Low Earth Orbit

Low earth orbits (LEO) are satellite systems used in telecommunication, which orbit between 400 and 1,000 miles above the earth's surface. They are used mainly for data communication such as email, video conferencing and paging. They move at extremely high speeds and are not fixed in space in relation to the earth.

LEO-based telecommunication systems provide underdeveloped countries and territories with the ability to acquire satellite telephone service in areas where it otherwise would be too costly or even impossible to lay land lines.

Low earth orbit is defined as an orbit within a locus extending from the earth’s surface up to an altitude of 1,200 miles. Attributing to their high speeds, data transmitted through LEO is handed off from one satellite to another as satellites generally move in and out of the range of earth-bound transmitting stations. Due to low orbits, transmitting stations are not as powerful as those that transmit to satellites orbiting at greater distances from earth’s surface.
Most communication applications use LEO satellites because it takes less less energy to place the satellites into LEO. Moreover, they need less powerful amplifiers for successful transmission. As LEO orbits are not geostationary, a network of satellites are required to provide continuous coverage.
However, as a result of the popularity of this type of satellite, studies reveal that the LEO environment is getting congested with space debris. NASA keeps track of the number of satellites in the orbit, and estimates that there are more than 8,000 objects larger than a softball circling the globe. Not all of these objects are not satellites, but rather pieces of metal from old rockets, frozen sewage and broken satellites.

Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in Low Earth Orbit (commonly called "LEO"). This orbit is almost identical to our previous baseball orbiting example, except that it is high enough to miss all the mountains and also high enough that atmospheric drag won't bring it right back home again.

Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in Low Earth Orbit (commonly called "LEO"). This orbit is almost identical to our previous baseball orbiting example, except that it is high enough to miss all the mountains and also high enough that atmospheric drag won't bring it right back home again.

Advantages and Disadvantages of LEO

Low Earth Orbit is used for things that we want to visit often with the Space Shuttle, like the Hubble Space Telescope and the International Space Station. This is convenient for installing new instruments, fixing things that are broken, and inspecting damage. It is also about the only way we can have people go up, do experiments, and return in a relatively short time.

There are two disadvantages to having things so close, however. The first is that there is still some atmospheric drag. Even though the amount of atmosphere is far too little to breath, there is enough to place a small amount of drag on the satellite or other object. As a result, over time these objects slow down and their orbits slowly decay. Simply put, the satellite or spacecraft slows down and this allows the influence of gravity to pull the object towards the Earth.

The second disadvantage has to do with how quickly a satellite in LEO goes around the Earth. As you can imagine, a satellite traveling 18,000 miles per hour or faster does not spend very long over any one part of the Earth at a given time. So what happens if we want a satellite to spend all of its time over just one part of the Earth? For instance, a weather satellite wouldn't be very effective for us in North America if it didn't have a long dwell time over us. (Dwell time = the time a satellite sits over one part of the globe.) Also, a communications satellite wouldn't work very well for us in North American if it spent most of its time over Africa or Asia.

There are two ways to accomplish this. One solution is to put a satellite in a highly elliptical orbit and the other is to place the satellite in a geosynchronous orbit.

http://www.techopedia.com/definition/8044/low-earth-orbit-leo

http://www.polaris.iastate.edu/EveningStar/Unit4/unit4_sub3.htm