Introduction

A satellite communications system can be broadly divided into two segments,a ground segment and a space segment. 

The space segment will obviously include the satellites, but it also includes the ground facilitiesneeded to keep the satellites operational, these being referred to as the tracking, telemetry, and command (TT&C) facilities. In many networks it is common practice to employ a ground station solely for the purpose of TT&C.

The equipment carried aboard the satellite also can be classified according to function. The payload refers to the equipment used to provide the service for which the satellite has been launchedThe payload refers to the equipment used to provide the service for which the satellite has been launched. The bus refers not only to the vehicle which carries the payload but also to the various subsystems which provide the power, attitude control, orbital control, thermal control, and command and telemetry functionsrequired to service the payload.

In a communications satellite, the equipment which provides the connecting link between the satellite’s transmit and receive antennas is referred to as the transponder. The transponder forms one of themain sections of the payload the other being the antenna subsystemsmain sections of the payload, the other being the antenna subsystems.

In this chapter the main characteristics of certain bus systems and payloads are described.

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Fig. shows the channeling scheme for the 12 transponders. The incoming, or uplink, frequency MHz band to the common receiver -500The input filter passes the full . GHz425 .6to 925 .5range is

while rejecting out-of-band noise and interference. The frequency conversion shifts the carriers to At thiGH24t73MH id t di f500hi h i lth d li k f b d At this GHz. 2 .4to 7 .3MHz wide, extending from 500 , which is also the downlink frequency band

point the signals are channelized into frequency bands which represent the individual transponder bandwidths.

MHz guard bands -4MHz for each transponder, which along with 36 A commonly used value is gp , gytransponder 12 MHz available bandwidth to accommodate 500 between channels allows the

channels.

A t d h dl d l t d i h TVA transponder may handle one modulated carrier such as a TV signal, or it may handle a number of separate carriers

simultaneously, each modulated by its own telephony or other baseband channel.