Satellite Communication Systems Overview

SATELLITE COMMUNICATION SYSTEMS OVERVIEW

Satellite communication enables two Stations on the earth to communicate through radio broadcast which are located far away from each other. The two stations use satellite as a relay station for their communication. One of the earth station will transmit to the satellite. This is called as Uplink. The satellite Transponder will convert the signal and sends it down to the other earth station. This is called a Downlink. The satellite communication is advantageous over terrestrial communication because of the huge coverage area and high bandwidth capabilities. The disadvantages of satellite communication are large propagation delay, communication cost and gradual increase in use of bandwidth. The basic factors to be considered in satellite communication are; elevation angle is the angle of the earth surface to the center line of the satellite transmission beam. This angle gives considerable effects on satellites coverage area. Ideally, elevation angle should be 0 degrees, so that the transmission beam will reach the horizon visible to the satellite in all directions. However, there is a minimum elevation angle of earth stations because of the environmental factors like objects blocking the transmission, atmospheric attenuation, and earth electrical background noise. Coverage angle is the measure of the portion of the earth surface visible to a satellite taking the minimum elevation angle into account. [1] It is denoted as,

R/(R+h) = cos (β + θ)/cos (θ)

Here ‘R’ is earth radius (6370 Km), ‘h’ is satellite orbit height, ‘Β’ is coverage angle and ‘?’ is the minimum elevation angle. There are impairments like free space loss, atmospheric attenuation caused by rain and fog will create considerable effects during transmission. [1]

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There are different type of satellite services namely, Fixed Satellite Services (FSS) to offers point to point communication. Broadcast Satellite Services (BSS) to provide satellite television and radio. Mobile satellite Services (MSS) will provide telephony. There are number of satellites orbiting in different orbits. [1]

The types of satellite orbits are (GEO) geostationary earth orbit, Medium earth orbit (MEO), Low earth orbit (LEO) Molniya and tundra orbit.GEO satellites are in orbit 35,863 km above the earth’s surface along the equator. Objects in Geostationary orbit revolve around the earth at the same speed so as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth. This satellite has large coverage area which almost one fourth of the earth’s surface. It has 24 hour continuous view of the particular regions of the earth, so it is used for broadcasting and multipoint applications. These GEO satellites have difficulty in broadcasting in near polar region.LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface.LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass. So a network of LEO satellites is necessary for LEO satellites to be useful. A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less time delay, which is better for point to point communication. A LEO satellite’s smaller area of coverage is less of a waste of bandwidth. Atmospheric drag affects LEO satellites, causing gradual orbital deterioration. A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earth’s surface.MEO satellites are similar to LEO satellites in functionality.MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours.MEO satellites have a larger coverage area than LEO satellites. Tundra and Molniya Orbit Satellites are used by Russia for decades. Molniya Orbit is an elliptical orbit. The satellite remains in a nearly fixed position relative to earth for eight hours. A series of three Molniya satellites can act like a GEO satellite. It is useful for operations in near Polar Regions. [1] International Telecommunication Union (ITU) has done the frequency allocation and planning for all satellite services. To facilitate efficient frequency planning, the whole world is divided into three regions. [1] They are,

• Region 1: Europe, Africa, Mongolia
• Region 2: Greenland, South and North America
• Region 3: Australia, Asia, South west pacific

Frequency band are allocated within these regions to provide various satellite services. Some of the satellite services are Broadcasting Satellite Services, Mobile satellite services, fixed satellite services, Navigational and meteorological satellite services. [1]

Different types of satellites use various frequency bands. They are given in the table below,

References:

1. courses.missouristate.edu/HuiLiu/csc690/slides/Satellite.ppt (http://search.missouristate.edu/advanced/courses.aspx)