SATELLITE COMMUNICATIONS

NET 425 D

Dr. Marwah Ahmed Networks and

Communication

Department

1

Outlines

20-Sep-16 Networks and Communication Department

2

Welcome message to NET 425 D course.

Curriculum.

Introduction

Orbital Mechanics

Curriculum

20-Sep-16 Networks and Communication Department

3

..\2.4 course syllabus.pdf

..\..\..\..\..\desktop\Dr. Marwah Feb

2015\College\feb 2015\ ملف العضو\العملية

pdfالتعليمية\وثيقة حقوق وواجبات الطالبة الأكاديمية.

Over View

Wide range of radiocommunication systems.

VISUALYSE SOFTWARE

GSO & Non-GSO

..\Satellite Communication\GSO and Non-GSO.SIM

Non-GSO ..\Satellite Communication\Non-GSO PFD.SIM

Non-GSO

GSO.SIM-Non\Satellite Communication\..

Mobile Satellite ..\Satellite Communication\Mobile Satellite Service Feeder Link.SIM

Teledesic 840 satellite constellation

..\Satellite Communication\Teledesic 840 satellite constellation.SIM

Background

20-Sep-16 Networks and Communication Department

11

Two development in the twentieth century changed the way people lived: the automobile and the telecommunications.

Telecommunication systems have now made it possible to communicate with virtually anyone at any time.

The origins of satellite communications can be traced to an article written by Arthur C. Clarke in the British radio magazine “Wireless World in 1945”.

20-Sep-16 Networks and Communication Department

12

Satellite Communication systems were originally

developed to provide long-distance telephone service.

In the late 1960s, launch vehicle had been developed

that could place a 500 kg satellite in geostationary

earth orbit (GEO), with a capacity of 5000 telephone

circuits, marking the start of an era of expansion for

telecommunication satellites.

For the first time, live television links could be

established across the Atlantic and Pacific oceans to

carry news and sporting events.

Background

ORBITAL MECHANIC

Notes

Concept and Orbital Elements

Newton’s Laws of Motion

Satellite Motion

Orbital Period

The angular orientation of the plane relative to

the cone determines whether the conic section is a

circle, ellipse, parabola, or hyerbola.

Orbital Mechanic

Orbital mechanics, also called flight mechanics, is the

study of the motions of artificial satellites and space

vehicles moving under the influence of forces such as

gravity, atmospheric drag, thrust, etc.

The root of orbital mechanics can be traced back to

the 17th century when mathematician Isaac Newton

put forward his laws of motion and formulated his

law of universal gravitation.

Newton’s Law

Newton's laws of motion describe the relationship between the motion of a particle and the forces acting on it.

1. The first law states that if no forces are acting, a body at rest will remain at rest, and a body in motion will remain in motion in a straight line. thus, if no forces are acting, the velocity (both magnitude and direction) will remain constant.

2. if a force is applied there will be a change in velocity, i.e. an acceleration, proportional to the magnitude of the force and in the direction in which the force is applied.

where F is the force, m is the mass of the particle, and a is the acceleration.

The third law states that if body 1 exerts a force on

body 2, then body 2 will exert a force of equal

strength, but opposite in direction, on body 1. This

law is commonly stated, "for every action there is

an equal and opposite reaction".

Figure 2.1 (p. 18)

Forces acting on a satellite in a stable

orbit around the earth (from Fig. 3.4 of

reference 1). Gravitational force is

inversely proportional to the square of

the distance between the centers of

gravity of the satellite and the planet

the satellite is orbiting, in this case the

earth. The gravitational force inward

(FIN, the centripetal force) is directed

toward the center of gravity of the

earth. The kinetic energy of the

satellite (FOUT, the centrifugal force) is

directed diametrically opposite to the

gravitational force. Kinetic energy is

proportional to the square of the

velocity of the satellite. When these

inward and outward forces are

balanced, the satellite moves around

the earth in a “free fall” trajectory: the

satellite’s orbit. For a description of

the units, please see the text.

Satellite Motion

When in stable orbit, two forces acting on the satellite are:

Centripetal force (Fin) due to the gravitational attraction of

the earth about which the satellite is orbiting. It attempts to

pull the satellite down to earth

Centrifugal force (Fout) due to the kinetic energy of the

satellite which attempts to pull the satellite into the higher

orbit

If these two forces equal, the satellite will remain in the stable

orbit.

Centripetal force acting on the satellite,

Fin = m x ( µ / r2 )

= m x (GME / r2 )

Centrifugal force acting on satellite,

Fout = m x a

= m x ( v2 / r )

Where a = centrifugal acceleration and

a = v2 / r

If the forces on satellite are balanced:

Fin = Fout

m x (µ / r2) = m x (v2 / r)

Therefore velocity of the satellite in circular orbit, v:

v = (µ / r)1/2 or (GME / r)1/2

Class Work!

1. Explain what the terms centrifugal and centripetal

mean with regard to a satellite in orbit around

the earth

2. Derive the accelerator of each of the forces

applied on the satellite.

Orbital period

20-Sep-16

24

Networks and Communication Department

Orbital Period

If the orbit is circular, the distance traveled by a

satellite in an orbit around the earth is 2Лr

Where r = radius of the orbit from the satellite to the

centre of the earth

re = 6378.137 km

Orbital Period

Orbital Period, T

T = (2Лr)/v

= (2Лr) x (µ / r)-1/2

= 2Л(r)3/2/ µ1/2

20-Sep-16 Networks and Communication Department

28

Johannes Kepler (1571- 1630) was a German

astronomer and scientist who developed his three

laws of planetary motion by careful observations of

the behavior of the planets in the solar system over

many years.

1- The orbit of any smaller body about a larger body

is always an ellipse, with the center of mass of the

larger body as one of the two foci.

Kepler’s Three Laws

20-Sep-16 Networks and Communication Department

29

2- The orbit of the smaller body sweeps out equal

areas in equal time.

3- The square of the period of revolution of the

smaller body about the larger body equals a constant

multiplied by the third power of the semimajor axis of

the orbital ellipse. That is,

𝑇2 = (4𝜋2𝑎3)𝜇

where 𝑇 is the orbital period, 𝑎 is the semimajor axis

of the orbital ellipse, and 𝜇 is Kepler’s constant.

Kepler’s Three Laws

Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt

Copyright © 2003 John Wiley & Sons. Inc. All rights reserved.

Figure 2.5 (p. 22) Illustration of Kepler’s second law of planetary motion. A satellite is in orbit about the planet earth, E. The orbit is an ellipse

with a relatively high eccentricity, that is, it is far from being circular. The figure shows two shaded portions of the elliptical

plane in which the orbit moves, one is close to the earth and encloses the perigee while the other is far from the earth and

encloses the apogee. The perigee is the point of closest approach to the earth while the apogee is the point in the orbit that is

furthest from the earth. While close to perigee, the satellite moves in the orbit between times t1 and t2 and sweeps out an

area denoted by A12. While close to apogee, the satellite moves in the orbit between times t3 and sweeps out an area

denoted by A34. If t1 – t2 = t3 – t4, then A12 = A34.