There are essentially three types of Earth orbits: high, medium and low Earth orbit. Satellites that orbit in a medium (mid) Earth orbit include navigation and specialty satellites, designed to monitor a particular region. Most scientific satellites, including NASA’s Earth Observing System fleet, have a low Earth orbit. On which orbit a satellite will be launched to, depends mainly on its application. The orbit types can be categorized according to their height. The orbit height of a satellite corresponds to the distance between the Earth’s surface and the satellite. It determines its speed as it rotates around the Earth. Due to Earth’s gravity, the pull of gravity is stronger for lower orbits than for higher orbits. Therefore, a satellite situated on a lower orbit will circle the Earth faster than a satellite situated on a higher orbit. High Earth orbit: it describes orbits situated at about 36000 km above the Earth’s surface (42164 km from the Earth’s center). At this exact distance, the speed of the satellite on the orbit matches the Earth’s rotation, i.e. the satellite needs 24 hours to complete a full rotation on the orbit, when the orbit is situated exactly above the equator. Such orbits are also called geosynchronous orbits, as the satellite moves at the same speed than the Earth and seems to stay in place over a specific location. Those orbits are mainly used for weather and communication satellites Medium Earth orbit: it describes orbits situated at about 20200 km of the Earth’s surface, or 26560 km of the Earth’s center. At this height, a satellite rotates twice around the orbit during one Earth’s rotation. This orbit is also called semi-synchronous and this is the orbit type used by Global Navigation Satellite Systems such as GPS and GLONASS. A further important medium Earth orbit is the Molniya orbit which allows the observation of the poles, otherwise nearly impossible with equatorial geosynchronous orbits. Low Earth orbit: this type of orbits are used from almost all dedicated scientific Earth Observation satellites. Most of them use a particular, nearly polar orbit inclination, meaning that the satellite rotates around the Earth nearly from pole to pole (instead of around the equator as it is the case for geosynchronous satellites). This rotation takes about 99 minutes, depending of the specific orbit inclination. During one half of the orbit, the satellite views the daytime side of the Earth, i.e. the illuminated side. At the pole, satellite crosses over and views the nighttime side of Earth. Back to the daylight side, the satellite can view the area adjacent to the region flown over in the last orbit path, due to the simultaneous Earth’s rotation. In 24 hours, satellites situated on these orbits view almost all the Earth twice, for optical satellites once in daylight and once in the dark. Radar satellites seen each Earth region twice, from two different illumination directions. These specific polar-orbits are called sun-synchronous, as the local solar time stays the same each time a satellite flies over a specific region. This has the advantage of providing an almost constant angle of sunlight for each region on the Earth’s surface viewed by the satellite over time and ensure repeatable sun illumination conditions; the angle will only vary seasonally due to the Earth revolution around the sun. Due to this consistency, images of a specific region would not show much illumination changes due to shadows or sunlight and image interpretation over time such as change detection or monitoring approaches are possible. Because a sun-synchronous orbit does not pass directly over the poles, there is a data gap over both poles where no data is acquired.
Discuss different types of satellite orbits
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