GPS is now generally 12 channels, which can receive 12 satellites at the same time. Early models, such as GARMIN 45C, were 8-channel. The GPS receiver can output 2D (that is, 2D) data when it receives signals from three satellites. It has only longitude and latitude, but no altitude. If it receives more than four satellites, it will output 3D data, which can provide altitude. However, due to the problem of the earth itself, it is not a standard circle, so there are some errors in the height data. Now some GPS receivers have built-in barometers, such as the SUMMIT and VISTA of etrex. 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 powered on for the first time, or the distance between the power on and the last power off location is more than 800KM, because the ephemeris stored in the receiver are not correct, it is necessary to relocate on the receiver.

　　The GPS receiver should be used in the open and visible sky, so it cannot be used indoors. The accuracy of the hand-held GPS is generally about 10 meters, which means that you can see whether to go left or right on a road. The accuracy mainly depends on the signal reception of satellites and the distribution of satellites that can receive signals in the sky. If several satellites are distributed dispersedly, the positioning accuracy provided by the GPS receiver will be relatively high.

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