1. GPS system composition

　　GPS gloabal Positioning System, which is built in the United States. It is mainly divided into three parts, the control station on the ground, the satellites flying in the sky, and the receiver in our hands.

　　Let's talk about the equipment first. The satellite is prepared by the United States. As for the main control station, of course, it is in the mainland of the United States, in Colorado. Three ground antennas, five monitoring stations, distributed globally. The main tasks are collecting data, calculating navigation information, diagnosing system status, and scheduling satellites. There are 27 satellites in the sky, 20,200 kilometers from the ground. 24 of the 27 satellites are operational and 3 are spare. These satellites have been updated with five models in three generations. The satellite transmits two signals: L1 and L2. L1: 1575.42MHZ, L2: 1227.60MHZ. The clock on the satellite uses a cesium atomic clock or a rubidium atomic clock, and plans to use a hydrogen atomic clock in the future. In hand, is the receiver. Large and small, various poses and expressions, there are pocket, backpack, vehicle, ship, airborne and so on. Common handheld receivers receive L1 signals, as well as dual-frequency receivers for precise positioning.

　　2. About GPS receiver

　　GPS is now generally 12 channels, which can receive 12 satellites at the same time. Earlier models, such as the GARMIN 45C, were 8 channels. The GPS receiver can output 2D (that is, 2-dimensional) data after receiving signals from 3 satellites, only latitude and longitude, but no altitude. If it receives more than 4 satellites, it will output 3D data, which can provide altitude. However, because of the problem of the earth itself, it is not a very standard circle, so there are some errors in the height data. Now some GPS receivers have built-in barometers, such as etrex's SUMMIT and VISTA. These machines synthesize the final altitude based on the altitude data obtained from the two channels, which should be more accurate.

　　When the GPS receiver is turned on for the first time, or the last time it was turned off is more than 800KM away from the start-up distance, because the ephemeris stored in the receiver is not correct, it must be repositioned on the receiver.

　　The use of GPS receivers should be under the open visible sky, so it cannot be used indoors. The accuracy of handheld GPS is generally about 10 meters, which means that a road can be seen to the left or the right. The accuracy mainly depends on the signal reception of satellites and the distribution of satellites that can receive signals in the sky. If the distribution of several satellites is relatively scattered, the positioning accuracy provided by the GPS receiver will be relatively high.

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