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This tutorial paper presents an introduction to the problems of communications between aerospace vehicles and stations on the ground. The possible types of aerospace-to-ground communication links are described and classified according to the mode of signal propagation and according to obtainable range. The suitability of carrier frequencies throughout the electromagnetic radiation spectrum from very-low radio frequencies to light frequencies is discussed for the various types of links. Considerations of the information needed for the selection of modulation and demodulation techniques are presented, and factors which influence the practical value of digital coding are introduced. A discussion of the mutual influence of various parameters on the transmitter power requirement concludes the general survey.