The working principle and specific application of GPS

The working principle of GPS is simply to use some basic principles of geometry and physics that we are familiar with. First, we assume that the position of the satellite is known, and we can accurately measure the distance between our location A and the satellite. Then point A must be located on a sphere with the satellite as the center and the measured distance as the radius. Further, we have measured the distance from point A to another satellite, then point A must be on a circle where the two spheres intersect. We can also measure the distance to the third satellite to determine that point A can only be at two points where the three spheres intersect. Based on some geographic knowledge, one of the unreasonable locations can be easily ruled out. Of course, you can also measure the distance from point A to another satellite, and you can accurately locate it. As mentioned above, to achieve precise positioning, two problems need to be solved:
One is to know the exact position of the satellite;
The second is to accurately determine the distance from the satellite to our location on earth. Let's see how to do this.

GPS navigation diagram

How to know the exact position of the satellite

Be sure to know the exact location of the satellite. First of all, it is necessary to optimize the design of the satellite's orbit through careful consideration, and the monitoring station should continuously monitor the operation status of the satellite through various means, and send control commands at appropriate times to keep the satellite in the correct orbit. The correct running track is compiled into an ephemeris, injected into the satellite, and sent to the GPS receiver via the satellite. By receiving the ephemeris of each satellite correctly, you can know the exact location of the satellite.

This problem is solved, and the next step is to accurately measure the distance between a user on the earth and the satellite. Satellites are far above the earth and in motion. We cannot measure with a ruler like we measure things on the ground. So how do we do it?

How to determine the distance from satellite to user

We have all learned this formula in the past: time X speed = distance. We also know from physics that the speed of radio waves is 300,000 kilometers per second, so as long as we know the time when the satellite signal reaches us, we can use the formula of speed times time equal to distance to find the distance. Therefore, the problem comes down to measuring the time of signal propagation.

To accurately measure the signal propagation time, two problems must be solved. One is the time base issue. That means having an accurate clock. Just like we measure the length of a table every day, we need a ruler. If the ruler itself is not standard, the measured length is not accurate. The other is to solve the measurement method.

Time base problem

The GPS system has very precise atomic clocks installed on each satellite, and is often calibrated by monitoring stations. Satellites send navigation information, as well as precise time information. The GPS receiver receives this information and synchronizes with its own clock to obtain an accurate time. Therefore, in addition to accurate positioning, the GPS receiver can also generate accurate time information.

Method for measuring satellite signal transmission time

In order to avoid using too many technical terms, we first make an inappropriate metaphor. We start the recorder at the same place and satellite to play the "Dongfanghong" music, then we should be able to hear one after the other two "Dongfanghong" music Impossible to hear, just hypothetically able to hear), but it must be out of sync. In order to make the two together, we delayed the time to start the ground recorder. When we heard that two pieces of music were in sync, the delay in starting the recorder was equal to the time for the pieces of music to travel from the satellite to the ground. Of course, radio waves are much faster than sound waves, and radio waves cannot be received by ears. So, what we actually broadcast is not the "Dongfanghong" music, but a binary code called pseudo-random code. Delay the pseudo-random code generated by the GPS receiver to synchronize with the codeword received from the satellite. The measured delay time is the time when the satellite signal is transmitted to the GPS receiver. So far, we have solved the determination of the distance from the satellite to the user. Of course, the above is still a very ideal situation. The actual situation is much more complicated than mentioned above, so we have to take some countermeasures. For example: the speed of radio waves is not always a constant. There will be a certain delay when passing through the ions in the ionosphere and the water vapor in the troposphere. Generally, we can use the typical ionospheric and tropospheric models to correct the weather data collected by the monitoring station. In addition, before the radio waves are transmitted to the receiver antenna, there will be multipath effects due to the refraction and reflection of various obstacles and the ground. When designing a GPS receiver, corresponding measures must be taken. Of course, this comes at the cost of increasing the cost of the GPS receiver. Although the atomic clock is very accurate, it is not without any errors. The clock in the GPS receiver cannot be set as an expensive atomic clock as on a satellite, so the fourth satellite is used to calibrate the clock of the GPS receiver. We mentioned earlier that one point can be located for every three satellites measured. Using the combination of the fourth satellite and the previous three satellites, other points can be measured. Ideally, all measured points should coincide. But in fact, they do not completely overlap. Using this, the GPS receiver's clock can be calibrated in turn. When measuring the distance, the mutual geometric positions of the satellites are selected, and the errors in the measurement are also different. For accurate positioning, you can measure more satellites and select a combination of satellites whose geometric positions are farther apart. The measurement error is smaller. When we mention the measurement error, there is another point to mention, is the US SA policy. The US government plans to provide two services in GPS design. One is the standard positioning service (SPS), which uses coarse code (C / A) positioning, with an accuracy of about 100m, and is provided for civilian use. The other is Precision Positioning Service (PPS), which uses precise code (P code) positioning, with an accuracy of 10m, and is provided to the military and licensed private users. As many tests have shown that the positioning accuracy of the SPS is higher than the original design, the US government has conducted a kind of interference called "Selective Availability (SA)" on the civil code for its own safety to ensure its The military system has the best effectiveness. Since the SA randomly adds error information to the navigation message through the satellite, the positioning accuracy of the civil signal C / A code is reduced to a two-dimensional root mean square error of about 100 meters.

Using differential GPS technology (DGPS) can eliminate most of the errors mentioned above and the interference caused by SA, thereby improving the overall accuracy of satellite navigation and positioning, and making the system error within 10 to 15 meters.

GPS technology errors

In the GPS positioning process, there are three parts of errors. One part is common to each user receiver, for example: satellite clock error, ephemeris error, ionosphere error, troposphere error, etc .; the second part is the propagation delay error that cannot be measured by the user or calculated by the correction model; The three parts are the inherent errors of each user receiver, such as internal noise, channel delay, and multipath effects. Using differential technology, the first part of the error can be completely eliminated, and the second part of the error can be mostly eliminated, which is related to the distance from the reference receiver to the user receiver. The third part of the error cannot be eliminated, and can only be improved by improving the technical specifications of the GPS receiver itself. The error brought about by the US SA policy is essentially an artificial increase in the errors of the first two parts, so the differential technology also overcomes the impact of the SA policy.

Differential GPS technology eliminates common error principle

If within 500 kilometers from the user, set up a reference receiver. It and the user receiver receive the signal of a certain satellite at the same time, then we can think that the signal passing to the two receivers is basically the same through the ionosphere and the troposphere, so the delay is also the same. Since the same satellite is received, the ephemeris error and the satellite clock error are also the same. If we know the three-dimensional coordinates by other methods (it can also be achieved with a highly accurate GPS receiver, its price is much higher than the general GPS receiver), then we can calculate from the measured pseudorange The error. By transmitting this error data to the user, the user can deduct the error from the measured pseudorange, and can achieve more accurate positioning.

GPS data processing software is an important part of the GPS user system. Its main function is to perform "rough machining" and "preprocessing" on the satellite measurement and recording data obtained by the GPS receiver, and to perform adjustment calculation, coordinate conversion and analysis on the processing results. Comprehensive treatment. Solve the three-dimensional coordinates of the measuring station, the coordinates, movement speed, direction and precise time of the measuring body.

GPS positioning technology is a developing high-tech, data processing technology is constantly being updated, and the processing software developed by various series of GPS receiver manufacturers has its own characteristics. Global Positioning System GPS is one of the most pioneering high-tech developments in recent years. Its global, versatile and all-weather navigation and positioning, timing and speed measurement advantages are bound to be more and more widely used in many fields. In developed countries, GPS technology has begun to be used in transportation and road engineering. At present, the application of GPS technology in China's road engineering and traffic management has just started. I believe that with the development of China's economy, the rapid construction of high-grade highways and the gradual in-depth study of the application of GPS technology, its application in road engineering will also More extensive and in-depth, and play a greater role. GPS navigation system combined with electronic map, radio communication network and computer vehicle management information system can realize many functions such as vehicle tracking and traffic management. These functions include: Vehicle tracking can use GPS and electronic map to display the actual location of the vehicle in real time. And arbitrarily zoom in, zoom out, restore, change pictures; can move with the target, keep the target always on the screen; also can achieve multi-window, multi-vehicle, multi-screen simultaneous tracking. Use this function to track and transport important vehicles and goods.

Provide travel route planning and navigation

Providing travel route planning is an important auxiliary function of the car navigation system, which includes automatic route planning and manual route design. In automatic route planning, the driver determines the starting point and destination, and the computer software automatically designs the best driving route as required, including the calculation of the fastest route, the simplest route, and the route with the least number of highway sections. The manual route design is that the driver designs the starting point, the ending point and the passing point according to his destination, and automatically establishes a route library. After the route planning is completed, the display can display the designed route on the electronic map, and at the same time display the vehicle running path and method.

Information query

Provide users with main objects, such as tourist attractions, hotels, hospitals and other databases, users can query on electronic maps as needed. Inquiry information can be displayed in the form of text, language and images, and its location is displayed on the electronic map. At the same time, the monitoring center can use the monitoring console to query the location of any target in the area, and the vehicle information will be displayed on the electronic map of the control center in digital form.

Traffic command

The command center can monitor the running status of vehicles in the area and make reasonable dispatches to the monitored vehicles. The command center can also talk to the tracked target at any time to implement management.

Emergency assistance

Through GPS positioning and monitoring management system, emergency assistance can be provided to vehicles in danger or accidents. The electronic map of the monitoring station displays help information and alarm targets, plans the optimal assistance plan, and alerts the on-duty personnel with alarm sound and light for emergency treatment.

The research and application of GPS technology in car navigation and traffic management engineering has just started in China, and foreign research in this area has already begun and has achieved certain results. The University of Calgary, Canada has designed a dynamic positioning system, which includes a strapdown inertial system, two GPS receivers and a microcomputer, which can measure the linear parameters of existing roads and serve the road management system. The United States has developed a road traffic management system for cities. The system uses GPS and GIS to build a road database. The database contains various current data, such as the exact location of the road, road conditions, and facilities along the road. Officially launched in 1995, it plays an important role in urban road traffic management. In recent years, various systems for vehicle guidance have been developed abroad. Among them, the real-time determination of vehicle position was mainly based on inertial measurement systems and wheel sensors. With the development of GPS and the advantages shown, there are alternatives to the first two methods. the trend of. The GPS positioning used for urban vehicle guidance is generally to set up a reference station in the city. The on-board GPS receives the information transmitted by the reference station in real time, and the real-time position can be calculated after differential processing. The current position and the target to be reached are on the road. By performing optimization calculations on the Internet, the optimal route to the goal can be displayed on the road electronic map to serve vehicles such as public security, fire fighting, emergency repair, and first aid.