Communications" for the magazine
Wireless World. This article, which the editors renamed "Extra-Terrestrial
Relays", was published in the October issue. In it Clarke described the properties of the geostationary orbit, a circular orbit in the equatorial plane of the earth such that a satellite appears to hover over a fixed point on the equator. The period of revolution is equal to the period of rotation of the earth with respect to the stars, or 23 hours 56 minutes 4.1 seconds, and thus by Kepler's third law the orbital radius is 42,164 km. Taking into account the radius of the earth, the height of a satellite above the equator is 35,786 km.
Clarke observed that only three satellites would be required to provide communications over the inhabited earth.
As a primary application of such a satellite system, Clarke proposed that satellites in geostationary orbit might provide direct broadcast television service similar to DBS systems like DirecTV -- a remarkable idea at a time when television was still in its infancy and it was not yet known whether radio signals could penetrate the ionosphere. He worked out a simple link budget, assuming a downlink frequency of 3 GHz, and estimated that the required transmitter output power for broadcast service to small parabolic antenna receivers would be about 50 watts.
Electric power would be provided by steam generators heated by solar mirrors, but advances in technology might make it possible to replace them by arrays of photoelectric cells. Batteries would be used to provide uninterrupted service during eclipses, which occur in two seasons centered about the equinoxes.
Clarke also estimated the mass ratio of a multistage launch vehicle necessary to deploy the satellite. However, he imagined the geostationary satellites to be outposts inhabited by astronauts to whom supplies would be ferried up on a regular