1 Introduction
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In foreign countries, the car GPS global positioning system is gradually becoming a necessary configuration for mid-to-high-end cars. Rockwell Corporation has developed an advanced turning and turning navigation and information system, which includes a small color display screen, direction sensor and GPS receiver. The system has the most effective road selection function, turning and turning steering function and vehicle precision. Multiple functions such as positioning. In Japan and European countries, GPS vehicle auto-navigation systems and GPS-based intelligent geographic information management systems have been installed in mid- to high-end vehicles.
With the continuous expansion of urban construction in China and the increasing number of urban vehicles, especially the traffic management, transportation services and traffic safety of vehicles are becoming more and more important. Therefore, the car GPS system has been more and more widely used.
The intelligent transportation system--ITS (interligent transport system) is combined with the radio communication system. The vehicle and the command center can communicate with each other to form a positioning navigation and traffic management system, which not only enables the vehicle to have the ability to locate and navigate. And can dispatch, manage and service a variety of vehicles. It mainly completes vehicle identification and positioning, automatic navigation, traffic monitoring, communication and information processing technology.
The system structure of ITS mainly includes three parts: in-vehicle system, communication system and ground monitoring center management system.
1. The vehicle system consists of a vehicle positioning unit, a transmitting and receiving unit for positioning information, a display unit, and an onboard computer.
The main functions of car GPS:
1) The vehicle positioning unit receives the positioning information from the satellite in real time by the GPS receiver, such as giving the real-time position and speed of the motor vehicle.
2) Receiving and receiving function of positioning information
The GPS receiver locates the information in real time and sends the positioning information to the monitoring center, and receives the scheduling instruction sent by the monitoring center.
3) The data display function displays the real-time position of the vehicle and the dispatching command sent by the ground monitoring command center.
4) Alarm function Once an emergency occurs, the alarm device is activated, and the monitoring center immediately displays information such as the vehicle situation, the location and time of the accident.
2. Communication System The communication system realizes information such as the position, status, and request service of the vehicle to the central management system or receives information and commands from the control center through the duplex communication unit for the motor vehicle and the ground monitoring center control management system.
3. The ground monitoring center management system consists of a central control management station, a computer, a dispatch management system, a database, a communication route management system, and a large-screen display.
The computer has a digital map, advanced AVLN system with address matching, map matching, best path calculation and driving instructions.
The large screen display can display information such as the location of the vehicle on a digital map.
2 Principle of concentrated differential technology
Generally, the differential GPS technology needs to be equipped with a GPS receiver in each car, and receives the correction number, which will cause the structure of the vehicle system to be complicated, and the work speed is slow. Therefore, a centralized differential technique can be employed in practical applications.
The centralized differential technology consists of two parts: the vehicle part and the control center management system. The vehicle part is equipped with a GPS receiver and a communication station. It only receives the GPS signal without considering the reception of the differential signal, and sends the GPS positioning signal to the ground monitoring through the communication station. At the center, the monitoring center concentrates on differential processing, recording, storage, and data.
The ground monitoring center is located at the base station and the coordinates are precisely known. A GPS receiver is installed at the reference point, and communication stations, computers, electronic maps, and displays are installed. When working, the GPS receiver on each vehicle sends its location, time and vehicle number together with the monitoring center. The GPS receiver of the monitoring center compares the measured three-dimensional position with the known coordinates of the station. Differential correction number. And send back to the vehicle, correct the position of the vehicle, calculate the real-time position of the vehicle, and display it on the screen after coordinate transformation.
The system works as shown
Second, the measurement method: 
Pseudorange differential technology is currently the most widely used differential technology. This technology is used in almost all commercial differential GPS receivers. The RTCM-104 recommended by the International Maritime Radio Committee also uses this technology. Therefore, from the perspective of the versatility and practicability of the system, the pseudo-difference technique is adopted in the centralized differential positioning system.
Let the exact coordinates of the base station (control center) be (X0, Y0, Z0), and the geocentric coordinates of the i-th satellite at a certain moment calculated from the ephemeris data are (Xi, Yi, Zi), then The distance from the satellite to the base station is 
3 error analysis:
Let J be the base station position, U be the user's vehicle equipment position, the distance between the base station and the subscriber station is n, the base station to satellite pseudorange is Ïj, and the user to satellite pseudorange is Ïu. The slant distance between the actual position of the satellite and the position where the GPS signal propagates is d, and the pseudorange measurement errors of the base station and the subscriber station are δÏj and δÏu, respectively. It can be seen from the figure that δÏj≠δÏu. Using the base station pseudorange differential error δÏj to correct the measured value of the user's onboard equipment, there is obviously a residual error that cannot be calibrated. The error value is:
It can be seen that the method can offset the common error between the vehicle and the base station.
If the position error of the satellite is σ=1000m, the actual distance from the base station to the satellite is R=20000Km. As the distance between the vehicle and the ground monitoring command center changes, the measurement error is as shown in the following table: 
It can be seen from the above table that for urban traffic vehicle management, as long as the distance between the vehicle and the ground monitoring command center does not exceed the range, the measurement accuracy is sufficiently high.
From the practical application point of view, the centralized differential technology has the characteristics of simple structure and small measurement error of the vehicle-mounted equipment, and the data communication can adopt the communication equipment of the original vehicle, as long as the communication conversion interface is added. Therefore, it has a good practical application effect.
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