A geostationary satellite is an Earth-orbiting satellite placed at an altitude of approximately 22,300 miles or 35,800 kilometers directly above the equator. It revolves in the same direction as the Earth rotates, taking about 24 hours to complete one orbit, which is the same amount of time it takes for the Earth to rotate once on its axis. This makes the satellite appear nearly stationary in the sky to a ground-based observer, hence the term "geostationary".

Geostationary satellites are equipped with transponders that receive, amplify, and retransmit radio signals to and from Earth. These signals can be used for a wide range of applications, including telecommunications, broadcasting, remote sensing, and navigation. Broadcasting television and radio programming is one of the most common uses of geostationary satellites.

To maintain their position in space relative to the Earths rotation, geostationary satellites use thrusters and reaction wheels to make small adjustments to their position as needed. These tools allow the satellite to keep its orbit at the same speed as the Earths rotation and remain in the same position above the Earths equator.

Geostationary satellites are useful because they are visible from a large area of the Earths surface, extending 81° away in latitude and 77° in longitude. They appear stationary in the sky, which eliminates the need for ground stations to have movable antennas. This means that Earth-based observers can erect small, cheap, and stationary antennas that are always directed at the desired satellite. However, geostationary satellites are directly overhead at the equator and appear lower in the sky to an observer nearer the poles. As the observers latitude increases, communication becomes more difficult due to factors such as atmospheric refraction, Earths thermal emission, line-of-sight obstructions, and signal reflections from the ground or nearby structures. At latitudes above about 81°, geostationary satellites are below the horizon and cannot be seen at all.