Satellite Communication, Slides of Mobile Computing

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Satellite Systems

◼ Introduction

◼ History

◼ Basics

◼ Orbit Types

◼ Routing

◼ Localization

◼ Handover

Introduction

◼ What is a satellite?

❑ A satellite is any object

that orbits or revolves

around another object.

For example, the moon is

a satellite of the earth &

the earth is a satellite of

the sun.

Introduction ◼ Advantages of satellite communication over terrestrial communication ❑ The coverage area of a satellite greatly exceeds that of a terrestrial system. ❑ Transmission cost of a satellite is independent of the distance from the center of the coverage area. ❑ Satellite to Satellite communication is very precise. ❑ Higher Bandwidths are available for use.

Introduction

◼ Disadvantages of satellite communication

❑ Launching satellites into orbit is costly.

❑ Satellite bandwidth is gradually becoming used

up.

❑ There is a larger propagation delay in satellite

communication than in terrestrial communication.

Typical Satellite Systems

Applications ◼ Traditionally

❑ weather forecasting

❑ radio and TV broadcast satellites

❑ military satellites

❑ satellites for navigation and localization (e.g., GPS)

◼ Telecommunication

❑ backbone for global networks

❑ connections for communication in remote places or

underdeveloped areas

❑ global mobile communication: extends the area of already

existing networks

Basics ◼ Inclination angle : angle 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 ◼ Elevation angle : angle between center of the satellite beam and the plane tangential to the earth’s surface ◼ Footprint : The area on earth where the signal of the satellite can be received ◼ Perigee : For non-circular orbits, the closest point to the earth ◼ Apogee : For non-circular orbits, the point farthest from earth ◼ Line of apsides : The line joining the perigee and apogee through the center of the earth. ◼ Ascending node : The point where the orbit crosses the equatorial plane going from south to north. ◼ Descending node : The point where the orbit crosses the equatorial plane going from north to south. ◼ Line of nodes : The line joining the ascending and descending nodes through the center of the earth ◼ Prograde orbit : An orbit in which the satellite moves in the same direction as the earths rotation. ◼ Retrograde orbit : An orbit in which the satellite moves in a direction counter to the earth's rotation.

Inclination Angle

Inclination Angle ◼ The inclination of the orbit determines the area covered by the path of the satellite. The greater the inclination, the greater the amount of surface area covered by the satellite.

Elevation Angle of a Satellite

Atmospheric Attenuation Depending on the elevation, the signal has to penetrate a smaller or larger percentage of the atmosphere. An elevation less than 10 degrees is considered useless for communication

The Van Allen radiation Belts ◼ The Van Allen radiation belts consists of ionized particles and are at heights of about 2 , 000 - 6 , 000 km ( inner belt) and about 15 , 000 – 30 , 000 km ( outer belt) respectively and make satellite communication very difficult in these orbits ◼ These regions are held captive by the magnetic influence of the earth ◼ Radiation is concentrated and closest to the earth at the poles (aurora) ◼ Satellite orbits are designed to spent as little time as possible in these belts or completely avoid them ◼ Satellites that travel in and around the belts may be damaged

GEO ORBIT GEO 35,786km

GEO

◼ Orbits at a height of 36 , 768 km, GEO satellites appear fix in sky;

they have a period of 24 hours and an inclination of 0 degree

◼ Advantages

❑ Three GEO satellites are enough to cover the entire earth; handover generally not required due to large footprints ❑ Senders ad Receivers use fixed antennas positions, no adjusting is needed ❑ Ideal for TV and radio broadcasting ❑ Lifetime about 12 to 15 years hence very cost effective ❑ Do not exhibit Doppler shift ( change in the apparent frequency of operations to and from satellite, caused by the relative motion of the satellite and earth station ) because relative movement is zero ❑ The frequency bands of a geosynchronous satellite can be reused by different methods for increasing the channel capacities ❑ The most common orbit used for communications satellites.