09-08-2016, 05:38 PM
Just a small correction to Eric's great post. The GPS satellites are in medium earth orbit (MEO), not low earth orbit (LEO). LEO encounters problems with interaction with the earth's atmosphere so orbital adjustments are needed, whereas MEO is above those effects although gravitational effects still require adjustments, just not as often. MEO also allows the satellites to orbit two times a day rather than several times a day.