How many satellites are in view at all times?

• A. Five
• B. Four
• C. Six
• D. Three

Answer: (A)

GPS receivers rely on signals from satellites that are visible above the horizon. Four satellites are enough to determine a 3D position plus the receiver’s clock error, but having more satellites in view provides redundancy, improves the geometry of the solution, and enhances reliability and integrity of the fix. In practice, from any location on Earth you can expect about five satellites to be in view at all times, with sometimes more depending on the moment and geometry. So five is the number you should plan on for a robust, continuous navigation solution. If only three or four were in view, the fix would be less reliable and more susceptible to errors, while six or more can occur but isn’t the minimum you should expect.