Satellite Communications

Abstract

AbstractSatellite communications have proliferated worldwide. The trend since about 1965 has been to use the geostationary orbit, where a satellite placed over the equator at the proper distance appeared to an earth station to be stationary. A great advantage for geostationary earth orbit (GEO) satellites is that “stationarity” eliminates the need for tracking in many circumstances. Delay is a principal disadvantage. This is due to the distance involved for a signal to reach a GEO satellite, about 125 ms.A new communication satellite series is evolving using low earth orbit (LEO) satellites. Here the distance to a satellite is greatly reduced (about 500–1500 km above the earth's surface), and hence there is much less delay, 1.6 to 5 ms. Expect LEO satellites to be contenders in two arenas: cellular telephony and data transport. Tracking requirements may be listed as a disadvantage, unless some form of omnidirectional antenna is used. Generally there are much higher elevation angles when compared to their GEO counterparts. This must be listed as an advantage. A brief discussion of LEO satellite systems is provided at the end of the chapter.A large portion of the communication satellites over the Western Hemisphere supports entertainment, particularly TV, supplying service to CATV head‐ends, motels/hotels, and the like. The original intent of GEO satellites was to relay over medium and long distances, multichannel telephony. This is now in decline. Fiber optic cable systems with their far‐reduced propagation delay and nearly infinite bandwidth have caused this decline. One may state that communication satellites have limited bandwidth whereas fiber‐optic cables have nearly unlimited bandwidth.Systems using very small aperture terminals (VSATs) have great promise, and many such systems are in operation. VSATs are commonly used for data connectivities, usually from outstations to a centralized hub.There are three important issues of concern to a satellite system designer: coexistence of satellite communications with terrestrial radio communications (in particular, line‐of‐sight microwave); shortage of desirable frequency assignments for use with geostationary satellites, forcing migration to higher frequencies such as the 30/20‐GHz band with the associated propagation limitations; and the actual orbital crowding where physical separation between GEO satellites is down to 2 degrees.The objective of this chapter is to answer the needs of the satellite link and system designer considering the aforementioned concerns. There is notable lack of standardization, and some sort of standards are set inside a particular system and rarely cross system boundaries. INTELSAT (International Telecommunication Satellite [consortium]) is the largest system operation in the world. This group has set its own standards. If a user wishes an earth station to interoperate with an INTELSAT satellite, that earth station must meet certain standard requirements established by INTELSAT. One of the goals of this chapter is to provide a brief outline of INTELSAT requirements.