The Global Positioning System: How many satellites are required to calculate a position?
When you use your GPS receiver you are asking it to find your position on the earth. There are four components required to answer that question.
Three of them are geometric, usually symbolized as X, Y and Z. These are your old friends, Cartesian coordinates. And there is a fourth - time - symbolized T. This is important to know because the time it takes the GPS signal to leave the satellite and reach you provides the distance between you and the satellite, the key to the whole thing in a way.
Since there are four unknowns you have to have at least four GPS signals to answer the question, "Where am I?" It's just like those wonderful simultaneous equation you had so much fun with in school. Four unknowns requires four equations for resolution.
Jan Van Sickle, Ph.D.
GPS, GNSS and GIS