1 Introduction to GPS Positioning System 1.1 Introduction to GPS Technology System GPS (Global Positioning System), generally translated as "Global Positioning System", is deployed by the US Department of Defense. Its primary task is to provide global coverage for the US military and its allies. Uninterrupted positioning, navigation and other data within the scope. Secondly, it also provides data services such as positioning, navigation, speed measurement, and timing for civilian and commercial use. GPS can provide high-precision, all-weather, continuous, real-time 3D positioning, 3D speed measurement and time reference for any number of users anywhere in the world. Because of its high efficiency, high precision, multi-function, easy to move, and low price in positioning, navigation, speed measurement, and timing, this system is widely used in earth science.
The GPS system includes GPS satellites, GPS monitoring stations, and user receiving equipment and GPS application software. The GPS system currently has a total of 24 satellites distributed in six fixed orbits that orbit the Earth. The track is about 20400km from the ground. Each star sends a precise information about time and its position to the ground in a 12h cycle.
Since the distance from any point on the earth to the satellite is not equal, and there is a corresponding set of relatively determined data, in practical applications, when the handheld receiver receives the data signal at the test point, the group can be used. The time difference of the arrival of the data is used to calculate the distance of the point from the satellite, and the relative position of the point is determined to achieve the purpose of positioning. According to the calculation formula, the positioning has two-dimensional and three-dimensional points. The two-dimensional positioning needs to receive at least three satellite ephemeris; and the three-dimensional positioning needs to receive at least four satellite ephemeris.
Because the handheld receiver can provide positioning information with accuracy within 25 meters and speed information within 5m/s, it is too accurate. In order to ensure military security, the US government has adopted SA (Selective Availability) for GPS satellites. The technology handles the random interference of the ephemeris signals sent by the satellite to the ground, reducing its accuracy to 100 meters. However, this can not meet some applications with high precision requirements, so it uses a technology called DGPS (Differential GPS) to make up. Its accuracy can be corrected to reach about 50M.
For the purpose of military and political purposes, and for the sake of commercial interests, the US government has been adjusting its GPS policy in recent years in accordance with the development of the international situation. The most important thing is that the SA policy is intended to be terminated early. At the end of March 1996, US Vice President Al Gore announced that "the United States will suspend the implementation of the SA policy within ten years and continue to provide users with standard location services (GPS) free of charge", which is a major policy change of the US government. The US military recently believes that this end time may be advanced as the United States uses GPS fine code positioning (DPS) accuracy and anti-jamming capabilities. Cancellation of SA, the accuracy of the C/A code air signal used by civil users will be significantly improved substantially, and the user's real-time single-point positioning accuracy will be improved from the current 50M to about 20M.
In the digital earth plan, the US Vice President Al Gore publicly announced that it will establish a 1M resolution database, and the level of satellite technology will be improved, and the positioning accuracy of GPS will be greatly improved.
1.2 Other satellite positioning system Navigation satellite system In addition to the US GPS satellite system, it can be compared with the Russian GLONASS satellite system and a system of 24 satellites. Due to financial difficulties, lack of maintenance and supplement, there may be 19 available. With the recovery of the Russian economy and military needs, it will be improved and improved. The GLONASS system is open and conducive to use. Many GPS manufacturers have actively studied the dual-system application software of GPS and GLONASS in order to improve the performance and accuracy of GPS receivers. The GLONASS system has been fully utilized and has achieved initial success. For example, the JAVAD GPS receiver in the United States uses super-integration technology to integrate 40 common channels into the chip, which narrows the difference between GPS and GLONASS without any reason, and combines them to increase the number of observation satellites.
European GNSS system: The European strategy is to replace land-based navigation with GPS satellite or space-based navigation as much as possible to achieve maximum cost-benefit ratio. However, he also firmly believes that he cannot rely on the satellite system controlled by other countries' military to realize his own navigation. Therefore, he is actively establishing his own satellite navigation system GNSS. Its goal is to take a two-step process. First, develop a folk GNSS-1, its main content. It is an enhancement to the existing GPS and GLONASS satellite bases, that is, the use of geostationary satellites to provide services for navigation in Europe, namely the EGNOS program, which was launched in 1995, achieves initial operational capability in 1999, and achieves full operational capability in 2002. . The second goal was to build GNSS-2, which gradually expanded from a regional to a global system. Japan is also actively planning to establish Japan's Multi-Function Satellite Augmentation System (MSAS). The research and application of GPS development in China is constantly deepening and widening. In particular, the national permanent GPS tracking network and corresponding communication network and data processing facilities have been established, and it has developed into a comprehensive service system for GPS in China, providing services for national economic construction, national defense construction and social progress. The manufacture and production of GPS receivers are also from scratch, the level of technology is constantly improving, and the price is much lower than that of imported similar products. In the near future, China will also have its own satellite navigation and positioning system.
2. Application of GPS technology system in navigation electronic map
2.1 Application examples of car navigation electronic maps Driver Scott has borrowed a car with a "never get lost" device in San Francisco. That device is an information display connected to a GPS receiver that is placed in the cab where it is not noticed. He enters his current address into the machine, and a color map is immediately displayed on the screen. The brightest route is the best route to his residence, a thick yellow arrow indicating the direction of the car, and a nasal sound. The voice also told him from time to time: "The next intersection turns left." He checked the location of his car while driving. On the map, the car is represented by a triangular figure that is moving along that bright line...
2.2 Application principle and application mode of car navigation electronic map
2.2.1 Application principle of car navigation electronic map The electronic navigation system in GIS and the real-time positioning technology of GPS receiver are used to form various electronic navigation systems of GPS+GIS.
2.2.2 Application mode of car navigation electronic map
There are two main application modes of car navigation electronic maps: 1 GPS stand-alone positioning + vector electronic map. The system automatically calculates and displays the best route based on the target location (input at work) and the current location of the vehicle (as determined by GPS), guiding the driver to the destination as quickly as possible, and prompting the driver in a multimedia manner. Making a vector map database takes a lot of money. 2 GPS differential positioning + vector electronic map. The system can achieve positioning accuracy of 1~3M through two GPS pseudorange differential technologies between the fixed station and the mobile vehicle. When the two-way communication mode is adopted, the automatic navigation system of the vehicle and the ship can be formed, and the mobile vehicle can be mounted on the ship. The GPS positioning results are transmitted to the control center in real time and displayed on the electronic map to form a traffic network monitoring and command system. In order to prevent insufficient GPS satellite signals from being received when the building blocks are blocked, a low-priced piezoelectric oscillation gyro is installed in addition to the GPS receiver. The Kalman filter algorithm is used to simultaneously process the GPS, odometer and gyroscope data for real-time positioning of the carrier.
3. Introduction to GPS positioning process
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GPS combined with electronic map can realize urban traffic management, vehicle dispatch management, automatic guidance and monitoring of public security, bank vehicles, ports and river vessels, and has great application potential. According to the vector map made by the topographic map, the GPS coordinates must be coordinately converted to match correctly. The coordinate matching problem will be discussed in detail from the aspects of GPS positioning coordinate system, WGS-84 geodetic coordinates, map projection, and plane coordinate transformation. The GPS positioning process mainly has the following steps:
(1) Determine the position of the user's cosmic rectangular coordinate system, that is, the user's X, Y, and Z positions.
(2) Conversion of the cosmic rectangular coordinate system to the WGS-84 geodetic coordinate system, and the user's WGS-84 geodetic coordinate positions λ, φ, h are obtained.
(3) Coordinate projection conversion, that is, the spherical coordinates λ, φ, h are converted into planar electronic map projection coordinates, such as Gauss-Krüger projection coordinates.
(4) Two-dimensional planar similarity transformation, that is, through translation, rotation, and scaling operations, to achieve registration with the electronic map.
All of the above four processes are automatically calculated by the computer program, and the specific algorithm is omitted here.
4 Composition and function of vehicle automatic navigation system based on GPS and electronic map 4.1 Composition of vehicle automatic navigation system based on GPS and electronic map
The whole GPS electronic map vehicle dynamic guidance system is composed of the following figure, which is composed of a main control computer, a liquid crystal display, a voice alarm, a remote controller, a combined navigation processor, a GPS sensor, a rate gyroscope, an optical drive, and the like. The main control computer can be a general purpose computer or a dedicated processor depending on the user's needs.
4.2 Function of Vehicle Autopilot System Based on GPS and Electronic Map This system can realize real-time tracking display, optimal path selection and guidance, display navigation information, map retrieval and voice prompt alarm in the electronic map of sports carriers such as cars and ships. , vector graphics layered display and zoom display; can meet the guidance and monitoring of urban vehicles, ports, rivers, and marine vessels, GPS + track estimation combined navigation function can be correctly guided even under the conditions of abnormal signals. The electronic map is stored on a disc and can store large-capacity vector electronic maps. The vector electronic map generates a dot matrix form stored in the host memory, which can achieve the smoothing effect of map retrieval and vehicle tracking. When the boat and ship reach the edge of the map, the next new vector image will be automatically transferred from the disc for automatic switching.
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