As we know that satellites are the artificial moving devices which move around the earth with earth speed, means these satellites are synchronized with earth speed. So satellite communication means that powerful point to point communication system through which at we can communication around the globe at high speed. Basically, in satellite communication, the satellite is an RF repeater which receives information signals from the earth station.
The satellite amplifies the information signal taking care of attenuation in the signal and then transmits it to the earth station again. Considering the limitation of “line of sight” communication the transmission antenna is mounted on a satellite. These satellites are three in number transmitted into space at the altitude of 22300 miles or 35900km around the globe, which are connected to each other at 1200 angle which cover3600. Although satellite communication system is very expensive the advantage of communicating all over the world at high speed is more important.
In the following figure a satellite communication system is mentioned, where the role of a transponder is clearly magnified.
Satellite communication system can be consists of the following possible parts
- Input filter: This is the first part of satellite communication which receives uplink frequencies (5.9GHz to 6.4GHz) from the earth station. It filters the unwanted distortion from the information signal and drives only 6GHz frequencies to the amplifier.
- 6 GHz amplifier: This amplifier receives 6GHz uplink weak frequencies from the input filter. It amplifies these frequencies and transmits it to the mixer.
- 2.2GHz Local oscillator: Local oscillator generates 2.2GHz frequencies so that the 6GHz amplifier providing frequencies are converted to 4GHz in the mixer.
- Mixer: This part of the satellite receives 6GHz uplink frequencies from the amplifier and 4GHz frequencies from the local oscillator. The mixer mixes it up the frequencies of an amplifier and local oscillator. In result, it converted into 4GHz intermediate frequency (IF). The amplitude of these IF frequencies are so weak so they are amplified through a 4GHz amplifier which is situated just after the mixer.
- Transponder: That equipment which receives the information signal, process it and then retransmit it on different frequencies called transponder. This is the most important part of the satellite communication through which 4GHz information signals are passing through different processes converted into downlink frequency (3.7 to 4GHz) and then transmit it towards the earth station.
A Transponder is Consists of the Following Parts
1) Input multiplexer
2) Traveling wave tube (TWT)
3) Output multiplexer
The input multiplexer receives 4GHz from the amplifier. Which multiplex the channels and frequency band. Then the signal is given to the TWT amplifier which is a specially designed high power amplifier for the microwaves. The output multiplexer converts these signals into downlink frequencies (3.7GHz to 4.2GHz) and transmits it towards the earth station.
Now a day the telephone communication, television transmissions and internet facilities are accessible due to the satellite communication. Satellite communication is now more used for conveyance and spying purposes.