Meaning and Orbital Characteristics
A geosynchronous orbit (GSO) is an Earth-centred orbit in which a satellite has an orbital period equal to Earth’s rotational period, approximately 23 hours 56 minutes 4 seconds.
As a result, the satellite returns to the same position in the sky at the same sidereal time each day.
Key characteristics:
- orbital period matches Earth’s rotation;
- typical semi-major axis is about 42,164 km from Earth’s centre;
- corresponding altitude for a circular orbit is about 35,786 km above the equator;
- the orbit may be circular or elliptical;
- it may have zero or non-zero inclination.
A geosynchronous satellite is therefore not necessarily stationary relative to an observer on Earth.
Geosynchronous vs Geostationary Orbit
A geostationary orbit (GEO) is a special type of geosynchronous orbit.
For an orbit to be geostationary, it must be:
- circular;
- located directly above the equator;
- have zero inclination;
- move in the same direction as Earth’s rotation;
- possess the same orbital period as Earth.
Hence:
Every geostationary orbit is geosynchronous, but every geosynchronous orbit is not geostationary.
If a geosynchronous orbit has inclination or eccentricity, the satellite appears to move in the sky, often tracing a figure-eight or analemma pattern when viewed from Earth.
Geostationary satellites, by contrast, appear fixed over a single longitude, making them particularly suitable for continuous regional coverage.
Applications, Advantages and Limitations
Geosynchronous and geostationary satellites are widely used for:
- telecommunications and broadcasting;
- weather monitoring;
- satellite television;
- disaster warning;
- meteorological observations;
- military communications;
- continuous surveillance of large geographical regions.
Indian satellites using geostationary or geosynchronous regimes include systems such as INSAT, GSAT and meteorological satellites.
Major advantages are:
- continuous coverage of roughly the same region;
- relatively few satellites needed for wide-area communication;
- ground antennas can remain pointed in a fixed direction for geostationary satellites.
Major limitations include:
- high launch-energy requirement;
- substantial signal latency because of the large distance from Earth;
- weaker coverage of polar regions;
- congestion of valuable orbital slots and radio-frequency spectrum;
- need for periodic station-keeping because gravitational perturbations gradually disturb the orbit.
The International Telecommunication Union (ITU) coordinates orbital positions and associated radio-frequency use to reduce harmful interference among geostationary satellite systems.
