Mobile and Wireless Networks

Course Instructor: Dr. Safdar Ali

Satellite Communication

INTRODUCTION

INTRODUCTION

Communication satellite are off-course only one

means of telecommunication transmission.

The traditional means include copper wire and

microwave point- to-point links. Newer techniques

involves use of optics either point-to-point infrared

or fiber optics. Point-to-point radio system such as

short wave radio may also be used.

Why Satellite ??

INTRODUCTION

Inter- continent communication is difficult by using

the guided mediums.

The same feature enables satellites to provide

communications links to remote communities in

sparsely populated areas that are difficult to access

by other means.

INTRODUCTION

Very large areas of the earth are visible from a

satellite, the satellite can form the star point of a

communications net, simultaneously linking many

users who may be widely separated geographically.

INTRODUCTION

The information transferred most often correspondence

to voice (telephone), video (Television) and digital data.

Satellite

Disadvantages??

DISADVANTAGES OF SATELLITES

COMMUNICATION

Launching satellites into orbit is costly.

There is a larger propagation delay in satellite

communication than in terrestrial communication.

What is Satellite ??

INTRODUCTION

A communication satellite is basically an electronic communication package placed in orbit whose prime objective is to initiate or assist through space.

INTRODUCTION

Satellite contains several transponders which

listens to some portion of spectrum, amplifies the

incoming signal and broadcasts it in another

frequency to avoid interference with incoming

signals.

INTRODUCTION

ELEMENTS OF SATELLITE COMMUNICATIONS

The basic elements of a communication satellite

service are divided between;

Space Segment

Ground Segment

ELEMENTS OF SATELLITE COMMUNICATIONS

The space segment consist of the spacecraft &

launch mechanism.

Ground segment comprises the earth station and

network control center of entire satellite system.

INTRODUCTION

TELSTARECHO 1

INTRODUCTION

Asiasat 2SYNCOM 2

EXAMPLES OF SATELLITE RADIO SERVICES

Fixed Satellite Service FSS

Mobile Satellite Service MSS

Broadcast Satellite Service BSS

Radio Navigation Sat. Serv. RNSS

Radio location Sat. Service RSS

Space Operation Service SOS

Earth observation Sat. Serv. ESS

. . .

In total more than 18 radio services

SATELLITE OPERATING FREQUENCY BANDS

APPLICATIONS

Communication (truncking call)

Teleconference

TV Broadcasting

Data communication

Weather telecast

Navigation

GPS

Security/Calamity monitoring

Orbital Mechanics

THE ORIGIN OF SATELLITE

The concept of using object in space to reflect

signals for communication was proved by Naval

Research Lab in Washington D.C. when it use the

Moon to establish a very low data rate link between

Washington and Hawaii in late 1940’s.

Russian started the Space age by successfully

launching SPUTNIK the first artificial spacecraft to

orbit the earth, which transmitted telemetry

information for 21 days in Oct. 1957.

THE ORIGIN OF SATELLITE

The American followed by launching an

experimental satellite EXPLORER In 1958.

In 1960 two satellite were deployed “Echo” &

“Courier”

In 1963 first GEO “Syncom”.

The first commercial GEO (Intelsat & Molnya) in

1965 these provides video (Television) and voice

(Telephone) for their audience

ORBITAL MECHANICS

Satellite technology has progressed tremendously ,since Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World.

When Clarke wrote, there were no satellite in orbit nor rockets were powerful enough to launch them.

After 20 years, his idea was proven, when early bird was launched.

It cost roughly $ 25,000 per kg to get a geostationary satellite in orbit.

KU- BAND SATELLITE IN GEO

MAJOR PROBLEMS FOR SATELLITE

Positioning in orbit in-term of Frequency & Orbit Selection

Stability

Power

Harsh environment

Interference Problem

High initial investment

New investment require in Ground Segment

Short life time

Spectrum crowding

Regulatory aspects (landing rights etc.)

Launch vehicle reliability

ELECTROMAGNETIC SPECTRUM

TYPES OF SATELLITE ORBITS

Based on the inclination, i, over the equatorial

plane:

Equatorial Orbits above Earth’s equator (i=0°)

Polar Orbits pass over both poles (i=90°)

Other orbits called inclined orbits (0°<i<90°)

Based on Eccentricity

Circular with centre at the earth’s centre

Elliptical with one foci at earth’s centre

TYPE OF ORBITS

LEOs - Low Earth Orbit

MEOs - Medium Earth Orbit

GEO - Geostationary Earth Orbit

GEOSTATIONARY EARTH ORBIT (GEO)

Originally proposed by Arthur C. Clarke

Circular orbits above the equator

GEO

Orbital height above the earth about 23000

miles/35786.16km

Round trip time to satellite about 0.24 seconds

GEO

GEO satellites require more power for communications

The signal to noise ratio for GEOs is worse because of the distances involved

A few GEOs can cover most of the surface of the earth

Note that polar regions cannot be “seen” by GEOs

GEO

Since they appear stationary, GEOs do not require

tracking

GEOs are good for broadcasting to wide areas

GEO

Angular separation about 2 degrees - allows 180

satellites

THE VIEW FROM 36,000KM

SOME GEO’S ABOVE US

Optus

AsiaSat

PAS

Intelsat

Inmarsat

Palapa

LEO

Low Earth Orbit

200-3,000 km

High orbit speed

Many satellites

Predominately mobile

Iridium, Globalstar

(space shuttle orbit)

MEO

Medium Earth Orbit

6,000 – 12,000km

About 12 satellites

Voice and mobile

ICO (Odyssey), Orbcomm,

Ellipso

WHY SATELLITE REMAINS IN THE ORBIT

SATELLITE ORBITS AND PERIODS

SATELLITE ORBITS

ASSIGNMENT # 2

Why Modulation??

Why do we use Decibel in Telecommunication??

Difference between the optical and radio horizon??

What is Multi-level signalling ???

Kepler’s Laws

KEPLER’S LAWS

LAW 1: The orbit of a planet about the Sun is

an ellipse with the Sun's center of mass at one

focus (1609)

LAW 2: A line joining a planet and the sun

sweeps out equal areas in equal intervals of

time (1609)

LAW 3: The squares of the periods of the

planets are proportional to the cubes of their

semi-major axes (1619)

MAJOR AND MINOR AXIS

KEPLER’S FIRST LAW

Kepler’s first law states that the path followed by a

satellite around the primary will be an ellipse.

An ellipse has two focal points shown as F1 and

F2.

KEPLER’S FIRST LAW

The center of mass of the two-body system, termed

the barycenter, is always centered on one of the

foci.

Because of the enormous difference between the

masses of the earth and the satellite, the center of

mass coincides with the center of the earth, which

is therefore always at one of the foci.

KEPLER’S FIRST LAW

The semi-major axis of the ellipse is denoted by a, and the semi-minor axis, by b. The eccentricity e is given by:

For an elliptical orbit, 0 < e < 1. When e= 0, the orbit becomes circular.

Earth’s orbit has an eccentricity of 0.017 (nearly circular)

KEPLER’S SECOND LAW

Kepler’s second law states that, for equal time

intervals, a satellite will sweep out equal areas in its

orbital plane, focused at the barycenter.

KEPLER’S SECOND LAW

Assuming the satellite travels distances S1 and S2

meters in 1 s, then the areas A1 and A2 will be

equal.

KEPLER’S SECOND LAW

The average velocity in each case is S1 and S2

m/s, and because of the equal area law, it follows

that the velocity at S2 is less than that at S1.

KEPLER’S SECOND LAW

APOGEE AND PERIGEE

Satellites go faster at Perigee than at Apogee.

Apogee: A point for a satellite farthest from the

Earth.

Perigee: A point for a satellite closest from the

Earth.

KEPLER’S THIRD LAW

Kepler’s third law states that the square of the

periodic time of revolution of the smaller body about

the larger body equals a constant multiplied by the

third power of the semi major axis of the orbital

ellipse.

Kepler's third law of orbital motion gives us a

precise relationship between the speed of the

satellite and its distance from the earth.

KEPLER’S THIRD LAW

Satellites in circular orbits travel at a constant

speed. Simple. We just specify that speed, and

we're done.

Satellites in non-circular (i.e., eccentricity > 0) orbits

move faster when they are closer to the earth, and

slower when they are farther away.

KEPLER’S THIRD LAW

The common practice is to average the speed. You

could call this number "average speed", but

astronomers call it the "Mean Motion“.

Mean Motion is usually given in units of revolutions

per day.

NUMERICAL

The earth rotates once per sidereal day of 23 h 56

min 4.09 s. Show that the radius of the GEO is 42,

164.17 km.

NUMERICAL

The Space Shuttle is an example of a low earth

orbit satellite. Sometimes, it orbits at an altitude of

250 km above the earth’s surface, where there is

still a finite number of molecules from the

atmosphere. The mean earth’s radius is

approximately 6378.14 km. Using these figures,

calculate the period of the shuttle orbit when the

altitude is 250km and the orbit is circular. Find also

the linear velocity of the shuttle along its orbit.

WHAT IS A SATELLITE ?

A celestial body –In astronomical term, e.g. Moon

A space vehicle launched by humans and orbits the

earth or another celestial body –In aerospace terms

Communication Satellite –provides communication

and other services to variety of consumers

It is a microwave repeater in the sky

A satellite radio repeater is called a transponder

A satellite may have one to many transponders

WHAT IS A SATELLITE SYSTEMS ?

A Satellite System may consist of :

one or more satellite space vehicles,

a ground based control earth station,

and a network of user earth stations that provides the

interface facilities for the transmission and reception of

terrestrial communications traffic through the satellite

systems.

WHAT IS A SATELLITE SYSTEMS ?

Transmissions to and from the satellites are

categorized as either bus or payload.

The payload is the actual user information conveyed

through the system.

The bus includes control mechanisms that support the

payload operation.

TYPES OF SATELLITES

Passive Satellites

Active Satellites

PASSIVE SATELLITES

Simplest type of satellite is a passive reflector

It simply “bounces” signals from one place to another.

It reflects signals back to earth as there are no gain devices on board to amplify or modify the signals.

The passive satellites used in the early years of satellite communications were both artificial as well as natural.

Moon became the first passive satellite in 1954 when the U.S Navy successfully transmitted the first message over an Earth-to-moon-to-Earth communication system.

PASSIVE SATELLITES

But moon proved to be unreliable communication

satellite as it is above the horizon only half of the

time and its position relative to earth is constantly

changing.

Radio beacon transmitters are required for tracking

and ranging purposes.

Beacon are used so that an earth station can lock

on to and use to determine the exact location of a

satellite so the earth station can align its antennas.

ARTIFICIAL PASSIVE SATELLITE

The first artificial passive satellite

Echo-I of NASA was launched in

August 1960.

Echo-I was 100-ft. diameter

inflatable plastic balloon with

aluminum coating that reflected

radio signals transmitted from

huge earth station antennas.

ARTIFICIAL PASSIVE SATELLITE

Echo-I had an orbital height of 1000 miles. Earth

Stations across US and Europe picked up the

signal and contributed a lot in motivating research

in communication satellite.

DISADVANTAGE OF PASSIVE SATELLITE

Moon proved to be unreliable communication satellite

as it is above the horizon only half of the time and its

position relative to earth is constantly changing.

Earth Stations required high power (10 kW) to transmit

signals strong enough to produce an adequate return

echo.

Large Earth Stations with tracking facilities were

expensive.

DISADVANTAGE OF PASSIVE SATELLITE

Control of satellites not possible from ground.

As little as 1 part in every 10 x 18 of an earth

station’s transmitted power is actually returned to

earth station receiving antennas.

A global system would have required a large

number of passive satellites accessed randomly by

different users.

ACTIVE SATELLITES

It is capable of receiving, amplifying, reshaping,

regenerating and retransmitting the information.

Has sophisticated electronic equipment on-board.

ACTIVE SATELLITES

Advantages of active satellites are:

Require lower power earth station

Directly controlled by operators from ground.

Disadvantages of active satellites are:

Disruption of service due to failure of electronics

components on-board the satellites

Requirement of on-board power supply

Requirement of larger and powerful rockets to launch

heavier satellites in orbit