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This is a description of the Space Foundation methodology for classifying different satellite orbits with Earth at their focus.
According to the Union of Concerned Scientists, a total of 1,381 satellites were still operational at the end of 2015. These satellites are mostly located in low Earth orbit (LEO) between 200 and 2,000 kilometers (124–1,242 miles) of altitude. LEO is home to 759 active satellites, or 55% of the total. This family of orbits remains the main orbital location of satellites due to the wide range of missions it allows and to the low energy required to reach it, which typically results in lower launch costs.
According to the Union of Concerned Scientists, a total of 1,235 satellites were active at the end of 2014. These satellites are mostly located in low Earth orbit (LEO) between 200 and 2,000 kilometers (124–1,242 miles) of altitude. LEO is home to 655 active satellites, or 53% of the total.
There were ## active satellites in orbit at the end of 2013, all performing various missions depending on their configuration and orbit. Of all active satellites, ## (approximately ##% of the total) were in geosynchronous equatorial orbit (GEO), an orbit 35,790 kilometers (22,240 miles) above the Equator, which allows satellites to circle the Earth exactly once per day, thus appearing to be fixed above a single point on the Earth’s surface, which is valuable in communications applications.
The closer proximity to the Earth also greatly reduces signal delay from a LEO satellite to ground stations and allows for smaller receivers on the ground. While minimizing signal delay is not vital for DTH services or corporate networks, it makes the orbit ideal for voice traffic being sent directly to handheld devices. These lower orbits are challenging since the satellites constantly move in and out of view of individual ground receivers. If it is necessary to maintain a continuous link despite the movement of the satellites, a fleet of spacecraft is required to form a constellation.
The closer proximity to the Earth also greatly reduces signal delay from a LEO satellite to ground stations and allows for smaller receivers on the ground. While these attributes are beneficial, these lower orbits are challenging in that these satellites constantly move in and out of view of individual ground receivers. If it is necessary to maintain a continuous link, a fleet of spacecraft is required to form what is called a satellite constellation. LEO is home to communications constellations belonging to mobile satellite services companies such as Iridium and Globalstar.
Satellites provide a perspective of the Earth that cannot be matched by ground-based technology. In the early days of the Space Age, satellites served little purpose beyond demonstrating that they were in orbit. Decades of experience and technological advancement have yielded sophisticated craft that perform multiple essential missions for militaries, government agencies, and companies around the world. Modern satellites are specialized vehicles designed typically to serve a single specific mission, such as communications, meteorology, remote sensing, scientific measurements, navigation, or reconnaissance.
As geostationary (GSO) satellites remain in an essentially fixed position relative to the ground at all times, they can communicate with fixed ground stations continually. Medium Earth orbit (MEO) and low Earth orbit (LEO) satellites appear to “fly by” overhead and have a shorter communication window for a given station, from ten minutes for a LEO satellite to two hours or more for a MEO satellite. Satellites in highly elliptical orbits may be in communication with a fixed ground station for up to eight hours.