primary application of the GPS is military and
related navigation.
 GPS is used by all services for ships, aircraft, and
ground troops.
 Most civilian applications also involve navigation, which
is usually marine or aviation-related.
© 2008 The McGraw-Hill Companies
63
17-6: Global Positioning System
GPS Applications
 Commercial applications include surveying,
mapmaking, and construction.
 Vehicle location is a growing application for trucking and
delivery companies, taxi, bus, and train transportation.
 Police, fire, ambulance, and forest services also use
GPS.
 A new hobby called geocaching uses GPS receivers.
In this sport, one team hides an item or “treasure” and
then gives the other team coordinates to follow to find
the treasure within a given time.
© 2008 The McGraw-Hill Companies