Satellite communications introduction
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Parabolic antennaA parabolic antenna for Erdfunkstelle Raisting the biggest facility for satellite communication in the world, based in Raisting, Bavaria, Germany. A parabolic antenna is a high-gain reflector antenna used for radio, television and data communications, and also for radiolocation (radar), on the UHF and SHF parts of the electromagnetic spectrum. The relatively short wavelength of electromagnetic radiation at these frequencies allows reasonably sized reflectors to exhibit the desired highly directional response for both receiving and transmitting. With the advent of TVRO and DBS satellite television, the parabolic antenna became a ubiquitous feature, not only in rural locales where CATV and terrestrial signals were limited or non-existent, but also in urban and suburban regions, where the aforementioned services compete with CATV and broadcast media. Extensive terrestrial microwave links, such as those between cellphone base stations, and wireless WAN/LAN applications have also proliferated this antenna type. Earlier applications included ground-based and airborne radar and radio astronomy. Satellite TransponderA transponder is a broadband RF channel used to amplify one or more carriers on the downlink side of a geostationary communications satellite. It is part of the microwave repeater and antenna system that is housed onboard the operating satellite. Examples of these satellites include AMC 4 and Telstar 5, located at 101 and 97 degrees west longitude, respectively. These satellites and most of their cohorts in the geostationary orbit have bent-pipe repeaters using C and Ku bands; a bent pipe repeater is simply one that receives all signals in the uplink beam, block translates them to the downlink band, and separates them into individual transponders of a fixed bandwidth. Figure 1 shows the basic concept. Each transponder is amplified by either a traveling wave tube amplifier (TWTA) or a solid state power amplifier (SSPA). Satellites of this type are very popular for transmitting TV channels to broadcast stations, cable TV systems, and directly to the home. Other applications include very small aperture terminal (VSAT) data communications networks, international high bit rate pipes, and rural telephony. Integration of these information types is becoming popular as satellite transponders can deliver data rates in the range of 50 to 150 Mbps. Achieving these high data rates requires careful consideration of the design and performance of the repeater. 
Block diagram of a basic satellite transponder 
Figure 1. Elements of a basic Satellite link showing ground stations and the satellite transponder. The nature and location of the various system impairments are also shown.The most significant impairments to digital transmission come about in the filtering, which constrains bandwidth and introduces delay distortion, and the power amplification, which produces AM/AM and AM/PM conversion. These effects will be discussed in detail later in this article. For maximum power output with the highest efficiency (e.g., to minimize solar panel DC supply), this amplifier should be operated at its saturation point. However, many services are sensitive and susceptible to AM/AM and AM/PM conversion, for which backoff is necessary. With such an operating point, intermodulation distortion can be held to an acceptable level; however, backoff also reduces downlink power. The transponder itself is simply a repeater. It takes in the signal from the uplink at a frequency f1, amplifies it and sends it back on a second frequency f2. Figure 2 shows a typical frequency plan with 24-channel transponder. The uplink frequency is at 6 GHz, and the downlink frequency is at 4 GHz. The 24 channels are separated by 40 MHz and have a 36 MHz useful bandwidth. The guard band of 4 MHz assures that the transponders do not interact with each other. Advantages of Satellite communication Directory: All%20Toolkits All%20Toolkits -> E-government Transition Framework Enterprise Strategy and Policy Division All%20Toolkits -> Cell Phones Operating System Security Comparison All%20Toolkits -> Sen958 Android Phone Application Development All%20Toolkits -> Apache and Apache Tomcat Tony Collings, February 2007 All%20Toolkits -> 10 Innovative Speech Applications All%20Toolkits -> Career profile All%20Toolkits -> The Effect of Information Technology on Decision Making Download 124.12 Kb. Share with your friends:
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