How is GPS located? The popular point of positioning principle

Twenty-four GPS satellites orbit the earth at a height of 12000 km from the ground, with a period of 12 hours, so that more than four satellites can be observed at any time and at any point on the ground.

Because the position of the satellite is accurately known, in GPS observation, the distance from the satellite to the receiver can be composed of three equations by using the distance formula in three-dimensional coordinates and three satellites, and the position (x, y, z) of the observation point can be solved. Considering the error between satellite clock and receiver clock, there are actually four unknowns, X, Y, Z and clock difference, so it is necessary to introduce a fourth satellite to form four equations to solve, so as to get the latitude, longitude and elevation of the observation point.

In fact, receivers can often lock on more than four satellites. At this time, the receiver can be divided into several groups with four satellites in each group according to the constellation distribution of satellites, and then the group with the smallest error can be selected by algorithm to locate, which improves the accuracy.

Due to the error of satellite orbit and satellite clock and the influence of atmospheric troposphere and ionosphere on signals, the positioning accuracy of civil GPS is only 10 meter. In order to improve the positioning accuracy, differential GPS(DGPS) technology is widely used to establish a reference station (differential substation) for GPS observation. By comparing the precise coordinates of the known reference station with the observed values, a correction number is obtained and released to the public. After receiving the correction number, the receiver compares it with its own observation value to eliminate most errors and get a more accurate position. Experiments show that the positioning accuracy can be improved to 5 meters by using differential GPS.

What is a GPS module?

The GPS module is a GPS signal receiver, which can be connected with a computer or a mobile phone through wireless Bluetooth or wired mode, and transmit the received GPS signal to the GPS software in the computer or the mobile phone for processing. We often say that the GPS positioning module is called the user part. It receives and demodulates the broadcast C/A code signal of the satellite like a "radio", and the intermediate frequency is 1575.42MHz. The GPS module does not broadcast the signal, so it belongs to passive positioning.

The key to the application of GPS module lies in the formulation of serial communication protocol, that is, the input and output protocol format related to the module. It mainly includes data types and information formats, in which the data types mainly include binary information and NMEA National Institute of Oceanography data information. Both kinds of information can be communicated with GPS receiver through serial port.

The GPS module calculates the pseudo-range from each satellite, and uses the distance intersection method to calculate the longitude, latitude, altitude and time correction of the receiver, which is characterized by high point speed but large error. The initial positioning module needs at least 4 satellites to participate in the calculation, which is called 3D positioning. Three satellites can achieve 2D positioning, but the accuracy is not good. The GPS module continuously outputs positioning information and auxiliary information in NMEA format through serial port for the receiver to choose and apply.