11.1 INTRODUCTION

The concept of communication satellites was put forth in the 1940s by science fiction writer Arthur C.Clarke. Later in 1955, J.R.Pierce published a paper entitled Orbital Radio Relays, estimating technical feasibility. In 1957, Russia launched the first satellite, the low-altitude Sputnik spacecraft. Just a few months later, the United States launched the Explorer satellite. In 1960, a passive reflector, the ECHO balloon was launched by the United States into a circular orbit 1600 km above the earth. This 30-m diameter balloon constructed of lightweight mylar with an aluminized surface provided communications between Bell Laboratories in New Jersey and Jet Propulsion Laboratories in California by bouncing radio signals off the balloon during times of mutual visibility. Large parabolic antennas up to 30 m in diameter, high-power transmitters with several kilowatts output and receivers with low noise maser front ends were necessary to obtain one high-quality voice circuit between the east and west coast of the United States.

The use of active repeaters (devices on board the satellite which would receive weak signals, amplify them, and send them back down towards the earth) was the first of two major advances in technology that accelerated the evolution of communication satellites to the form we know today. The first satellite to use active repeaters with real time transmission and bandwidths sufficient to carry television was the TELSTAR satellite of the AT&T Company. This satellite, launched in...