The original manuscript was received on July 26, 2001, and the revised manuscript was received on September 13, 2001.
The vehicle monitoring and dispatching system (VMDS) is one of the important contents of the intelligent transportation system (ITS). It is also one of the most urgently demanded applications in the field of transportation management in China, with great market potential and economic benefits. It can be widely applied to the real-time positioning, monitoring and operation scheduling of urban taxis, buses, freight cars and other special vehicles, greatly improving the efficiency and safety of vehicle transportation management. This paper proposes to develop VMDS based on GPS, GIS and GSM public digital mobile communication network, which integrates the latest technology in positioning technology, communication technology and information collection, management technology, etc. .
2 The main content of the vehicle monitoring and dispatching system involves (1) vehicle positioning; (2) two-way communication; (3) system spatial data (traffic electronic map) and attribute data collection, input, coordinate conversion and management; (4 ) Visualization of positioning information and dynamic update of system data; (5) System function development (such as monitoring, scheduling, etc.).
Vehicle positioning technology is generally divided into vehicle autonomous positioning and combined vehicle positioning. Vehicle autonomous positioning is typically represented by Terrain Radio Frequency (TRF), Signal Pole (SP), Dead Reckoning (DR) and GPS positioning; combined vehicle positioning is currently mainly based on GPS / DR GPS (DGPS) / electronic map combination positioning.
After the United States canceled the SA (Selected Availability) policy of artificially limiting positioning accuracy on May 1, 2000, the civilian single-frequency dynamic vehicle GPS positioning accuracy has been increased from 100m to about 10m, which can fully meet the needs of vehicle monitoring, dispatching and command management. Table 1 gives the requirements of ITS on vehicle positioning accuracy. At the same time, GPS has the characteristics of providing three-dimensional position, three-dimensional speed and time information in real time all-weather, which is unmatched by other positioning methods. This is the fundamental reason why the system uses GPS for vehicle positioning.
Table 1 Accuracy requirements for vehicle positioning by ITSService types Navigation and route guidanceAutomatic vehicle monitoringAutomatic vehicle identificationPublic safety resource management Accident or emergency responseAnti-collision vehicle command and control System communication must use radio networking. At present, the development of foreign GPS vehicle monitoring systems in communication is roughly divided into two modes, one is the combination of GPS and conventional communication systems, GPS and trunked radio (TrunkedRadio) systems, or GPS and CDPD (CellularDigitalPacketData); the other is GPS and GSM , GPS and satellite network communication combination. The cluster system is suitable for short-time conversations, and has the characteristics of simplex, shared information, etc. It has obvious shortcomings when it is used for vehicle monitoring and dispatching: First, it has small coverage, lack of roaming, and difficulty in capacity expansion; Reliability of data transmission, high investment in network construction and maintenance at the same time. The communication link of the satellite network is complicated. If it is used in VMDS, the input cost is too high, and the product is difficult to popularize. In contrast, GSM is the best choice. It has the following advantages: (1) High degree of standardization, 1 = 1 open, strong networking ability, capable of international roaming; supports point-to-point two-way short message service; confidentiality, good security performance, with authentication and encryption functions; especially GSM network The supported digital phone compatibility function is more suitable for vehicle monitoring and dispatching without basic investment. In China, the GSM public digital mobile communication network is the digital mobile cellular communication system with the largest coverage, the highest system reliability, and the largest number of telephones. The use of GSM to transmit GPS information and other information is the most economical, reasonable and effective communication method. Its use can give full play to the characteristics and resource advantages of the business services of various systems, and achieve the positioning, monitoring and scheduling of mobile targets with minimal investment and operating expenses.
The background map (also called spatial data) and related attribute data required for VMDS vehicle positioning are organized and managed by GIS. As the geographic information management platform of ITS, GIS is increasingly proven to be an effective technology for storing, managing and analyzing geographic information (graphic data and attribute data). At present, the latest development direction of GIS is networking and componentization, which is a good way to solve the problem of massive information management. It can be considered that VMDS is the secondary development result of GIS. Therefore, choosing the appropriate GIS basic platform is also a key to this development. After comprehensive analysis of the various aspects of the situation, the system selects MapInfo4 and MapX4.01 components for development.
2 System design 2.1 System hardware and software environment Hardware environment: GPS receiver, high-end microcomputer, total station, data conversion equipment, mobile phone, A0 format digitizer or A3 format scanner, etc.
14. GEOSCAN scanning vectorization software, electronic tablet mapping software, Visual Basic 6.0, Access relational database, etc.
Schematic diagram of the software and hardware environment developed for this system.
Attribute number geographic graphics number information system platform database torsion field collection> f total station + 1i electronic hand-held board J * (digitizer) scanner) Windows98 operating system eight / letter] VMDS developed software and hardware environment system overall structure System composition The system consists of three parts, namely mobile vehicle, GSM communication network and control center. Among them, the mobile vehicle is equipped with a GPS receiver, an in-vehicle mobile phone and a data converter, which are responsible for collecting vehicle positioning data, and after processing by the data converter, the in-vehicle mobile phone sends out via GSM. The control center is mainly composed of mobile phones, data converters and computers. The system software is installed on the central control computer, which receives the GPS positioning information in real time, performs transformation, matching and other processing, and then displays it on the computer screen in a corresponding manner. The manager can always know the digitized map of the position of the mobile vehicle and can understand the situation at any time through the on-site listening function. The control center can also send dispatch instructions to the designated vehicle through the system and telephone at any time, and have a dialogue with the scene. The system composition is as shown.
3 The key technology of system development 3.1GIS is used to make the traffic electronic map of VMDS The traffic electronic map mentioned here is the background map used for displaying GPS positioning data in the system, that is, the GIS graphic database. Its production procedures include data collection, conversion (data format and coordinate system conversion) and access to GIS graphics library. There are three situations for the collection of graphic data, namely actual measurement on site, digitization of existing drawings and ready-made digital graphic data. The process is as shown.
Existing drawings digitized GIS graphics data acquisition on-site measured mapping currently mainly has two modes: theodolite notation and total station electronic tablet mapping. On-site measurement and mapping were carried out with a total station and an electronic tablet to supplement the problem of imperfection of existing electronic graphic data. Electronic tablet mapping involves control measurement and detail measurement. Control surveys are generally completed by laying attached wires or closed wires along the street or highway. The total measurement of the broken part is measured by the polar coordinate method. It is worth noting that the electronic tablet mapping is to measure while measuring. The runner should pay attention to the rationality and density of the running point, understand the characteristics of computer mapping, and thus improve the overall accuracy of the graphics.
There are also two modes for digitizing existing drawings, that is, the digitalization of the hand-tracking digitizer and the vectorization of the scanner scan. The development of this system involves on-site surveying and digitization of hand-tracking of existing maps. Before digitizing the hand-held tracking digitizer, it is necessary to study and determine the number of layers, layer names, colors, codes, etc. according to the information on the map and the requirements of this system in advance. When digitizing, two steps of initialization work should be done: (a) select at least three control points on the drawing for the positioning of the drawing to ensure its geographic accuracy and thus the accuracy of the entire database; (b) set the threshold, ie The minimum distance from any two points in the digitizing process. Points greater than this value will not enter the database during the digitization process, ensuring the accuracy of the digitization results. Whether it is an electronic tablet mapping or a hand-held digital map, it will eventually need to be converted through a file format, usually converted into a public interface graphic file grid. * .tab file is the graphic data file format of Maplnfo and MapX. Specifically in the conversion, we must pay attention to the conversion of the coordinate system and the setting of the layer in the conversion process.
VMDS data communication The data communication of this system mainly refers to the process that the central computer sends signals to the mobile vehicle through the data converter and mobile phone, and then the mobile vehicle sends the information back to the control center through the GSM network. The GPS receiver and the central computer communicate through the RS232 interface. Due to space limitations, only the communication driver for the central computer is given here.
This system is programmed in VB6 environment. The communication of the central computer is completed by the MSComm control of VB6. The control provides two serial communication methods, namely event-driven and query.
Event-driven is when the serial port receives or sends a specified amount of data, or when the state changes, the control will trigger the OnComm event (Events), to check the CommEvent attribute (Features), execute the corresponding method (Methods). Event-driven has the advantages of timely program response and high reliability. Query mode is to query some property values ​​(such as InBufferCount value) of the MSComm control after each important program to detect events and communication errors. This is commonly used for small programs.
This system uses event-driven. When the system starts to run, open the serial port and complete the initial settings (see program list 1).
When the central computer issues an instruction or receives positioning data from a mobile vehicle, the OnComm event is triggered and processed by two important methods, comReceive and comEvSend (see Listing 2), thus completing the system's communication function.
GPS positioning information processing and display GPS positioning information is a short sentence with rich information. At present, most GPS receivers can output data in the form of standard ASC codes. It has a variety of data sentence patterns, and each sentence pattern starts with the program list number 1, and outputs Greenwich Mean Time (UTC), longitude, the number of received stars for positioning, geometric accuracy, and open line height. , Track direction, magnetic difference, check code, speed and other information. The data in each sentence is separated by a comma. Users can extract their useful data through programming according to actual needs. For example, the GGA data sentence is the source of information, GGA is the sentence identifier, followed by UTC time, latitude, longitude, speed, course, date, etc. When the computer serial port receives positioning data, it triggers the OnComm event, accumulates characters, and uses * Chr (10) * as the end mark. Then extract the required information in the cumulative field, see the program list 2. Due to space limitations, only the main term CasecomEvSend 'is provided to process the data object to provide multiple methods to achieve the point display on the layer. Highway transportation technology, 2 Changdan, Wang Jing, fruitful. Modern mobile communication technology and organization. Journal of China Highway, 2000, 13 (3).
Fan Yizhi. VisualBasic and RS232 serial communication control. Beijing: China Youth Press, 2001. (Continued from page 208)
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