Who Owns the Largest Cellular Network in the U.S.?

onyx00 asks: "Cellular phones are everywhere nowadays. It seems that anywhere you go almost everyone has a cellular phone. Originally the high cost of air-time hindered their success, but now even ten-year-olds can afford to have one. With the many different cellular companies however, I wonder who owns the largest segments of the cellular networks? Some companies offer nationwide service, but is this by their own network, or by use of others? Who really "owns" the cellular network? Do any companies actually use satellites to help link their cellular segments?"