Bachelor Degree in Satellite Communications, First Chapter

 

Overview of Satellite Systems:

1.1  Introduction

The use of satellites in communications systems is very much a fact of everyday life, as is evidenced by the many homes equipped with antennas, or “dishes,” used for reception of satellite television. What may not be so well known is that satellites form an essential part of telecommunications systems worldwide, carrying large amounts of data and telephone traffic in addition to television signals.

Satellites offer a number of features not readily available with other means of communications. Because 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. The same feature enables satellites to provide communications links to remote communities in sparsely populated areas that are difficult to access by other means. Of course, satellite signals ignore political boundaries as well as geographic ones, which may or may not be a desirable feature.

To give some idea of cost the construction and launch cost of the Canadian Anik-E1 satellite (in 1994 Canadian dollars) was $281.2 million, and that of the Anik-E2, $290.5 million. The combined launch insurance for both satellites was $95.5 million. A feature of any satellite system is that the cost is distance insensitive, meaning that it costs about the same to provide a satellite communications link over a short distance as it does over a large distance. Thus a satellite communications system is economical only where the system is in continuous use and the costs can be reasonably spread over a large number of users.

Satellites are also used for remote sensing, examples being the detection of water pollution and the monitoring and reporting of weather conditions. Some of these remote sensing satellites also form a vital link in search and rescue operations for downed aircraft and the like.

A good overview of the role of satellites is given by Pritchard (1984) and Brown (1981). To provide a general overview of satellite systems here, three different types of applications are briefly described in this chapter: (1) the largest international system, Intelsat, (2) the domestic satellite system in the United States, Domsat, and (3) U.S. National Oceanographic and Atmospheric Administration (NOAA) series of polar orbiting satellites used for environmental monitoring and search and rescue.

1.2  Frequency Allocations for Satellite Services

Allocating frequencies to satellite services is a complicated process which requires international coordination and planning. This is carried out under the auspices of the International Telecommunication Union (ITU).

To facilitate frequency planning, the world is divided into three regions:

1.      Region 1: Europe, Africa, what was formerly the Soviet Union, and Mongolia.

2.      Region 2: North and South America and Greenland.

3.      Region 3: Asia (excluding region 1 areas), Australia, and the south-west Pacific.

Within these regions, frequency bands are allocated to various satellite services, although a given service may be allocated different frequency bands in different regions. Some of the services provided by satellites are:

1.      Fixed satellite service (FSS).

2.      Broadcasting satellite service (BSS).

3.      Mobile satellite services.

4.      Navigational satellite services.

5.      Meteorological satellite services.

There are many subdivisions within these broad classifications; for example, the FSS provides links for existing telephone networks as well as for transmitting television signals to cable companies for distribution over cable systems. Broadcasting satellite services are intended mainly for direct broadcast to the home, sometimes referred to as direct broadcast satellite (DBSS) service [in Europe it may be known as direct-to-home (DTH) service]. Mobile satellite services would include land mobile, maritime mobile, and aeronautical mobile. Navigational satellite services include global positioning systems (GPS), and satellites intended for the meteorological services often provide a search and rescue service.

I’ll list the frequency band designations in common use for satellite services. The Ku band signifies the band under the K band, and the Ka band is the band is the band above the K band. The Ku band is the one used at present for DBS, and it is also used for certain FSS. The C band is used for FSS, and no DBS is allowed in this band. The very high frequency (VHF) band is used for certain mobile and navigational services and for data transfer from weather satellites. The L band is used for mobile satellite services and navigation systems. For the FSS in the C band, the most widely used subrange is approximately 4 to 6 GHz. The higher frequency is nearly always used for the uplink to the satellite, for reasons that will be explained later, and common practice is the denote the C band by 6/4 GHz, giving the uplink frequency first. For the direct broadcast service in the Ku band, the most widely used range is approximately 12 to 14 GHz, which is denoted by 14/12 GHz. Although frequency assignments are made much more precisely, and they may lie somewhat outside the values quoted here (an example of assigned frequencies in the Ku band is 14,030 and 11,730 MHz), the approximate values stated are quite satisfactory for use in calculations involving frequency).

Care must be exercised when using published references to frequency bands, because the designations have been developed somewhat differently for radar and communications applications; in addition, not all countries use the same designations.

The official ITU frequency band designations for completeness. However, in this text the designations will be used, along with 6/4 GHz for the C band and 14/12 GHz for the Ku band.

~To Be Continued.

~ Dr. Sam Fouad,

2025.