ABSTRACT
The core technology is the Global Navigation Satellite System (GNSS) that can precisely fix positions anywhere on/near Earth. In fact, GNSS is a global infrastructure that enables many different location-sensitive applications, making economies more efficient and effective. At present, there are four GNSSs: the US GPS (Global Positioning System ), EU Galileo, Russian GLONASS, and Chinese BeiDou. Nowadays in the US, GPS is widely used for road navigation. Compared to looking for street signs, highway exits, house numbers, and following maps, GPS navigation makes life so much easier and safer, especially when traveling at night or in an unfamiliar area. GPS satellites fly in medium Earth orbit at an altitude of approximately 20,200 kilometers. A GNSS uses time to determine location. Satellites serve as lighthouses in the sky with atomic clocks to deduce accurate locations of objects on or near Earth.
