Design of Vehicle GPS GSM Dual Positioning Terminal in Vehicle Monitoring System

Abstract: With the development of China's transportation industry, tracking and monitoring of vehicles is becoming more and more important in logistics and other industries. In the Vehicle Monitoring System, the vehicle positioning terminal mostly uses GPS technology for positioning, while the GPS positioning has high positioning accuracy but has many defects of blind spots. The location service based on GSM network can make up for the shortcomings of GPS positioning blind spots, so that it can effectively overcome the shortcomings of GPS positioning blind spots. Therefore, the positioning terminal integrates the advantages of GPS and GSM network positioning methods. The experimental results show that the positioning terminal can achieve accurate, real-time, all-weather, no blind spot positioning, and greatly improve the positioning performance.
Keywords: GPS positioning; GSM positioning; positioning terminal; STK card

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l Introduction With the expansion of urban construction in China and the increasing number of vehicles, GPS positioning and monitoring systems have been widely used in transportation management, rational dispatching, and safety management. The application of GPS positioning technology provides specific real-time positioning capability for navigation and positioning of vehicles, ships and other vehicles. The car GPS receiver allows the driver to know his or her specific location at any time. The GPS positioning information is sent to the monitoring command center through the short message form, and the monitoring command center can match the positioning information with the electronic map to grasp the specific position of each vehicle in time. However, GPS satellite positioning requires a funnel-shaped 15° clearance perspective for the sky and is greatly affected by the cloud layer. For gps tracking and navigation in transportation and other industries, it is necessary to track the vehicle in real time. Therefore, when the GPS satellite signal is blocked by buildings and trees, the GPS cannot perform normal navigation tracking and positioning.
The location service based on the GSM network can make up for this shortcoming of GPS positioning, and the GSM network coverage is wide and there is no blind spot and the location information can be transmitted to the far end. Based on the above considerations, this paper proposes a dual positioning terminal that integrates GPS and GSM network positioning. The terminal combines the advantages of GPS and GSM network positioning, and overcomes the disadvantages of more GPS location blind spots and lower GSM network positioning accuracy. The performance is significantly improved.

2 System Overview
2.1 Positioning Principle GPS (Global Positioning System) is a new generation of satellite navigation system in the United States, providing users with a global, all-weather, continuous satellite radio navigation system that provides real-time 3D position, 3D speed and High precision time information. The GPS receiver converts its position and time information by receiving navigation information broadcast by any satellite in its line of sight.
Both China Mobile and China Unicom have opened location services. As long as the SIM card supporting STK (SIM TOOL KIT) is selected and the local network has opened the location service, the location information can be obtained through the STK command. The STK command is a set of commands for developing value-added services, a small programming language that allows the smart card-based user identity module SIM to run its own application software. The STK card is a SIM card with STK function. The STK command will be slightly different depending on the manufacturer of the GSM module. For details, refer to the manufacturer's STK Command User Manual.
The GSM positioning method is similar to the GPS positioning. In the GSM network, the positions of the mobile base stations are known, and the distance from the positioning terminal to the base station is determined according to parameters such as the strength of the signal when the positioning terminal and each base station communicate, according to the positions of the three base stations. The position coordinates of the positioning terminal can be obtained by the distance from the base station to the positioning terminal.
2.2 Introduction to the monitoring system The positioning system consists of two parts: the monitoring center and the vehicle positioning terminal. The GSM module in the vehicle positioning terminal not only communicates with the monitoring center through short messages, but also obtains the location service information from the mobile operator through sTK. The positioning terminal transmits the GPS location information or the positioning information obtained from the GSM network to the monitoring center in the form of a short message. The monitoring center integrates the GIS (Geographic Information System) map, and relies on the positioning information in the received short message to complete the matching of the terminal with the electronic map and some navigation instructions. The overall framework of the monitoring system is shown in Figure 1.

3 car dual positioning terminal hardware design
3.1 Vehicle terminal hardware components The vehicle positioning terminal mainly includes GSM module, power module, voice module, LCD display module and GPS-OEM board. The positioning terminal uses a single chip microcomputer as a control chip. The GSM module uses the TC39I GSM engine module of Siemens. In the vehicle terminal, the TC39I GSM engine module not only obtains the positioning information through the GSM network, but also communicates with the monitoring center through the form of short messages. The GPS-OEM board adopts ASHTECH ACl2OEM board, which is the most complete, smallest size and lowest power professional-grade OEM board produced by THALES. ACl2 not only combines GPS navigation timing, but also supports remote differential operation and has a tracking satellite-based augmentation system with accurate carrier phase measurement performance. The GPS-OEM board is a large-scale integrated chip that integrates the main components of the GPS receiver and is integrated on a single circuit board. Its structure is shown in Figure 2.

3.2 GPS-OEM board working principle GPS-OEM board is mainly composed of inverter, frequency counter, signal channel, microprocessor and storage unit. After the initial self-test, the GPS-OEM board will automatically receive the GPS RF signal from the antenna, convert it in the low-noise inverter, change the RF signal from the L-band into a low-frequency signal, and then measure the GPS signal from the satellite through the signal channel. The navigation message sent by the GPS satellite is interpreted by the propagation time of the receiving antenna, and finally the central processor calculates the three-dimensional coordinates, speed, time and other information of the measured point, and finally outputs the serial data via the I/O port.
The input and output of the GPS-OEM board follow the serial communication protocol, and can output data information in the form of ASCII code conforming to the NMEA_0183 standard. The data structure is 8 data bits, 1 start bit, 1 stop bit, and no parity bit. The user can initialize the GPS-OEM board by inputting the sentence, set the data format, baud rate and output statement type and other information, and the output statement is to output various GPS data information to the user.
3.3 Overall hardware design GSM module and GPS-OEM board adopt serial data communication mode and MCU has two serial ports, which realize communication between UARTl and UARTO respectively, and the data transmission control is simple and reliable.
The 232 level conversion section is used for TTL-RS232 level conversion. The power module provides a stable voltage output for the processor, GSM engine module, and GPS module. The main function of the external control part is to accept external control commands such as manual alarms. The GPS-OEM board is used to provide location information, time information and other data. The microcontroller periodically extracts location information from the GPS-OEM board or relies on the STK card to extract location information from the GSM network operator through the GSM engine module, and then sends short messages through the GSM module. The form sends the data to the monitoring center, the GSM module and receives and sends short messages to the monitoring center. The received short message can be displayed on the LCD screen by a single-chip processing, and can be converted into a voice prompting user by a language module. The hardware structure is shown in Figure 3.

Using the keyboard to roughly configure the measured longitude, latitude, and time can shorten the first positioning time and achieve rapid positioning. And you can change the displayed positioning information through keyboard input. The information you can select is: latitude and longitude, Beijing time, altitude, and so on. Since the membrane button type keyboard only needs one I/O port, which saves resources and shortens the program amount, the membrane button type keyboard is selected in the design.
In the hardware design, since the GSM module and the GPS module both contain RF modules, in order to avoid interference, the two modules should be as far away as possible. It can be placed on both sides of the PCB or at both ends of the diagonal. This has proven to be effective.

4 Software design of positioning terminal The system software design mainly includes initialization module, data processing module and man-machine dialogue module. Considering that GPS is more accurate than GSM, the positioning of GPS is mainly based on GSM positioning. The vehicle terminal is positioned by GSM when the GPS blind spot area (such as a garage, a boulevard, etc.) cannot be located. The main program flow is shown in Figure 4.

4.1 Initialization module Mainly to initialize the MCU, LCD module and ACl20 OEM board after power-on. For the MCU, set its serial port working mode, interrupt working mode and baud rate; for the LCD module, set the boot screen and display mode; for the ACl20 OEM board, you need to set the baud rate to successfully complete the serial port communication.
4.2 Data Processing Module This module is mainly responsible for processing the data received from the GPS-OEM board and the data input from the keyboard. The refresh rate of the output statement of the ACl20 OEM board is 1 Hz. In order to ensure the reliability and real-time performance of the data transmission, the utilization of the MCU is improved, and the interrupt mode is used instead of the query mode.
4.3 Human Machine Dialogue Module This module mainly deals with keyboard input and display module operations. In the initial configuration process of the GPS-OEM board, the statement setting of the OEM board needs to be configured according to actual needs, and another important work is to complete the data communication and processing between the MCU and the liquid crystal module.

5 Conclusion Because GPS positioning is accurate, GSM network coverage is wide and there is no blind spot. Therefore, the terminal prefers GPS positioning. When GPS cannot be located, starting GSM network positioning to locate the vehicle terminal can overcome the shortcomings of pure GPS positioning or GSM positioning. The cost is lower. Practice has proved that the positioning terminal can enable GSM network positioning in places such as high-rise buildings, avenues, and garages that cannot be located due to loss of GPS satellite signals, thereby improving the stability of positioning. It is a system that can provide real-time, accurate and interactive information. It can basically achieve accurate and blind spot-free positioning and ensure the positioning effect. The system is fully feasible in terms of technology, economy and environmental protection.