What is a GPS module?
The GPS module is a GPS signal receiver. It can be connected to a computer or mobile phone via wireless Bluetooth or wired connection, and it transmits the GPS signals it receives to the GPS software in the computer or mobile phone for processing. The GPS positioning module we often refer to is called the user part. It receives and demodulates the satellite's broadcast C/A code signal, with a frequency of 1575.42 MHz, just like a "radio". The GPS module does not broadcast signals and is a passive positioning device.
The key to the application of the GPS module lies in the formulation of the serial communication protocol, that is, the relevant input and output protocol format of the module. It mainly includes data types and information formats, among which the data types mainly include binary information and NMEA (National Marine Electronics Association) data information. These two types of information can be communicated with the GPS receiver through the serial port.
The GPS module calculates the pseudo-distance with each satellite and uses the distance intersection method to obtain the four parameters of the receiver, namely longitude, latitude, altitude and time correction. The advantage of this method is its fast point positioning speed, but the error is large. For the initial positioning module, at least 4 satellites need to be involved in the calculation, which is called 3D positioning. With 3 satellites, 2D positioning can be achieved, but the accuracy is not good. The GPS module continuously outputs positioning information and auxiliary information in NMEA format through the serial communication port, which can be selected and applied by the receiver.
The principle of GPS module positioning
The 24 GPS satellites orbit the Earth at an altitude of 12,000 kilometers, completing one orbit every 12 hours. This enables any point on the ground to simultaneously observe more than 4 satellites at any given time.
Since the position of the satellite is precisely known, in GPS observations, the distance from the satellite to the receiver can be calculated using the distance formula in the three-dimensional coordinate system. With the help of three satellites, three equations can be formed to solve for the position of the observation point (X, Y, Z). Considering the error between the satellite's clock and the receiver's clock, there are actually four unknowns: X, Y, Z and the clock difference. Therefore, a fourth satellite needs to be introduced to form four equations for solution, thereby obtaining the longitude, latitude and elevation of the observation point.
In fact, the receiver can lock onto more than four satellites. At this point, the receiver can divide the satellites into several groups according to their constellation distribution, with each group consisting of four satellites. Then, through algorithms, it can select the group with the smallest error for positioning, thereby improving the accuracy.
Due to the errors in the satellite's orbit and clock, as well as the influence of the troposphere and ionosphere on the signal, the positioning accuracy of civilian GPS is only 10 meters. To improve the positioning accuracy, the differential GPS (DGPS) technology is commonly adopted. A reference station (DGPS station) is established for GPS observations. Using the precise coordinates of the known reference station and comparing them with the observation values, a correction factor is obtained and released to the public. After the receiver receives this correction factor, it compares it with its own observation values, eliminating most of the errors, and obtaining a more accurate position. Experiments show that using DGPS, the positioning accuracy can be improved to 5 meters.
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