Space Communications

The first satellite, Sputnik 1, carried a radio transmitter. In 1957 the only way to tell if the satellite really had made it into space and was orbiting Earth was to listen for the bleeps that its radio trans­mitted as it passed overhead.

The frequencies of radio signals used to communicate with spacecraft have to be chosen carefully because some radio signals will not travel through Earth’s atmosphere into space. Radio signals used for space communications today are mainly in the super high frequency (SHF) band. Radio signals in this band have frequencies from 3 gigahertz to 30 gigahertz. (A gigahertz is equal to 1 bil­lion hertz, or waves per second.)

In the early days of spaceflight, a ground station could communicate with a spacecraft only while it was above the horizon. When it passed over the ground station and disappeared below the hori­zon again, contact was lost. Ground sta­tions had to be set up all around the world to stay in contact with early manned spacecraft.