How iGPS device works

For a GPS device to calculate a location (its latitude and longitude), it needs to know where the satellites are and how far away they are. Supposing it gets a fix on one satellite first and it calculates the distance to the satellite as 12,000 kms. By that logic, it could be anywhere on the surface of an imaginary sphere with a radius of 12,000 kms and with the satellite at the center. Once four satellites get locked on, all these imaginary spheres will intersect at only one common point: the current location. There are variations too: A-GPS, or Assisted GPS uses GSM cell locations to augment tracking; WAAS or Wide Area Augmentation System uses satellites and ground stations that provide GPS signal corrections, increasing accuracy. There can be an unlimited number of simultaneous GPS users. This is because a GPS device only receives signals transmitted from the satellites to coordinate a location. They do not communicate with the satellites.