Chapter 9
Satellite Communication
Chapter Outline
9.1 Introduction
9.2 Satellite Communication 9.3 Types of Satellites
9.4 Components of a Satellite System 9.4.1 Earth Station and Its Subsystem 9.4.2 Spacecraft and Its Subsystem 9.5 Satellite Systems
9.6 Frequency Bands
9.7 Multiple Access Format (MAF)
9.7.1 Frequency Division Multiple Access (FDMA) 9.7.2 Time Division Multiple Access (TDMA) 9.7.3 Code Division Multiple Access (CDMA)
9.7.4 Demand-Assignment Multiple Access (DAMA) 9.8 Footprint and Spot Beam
9.9 Satellite Orbits
9.9.1 Low Earth Orbit 9.9.2 Medium Earth Orbit 9.9.3 Highly Elliptical Orbit
9.9.4 Geosynchronous Earth Orbiters 9.10 Key Points
9.11 Exercises
9.1 Introduction
• Satellite link is one of the important components of a broadband communication system.
• It became the backbone of long distance communication irrespective of geographical conditions.
• Satellites have passed the age when their use was restricted to outer space experiments and remote sensing. Today many satellites are multipurpose satellites which are used for communication, meteorological data collection, search and rescue, global positioning systems, etc.
• Satellite communication has transformed the world into a
global village.
History of Communication Satellites
• In the 1950s and early 1960s, people tried to set up communication systems by bouncing signals off metallized weather balloons. Unfortunately, the received signals were too weak to be of any practical use. Then the U.S. Navy noticed a kind of permanent weather balloon in the sky, the moon and built an operational system for ship-to-shore communication by bouncing signals off it.
• Further progress in the celestial communication field had to
wait until the first communication satellite was launched. The
key difference between an artificial satellite and a real one is
that the artificial one can amplify the signals before sending
them back, turning a strange curiosity into a powerful
communication system.
9.2 Satellite Communication
Figure 9.1: Satellite block diagram, (a) Ideal, (b) Repeater
Advantages and Disadvantages of Satellite Communication
Advantages:
• It is capable of transmitting signal long distances without using relay.
• All the terrestrial relays are point-to-point whereas satellite relays are point-to- multipoint.
• Satellite circuits can be installed rapidly.
• Satellite coves a large geographical area, thus it provides a high coverage area.
• The mobile communication, Cable TV can be easily achieved by satellite systems.
• The satellite costs are independent of distances.
Disadvantage :
• High free space loss.
• To lunch geo-stationary satellites in space powerful launch vehicles
are required and the high cost of launching can affect the total cost
for communication.
9.3 Types of Satellites
Active Satellites Passive Satellites Satellites
Figure 9.2: Categories of satellites
Active satellites are used for processing and transmitting signals.
Note
9.4 Components of a Satellite System
9.4.1 Earth Station
Major Subsystems of an Earth Station
• Antenna
• Transmitter
• Receiver
• Tracker
Categories of Earth Stations
• Receive only
• Transmit only
• Transmit and receive
9.4.2 Spacecraft and its Subsystems
• Altitude and Orbit Control System (AOCS)
• Attitude Control System (ACS):
• Orbit Control System:
• Telemetry, Tracking and Command (TTC)
• Power System
• Antenna Subsystem
Very Small Aperture Terminal (VSAT)
Spacecraft
Figure 9.3: VSAT mini-hub network configuration
Ultra Small Aperture Terminals (USATs)
The USAT consists of following elements:
• A 14 inch or 24 inch offset fed antenna.
• 30 GHz solid state power amplifiers, ranging from 0.25 watts to 4 watts.
• 4 dB noise figure receivers.
• A 70 MHz user interface.
• Up and down converter equipment for frequency translation to
Ka-band.
9.5 Satellite Systems
Figure 9.4: Ground-to-ground
Downlink
9.5 Satellite Systems
Spacecraft (S1)
Earth Station
(A) Earth Station
(B)
Uplink Downlink
Spacecraft (S2) Crosslink
Figure 9.5: Ground cross link ground
The link between the satellites in space is called crosslink.
Note
9.5 Satellite Systems
Spacecraft (S1)
Earth Station (A) Uplink
Crosslink
User terminal
9.6 Frequency Bands
Table 9.1: Frequency bands for satellite
9.7 Multiple Access Format (MAF)
Multiple Access Format is simply a form of carrier wave multiplexing that allows many uplink carriers to pass their waveforms through the satellite electromagnetically, so that they are separable during the downlink retransmission.
Types of Multiple Access Formats
There are four types of Multiple Access Format, they are -
• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division Multiple Access (CDMA)
• Demand-Assignment Multiple Access (DAMA)
9.7.1 Frequency Division Multiple Access
Figure 9.7: FDMA system model
9.7.2 Time Division Multiple Access
Figure 9.8: TDMA system model
FDMA and TDMA are two basic types of narrow band access technologies
Note
9.7.3 Code Division Multiple Access (CDMA)
Figure 9.9: CDMA system model
9.8 Footprint and Spot Beam
Spot beam
Figure 9.10: Spot beam
9.9 Satellite Orbits
a. Equatorial-orbit
satellite b. Inclined-orbit
satellite c. Polar-orbit satellite
Figure 9.11: Satellite orbits
Categories of satellite orbits
Satellite Orbits
Circular Orbits Elliptical Orbits
Figure 9.12: Categories of satellite orbits
Elliptical orbits
Figure 9.13: Elliptical orbits
Circular Orbit Satellites
Figure 9.14: Taxonomy of circular orbit satellites Satellites
MEO HEO GEO
LEO
Satellite Orbit Altitudes
Figure 9.15: Satellite orbit altitudes
There is only one orbit for GEO satellites.
Note
9.9.1 Low Earth Orbit
Crosslink
Spacecraft Spacecraft
Figure 9.16: LEO satellite system
Iridium System
Iridium is designed to provide direct worldwide voice and data
communication using handheld terminals, a service similar to cellular telephony but on a global scale.
Note
Globalstar is used for telephone and low-speed data communications
Note
Teledesic
Figure 9.18: Teledesic
Teledesic has 288 satellites in 12 polar LEO orbits, each at an altitude of 1350 km.
Note
9.9.2 Medium Earth Orbit
Galileo Positioning System
Galileo is intended to provide:
• Greater precision to all users than is currently available.
• Improved coverage of satellite signals at higher
latitudes, northern regions such as Scandinavia will be benefited.
• A global positioning system that can be relied upon,
even in times of war.
9.9.2 Medium Earth Orbit
Global Positioning System
A
B
C
Position of the person
Figure 9.19: Triangulation
9.9.2 Medium Earth Orbit
Global Positioning System
Figure 9.20: Global positioning system
9.9.3 Highly Elliptical Orbit
Figure 9.23: HEO satellite orbit
9.9.4 Geosynchronous Earth Orbit
Figure 9.21: Satellites in geosynchronous orbit
Direct Broadcast Satellite (DBS) Service
Figure 9.22: Block diagram of a DBS system
Performance characteristics of circular orbiting satellites
TABLE 9. 2 Performance characteristics of circular orbiting satellites
