 the data. A typical receiver block diagram is shown in Figure 7.11. Circuitry up to the intermediate frequency stages is similar to that of a normal radio receiver, using the superhet principle. Even SO the IF stage must be sufficiently wide to accommodate the 1.5 MHz signal. After the IF stages the similarities end because the signal enters an orthogonal frequency division multiplex decoder. This reconstitutes the raw data which is further processed as shown to produce the audio for amplification, or data for display or other uses, including being linked into PCs, etc. 8 Satellites Satellites have become an accepted part of today's technology. They are used for a variety of purposes from providing links for long-distance telecommunications, to direct broadcast television and radio, gathering weather and geological information, as well as a host of other facilities. To reach where we are today has taken a tremendous amount of develop- ment and investment since the idea of communications satellites was first proposed by Arthur C. Clarke in an article in the British magazine entitled Wireless World in 1945. Even before this, Sir Isaac Newton proposed the ideas of satellites, sketching ideas for placing an 'artificial moon' in orbit by firing it from a canon placed on the top of a mountain. It was not until 4 October 1957 that the first satellite was launched. Named Sputnik, it weighed just over 80 kg and was placed into a low earth orbit, circling the earth every 96 minutes. This meant that despite its relatively small size people could actually see it on earth with the naked Since Sputnik the development of satellites has continued at a swift pace. It did not take long before other satellites followed, each one enabling the technology to develop a little further. The electronics to enable all the functions in today's highly sophisticated satellites to be performed has been developed along with the launch vehicles capable of putting these satellites, often as heavy as a ton, into space. Both of these elements of t