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0 PrefaceIn recent years, mobile vehicle real-time positioning technology based on GPS system and GSM wireless communication network has developed rapidly and is widely used in public security, fire protection, banking, logistics management and other industries. These special industries hope to real-time positioning and remote monitoring of mobile vehicles. Scheduling. In addition, civil vehicles, especially the taxi industry, are paying more and more attention to vehicle safety issues. The vehicle-mounted system can realize the functions of anti-theft, anti-robbery and automatic control of vehicles. The GPS/GSM vehicle monitoring and management system can transmit data by means of GSM short message service (SMS) or GPRS. The former has the characteristics of convenience, speed and low cost, and has received widespread attention in the vehicle system. The XT55 GPS/GSM integrated module and single-chip microcomputer system are selected to construct the vehicle-mounted equipment, and the SMS and monitoring center are used to realize the bidirectional transmission of information.
1 GPS / GSM vehicle equipment composition
As shown in Figure 1, the GPS/GSM-based vehicle monitoring and management system is mainly composed of three parts: the vehicle terminal part (GPS receiver, GSM communication machine, microprocessor and human-computer interaction control unit, etc.) and transmission medium (GPS). Satellite constellation and GSM wireless communication network, etc.) and monitoring center (GIS management system, GPS receiver, GSM communication machine and central processing unit, etc.). The vehicle terminal uses GPS to obtain the current position and time information of the user, and sends it to the monitoring center through the GSM network in the form of SMS, and the monitoring center sends a control command to the vehicle through the GSM network.
The hardware structure of the vehicle terminal equipment is shown in Figure 2. The system designed in this paper uses SIMENS's low-power wireless module XT55. The module adopts ultra-compact design, integrated with tri-band GSM/GPRS communication module and GPS-OEM module. The GPS-OEM board receives the satellite signal from the GPS antenna unit, and obtains the user's position and time through processing, and sends it to the microprocessor through the serial port 0 for processing. The obtained data is sent to the GSM module via the serial port 1, and then transmitted by the GSM antenna. Transferred to the monitoring center via the GSM wireless communication network. In addition, the system also includes GPS antenna and low noise amplifier (LAN), GSM antenna, SIM card slot, phone handle, LCD display, buttons and other units.
2 system signal flow design
2.1 GPS signal flow design
The GPS-OEM board uses the NMEA0183 format to output ASCII data information. The NMEA0183 statement has multiple data patterns, each containing multiple pieces of information. We are concerned with the user's location, speed and time information, which can be used to identify the data read and extract it. Take GPRMC as an example, the format is as follows:
Where: GP is the information source, RMC is the sentence identifier, followed by UTC time, state (A=location, V=navigation), latitude, latitude direction (N or S), longitude, longitude direction (E or W ), rate, azimuth (two-dimensional direction pointing), UTC date, magnetic difference, magnetic heading, etc.
When the system works, the received GPS positioning information can be saved in a structure, and the positioning information is sent back to the monitoring center through GSM at a certain time interval according to the command issued by the monitoring center. The flowchart of the interrupt receiving program of the GPS positioning information is shown in FIG. 3.
2.2 GSM signal flow design
GSM signal processing includes two types of information transmission and information reception. The received GSM signal, that is, the control signal, is sent by the user or the monitoring center to the in-vehicle system to obtain the carrier navigation parameter or implement the system expansion function. Figure 4 shows the received signal flow. The transmitted GSM signal contains navigation parameters that reflect the status of the carrier, as well as some meaningful text, such as alarm information. Figure 5 shows the signal transmission process.
3 GSM communication service
One of the key technologies of in-vehicle systems is the encoding and decoding of short messages. The specifications related to short message transmission mainly include GSM03.38, GSM03.40 and GSM07.05. The first two describe the technical implementation of SMS, while the latter specifies the interface standard for SMS. There are 3 ways to send and receive SMS messages: Block Mode, Text Mode and PDU Mode. PDU Mode is the default encoding mode of the mobile phone. Any character set can be used. The data to be transmitted is encoded to obtain the PDU string (ASCII code). The PDU string contains the displayable message and the SMS service center number, target number, reply number, and code. Other information such as method and service time.
3.1 "Peer-to-peer" SMS service mode
The so-called "peer-to-peer" short message service mode, that is, the in-vehicle unit and the monitoring center both use a mobile phone-like short message receiving and transmitting communication module, and the in-vehicle unit short message receiving and transmitting communication module sends a short message including the GPS information and the destination station to be sent to the short message service. The center (SMSC) is then forwarded by the SMSC to the monitoring center SMS transceiver module; and the monitoring center SMS transceiver module also includes the SMS sent by the central processing unit and the GPS differential correction data together with the destination station to be sent to. It is sent to the SMSC, and then forwarded by the SMSC to the in-vehicle unit short message transceiver communication module.
3.2 GSM short message encoding and its program implementation
The short message uses a Text mode or a PDU mode based on the AT command. Common AT commands are shown in Table 1.
The short message is sent with text Text mode and PDU mode. Any coding method can be used when using the PDU mode. The text uses 16 b Unicode encoding to display text information, which can be used on most mobile phones. When sending or receiving a Chinese short message, the Chinese code of GB2312 needs to be converted into a Unicode code of the code page of CP936. When encoding English characters, each character is represented by 7 b. To transmit this short message, these 7 b are converted into 8 b, and the source string is encoded into a group of 8 characters, among the characters in the group. compression. The signal flow of the SMS encoding (decoding) program is shown in FIG. 6.
4 Conclusion
Vehicle-mounted mobile terminals are an important part of the Intelligent Transportation System (ITS). This paper uses GPS-OEM, GSM communication module, single-chip microcomputer and other peripheral devices to build a terminal system based on short message mode, which realizes the reasonable scheduling of the target vehicle by the monitoring center, and enables the user to effectively control the vehicle, so as to achieve real-time control of the vehicle. Positioning, remote monitoring, security alarms and automatic control. The equipment developed at the same time has the advantages of low cost, small size, simple use and maintenance, and high reliability.
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