Why does GPS not work near buildings and other tall objects?

Your device requires signals from GPS satellites in order to work, so it needs a clear view of the sky. Dense materials (such as concrete, rock, steel, heavy wood), tall buildings, terrain features (like mountains, ridges, and cliffs), and large trees can block or degrade GPS signals and may make it impossible to determine your location.

GPS satellites are not always immediately overhead and could be anywhere in the skies at any time, even low on the horizon; however, there are enough satellites in orbit so that four or more will be view of your device if not blocked by large objects, terrain, or dense materials.