Communication Transponders

  Hello Dear Reader,

Here in this post, I will give an idea about different types of Communication Transponders and it's working principles.

The use of orbiting satellites as an integral part of the worldwide communication system has progressed rapidly over the last several decades. As the technology and hardware of such systems continue to make significant advancements, it is expected that satellite will continue to play an ever-increasing role in the future of long-range communication. Each new generation of a satellite is more technologically sophisticated than its predecessors, and each undoubtedly will have a significant impact on the development and capabilities of the military, domestic, and international communication systems.

A communication satellite is basically an electronic communication package place in orbit around the earth. The prime objective of the satellite is to aid communication transmission from one point on or near earth to another like voice(telephone) and TV Broadcast. In modern systems, however, the earlier classification of communication services into FSS, BSS, and MSS has been expanded to include emerging services like DTH, Digital broadcasting, telehealth and telemedicine, distant learning, interactive multimedia & TV data relay, disaster management, and satellite navigation.

Fig-1: Block Diagram Of Bent pipe Repeater

Fig-1 shows a generalized block diagram of a typical Bent pipe repeater. The path of each channel from the receiving antenna to the transmitting antenna is a 'transponder'( known so as it transmits the downlink by responding to the uplink signal from the ground). The RF front end receives the modulated carries in the form of electromagnetic fields from the ground transmitting antenna to the satellite antenna. The satellite collects the impinging electromagnetic field, amplifies the uplink carrier while filtering off as much receive and retransmit an amplified version of the same uplink waveform at the same satellite will cause unwanted feedback, or ring around, from the downlink antenna back into the receiver. For this reason, satellite repeaters must involve some form of frequency translation prior to the power amplification.

The translation shifts the uplink frequencies to a different set of downlink frequencies so that some separation exists between the frequency bands. This separation allows frequency filtering at the satellite uplink antenna to prevent rinng around from the transmitting(downlink) frequency band. Frequncy band separation also allows the same antenna to be used for both receiving and transmitting simplifying the satellite hardware. Also, the available bandwidth is usually divided equally between the number of transponders operating in that bandwidth. Thus the main function of a transponder is to transpose the uplink frequency(receive) to downlink frequency(transmit) along with a) receiver function b) channelization function and c) transmitter function. All of thesefunctions must be performed within the power, mass and environmental constraints imposed by the overall spacecraft design.

A transponder has interfaces with a satellite platform or bus which includes power systems, thermal systems, telemetry, and telecommand system, etc.

Fig-2: MSS Forward and Return Link Transponder

Space segment for Mobile Satellite Services (MSS) consists of two transponders [Fig-2], one for the forward link (CXs) and one for the return link (SxC) along with platform or bus. The communication link between two mobile users will be established via hub station making it a double-hop. The communication link from the hub station to the user terminal is called Forward Link while the link from a user terminal to a hub is known as Return Link.

The establishment of communication between two mobile stations in Fig-3.

Fig-3: Establishment of Communication between two mobiles through a Hub

A satellite that electronically operates on the received uplink to reformat it in some way prior to retransmission is called a processing satellite. In more sophisticated processing satellites the uplink carrier waveforms are actually reformatted or restructured, rather than merely frequency translated, to form downlink. Fig-4 show the typical schematic of on an onboard processing payload.

Fig-4: Basic Elements of a Regenerative Transponder

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