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09 Mar 2008 03:50:23 | Gary Davis
Satellite Orbits By Gary Davis
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Satellite TV. Any object that moves around the earth has an
orbit. The orbit is defined by 3 factors. The first is the shape
of the orbit, which can be circular or elliptical. The second is
the altitude of the orbit. The altitude is constant for a
circular orbit but changes constantly for an elliptical orbit.
The third factor is the angle the orbit makes with the equator.
An orbit that brings the satellite over the poles or close to it
has a large angle. An orbit that makes the satellite stay close
to the equator has a small angle. Orbits depend on the
mission the satellite was built for. The following orbits can be
defined: Low Earth Orbit
Satellites in low earth orbit (LEO) orbit the earth at altitudes
of less than 2000 km (1242 miles). Satellites in LEO can get
much clearer surveillance images and require much less power to
transmit their data to the earth. Medium Earth Orbit At an altitude of
around 10.000 km (6.000 miles) a satellite is in medium earth
orbit (MEO). This altitude balances the advantages and
disadvantages of LEO and GEO. MEO’s are used generally for
navigational satellites and communications satellites.
Geostationary Orbit A satellite in geostationary orbit
orbits the earth in exactly 1 day and is placed above the
equator. The angle with the equator is 0 degrees. As a result
the satellite seems to stand still as seen from the earth. These
satellites are used for communications and Satellite TV.
Polar Orbit An orbit that goes
over both the North and the South Pole is called a Polar Orbit.
The angle with the equator is 90 degrees. The advantage of these
orbits is that they go over the poles. This may sound funny, but
in reality most satellites never “see” the poles. Most polar
orbits are in LEO, but any altitude can be used for a polar
orbit. Polar orbits are used a lot by navigation satellites
which have to provide navigational information all over the
world, including the poles. Sun
Synchronous Orbit This is a very interesting type of
orbit. Satellites in Sun Synchronous Orbit pass over the same
locations on earth at the same time each day. Suppose a
satellite in sun synchronous orbit passes over your house at 3
pm. The next time the satellite will pass over your house is 24
hours later at the same time the next day. In order to do
this the satellite has a very special orbit. Not only does it
orbit the earth, but the plane of the orbit changes as well. It
has to do this because each day the earth moves a bit through
space around. After 3 months, the earth has moved 90 degrees of
its orbit around the sun. If the satellites orbit wouldn’t have
moved with it, it would show up 6 hours later than planned.
(Actually the earth spins around its axis in approx. 23 hours,
56 minutes and 3 seconds and not in 24 hours. Because of the
earth’s orbit around the sun, 1 day lasts 24 hours. A star day
however is approx. 23 hours, 56 minutes and 3 seconds long.)
However, there is no need to actively change the plane of the
orbit. The earth is not a perfect sphere but is a little bit
wider around the equator. This is caused by the spinning of the
earth. The gravitational difference this causes changes the
orbit of a satellite. When the angle with the equator is chosen
correctly (approx. 8 degrees of a polar orbit), an orbit is a
sun synchronous orbit. A very special type of sun
synchronous orbit is called the dawn-to-dusk orbit. This orbit
is above the earth where the sun comes up or goes down. A
satellite in this orbit never enters the shadow of the earth but
always receives sunlight. Orbit Decay
and Reentry The earth’s atmosphere doesn’t stop at a
certain altitude but really fades out into space. The higher you
get the less thick it is until eventually there is no more
atmosphere. Generally we say that the atmosphere is about 100 km
(62 miles) thick, but in reality it extends much further out
into space. Satellites experience friction from the atmosphere
up to altitudes of 1000 km (620 miles). Due to the friction,
a satellite will loose speed and with that the altitude will
decrease until eventually the satellite will fall out of orbit
back to the earth. Depending on the altitude this happens sooner
or later. A satellite at an altitude of 200 km (124 miles) will
stay in orbit for just a couple of months. At 300 km (186 miles)
a satellite can stay in orbit for a couple of years. Above 1000
km (620 miles) a satellite can stay in orbit for thousands of
years. Letting Satellites fall back to earth and burn up in
the atmosphere is also a way of disposing of satellites.
About the author:
Gary Davis is owner of Dish Network
Satellite TV, has several years experience in the Satellite
TV Industry and has written several articles on satellite TV.
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