Recent years have seen a number of proposals for the use of satellites by aviation, either for navigation or for air/ground communications and the question is often asked why commercial aviation, one of the most technologically advanced industries in the world, is so reluctant to utilize satellite techniques. So far, these proposals have not succeeded because they failed to satisfy a genuine operational requirement and were not, by any standard, cost effective. Up to the present time the systems which have evolved over the years have performed the required tasks reasonably adequately and would appear, given the normal process of development, to be able to continue to do so for some years to come.