The Four-Satellite Rule
The GPS constellation is engineered so that — from any open spot on Earth, at any time — at least four satellites are above the horizon. Four is the minimum required to solve for your three-dimensional position plus the receiver's clock error. Modern phones often see 8–12 GPS satellites simultaneously, and even more when combined with Galileo, GLONASS and BeiDou.
Urban Canyons and Cold Starts
Coverage degrades in deep urban canyons, dense forests and indoors. That's why assisted-GPS (A-GPS) was invented: your phone downloads the constellation almanac over cellular instead of waiting 12.5 minutes to receive it from a slow 50 bps satellite downlink. Deep dive in our GPS technology guide.