Lec satellite communication

2

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.
 An object which has been placed into orbit. Such
objects are sometimes called artificial satellites to
distinguish them from natural satellites such as the
Moon.
 An object launched to orbit Earth or another celestial
body

3

Sputnik -was the first Earth-orbiting
artificial satellite. It was launched into
an elliptical low Earth orbit by the
Soviet Union on 4 October 1957, with
Sergei Korolev as chief designer
- it was about the size of a
basketball, weighed only 183 pounds,
and took about 98 minutes to orbit the
Earth on its elliptical path
- Sputnik 1 helped to identify the
density of high atmospheric layers
through measurement of its orbital
change and provided data on radio-
signal distribution in the ionosphere

4

Sputnik 2 was launched on
November 3, 1957 and
carried the first living
passenger into orbit, a dog
named Laika.
Laika (, literally meaning
"Barker") was a Soviet space
dog (c. 1954–November 3,
1957) who became the first
animal to orbit the Earth and
the first orbital death

5

Laika, a stray, originally named Kudryavka (Little Curly),
underwent training with two other dogs, and was eventually
chosen as the occupant of the Soviet spacecraft Sputnik 2
that was launched into outer space on November 3, 1957.

6

Explorer 1 - was the first Earth
satellite of the United States as part
of the program for the International
Geophysical Year and in response
to the launch of the Soviet satellite
Sputnik 1.
- was designed and
built by the Jet Propulsion
Laboratory (JPL), while the Jupiter-
C rocket was modified by the Army
Ballistic Missile Agency (ABMA) to
accommodate a satellite payload

7

AGILA II
 (also known as Mabuhay 1), named after
the critically endangered Philippine eagle is
a communications satellite launched in
August 19, 1997.
 It provides telecommunications services for
the Mabuhay Philippines Satellite
Corporation. Built by Space
Systems/Loral, the satellite provides the
most powerful coverage in the Asia-Pacific
region. Its control station is located at the
MPSC Space Center in the Subic Bay
Freeport Zone

8

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.

9

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.

10

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. Satellite orbits. (a) Circular orbit. (b)
 A satellite rotates in an orbit that Elliptical orbit.
forms a plane that passes through the
center of gravity of the earth called
geocenter.

11

Satellite Orbits

Figure 17-2: The orbital plane passes through the geocenter.

12

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).

13

Satellite Orbits

Elliptical orbit showing apogee and perigee.

14

Satellite Orbits
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.

15

Satellite Orbits
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.

16

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.

17

Satellite Orbits

(a) Angle of inclination. (b) Ascending and descending orbits.

18

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.

19

Satellite Orbits

Angle of elevation.

20

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.

21

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.

22

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.

23

Satellite Communication Systems

 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.

24


 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.

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 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.

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Using a satellite as a microwave relay link.

27


 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.

28
Satellite Communication
Systems
 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

29

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.

30

 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.

31


General block diagram of a communication satellite.

32

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.

33

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.

34

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.

35

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.

36

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.

37

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

38

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.

39

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.

40

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.

41

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.

42

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.

43

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.

44

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.

45

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.

46

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.

47

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.

48

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.

49

 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.

50

 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.

51

 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.

52

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.

53

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.

54

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.

55


The GPS space segment.

56

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.

57

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 S-
band uplink once per day.

58

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.

59

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.

60


: A GPS receiver.

61


How triangulation works to locate a GPS receiver.

62

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.

63

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.

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