The design of swept-frequency topside sounders

Echo sounding of the ionosphere from below has, for four decades, been a fruitful way of obtaining information about the upper atmosphere and the effects of solar emissions. In the last few years, earth satellites have been used as platforms from which to sound the ionosphere from above. This technique has opened new areas of ionospheric research and has the great advantages of world-wide coverage and rapid movement from one observation site to another. The paper is concerned with the design and engineering aspects of the topside sounders in the Alouette and ISIS satellites. It is shown how the requirements of the experiment and the limitations of the spacecraft and its environment determine the design of the instrumentation. The evolution of the swept-frequency sounders is traced from the first sounder, flown in the Alouette I satellite, to the more sophisticated ones in Alouette II and ISIS-I, and engineering problems of particular interest or difficulty are described in detail. Finally, there is a discussion of how these techniques might be extrapolated in a sounder designed to explore very distant regions in the earth's magnetosphere.