The Global Positioning System (GPS) consists of 24 communication satellites distributed on six tracks. These satellites orbit the earth twice a day, with an angle of about 55 degrees to the equator. These 24 satellites continuously transmit coded positioning and timing information at high frequencies (1500 MHz range).
The GPS terrestrial receiver receives the signal and uses the coded information to calculate the position of the Earth coordinate system. The receiver determines the position by calculating the time it takes for the wireless signal transmitted from each satellite to reach the receiver. Multiply the time by the speed of light to get the distance from the receiver to each satellite: distance = speed of light x time. Time can be obtained by the exact code matching technique in the GPS receiver. The position of each satellite is encoded in the transmitted signal. With this data, the receiver can calculate the three-dimensional coordinates of the location on the earth.
Differential (DGPS) is a base station where a high-precision GPS receiver is installed at a location where it is accurately measured. The position measured by the base station receiving the GPS satellite signal is compared with the known position of the own station to obtain the position measurement error or pseudorange measurement error, and these errors are transmitted as correction values to the surrounding space. The GPS user receiver near the base station receives the correction signal from the base station and corrects its own GPS measurement value, thereby greatly improving the positioning accuracy.