Telecommunications

Abstract

Radio communications to and from a spacecraft are its lifeline, with which commands are received, and scientific and engineering data are sent to Earth. Huge and sophisticated parabolic antennas are used on the ground to detect very weak signals with a large signal-to-noise ratio and to send into space powerful, highly collimated beams. Special electromagnetic links also provide very precise measurements of distance and velocity, used for navigation and research. Similar techniques are also used in radar ranging to the Moon, passive artificial satellites, asteroids and planets (ranges to the Moon and artificial satellites may also be obtained in the optical band). In this chapter we discuss the energetics of radio communications in space and introduce spectral theory, a tool necessary to describe the structure of the noise. Particular attention is devoted to coherent propagation and to the effect of refractive and dispersive media.