精选优质文档-倾情为你奉上Unit 20 Understanding the GPS（认识 GPS）What Is GPS?（什么是 GPS）The global Positioning System (GPS) operated by the U.S. Department OF Defense (DOD)is a satellite-based system that can be used to locate positions anywhere on the earth. GPS provides continuous (24hours/day),real-time,3-dimensional positioning, navigation and timing worldwide. Any person with a GPS receiver can access the system, and it can be used for any application that requires location coordinates. The GPS sy
