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what is low earth orbit?

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Low earth orbit

LEO  basics

With Low Earth Orbit extending from 200 km to 1200 km it means that it is relatively low in altitude, although well above anything that a conventional aircraft can reach. ,However LEO is still very close to the Earth, especially when compared to other forms of satellite orbit including geostationary orbit.

The low orbit altitude of leads to a number of characteristics:

  • Orbit times are much less than for many other forms of orbit. The lower altitude means higher velocities are required to balance the earth's gravitational field. Typical velocities are very approximately around 8 km/s, with orbit times sometimes of the order of 90 minutes, although these figures vary considerably with the exact details of the orbit.
  • The lower orbit means the satellite and user are closer together and therefore path losses a less than for other orbits such as GEO
  • The round trip time, RTT for the radio signals is considerably less than that experienced by geostationary orbit satellites. The actual time will depend upon factors such as the orbit altitude and the position of the user relative to the satellite.
  • Radiation levels are lower than experienced at higher altitudes.
  • Less energy is expended placing the satellites in LEO than higher orbits.
  • Some speed reduction may be experienced as a result of friction from the low, but measurable levels of gasses, especially at lower altitudes. An altitude of 300 km is normally accepted as the minimum for an orbit as a result of the increasing drag from the presence of gasses at low altitudes.


 Applications for LEO satellites

A variety of different types of satellite use the LEO orbit levels. These include different types and applications including:

  • Communications satellites - some communications satellites including the Iridium phone system use LEO.
  • Earth monitoring satellites use LEO as they are able to see the surface of the Earth more clearly as they are not so far away. They are also able to traverse the surface of the Earth.
  • The International Space Station is in an LEO that varies between 320 km (199 miles) and 400 km (249 miles) above the Earth's surface. It can often be seen from the Earth's surface with the naked eye.

Advantages and Disadvantages of Operating from LEO

Using LEO as an operational zone has certain advantages. It is closer to the Earth's surface and much cheaper and convenient to place a satellite, perform experiments, fix and install new equipment. After the operations are finished, the return trip lasts for a very short time.

However, there are disadvantages as well. The amount of atmospheric gases is low but still significant in these altitudes. These gases cause atmospheric drag that result in orbital decay over time. The satellite will eventually slow down and be pulled back by the Earth's gravity. Another serious problem is the dwell time, or the time a satellite remains above a region. LEO satellites orbit the Earth too quickly ( ~18,000 miles per hour ) and this may cause problems during the operation of a weather or communications satellite over a certain part of the globe.

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