Satellite Communications
Chen, Zhi Ning
Contacts:
„e-mail: chenzn@icr.a-star.edu.sg
„URL: http://www.icr.a-star.edu.sg/~chenzn
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About This Course:
Introduction to satellite communication systems
Description of satellite system /Types of orbits
Satellite launchers / Radio regulations
Various satellite applications
Link analysis, propagation and interference
Calculation of link budget using G/T, C/N ratio, EIRP
Effects of various propagation phenomenon like rain, ionosphere
Terrestrial and satellites interference
Earth station technology
Earth Station design/ Antenna
RF sub-system / Communication sub-system
Network interface/ Monitoring and control
Satellite communication payload
Mission and characteristics
Conventional, multibeam and regenerative transponder
Antenna coverage
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To discuss the important issues of SCS at a system level.
Target & Main Reference Books
To learn the elements of satellite communication systems (SCS)
„To master the basic principle of the relevant designs of SCS
„
[1] Satellite Communication Systems-Systems,
Techniques and Technology
Systems
(3rd Edition) By G. Maral & M. Bousquet
November 2000, John Wiley & Sons
[2] Digital Satellite Communications
(2nd Edition) By Tri T. Ha
1990, McGRAW-HILL
[3] Satellite Communications
(3rd Edition) By D. Roddy
2001, McGRAW-HILL
[4] Satellite Communication Systems Engineering
(2nd Edition)By W. L. Pritchard, H. G. Suyderhoud, & R. A. Nelson
1993, P T R Prentice Hall
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Contents
„
Introduction to satellite communication systems
„
Satellite orbits (Geostationary Orbit)
„
Propagation and Antennas
„
Link analysis and interference
„
Earth station technology
„
Satellite communication payload
„
Satellite communication techniques
„
Satellite multiple access techniques
„
Emerging satellite technologies
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1 Introduction
1.1 Satellite Communication Systems
& Applications
basic elements and applications
1.2 Some Issues in System Design
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1.1 Satellite Communication Systems &
Applications
An SCS is a type of wireless communication systems.
Features:
•Providing coverage over large geographical areas
•Resulting in interconnectivity between various
communication resources
•Realising seamless coverage to combine satellite mobile
communication systems with terrestrial mobile systems
It is important to note the common and different characteristics between the SCS and
other wireless communication systems.
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Basic Systems
System
Space
segment
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Ground
segment
Technology
missile
+ microwave
Different from other terrestrial wireless communication systems, the SCS
basically comprises Space and Ground segments. The communication will be
built up between the space and ground segments.
The characteristics of each segment depend on their applications such as fixed,
mobile or direct broadcasting systems.
With the missile technology, the space segments can be sent to a proper
position in space.
To reduce sizes of the segments, especially the space segments, the segments
are made by microwave technology.
The microwave technology also allows the use of high frequency to lower path
losses.
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Satellite
Basic Systems
downlink
Main satellite services:
•Fixed satellite service, FSS
•Mobile satellite service, MSS
•Direct broadcast system, DBS
TT&C
Ground
Station
uplink
Telemetry Tracking and Command (TT&C)
To control satellite and monitor satellite’s performance.
Earth
Station
A typical FSS system
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Satellite
Basic Systems
Mobile Satellite System (MSS)
Gateway
Aircraft
In an MSS, the ground segments consist several
types of mobile terminals, which can
communicate each other directly and via a
satellite. Also, they can be connected to fixed
telecommunication networks through a satellite.
Usually, the MSS can be categorized into three
classes: maritime, aeronautical and land.
Ship
Fixed
Network
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satellite
Basic Systems
Direct-to-home
Reception
Gateway
Direct Broadcast System (DBS)
A large earth station transmits the programmes
to a high-power satellite. The latter broadcast
the programmes to small terminals on the earth.
Community
Reception
Program
Source
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Geostationary orbit is 35,786km over
equator. The satellites orbiting the
Earth in such an orbit rotate in unison
with the Earth.They appear almost
stationary to the users on the Earth.
So, this minimizes the operational
requirements of earth stations by
simplifying tracking systems of
ground terminals.
Basic Elements
Molniya Orbit
Space Segment
Ground Segment
Any satellite communication system
consists of at least one satellite and several
terminals on the earth. The basic elements
include the satellite, earth stations, and the
orbit, where the satellite is moving on.
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Basic Elements
Space
Segment
Satellites
Receiving signals from an earth station
Changing frequency of the carrier
Amplifying the signals
Transmitting the signals back to other earth stations
Payloads
Platform
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receiving antennas
transmitting antennas
all the electronic equipment
Electric power supply
Temperature Control
Altitude and orbit control
Propulsion equipment
TT&C equipment
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VSAT: very small aperture terminals
Basic Elements
Ground
Segment
Terrestrial Networks
All Earth Stations
Connecting the end-users
to earth stations
Transmitting signals to satellites
Receiving signals from satellites
Changing frequencies of the signals
FSS: several kinds of fixed earth stations
large station(11-30m antennas) or VSAT (1-3m antennas)
Earth
Stations
OF
MSS: several kinds of mobile terminals connected to the fixed
telecommunication networks via fixed earth stations
(maritime, aeronautical, and land base)
DBS: gateways plus small terminals with 30-100cm antennas
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Basic Elements
HPA
Earth
Station
upconverter
baseband
processing
feed
system
LNA
drive
motors
modulator
downconverter
tracking
system
interface
demodulator
from/to
terrestrial
systems
A typical earth station usually include the elements used in any wireless communication systems.
They are roughly classed into two parts, transmitting and receiving parts.
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1.2 Some Issues in System Design
Signal
Quality
Multipoint Data
Transmission
Network
Services
Voice, data,video
Satellite
Broadcasting
Network
Cost
Technology
Data
Collection
Network
network
To be discussed
Link
Design
System
Design
Selection
of
radio
frequency
•Application
•Propagation
•Technology
•Regulation
•Spectrum
•Modulation
•Coding
•...
The target of system design is the high signal quality. The important issues include kinds of services, cost
of system, status of technology, network in use. The most important consideration is the selection of the
radio frequency. The selection of the RF is strongly dependent on these factors.
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Frequency Allocations for Satellite
Electromagnetic Spectrum for RF
Frequency
3 Hz-30 kHz
30 -300 kHz
0.3 -3 MHz
3 -30 MHz
30 -300 MHz
0.3 -3 GHz
3 -30 GHz
30 -300 GHz
100 -1000 GHz
Wavelength, m
108-104
104-103
103-102
102-10
10-1
1-10-1
10-1-10-2
10-2-10-3
3×10-5-3×10-9
Designation
Very low frequency (VLF)
Low frequency (LF)
Medium frequency (MF)
High frequency (HF)
Very high frequency (VHF)
Ultra high frequency (UHF)
Super high frequency (SHF)
Extremely high frequency (EHF)
Infrared, visible light, ultraviolet
This is electromagnetic frequency spectrum. Of them, the bands of 3-300GHz is used for various radio
applications, such as radar and communications.
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Frequency Allocations for Satellite
Frequency spectrum for Satellite Communications
B and
L
S
C
X
Ku
K
Ka
M illim eter
F requency range, G H z
1-2
2-4
4-8
8-12
12-18
18-27
27-40
40-300
Particularly, these frequency bands are allocated for satellite systems.
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Frequency for uplink is always higher than frequency for downlink! Why?
Frequency Allocations for Satellite
Frequency Allocations for Satellite Services
FSS
- C band: ~6 GHz uplink , ~4 GHz downlink - INTELSAT
- X band: ~ 8 GHz uplink , ~7 GHz downlink- government
- Ku band: ~ 14 GHz uplink , ~12 GHz downlink- EUTELSAT, Telecom I & II -INTELSAT
- Ka Band: ~ 30 GHz uplink , ~20 GHz downlink- experimental
- Each allotment consists of an orbital position/bandwidth in the bands mentioned/service area
VSAT – Very Small Aperture Terminals (C and Ku band)
MSS
-L band ~ 1.6 GHz uplink, ~1.5 GHz downlink for the mobiles
-Ku band for Network Control Centre and Hubs.
DBS
- Ku band: ~12 GHz downlink
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International Organizations for
Satellite Communications
International Telecommunication Union (ITU)
- United Nations (UN) organization for telecommunications
CCIR= International Radiocommunications Consultative Committee
CCITT= International Telegraph and Telephone Consultative Committee
Technical & Operational matters, produce Reports & Recommendations.
WRAC= World & Regional Administrative Radio Conferences
Discuss certain topics and carry out total & partial revision of the
administrative regulations like Radiocommunications Regulations (RR)
IFB= International Frequency Registration Board
Register frequency assignments to radio stations given by the countries
and verify that the they conforms to the international frequency assignments
ITU divided the Earth into 3 regions for frequency allocation
1. Region 1- Europe, Africa, ex-Soviet Union, Mongolia
2. Region 2- North & South America, Greenland
3. Region 3- Asia(excluding region 1 Areas), Australia & South West Pacific (Singapore)
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Conclusions
•Brief introduction of SCS
•Basic elements of SCS
•Main applications of SCS
•Allocations of frequency for satellite
•International organizations for SCS
We just snapshot the satellite communication systems and their main applications.
Compared to other wireless communication systems, the key features of the SCS are their
radio link and operation frequency. Therefore, the system design strongly depends on the
link design. In the following lectures, we are going to discuss the issues related to the link
design.
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GPS
Global Positioning System!
What is GPS?
Why GPS?
How does GPS work?
….
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What is GPS?
The GPS is a worldwide radio-navigation system formed from a constellation of 24
satellites and their ground stations.
GPS uses these "man-made stars" as reference points to calculate positions accurate to a
matter of meters. In fact, with advanced forms of GPS you can make measurements to
better than a centimeter!
In a sense it's like giving every square meter on the planet a unique address.
GPS receivers have been miniaturized to just a few integrated circuits and so are
becoming very economical. And that makes the technology accessible to virtually
everyone.
These days GPS is finding its way into cars, boats, planes, construction equipment, movie
making gear, farm machinery, even laptop computers. Soon GPS will become almost as
basic as the telephone. Indeed, it just may become a universal utility.
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Why GPS?
Trying to figure out where you are and where you're going is probably one of
man's oldest pastimes.
Navigation and positioning are crucial to so many activities and yet the process
has always been quite cumbersome.
Over the years all kinds of technologies have tried to simplify the task but
every one has had some disadvantage.
The result is the Global Positioning System, a system that's changed
navigation forever
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How does GPS work?
Here's how GPS works in five logical steps:
1.The basis of GPS is "triangulation" from satellites.
2.To "triangulate," a GPS receiver measures distance using the travel time of radio
signals.
3.To measure travel time, GPS needs very accurate timing which it achieves with some
tricks.
4.Along with distance, you need to know exactly where the satellites are in space. High
orbits and careful monitoring are the secret.
5.Finally you must correct for any delays the signal experiences as it travels through
the atmosphere.
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How does GPS work?
Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference
points for locations here on earth. That's right, by very, very accurately measuring our distance
from three satellites we can " triangulate " our position anywhere on earth. Forget for a moment
how our receiver measures this distance. We'll get to that later. First consider how distance
measurements from three satellites can pinpoint you in space.
Suppose we measure our distance from a satellite and find it to be 11,000 miles. Knowing that
we're 11,000 miles from a particular satellite narrows down all the possible locations we could
be in the whole universe to the surface of a sphere that is centered on this satellite and has a
radius of 11,000 miles.
Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away.
That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000
miles from the second satellite. Or in other words, we're somewhere on the circle where these
two spheres intersect. If we then make a measurement from a third satellite and find that we're
13,000 miles from that one, that narrows our position down even further, to the two points
where the 13,000 mile sphere cuts through the circle that's the intersection of the first two
spheres. So by ranging from three satellites we can narrow our position to just two points in
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How does GPS work?
To decide which one is our true location we could make a fourth measurement. But usually
one of the two points is a ridiculous answer (either too far from Earth or moving at an
impossible velocity) and can be rejected without a measurement. A fourth measurement
does come in very handy for another reason however, but we'll tell you about that later.
Next we'll see how the system measures distances to satellites.
1.Position is calculated from distance measurements (ranges) to satellites.
2.Mathematically we need four satellite ranges to determine exact position.
3.Three ranges are enough if we reject ridiculous answers or use other tricks.
4.Another range is required for technical reasons to be discussed later.
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Web sites related to GPS:
http://www.trimble.com/index.htm
http://www.allgps.com/
……
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Problems:
•Describe basic elements and functions of a satellite communications system,
such as fixed, mobile and direct broadcast system.
•What are the important considerations in the design of satellite
communication systems?
•Describe the GPS.
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