

When a satellite is launched, it is placed in orbit around the Earth. The Earth's gravity holds the satellite in a certain path as it goes around the Earth, and that path is called
an "orbit." 

There is only one main force acting on a satellite when it is in orbit, and that is the gravitational force exerted on the satellite by the Earth. This force is constantly pulling
the satellite towards the centre of the Earth. 

A satellite doesn't fall straight down to the Earth because of its velocity. Throughout a satellites orbit there is a perfect balance between the gravitational force due to the
Earth, and the centripetal force necessary to maintain the orbit of the satellite. 

The formula for centripetal force is: F = (mv^{2})/r The formula for the gravitational force between two bodies of mass M and m is = (GMm)/r^{2} 

The most common type of satellite orbit is the geostationary orbit. This is described in more detail below, but is a type of orbit where the satellite is over the same point of Earth
always. It moves around the Earth at the same angular speed that the Earth rotates on its axis. 

We can use our formulae above to work out characteristics of the orbit. 

These launch vehicles are used to send our satellites into space 

(mv^{2}/r) = (GMm)/r^{2}v^{2}/r = (GM)/r^{2Now, v = (2pr)/T}=> (((2pr)/T)^{2})/r = (GM)/r^{2}=> (4p^{2}
r)/T^{2} = (GM)/r^{2}=> r^{3} = (GMT^{2})/4p^{2} 

We know that T is one day, since this is the period of the Earth. This is 8.64 x 10^{4} seconds. We also know that M is the mass of the Earth, which is 6 x 10^{24}
kg. Lastly, we know that G (Newton's Gravitational Constant) is 6.67 x 10^{11} m^{3}/kg.s^{2}So we can work out r.
r^{3} = 7.57 x 10^{22
}Therefore, r = 4.23 x 10^{7} = 42,300 km.
So the orbital radius required for a geostationary, or geosynchronous orbit is 42,300km. Since the radius of the Earth is 6378 km
the height of the geostationary orbit above the Earth's surface is ~36000 km. 

There are several kinds of orbits. Here are three of them. There are many different types of orbits used for satellite telecommunications, the geostationary orbit described above is
just one of them. Outlined below are the most commonly used satellite orbits. The orbits are sometimes described by their inclination  this is the angle between the orbital plane and the
equatorial plane. 

Geostationary Orbit 

The most common orbit used for satellite communications is the geostationary orbit (GEO). This is the orbit described above – the rotational period is equal to that of the Earth. The
orbit has zero inclination so is an equatorial orbit ( located directly above the equator ). The satellite and the Earth move together so a GEO satellite appears as a fixed point in the sky from
the Earth. The advantages of such an orbit are that no tracking is required from the ground station since the satellite appears at a fixed position in the sky. The satellite can also provide
continuous operation in the area of visibility of the satellite. Many communications satellites travel in geostationary orbits, including those that relay TV signals into our homes. 

However, due to their distance from Earth GEO satellites have a signal delay of around 0.24 seconds for the complete send and receive path. This can be a problem with telephony or
data transmission. Also, since they are in an equatorial orbit, the angle of elevation decreases as the latitude or longitude difference increases between the satellite and Earth station. Low
elevation angles can be a particular problem to mobile communications. 

Low Earth Orbit/Medium Earth Orbit 

A Low Earth Orbit (LEO), or Medium Earth Orbit (MEO) describes a satellite which circles close to the Earth. Generally, LEOs have altitudes of around 300 – 1000 km with low
inclination angles, and MEOs have altitudes of around 10,000 km. 

A special type of LEO is the Polar Orbit. This is a LEO with a high inclination angle (close to 90 degrees). This means the satellite travels over the poles. 

Satellites that observe our planet such as remote sensing and weather satellites often travel in a highly inclined LEO so they can capture detailed images of the Earth's surface due
to their closeness to Earth. A satellite in a Polar orbit will pass over every region of Earth so can provide global coverage. Also a satellite in such an orbit will sometimes appear overhead
(unlike a GEO which is only overhead to ground stations on the equator). This can enable communication in urban areas where obstacles such as tall buildings can block the path to a satellite.
Lastly, the transmission delay is very small. 

Any LEO or MEO system however, for continuous operation, requires a constellation of satellites. The satellites also move relative to the Earth so wide beam or tracking narrow beam
antennas are needed. 

Elliptical Orbits 

A satellite in elliptical orbit follows an ovalshaped path. One part of the orbit is closest to the centre of Earth (perigee) and another part is farthest away (apogee). A satellite
in this type of orbit generally has an inclination angle of 64 degrees and takes about 12 hours to circle the planet. This type of orbit covers regions of high latitude for a large fraction of
its orbital period. 





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