The main purpose of GPS satellite positioning

　　Among the many application fields of GPS satellite positioning, vehicle application accounts for the largest proportion in each application, accounting for about 50% of the total. The GPS vehicle applications are generally divided into two categories: GPS vehicle tracking systems and GPS vehicle navigation systems.

　　1. GPS vehicle tracking system

　　GPS vehicle tracking system in GPS satellite positioning is divided into three categories: anti-theft alarm (including emergency rescue and medical assistance), dispatch command and fleet management (logistics and transportation).

　　The GPS vehicle tracking system integrates GPS technology, modern communication technology and geographic information technology (GIS).

　　Using GPS technology, the GPS receiver in the GPS vehicle terminal obtains vehicle information by receiving and processing GPS constellation signals to locate the vehicle, including the vehicle's dynamic coordinate position (longitude, latitude and altitude), time, status, etc.

　　GPS vehicle terminals generally use DGPS technology, that is, differential GPS technology, to improve the accuracy of vehicle positioning. The GPS receiver of the GPS vehicle terminal receives a vehicle positioning information, the fixed GPS receiver of the monitoring center receives a monitoring center positioning information, and the monitoring center positioning information is downloaded to the GPS vehicle terminal through the wireless communication link, and the GPS vehicle terminal comprehensive analysis The precise positioning information of the vehicle is obtained from the positioning information received by the GPS receiver itself and the monitoring center positioning information received through the wireless communication link.

　　Using modern communication technology, a wireless communication link is established between the monitoring center and the monitored vehicle. Through the wireless communication link, the two-way transmission of voice and data signals between the monitoring center and the monitored vehicle. The GPS vehicle terminal uploads the vehicle information to the monitoring center. The monitoring center downloads the control information of a specific vehicle to the monitored vehicle, and the monitoring center staff need to call the vehicle driver to confirm the information, etc.

　　From the perspective of the existing wireless communication technology, the optional communication networks include: GSM cellular mobile network, cluster or conventional network, satellite communication network (Iridium system) and so on.

　　The vehicle monitoring system based on conventional communication is relatively simple in design, networking and use, but its scope of action is small. It can roam across the country, has a large user capacity and supports both data and voice. It is very suitable for the public network communication platform of the GPS vehicle monitoring system. This method can also greatly save the cost of using the voice channel and overcome the previous busy channel. However, there is still a certain delay in places where there is no service area and no short message service, and its calling function cannot be compared with that of the trunking system; the trunking system communication method is used to carry out In point-to-multipoint communication, it is suitable for a dedicated wireless dispatching command network, which has incomparable advantages for some special units, especially for established radio stations, with fewer vehicles, mainly for urban dispatching, and high density requirements. However, its network construction cost and coverage cannot be compared with the GSM system.

　　For the public GPS vehicle monitoring system facing the society, with large capacity, wide coverage and complex business, GSM is the first choice.

　　At present, GSM networks in various places have opened short message (SM) services. The principles and characteristics of GSM and short message services are as follows: The advantages of GSM (Group Special Mobile) digital cellular mobile communication: high spectral efficiency, good security and stability, High integration, large capacity, open interface, strong anti-noise performance, flexible business, wide coverage, easy to achieve nationwide networking, good signal quality in the coverage area under all conditions, including high-noise environment, no cell interference and roaming Good performance and high reliability of mobile service data.

　　GSM also has the transmission point-to-point short message service, and can distinguish between the "mobile station called point-to-point short message service" (SMS-MT/PP) that receives short messages and the "mobile station that enables a GSM user to send short messages to another GSM user". Call Point-to-Point Short Message Service (SMS-MO/PP) and another short message service, Cell Broadcast Short Message Service (SMS-CB), which can send general short messages to a given geographical area at regular intervals Broadcast to all users. The short message service is suitable for the service requirements of transmitting information of less than 160 characters. It uses the public control channel of GSM, and does not affect the transmission of short messages during the call. The advantages of the short message service: fast transmission speed, low price for single data transmission, and does not occupy a voice communication channel. Disadvantages: increase the burden of the control channel, increase the time occupied by a single user of the control channel, and the system needs to be modified accordingly. Due to the many advantages of the short message service, and the data that the GPS system needs to transmit each time is less than 40 bytes, the short message service of the GSM system is more suitable for the "GPS vehicle monitoring system" than other communication methods in terms of communication methods. requirements. Except for a few blind spots in the mountains, other places are covered seamlessly. In terms of transmission delay, the mobile phone to mobile phone is 20 seconds to 30 seconds (the local transmission and remote transmission delay are basically the same), and the mobile phone to the short message center and the short message center to the mobile phone are 3 seconds to 5 seconds.

　　Using GIS technology, the monitoring center analyzes the received vehicle information, converts the vehicle's coordinate position into a geographic location, displays the vehicle's position and running track on the electronic map, and records the vehicle's position, speed, time and status, etc. The owner may be interested in Information. At the same time, the monitoring center also sends the corresponding control information to the monitoring vehicle in combination with the relevant information of the specific vehicle in the system, and then downloads it to the monitored vehicle through the wireless communication link. The control part of the GPS vehicle terminal controls the vehicle according to the received control signal.

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