The first artificial satellite of Earth was placed in orbit almost 40 years ago. That event, in October 1957, had great political and technological significance. Almost overnight, thoughtful people began to discuss the possible uses of Earth satellites. Two great applications of space technology have emerged since then. Communication satellites in synchronous orbit now play a prominent role in our lives. They complement terrestrial links and offer communication services not previously available. Equally profound has been the development of a worldwide navigation service based on the constellation of Global Positioning System satellites.