Satellite Transponder

A transponder is the satellite subsystem that receives the uplink, amplifies it, changes its frequency and retransmits it toward Earth.

Bent-pipe transponder: receive antenna $\to$ LNA $\to$ frequency converter $\to$ channel filter $\to$ HPA $\to$ transmit antenna. A regenerative transponder additionally demodulates and remodulates.

Signal path: receive the weak uplink $\to$ low-noise amplify (LNA) $\to$ convert uplink to downlink frequency $\to$ channel-filter the assigned bandwidth $\to$ high-power amplify (HPA) $\to$ retransmit to the coverage area.

Type	Behaviour
Bent-pipe (transparent)	Amplifies and frequency-converts without demodulating
Regenerative	Demodulates, processes/decodes, remodulates and retransmits

Bent-pipe versus regenerative transponder. Regeneration removes uplink noise before the downlink but needs onboard processing.