A communication satellite is like an international messenger, acting as a relay station for radio communication. Collect all kinds of "letters" from underground and then "deliver" them to users in different places. Because it "stands" at an altitude of 36,000 kilometers, its "delivery" coverage is particularly large, and a satellite can be responsible for the communication of the earth's surface13. If three communication satellites are evenly placed in the geostationary orbit, except the north and south poles, global communication can be realized.