Since the advent of the Global Positioning System (GPS), it has attracted many users with its high precision, all-weather, global coverage, convenience and flexibility. It can provide accurate geographic location, vehicle speed and accurate time information anywhere in the world and near-earth space, and has been greatly applied in the logistics, automobile, and personnel positioning industries.
The basic principle of GPS positioning is to measure the distance between a satellite with a known position and a GPS receiver on the ground, and then the receiver communicates with at least 4 satellites to calculate the distance to these satellites to determine its position on the earth. The positioning accuracy of ordinary GPS is ≥ 1 meter, and the signal error has a 50% probability of reaching more than 2 meters. People who have been pitted by mobile phone GPS navigation must have experienced this. In addition, GPS cannot support precise height setting, and the error may be as high as ten meters.

How does GPS positioning error occur?

1. Atmospheric influence: The refraction effect of the ionosphere and troposphere on electromagnetic waves in the atmosphere changes the propagation speed of GPS signals, which delays GPS signals.
2. Satellite ephemeris error: Due to the complicated external force during the operation of the satellite, the ground control station and receiving terminal cannot determine and master the law, so the error cannot be eliminated.
3. Satellite clock offset: Satellite clock offset refers to the difference between GPS satellite clock and GPS standard time. The satellite uses a cesium atomic clock, so the time of the two may not be synchronized, just like your watch is not synchronized with the time of the wall clock in your living room.
4. Multi-path effect: GPS signals may also be received after being reflected on different obstacles. This is the so-called "multi-path effect".

The current positioning accuracy provided by the GPS system is better than 10 meters, and in order to obtain higher positioning accuracy, we usually use differential GPS technology: a GPS receiver is placed on a reference station for observation. According to the known precise coordinates of the base station, the distance correction number from the base station to the satellite is calculated, and the base station sends this data out in real time. While the user receiver is making GPS observations, it also receives the corrections sent by the base station and corrects the positioning results, thereby improving the positioning accuracy.

Differential GPS is divided into two categories: pseudorange differential and carrier phase differential.

1. Pseudorange difference principle

The pseudorange is the most widely used difference. Pseudorange measurement is to determine the distance from the satellite to the receiver, that is, the distance obtained by multiplying the propagation time of the ranging code signal transmitted by the satellite to the GPS receiver by the speed of light. The pseudo-range single-point positioning method uses a GPS receiver to measure the pseudo-range with more than 4 GPS satellites at a certain moment, and obtains the satellite's instantaneous coordinates from the satellite navigation message, and then uses the distance rendezvous method to find the antenna in the WGS- Three-dimensional coordinates in the 84 coordinate system. On the base station, observe all satellites. According to the known coordinates of the base station and the coordinates of each satellite, find the true distance of each satellite to the base station at every moment. Then compare with the measured pseudorange to get the pseudorange correction number, and transmit it to the user receiver to improve the positioning accuracy. This difference can achieve meter-level positioning accuracy, such as the "beacon difference" widely used in coastal areas.

2. Carrier phase difference principle

Carrier phase differential technology is also known as RTK (Real Time Kinematic) technology. Carrier phase measurement and carrier phase positioning are to determine the phase delay between the GPS satellite carrier signal and the receiver antenna. The GPS satellite carrier modulates the ranging code and navigation message. After the receiver receives the satellite signal, it first removes the ranging code and satellite message on the carrier to regain the carrier, which is called rebuilding the carrier. The GPS receiver compares the satellite reconstructed carrier with the local oscillator signal generated by the oscillator in the receiver through a phase meter to obtain the phase difference. Carrier phase differential technology is a differential method for real-time processing of carrier phase observations at two stations. That is, the carrier phase collected by the base station is sent to the user receiver, and the coordinate is calculated by the difference. The carrier phase difference can make the positioning accuracy reach the centimeter level. It is widely used in fields that require high-precision positions dynamically.
Generally, the GPS positioning system of mobile phones we use is affected by atmospheric ionosphere, troposphere, and satellite clock errors. The positioning accuracy is usually at the meter level, while RTK controls the positioning accuracy at the centimeter level through measurement. The effective application of RTK helps to achieve precise operations, such as shared vehicle parking management, car unmanned driving, drones, robot positioning, surveying and geographic information, safety engineering, unmanned agriculture, machinery control, etc.