SATELLITE COMMUNICATION – AN INTRODUCTION
Contents 1.1 Introduction 1.2 Basics 1.3 Applications of Satellites o Weather Forecasting o Radio and TV Broadcast o Military o Navigation o Global Telephone o Connecting Remote Areas o Global Mobile Communication 1.4 Frequency Allocation of Satellites 1.5 Types of Orbits o GEO o LEO o MEO o Sun Synchronous Orbit o Hohmann Transfer Orbit o Prograde Orbit o Retrograde Orbit o Polar Orbits 1.6 Examples o INTELSAT o U.S. Domsats o Polar Orbiting Satellites 1.7 Summary 1.8 Exercise
Satellites are specifically made for telecommunication purpose. They are used for mobile applications such as communication to ships, vehicles, planes, hand-held terminals and for TV and radio broadcasting.
2 They are responsible for providing these services to an assigned region (area) on the earth. The power and bandwidth of these satellites depend upon the preferred size of the footprint, complexity of the traffic control protocol schemes and the cost of ground stations. A satellite works most efficiently when the transmissions are focused with a desired area. When the area is focused, then the emissions don‟t go outside that designated area and thus minimizing the interference to the other systems. This leads more efficient spectrum usage. Satellite‟s antenna patterns play an important role and must be designed to best cover the designated geographical area (which is generally irregular in shape). Satellites should be designed by keeping in mind its usability for short and long term effects throughout its life time. The earth station should be in a position to control the satellite if it drifts from its orbit it is subjected to any kind of drag from the external forces.
Satellites orbit around the earth. Depending on the application, these orbits can be circular or elliptical. Satellites in circular orbits always keep the same distance to the earth‟s surface following a simple law: The attractive force Fg of the earth due to gravity equals m·g (R/r) 2 The centrifugal force Fc trying to pull the satellite away equals m·r·ω2 The variables have the following meaning: m is the mass of the satellite; R is the radius of earth with R = 6,370 km; ri s the distance of the satellite to the centre of the earth; g is the acceleration of gravity with g = 9.81 m/s2; ω is the angular velocity with ω = 2·π·f, f is the frequency of the rotation. To keep the satellite in a stable circular orbit, the following equation must hold: Fg = Fc, i.e., both forces must be equal. Looking at this equation the first thing to notice is that the mass m of a satellite is irrelevant (it appears on both sides of the equation). Solving the equation for the distance r of the satellite to the centre of the earth results in the following equation:
3 The distance r = (g·R2/(2·π·f)2)1/3 From the above equation it can be concluded that the distance of a satellite to the earth‟s surface depends on its rotation frequency. Important parameters in satellite communication are the inclination and elevation angles. The inclination angle δ (figure 1.1) is defined between the equatorial plane and the plane described by the satellite orbit. An inclination angle of 0 degrees means that the satellite is exactly above the equator. If the satellite does not have a circular orbit, the closest point to the earth is called the perigee.
Figure 1.1: Angle of Inclination The elevation angle ε (figure 1.2) is defined between the centre of the satellite beam and the plane tangential to the earth‟s surface. A so called footprint can be defined as the area on earth where the signals of the satellite can be received.
Figure 1.2: Angle of Elevation
1.3 APPLICATIONS OF SATELLITES
1.3.1) Weather Forecasting Certain satellites are specifically designed to monitor the climatic conditions of earth. They continuously monitor the assigned areas of earth and predict the weather conditions of that region. This is done by...