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.
Satellites in use have four basic applications: communications, remote sensing, navigation, positioning and timing, and scientific experimentation (generally using a suite of sensors with variations of some or all of the other types). The sidebar describes the various satellite orbits.
According to the Union of Concerned Scientists, a total of ## 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 ## active satellites, or ##% 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.
In 2012, ## satellites were successfully launched on behalf of ## different countries and international organizations. There were ## active satellites in orbit at the end of 2012, according to the Union of Concerned Scientists. Of these, ## satellites (approximately ##% of the total) were in GEO. An additional ## satellites (##% of the total) operate in LEO, with most flying at an altitude of 600–900 kilometers (370–560 miles). Between these extremes is medium Earth orbit (MEO), in which there are ## satellites (##% of the total). The ##remaining satellites (##% of the total) are in highly elliptical orbits (HEO); the high and low altitudes of their orbits are quite far apart—sometimes tens of thousands of kilometers.
In 2013, ## satellites were successfully launched on behalf of ## different countries. Of these, ## were “microsatellites,” with a mass of less than 91 kilograms (200 pounds). These satellites often carry scientific payloads or serve as demonstrations of new technologies. They usually operate in LEO, have a short life cycle, and are launched together with a larger primary payload. In most cases, three to six microsatellites piggyback on the primary payload, although one Minotaur launch in November 2013 carried ## such spacecraft, and a Dnepr launch two days later deployed ## more.
Ground stations serve as the links between Earth and space, tying together terrestrial and space infrastructure. They track satellites and spacecraft, as well as communicate with and control satellites, probes, crewed space vessels, and space stations.
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This is a description of the Space Foundation methodology for classifying different satellite orbits with Earth at their focus.
Definitions of the various intelligence disciplines for an understanding of how they may apply to satellite collection operations.