Orbits of the Earth
Low Earth Orbit (LEO) is commonly accepted as between 124 and 1,240 miles above the Earth’s surface. Spacecraft in LEO make one complete revolution of the Earth in about 90 minutes (the “orbital period”). The International Space Station (ISS) orbits between 198.6 and 215.5 miles above the Earth’s surface. The Space Shuttle and Hubble Space Telescope also operate in this region. The proximity to the Earth’s surface provides a high level of detail and ability to detect distinct objects (known as “spatial resolution”). Most remote sensing satellites, as well as national security assets are placed in LEO.
Medium Earth Orbit (MEO) is the region of space around the Earth above LEO (1,240 miles) and below geosynchronous orbit (22,240 miles). In MEO, orbital periods range from about two to 12 hours. The most common use for satellites in this region is for navigation, such as the United States’ Global Positioning System (GPS) (12,550 miles), Russian Glonass (11,870 miles) and European Galileo (14,430 miles) constellations. Communication satellites that cover the North and South Poles are also put in MEO.
Geosynchronous Orbit (GEO) is the region in which satellites orbit at approximately 22,240 miles above the Earth’s surface. At this altitude, the orbital period is equal to the period of rotation of the Earth, so the satellite appears to move neither east nor west. When a geosynchronous orbital path remains above the Earth’s equator at all times, the satellite is in a geostationary orbit and therefore appears completely stationary relative to the surface of the Earth. This is convenient for communication satellites. In addition, geostationary satellites provide a “big picture” view, enabling coverage of weather events. This is especially useful for monitoring large, severe storms and tropical cyclones. However, instruments and sensors on satellites in geostationary orbit have poor spatial resolution due to the distance from the Earth.
Polar Orbit refers to near-polar inclination and an altitude of 435 to 500 miles. Satellites in polar orbit pass over the equator and every latitude on the Earth’s surface at the same local time each day, meaning the satellite passes overhead any location at essentially the same time throughout all seasons of the year. This orbit enables regular data collection at consistent times and is useful for long-term comparisons. Weather and environmental monitoring satellites are often placed in polar orbits. Many national security assets in LEO also follow polar orbits.
GEO Transfer Orbit (GTO) is an elliptical orbit of the Earth, with the perigee in LEO and the apogee in GEO. This orbit is a transfer path after launch to LEO by launch vehicles carrying a payload to GEO.
Highly Elliptical Orbits (HEO) are characterized by a relatively low-altitude perigee and an extremely high-altitude apogee. These extremely elongated orbits are useful for communication satellites because of long dwell times over a point in the sky; visibility near apogee can exceed 12 hours. For example, some satellite radio providers use HEO orbits to keep two satellites positioned above North America, while another satellite quickly sweeps through the southern part of its 24-hour orbit.