Page 197 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 12: Space-based Wireless WANs

The most common type of satellite orbit is the geostationary orbit. This is described in more detail Notes
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.
(mv /r) = (GMm)/r 2
2
→ v /r = (GM)/r 2
2
Now, v = (2πr)/T.
→ (((2πr)/T) )/r = (GM)/r 2
2
2
2
→ (4π r)/T = (GM)/r 2
→ r = (GMT )/4π 2
2
3
We know that T is one day, since this is the period of the Earth. This is 8.64 x
10 seconds. We also know that M is the mass of the Earth, which is 6 x 10 kg.
24
4
Lastly, we know that G (Newton’s Gravitational Constant) is 6.67 x 10 m /kg.s 2
3
-11
So we can work out r.
r = 7.57 x 10 22
3
Therefore, r = 4.23 x 10 = 42,300 km.
7
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 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)
(Figure 12.1). 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.

Figure 12.1: Diagram of Geostationary Orbit

Source: http://www.satcom.co.uk/article.asp?article=11

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