Which statement correctly describes a satellite transponder used in communications satellites?

Introduction:
A satellite transponder is the fundamental “bent-pipe” element aboard a communications satellite. It listens to uplink signals, conditions them (filtering, frequency translation, amplification), and retransmits them back toward earth on a designated downlink frequency. Understanding this basic operation helps differentiate correct and incorrect statements about satellite links.

Given Data / Assumptions:

• We consider a conventional bent-pipe communications satellite (not regenerative on-board processing).
• Standard operation involves receive chain, frequency conversion, amplification, and transmit chain.
• Uplink and downlink use different frequency bands to avoid interference.

Concept / Approach:
A transponder includes a receiver (LNA, filtering), a frequency converter/translator, and a power amplifier feeding the transmit chain. It does not “echo” without change; it performs translation and amplification. Also, for most services, the satellite receives at a higher uplink frequency and transmits at a lower downlink frequency (e.g., C-band, Ku-band), contrary to the misleading claim about the direction of frequency change.

Step-by-Step Solution:

Verification / Alternative check:
Industry diagrams show transponders converting uplink to a different downlink frequency while applying gain and filtering, confirming the receiver-transmitter characterization.

Why Other Options Are Wrong:

• Echo without change: ignores frequency translation and amplification.
• Transforms code: regenerative satellites can process, but basic transponders do not change message coding.
• Lower RX, higher TX: reversed; satellites typically receive higher-frequency uplinks and transmit lower-frequency downlinks.
• None of the above: one option is correct.

Common Pitfalls:
Assuming satellites behave like passive mirrors; confusing regenerative processing with bent-pipe relaying.

Final Answer:
A satellite transponder contains a receiver and a transmitter that relay microwave transmissions between earth stations.