Navigation by Satellite: The Space Segment

Abstract

The basing of transmitters on satellites for communication, time dissemination, and positioning has the obvious advantage of line-of-sight radio communication over wide geographical areas. Surface communication, as we saw in the last chapter, suffers from uncertainties in the propagation velocity of radio waves over different surface conditions on land and sea, and the possibility of sky wave contamination. Indeed as far back as 1945, the futurist Arthur C. Clarke had published an article in the British magazine Wireless World [1] explicitly describing global radio communication using three artificial satellites in geostationary orbits. He made no claim of having “invented” stationary satellite orbits, that is, ones in which a satellite remains apparently fixed overhead above some point on the equator as the earth rotates about its axis; such a circumstance might easily have been noted by Newton. Rather he published his vision of their future application as overhead platforms for communication transmitters with global reach. What makes this article transcend mere science fiction, for which he is noted, is that he used his knowledge of orbital theory to compute the actual parameters of a geostationary orbit and what it might take to place an object in that orbit. That such a feat might not be so far-fetched he may have concluded from witnessing the powerful German V2 rockets come over London during the latter part of Second World War. Of course long before the war, going back as far as the Middle Ages rockets and rocket making began to appear in the history of warfare. In the USA in the early 1920s, 20 years before the V2, Robert Goddard carried out truly pioneering work on rocket propulsion using liquid propellants [2], work which presaged the large boosters later developed by NASA. So Clarke’s speculating about a geostationary communications satellite was not far-fetched.