Please help me in removing my mistake, since i see 3 satellite required to locate an place on earth and synchronization of receiver clock with gps satellite. [ I have assumed at any moment a place on earth will see certain satellite at a certain distance from it in the sky. That is a location can be known, if it can compute the distances of visible gps satellite. Information transmitted by each satellite include time of transmission and satellite identity ] If the receiver side clock is not synchronized with the atomic clock of the satellites, the distances from 3 satellite will not point to a single location on the surface of the earth. So the receiver clock adjust its time, so that the distances from the satellite fall on one location on earth, which is the exact location of the receiver and as well, the receiver clock get synchronized. So why do we require 4th satellite for. What am i missing? Thanks in advance for your help.