Satellite Orbits
A Dish Network satellite TV system is based off signals
transmitted and received by multiple satellites. But, the main
satellites in space are constantly moving in what is called an
orbit.
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. However, if
you're a Dish Network customer, you don't need to worry about
the Dish Network satellite falling to earth and disintegrating
into nothing. Unless a space alien shoots it down, the Dish
Network satellites are there to stay.
If you're interested in learning more about Dish Network and
satellite TV, go to http://www.dish-network-satellite-tv.ws/