Why are satellites used to relay radio waves?

The answer is quite simple. Basically, the earth is in the way. Imagine you have a signal that is sent out from China and is to be received in the US. It cannot travel directly to the US because the path is blocked by the earth. So the signal is relayed to a satellite above the China, and then that satellite relays the signal to a satellite above China, which is then received on the ground in the US.

It is because Radio Waves cannot travel directly through the Earth.

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