The denial of GPS services during Kargil War has driven ISRO to develop Indian Regional Navigation Satellite System (IRNSS), superior to GPS

The Global Positioning System (GPS), is a global navigation satellite system (GNSS) that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Each one transmits information about its position and the current time at regular intervals.  These signals, traveling at the speed of light, are intercepted by your GPS receiver. GPS receivers then calculate the distance of each satellite based on how long it took for the messages to arrive. It requires only three satellites to provide the location. The other satellites add to the accuracy. The more satellites there are in the sky, the more accurately GPS can tell you about your location.