This means that LEO satellites must be moving faster than MEO satellites. In a circle 35860km above the equator, there is an area where the satellites orbit the earth at exactly the same speed as the earth rotates beneath them. This means that those satellites appear to stand still as far as someone on earth is concerned. These satellites are the ones we receive our television over. Whether in low, medium or geosynchronous orbit, satellites use radio communication to send messages to the satellite and then, after possibly being sent to other satellites, that message is sent down to the receiver on earth.
Satellites can provide continuous communication coverage over most of the earth.
This system offers voice, facsimile, data and distress communication systems everywhere on the earth except the north and south poles. Dolphins and whales , for example are able to communicate over long distances using underwater sounds. They can tell where and what other objects in the water are using their sonar. In their songs , they also communicate with one another. Although radio doesn t work very well, particularly in salt water, humans can communicate underwater in similar ways to the dolphins and whales.