Geosynchronous orbit

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A geosynchronous orbit (short GSO) is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period.[1] The synchronization of rotation and orbital period means that for an observer on the surface of the Earth, the satellite appears to constantly hover over the same meridian (north-south line) on the surface, moving in a slow oscillation alternately north and south with a period of one day, so it returns to exactly the same place in the sky at exactly the same time each day

However, the term is often used[2] to refer to the special case of a geosynchronous orbit that is circular (or nearly circular) and at zero (or nearly zero) inclination, that is, directly above the equator. This is technically called a geostationary orbit. A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to ground observers. Communications satellites are often given geostationary orbits, so that the satellite antennas that communicate with them don't have to move, but can be pointed permanently at the fixed location in the sky where the satellite appears.

A semisynchronous orbit has an orbital period of 0.5 sidereal days, i.e., 11 h 58 min. Relative to the Earth's surface it has twice this period, and hence appears to go around the Earth once every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System.

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Orbital characteristics

All Earth geosynchronous orbits have a semi-major axis of 42,164 km (26,199 mi).[3] In fact, orbits with the same period share the same semi-major axis: a=\sqrt[3]{\mu\left(\frac{P}{2\pi}\right)^2} where a=semi-major axis, P = orbital period, μ = geocentric gravitational constant.

In the special case of a geostationary orbit, the ground track of a satellite is a single point on the equator. In the general case of a geosynchronous orbit with a non-zero inclination or eccentricity, the ground track is a more or less distorted figure-eight, returning to the same places once per solar day.

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