说明：  The Navstar Global Positioning System (GPS) is a space-based radio-positioning and timetransfer system. GPS provides accurate position, velocity, and time (PVT) information to an unlimited number of suitably equipped ground, sea, air and space users. Passive PVT fixes are available world-wide in all-weathers in a world-wide common grid system. Normally GPS contains features which limit the full accuracy of the service only to authorized users and protection from spoofing (hostile imitation).