Page 198 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes 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 (figure 12.2) is the Polar Orbit. This is a LEO with a high inclination angle
(close to 90degrees). This means the satellite travels over the poles.

Figure 12.2: LEO Orbit

Source: http://www.satcom.co.uk/article.asp?article=11
Figure 12.3: Polar Orbit

Source: http://www.satcom.co.uk/article.asp?article=11
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 (figure 12.3) will pass over every region of Earth
so can provide global coverage. Also a satellite in such an orbit will sometimes appear overhead

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