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Tuesday, November 22, 2016

Technology Updates: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.  Most satellites,  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. 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

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 around 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 swiveling 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.

Source and Credit :- http://www.space-airbusds.com/en/news2/do-you-know-how-a-communications-satellite-works.html

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