Summary
A medium Earth orbit (MEO) is an Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between above sea level. The boundary between MEO and LEO is an arbitrary altitude chosen by accepted convention, whereas the boundary between MEO and HEO is the particular altitude of a geosynchronous orbit, in which a satellite takes 24 hours to circle the Earth, the same period as the Earth’s own rotation. All satellites in MEO have an orbital period of less than 24 hours, with the minimum period (for a circular orbit at the lowest MEO altitude) about 2 hours. Satellites in MEO orbits are perturbed by solar radiation pressure, which is the dominating non-gravitational perturbing force. Other perturbing forces include: Earth's albedo, navigation antenna thrust, and thermal effects related to heat re-radiation. The MEO region includes the two zones of energetic charged particles above the equator known as the Van Allen radiation belts, which can damage satellites’ electronic systems without special shielding. A medium Earth orbit is sometimes called mid Earth orbit or intermediate circular orbit (ICO). Two medium Earth orbits are particularly significant. A satellite in the semi-synchronous orbit at an altitude of approximately has an orbital period of 12 hours and passes over the same two spots on the equator every day. This reliably predictable orbit is used by the Global Positioning System (GPS) constellation. Other navigation satellite systems use similar medium Earth orbits including GLONASS (with an altitude of ), Galileo (with an altitude of ) and BeiDou (with an altitude of ). The Molniya orbit has a high inclination of 63.4° and high eccentricity of 0.722 with a period of 12 hours, so a satellite spends most of its orbit above the chosen area in high latitudes. This orbit was used by the (now defunct) North American Sirius Satellite Radio and XM Satellite Radio satellites and the Russian Molniya military communications satellites, after which it is named.
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Graveyard orbit
A graveyard orbit, also called a junk orbit or disposal orbit, is an orbit that lies away from common operational orbits. One significant graveyard orbit is a supersynchronous orbit well beyond geosynchronous orbit. Some satellites are moved into such orbits at the end of their operational life to reduce the probability of colliding with operational spacecraft and generating space debris. A graveyard orbit is used when the change in velocity required to perform a de-orbit maneuver is too large.
Indian Regional Navigation Satellite System
The Indian Regional Navigation Satellite System (IRNSS), with an operational name of NavIC (acronym for Navigation with Indian Constellation; also, 'sailor' or 'navigator' in Indian languages), is an autonomous regional satellite navigation system that provides accurate real-time positioning and timing services. It covers India and a region extending around it, with plans for further extension.
Molniya orbit
A Molniya orbit (Молния, "Lightning") is a type of satellite orbit designed to provide communications and remote sensing coverage over high latitudes. It is a highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of 270 degrees, and an orbital period of approximately half a sidereal day. The name comes from the Molniya satellites, a series of Soviet/Russian civilian and military communications satellites which have used this type of orbit since the mid-1960s.
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