The system is also used for a wide range of geoscience applications. Gps signals are broadcast from a constellation of 24 or more earth orbiting satellites. As the gps signals are derived from the atomic frequency standards on board each satellite, they are widely used as a reference for time synchronization and frequency calibration and used in any weather conditions. These systems often use satellites, receivers and the software that allow users to determine the exact geographic position. The gps receivers convert sv signals into position, velocity, and time estimates. They have widely adopted in personal tracking, navigation and automatic vehicle located technologies etc.