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Do you know how a communications satellite works?

How do the signals travel? How are frequency bands chosen? What’s special about geostationary orbit? How are the orbital locations of the satellites regulated?

A communications satellite works like a relay station: signals transmitted by the ground stations are picked up by the satellite’s receiver antennas, the signals are filtered, their frequency changed and amplified, and then routed via the transmit antennas back down to Earth. In some cases the signal is first processed by digital computers on board the satellite, as for example for highly specific missions such as Inmarsat-4 or Skynet 5. Most satellites, however, are ‘transparent’, in that they retransmit the signal without modifying it – their role is simply to deliver the signal exactly to where it is required.

How do the signals travel?

The signals are delivered by carrier waves, modulated by frequency, amplitude, or other methods. Each signal has its own frequency and bandwidth. The larger the bandwidth, the more information the signal can carry.

Choosing frequency bands

To transmit a signal with lots of information (voice + image + data) a wide band must be used. Modern telecommunications media primarily use six frequency bands, designated by letters.Signals and frequencies

The data transmission rate depends directly on the bandwidth used to carry a signal, independently of the modulated carrier wave. Higher frequencies such as the Ka-band, however, can more easily accommodate large bandwidths, and thus transmit more information than L-band, for instance, where less bandwidth is available and there is greater competition amongst users.

The choice of frequency band depends on type of application and bandwidth requirements, propagation conditions, existing ground infrastructure and what ground equipment is necessary.

The higher the frequency, the more the beams that are generated can be targeted for a given antenna size: the energy is better concentrated and the same spectral band can be reused for non-adjacent zones (‘cells’).

Bande

Frequency range

Applications

L

1 to 2 GHz

Mobile telephony and data transmission

S

2 to 3 GHz

Mobile telephony and data transmission

C

3.4 to 7 GHz

Fixed telephone services, radio broadcast services, business networks

X

7 to 8.4 GHz

Government or military communications, encrypted for security reasons

Ku

10.7 to 18.1 GHz

High data-rate transmission, television, videoconferencing, business networks

Ka

18.1 to 31 GHz

High data-rate transmission, television, videoconferencing, business networks

Each user system is assigned a specific part – or ‘slot’ – of the overall band. Frequency bands are assigned according to standard guidelines set by the International Telecommunications Union (ITU), depending on the service to be provided, and coordination between operators must be maintained so as to avoid any interference between satellites.

Keeping pace with the Earth: geostationary orbit

Most communications satellites are positioned in geostationary orbit.

Geostationary orbit is a circular orbit, situated directly over the equator. A satellite positioned in geostationary orbit circles at the same speed and in the same direction as the Earth rotates, meaning that it stays ‘fixed’ in relation to a point on the ground. Geostationary orbit is at an altitude of about 36,000 km, or 22,380 miles (in fact, it is exactly 35,784 km) – a distance equal to six times the radius of the Earth – with an orbital period of 23 hours 56 minutes.

Why not exactly 24 hours? This is due to the small difference between the time the Earth takes to rotate Geostationary orbitaround its own axis and the length of one day. A day is slightly longer than the Earth’s period of rotation because at the same time the planet is orbiting around the sun: there are 365 days in a year, but the Earth completes 366 circles of the sun. This gives a difference of about four minutes between the ‘sidereal (or stellar) day’ and the ‘solar day’.

Geostationary orbit is therefore particularly well suited to communications applications, since the ground antennas, which must at all times be pointed towards the satellite, do not have to be equipped with a system for swivelling to track the satellite. A familiar example is a domestic satellite dish used to receive satellite television signals, which must always be aimed precisely at the point in the sky where the satellite is located.

 

 

Read more:

 A special dossier full of informative articles, animation and videos has been created about telecommunications satellites.

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