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|  | | | | | | Orbits
A fundamental principle behind telecommunication satellites is the type of orbit they use. Telecommunications satellites mainly use the Geostationary Earth Orbit (GEO). Other lower altitude orbits are also used, such as Low Earth Orbits (LEO).
The geostationary orbit
Geostationary Earth orbit was first proposed by the late British science and science fiction writer Arthur C. Clarke. Understanding GEO requires a recap of some basic physics.
If an object moves around the Earth with a velocity of more than about 10 kilometres per second, it becomes an artificial satellite. The satellite is able to resist the Earth’s gravitational attraction and will remain circling around the planet without falling back to the surface. The force needed to prevent the moving satellite from following its natural inclination to travel in a straight line is provided by the gravitational attraction of the Earth. The satellite will not need any propulsion and it will continue to orbit the Earth for months or years. Friction due to the upper layers of the Earth’s atmosphere may eventually slow the satellite down and cause it to burn up as it descends into the lower atmosphere.
If a satellite is launched with more energy, it will travel faster and the radius of its orbit will be larger. The larger orbit means that the angular motion of the satellite around the Earth is slower. As a reference, the Moon (Earth’s natural satellite) is 380 000 km from the Earth and its orbital period is 28 days.
If an artificial satellite orbiting in the plane of the equator is given the right amount of energy, its orbit will have a period of 24 hours. The satellite will circle around the Earth at exactly the same angular speed as the Earth rotates about its axis. When observed from the earth, the satellite will appear to remain stationary in the sky. The satellite is then said to be in ‘geostationary orbit’, which occurs at an altitude of 36 000 km. Since the satellite appears to be stationary, an antenna pointed at it will remain correctly pointed year after year, without any need for adjustment.
Satellites in geostationary orbit allow permanent communication links to be established by transmitting radio-frequency signals from fixed antennas. These signals are not very different, from the signals that are used to broadcast terrestrial television, but usually have a frequency between 3 and 50 times higher. The signal is received by the satellite, amplified and transmitted back to the Earth, allowing communications between points that are thousands of kilometres apart.
A particular property that makes geostationary satellites extremely attractive is their capacity to broadcast. Indeed, the signal that is re-transmitted by the satellite can be picked up by antennas anywhere in the coverage area of the satellite. This can be an area the size of a country, a region, a continent, or the face of the Earth that the satellite can see. But the most important effect is that anyone with a relatively small antenna (sometimes as small as 40 or 50 cm in diameter), can become a direct user of the satellite.
Low Earth orbits
Low Earth orbit satellites, such as the Space Shuttle, the International Space Station and many other scientific and Earth observation satellites, have a very high relative angular motion with respect to the Earth. In fact, they orbit the Earth in about 90 minutes at heights of just a few hundred kilometres.
A telecommunication satellite in low Earth orbit will, for example, appear in the southwest and move towards the northeast, being visible for about 10 to 20 minutes at a time. To guarantee continuity of service, a constellation of tens of satellites will have to be deployed. These are more or less equally distributed over the whole Earth.
LEO systems, as they are generically referred to, may require 48, 66, 77, 80 or even 288 satellites to cover the whole planet and provide the required services. Several of these systems have been deployed to provide communications to mobile terminals. They use a relatively low frequency: the L/S bands (between 1.5 and 2.5 GHz), which is in the same range as the frequencies used by GSM mobile telephony networks. The fact that the user's antennas are not very selective is a plus for them: no careful tracking of the satellite is then needed. Also, the low altitude decreases the delay due to signal transit time and reduces the power needed to establish communications.
Last update: 8 April 2009 | |
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