Q.With a diagram, explain the operation of a communication satellite system.

The onboard satellite device that receives, processes, and retransmits earth-station signals is called a transponder.

A transponder is the key onboard unit of a communication satellite. It:

1. Receives the uplinked signal transmitted from an earth (ground) station,
2. Amplifies the (typically very weak, after travelling thousands of km) received signal, and usually shifts its frequency (from the uplink frequency to a different downlink frequency, to avoid interference between the outgoing and incoming signals),
3. Retransmits (downlinks) the processed signal back toward the Earth, often broadcasting it over a wide coverage area/multiple directions so many ground stations can receive it. …

TV signals travel as line-of-sight space waves that can't follow Earth's curvature, so a satellite is used to 'see over the horizon' and relay the signal across long distances.

Television signals use very high frequencies (VHF/UHF range, tens to hundreds of MHz), which:

• are not reflected by the ionosphere (unlike lower-frequency sky waves used for AM radio), and
• cannot diffract/bend significantly around obstacles or the curvature of the Earth the way ground waves (low-frequency) can.

So TV signals must travel by space wave (line-of-sight) propagation - the transmitting and receiving antennas must effectively be able to 'see' each other. Because Earth is curved, a ground-based transmitting tower can only cover a limited range before the signal is blocked by the horizon (the range is limited by antenna height, roughly $d=\sqrt{2Rh}$, $R$ = Earth's radius, $h$ = antenna height).

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