Sketch block diagram of satellite earth station and state functions of each block
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Satellite earth station: The communication is established to the satellite through earth station. The earth station can be located on the ship at the sea, or it can be located on the space craft or actually on the earth. The location of the earth station is decided depending upon the ease of control of satellite and the function of the satellite. 

The type of earth station depends upon the (a) function of the station, (b) type of service, (c) frequency bands used, (d) transmitters, (e) receiver and (f) antenna characteristics.

The four major subsystems of any earth station are, receiver, antenna, transmitter and tracking equipment. The special earth station like TVRO ( TV receiver only or direct broadcast satellite receivers) does not have transmitting function. Some other earth stations are very special, like tracking and control of satellite.

The baseband signal is applied to the encoder. Encoder converts the format ready for modulation. The carrier is modulated by the encoded baseband signal. The modulated carrier is then upconverted to the uplink frequency of the satellite. The amplifier then amplifies this signal to high power level, ready for transmission. The signal is then passed through the polarization feed of the antenna. The signal received from the antenna is of different frequency (downlink frequency) and is very small in amplitude. This signal is amplified by the low noise amplifier. It is then down converted to the intermediate frequency by the down converter. This signal is then demodulated and decoded to get baseband signal
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The figure below shows the general block diagram of an earth station capable of transmission, reception and antenna tracking. The following are the major subsystems of the earth station –


Transmitter: There may be one or many transmit chains depending on the number of separate carrier frequencies and satellites with which the station must operate simultaneously. It consists of MUX, modulators and filters, HPA. Microwave transmitters are expensive devices that employ costly HPA’s such as TWTA and multi-cavity klystrons.


Receiver: There may be many receiver chains depending on the number of separate frequencies and satellites to be received and various operating conditions. The receiver subsystem consists of LNA and filters, down convertors, filters, demodulators and DEMUX equipment.


Antenna: Usually one antenna is used for both transmission and reception but not necessarily. Within the antenna subsystem are the antenna reflector and feed, separate feed systems to permit automatic tracking and a duplexer and MUX arrangement to permit simultaneous connection of many transmitters and receiver chains to the same antenna.


Tracking System: This comprises of control circuit and drive which are necessary to keep the antenna pointed at the satellite. Tracking system keeps antenna pointing in the direction of the satellite in spite of relative movement of the satellite and the station. 


Terrestrial Interface: This is the interconnection with whatever terrestrial system if any is involved. In case of small receive only and transmit only stations, the user may be at the earth station itself. 


Power Subsystem: This system includes the primary sources (the standard AC lines) for running the earth station. The subsystem operates power supplies which distribute a variety of dc voltages to the other equipment. The power subsystem also consists of emergency power sources such as diesel generators, batteries and inverters to ensure continuous operation during power failures. It often includes provision for no break changeover from one source to another.


Test Equipment: This includes the equipment necessary for routine checking of the earth station and terrestrial interface, possible monitoring of satellite characteristics and occasionally for the measurement of special characteristics. 

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