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.