A low Earth orbit (LEO) is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km. Given the rapid orbital decay of objects below approximately 200 km, the commonly accepted definition for LEO is between 160–2,000 km (100–1,240 miles) above the Earth's surface. The sideways speed needed to achieve a stable low earth orbit is about 7.8 km/s, but reduces with altitude.
LEO (Low Earth Orbit) satellite communication systems is a
recent development of systems for mobile satellite communications that now
exist, such as Inmarsat, AMSC. Mobile Satellite systems (satellites for mobile
communications) are in operation today use satellite traveling 36,000
kilometers on the surface of the Earth and have STP 24-hour time. With a career
that coincides with the equatorial zone, from a point on the Earth, the
satellite appears as if the movement Geostationary Earth Orbit (GEO).
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.
Space debris in LEO
Apart from the general congestion experienced in Low Earth
Orbit, the situation is made much worse by the general level of space debris
that exists.
There is a real and growing risk of collision and major
damage - any collisions themselves are likely to create further space debris.
The US Joint Space Operations Center currently tracks over 8
500 objects that have dimensions larger than 10 centimetres. However debris
with smaller dimensions can also cause significant damage and could render a
satellite unserviceable after a collision.
Human Use
The International Space Station is in a LEO that varies from
320 km (199 mi) to 400 km (249 mi) above the Earth's surface.
While a majority of artificial satellites are placed in LEO,
where they travel at about 7.8 km/s (28,080 km/h), making one complete
revolution around the Earth in about 90 minutes, many communication satellites
require geostationary orbits, and move at the same angular velocity as the
Earth. Since it requires less energy to place a satellite into a LEO and the
LEO satellite needs less powerful amplifiers for successful transmission, LEO
is still used for many communication applications. Because these LEO orbits are
not geostationary, a network (or "constellation") of satellites is
required to provide continuous coverage. Lower orbits also aid remote sensing
satellites because of the added detail that can be gained. Remote sensing
satellites can also take advantage of sun-synchronous LEO orbits at an altitude
of about 800 km (500 mi) and near polar inclination. ENVISAT is one example of
an Earth observation satellite that makes use of this particular type of LEO.
Sources:
( http://en.wikipedia.org/wiki/Low_Earth_orbit)
(http://www.helmigaala.net/low-earth-orbit-satellite-communication-systems.html)
(http://www.helmigaala.net/low-earth-orbit-satellite-communication-systems.html)
