Communications between a vehicle in outer space and Earth, using high-frequency electromagnetic radiation (radio waves). Provision for such communication is an essential requirement of any space mission. The total communication system ordinarily includes (1) command, the transmission of instructions to the spacecraft; (2) telemetry, the transmission of scientific and applications data from the spacecraft to Earth; and (3) tracking, the determination of the distance (range) from Earth to the spacecraft and its radial velocity (range-rate) toward or away from Earth by the measurement of the round-trip radio transmission time and Doppler frequency shift (magnitude and direction). A specialized but commercially important application, which is excluded from consideration here, is the communications satellite system in which the spacecraft serves solely as a relay station between remote points on Earth.