From Wikiwand: A visual example of a 24 satellite GPS constellation in motion with the earth rotating. Notice how the number of satellites in view from a given point on the earth& surface, in this example at changes with time.
Global Positioning System Overview - Providing an overview of Global Positioning System GPS. The official documents of GPS have a lot of details and they are difficult to digest. This website is user friendly and it provides a basic explanation on the design philosophy of GPS coupled with a lot of important parameters. More importantly, examples and pictorial illustrations are used such that readers can easily understand and grasp all the essential elements of GPS. Martin, EIE