说明：  GPS 主系统是居心叵测的老美发射运行的卫星系统，包含了27 颗能持续发送地理位置海拔高度和时间信号 的卫星，24 个正常使用，3 个备用，这些卫星平均分布运行在六个轨道上。一般来说，在地面上的GPS 接 收器能接收5～12 个卫星信号，而为了获得地面上的定位坐标，GPS 导航至少需要4 个卫星信号，三个用 来确定GPS 接收器的纬度、经度和海拔高度，第四个则提供同步校正时间
(GPS is the main system is very much ill-old US-run satellite launch system, contains 27 to continue to send location and time of the altitude of the satellite signal, 24 normal use, three back-up, the average distribution of these satellites orbit in six runs . Generally speaking, on the ground of the GPS receiver can receive 5 to 12 satellite signals, while on the ground in order to obtain location coordinates, GPS navigation needs at least four satellite signals, the three GPS receivers used to determine latitude, longitude and altitude, the fourth time to provide simultaneous correction)