1 Introduction
With the development of society and the advancement of communication technology, people have higher and higher requirements for mobile target monitoring. Not only need to know the location of the mobile target, but also real-time monitoring of the video image, alarm information, operating status and other data of the mobile target And processing. For example, the mobile monitoring and positioning system can assist in the management of long-distance buses. The command center determines the position of the bus by analyzing the GPS data sent by the bus and determines whether it is speeding. If the bus fails, encounters an accident or hijacks, it can notify the command center in time The command center then informs the transportation, public security, hospital and other departments for emergency treatment, and installing a camera in the passenger car can also effectively prevent criminal incidents while driving. On the other hand, the driver can understand the geographic information, weather conditions, traffic conditions, etc. ahead of the exercise through the command center. In addition, the mobile monitoring and positioning system has a wide range of application prospects in the fields of fishing boats going out to sea, urban taxi dispatching, public security patrols, and bank truck monitoring. The mobile monitoring and positioning system has developed along with the maturity of GPS technology and wireless communication network technology. In the early 1990s, GPS technology has just entered the practical stage, and GPS-based mobile tracking and monitoring systems have begun to market, but due to wireless The restriction of communication network technology has resulted in a small system construction scale and high input cost, so it has not been popularized. In recent years, various wireless communication private networks, trunked wireless networks, satellite data communication networks, and digital cellular mobile communication networks have developed rapidly, and their data carrying capacity has been significantly strengthened, and GPS technology has become more mature. The price of 12-channel GPS receivers on the market is also relatively low. On May 1, 2000, the US government announced the cancellation of the GPS SA policy, so that the general GPS receiver positioning accuracy can reach 25m. Due to the above reasons, mobile monitoring and positioning systems based on GPS and wireless communication networks have experienced rapid development in all countries of the world, and have shown strong development momentum, with huge market potential.
GPS technology and wireless communication network technology are the two key elements of the mobile monitoring and positioning system. Nowadays, GPS receivers or OEM cards have been commercialized. Their volume can be made very small, providing serial port standard data output, data interface is very convenient, and wireless Communication networks are diverse, and their construction costs and system capacities are also different. The available communication networks include cluster wireless networks, satellite data communication networks, and mobile communication networks.
2 Selection of mobile monitoring and positioning system communication network
Wireless communication network is an important part of mobile monitoring and positioning system. It can be divided into two categories: wireless private network and public wireless communication network. Among them, wireless private network, satellite communication network, and mobile communication network are relatively common forms.
2.1 Wireless private network
The wireless private network is a wireless communication network specially built for mobile monitoring and positioning systems. He has his own switching center, base station and monitoring platform. The disadvantage is that the networking cost is higher and the coverage area is limited. However, the wireless private network system is flexible and resistant to damage. It has strong capabilities and mobility, and is suitable for use in fields with high specificity such as flood control and disaster relief command systems and military combat command systems.
2.2 Satellite communication network
There are three types of satellite communications: geostationary satellite communications, medium-orbit satellite communications, and low-orbit satellite communications. Satellites function as relays. Earth stations are used as base stations or connected to users. The characteristic of satellite communication is that it covers a large area, can achieve global communication, and is insensitive to climate and transmission distance. It is especially applicable in areas where the general wireless network cannot cover, such as deserts and forests. In addition, in the maritime field, navigation and real-time monitoring of nautical ships through a monitoring and positioning system based on satellite communication network, when encountering pirates, shipwrecks, etc. It can be processed in a timely manner; in the field of aviation, the use of monitoring and positioning systems in the operation of civil aviation passenger aircraft can avoid aircraft collisions in the air. When a hijacking event occurs, you can use the camera on the aircraft to keep abreast of the status of the aircraft. The system constitutes an air command platform, etc. The disadvantage is that the system terminal equipment is complicated and the cost is high, which is not conducive to popularization.
2.3 Mobile communication network
In recent years, the development of mobile communication is in full swing, from the analog AMPS network to the digital cellular GSM network, to the CDMA mobile communication network, and now some countries in Japan have proposed the concept of the fourth generation mobile communication network (ie 4G), and began to develop and develop . Compared with wireless private network or satellite communication network, mobile communication network is more popular. Relying on mobile communication network to build a monitoring and positioning system has the advantages of large coverage area and low networking cost. Using the data access function of mobile communication network to develop value-added services, it also Can improve the resource utilization rate of the network. However, the data transmission rate of the mobile communication network is generally low. If the data transmission rate is increased, the original network needs to be transformed. For example, using CDPD (Packet Data Exchange) technology, packet data services can be provided on the analog cellular AMPS network at a channel rate of 19.2 kb / s. In the digital cellular communication network, the data rate of the GSM network is only 9.6kb / s. When the GSM network adopts GRPS (General Packet Radio Service) technology or EDGE (Enhanced Data Rate) technology, he will provide 115.2kb / s and 384kb / s data rate. In addition, in IS-95B, CDMAOne has enhanced mobile data service capabilities, and CDMA2000 can provide data rates up to 2Mb / s. The use of mobile communication network to build a mobile monitoring and positioning system is convenient, easy to maintain, and can cover all areas that can be reached by mobile phone signals. It is very suitable for applications such as vehicle and ship scheduling, 110 alarm command, and wireless train dispatching.
In addition to the several communication networks described above, there are also digital trunking communication networks, cordless telephone communication networks (including DECT systems, PHS systems, etc.), which can be used to transmit monitoring and positioning data.
3 Design of mobile monitoring and positioning system based on GSM
Through the comparison of the above several communication networks, the mobile communication network is the most economical and most popular method. The third-generation mobile communication network CDMA has a high data transmission capacity, but the CDMA network is still in the experimental network stage, and the GSM digital cellular mobile communication network has been very popular both domestically and internationally, with the most extensive market application and the largest coverage area , Is also the most mature in technology. Therefore, the use of GSM mobile communication network to build a mobile monitoring and positioning system is currently a more reasonable choice. There are three methods for transmitting monitoring and positioning data through the GSM mobile communication network: using voice channel transmission, using SMS (short message service) transmission, and using improved GSM high-speed data channel transmission.
3.1 Design method of voice channel communication using GSM
The data transmission using the GSM voice channel is the same as that on the ordinary telephone line, and the data communication is carried out through the modem modulation and demodulation at both ends of the transceiver. This method is mature in technology and has a wide application range. It is necessary to dial before data transmission, the call establishment time is longer, generally about 10s, and the communication cost is the same as the mobile phone fee, and the cost is higher. The mobile monitoring and positioning system established by this method is shown in Figure 1.
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