What is a car GPS navigation system? In short, it is a radio navigation and positioning system (GPS principle) that provides three-dimensional position, three-dimensional speed and other information around the world based on 24 global positioning satellites. So how does it work? Usually what we call a car GPS navigation system consists of three parts:
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The first is the ground control part, which consists of the main control station, the ground antenna, the overload station and the communication assistant system.
The second is the space part, which consists of 24 satellites and is distributed in 6 orbital planes.
The third is the user equipment part, which consists of a GPS receiver and a satellite antenna. Now the positioning accuracy of civilian use can reach within 10 meters (GPS navigator).
The basic working principle of car GPS navigation system
First, the distance between the satellite at the known location and the receiver of the user is measured, and then the data of multiple satellites is combined to know the specific location of the receiver. To achieve this, the position of the satellite can be detected in the satellite ephemeris based on the time recorded by the onboard clock. The distance from the user to the satellite is obtained by recording the satellite signal to the time elapsed by the user, and then multiplying it by the speed of light (due to the interference of the ionosphere of the atmosphere, this distance is not the true distance between the user and the satellite, but Pseudorange (PR): When the GPS satellite works normally, it will continuously transmit the navigation message with a pseudo-random code (referred to as pseudo-code) composed of 1 and 0 binary symbols.
There are two kinds of pseudo-codes used in the GPS system, namely the civil C/A code and the military P (Y) code. The C/A code frequency is 1.023MHz, the repetition period is one millisecond, the code spacing is 1 microsecond, which is equivalent to 300m; the P code frequency is 10.23MHz, the repetition period is 266.4 days, and the code spacing is 0.1 microsecond, which is equivalent to 30m. The Y code is formed on the basis of the P code, and the security performance is better. The navigation message includes information such as satellite ephemeris, working conditions, clock correction, ionospheric delay correction, and atmospheric refraction correction. It is demodulated from the satellite signal and transmitted on the carrier frequency with 50b/s modulation. The navigation message contains 5 sub-frames per main frame and is 6s long.
The first three frames are each 10 words; they are repeated every 30 seconds and updated every hour. The last two frames total 15000b. The contents of the navigation message mainly include telemetry code, conversion code, and the first, second, and third data blocks, and the most important one is the ephemeris data. When the user receives the navigation message, the satellite time is extracted and compared with his own clock to know the distance between the satellite and the user, and then the satellite ephemeris data in the navigation message is used to calculate the position of the satellite when transmitting the message. The user's position and speed information in the WGS-84 geodetic coordinate system can be known.
It can be seen that the role of the satellite part of the car GPS navigation system is to continuously launch navigation messages. However, since the clock used by the user's receiver is not always synchronized with the satellite onboard clock, in addition to the user's three-dimensional coordinates x, y, z, a Δt, that is, the time difference between the satellite and the receiver is introduced as an unknown number. Then use four equations to solve the four unknowns. So if you want to know where the receiver is, you must be able to receive at least 4 satellite signals.
The GPS receiver can receive time information accurate to the nanosecond level that can be used for timing; a forecast ephemeris used to predict the approximate location of the satellite in the next few months; and a broadcast ephemeris used to calculate the satellite coordinates required for positioning. The accuracy is from a few meters to tens of meters (different satellites, changing at any time); and GPS system information, such as satellite status.
The GPS receiver measures the distance from the satellite to the receiver. It is called pseudorange because it contains the error of the receiver satellite clock and the atmospheric propagation error. The pseudorange measured by the 0A code is called the UA code pseudorange, and the precision is about 20 meters. The pseudorange measured by the P code is called the P code pseudorange, and the precision is about 2 meters.
The GPS receiver decodes the received satellite signal or uses other techniques to remove the information modulated on the carrier and recover the carrier. Strictly speaking, the carrier phase should be referred to as the carrier beat frequency phase, which is the difference between the received satellite signal carrier phase affected by the Doppler shift and the phase of the receiver local oscillator generated signal.
Generally, the epoch time determined by the receiver clock is measured, and the tracking of the satellite signal is kept, and the phase change value can be recorded, but the initial phase values ​​of the receiver and the satellite oscillator at the start of observation are unknown. The phase integer of the epoch is also unknown, that is, the ambiguity of the whole week can only be solved as a parameter in data processing.
The accuracy of the phase observation is as high as millimeter, but the premise is that the whole-circumference ambiguity is solved. Therefore, the phase observation value can be used only when the relative positioning is performed and there is a continuous observation value, and the positioning accuracy is better than the meter level. Phase observations can be used.
The GPS observations include errors such as the clock difference of the satellite and the receiver, the atmospheric propagation delay, and the multipath effect. They are also affected by the satellite broadcast ephemeris error during the positioning calculation. Most of the common errors are caused by relative positioning. Offset or weaken, so the positioning accuracy will be greatly improved. The dual-frequency receiver can offset the main part of the ionospheric error in the atmosphere according to the observation of two frequencies. When the accuracy is high and the distance between receivers is far away (the atmosphere is obviously different) ), should use dual-frequency receiver.
Introduction to key indicators of car GPS navigation system
1. Tracking sensitivity. The tracking sensitivity of the car GPS navigation system is very important for the user, because when the user is in an occluded place such as an urban canyon, a tunnel, or an underground parking lot, the strength of the GPS satellite signal is greatly reduced. If the tracking sensitivity of the product is poor, the satellite signal will be lost and the position information will be lost.
2. Positioning accuracy. The positioning accuracy of the system specified in GB/T19392-2003 should be less than 50m (2DRMS measurement). Products with poor positioning accuracy in urban areas where road networks are relatively dense, GPS navigation is more difficult, so that the navigator can not identify "turning around", "left and right lanes", "viaduct", "crossing lanes", etc., such navigation It is a very serious flaw, it will make a wrong guide in the complex road segments that users need to navigate most.
3. Location update rate. The location update rate specified in GB/T19392-2003 should be less than 2s. The parameter is an interval between the location data refresh and the state in which the car GPS navigation system continuously moves at a certain speed.
4. Capture. In the state where the car GPS navigation system is continuously moving at a certain speed, from the time of starting to capturing, GB/T19392-2003 stipulates that when the system is first loaded, the distance required for the vehicle to travel during the capture process is not more than 20km. The time from start to capture should be less than 5 minutes.
5. Efficiency. Select the route with the starting point and the ending point within the range of 50km-500km, and calculate the route. The time from the start route calculation to the start route guidance should be no more than 1min; the response time of the car GPS navigation system to the user operation during the target retrieval process should be no more than 1min.
6, electromagnetic interference. This is a problem that consumers are easily overlooking. Because a large number of electronic devices are concentrated in the vehicle, the electromagnetic interference generated by the GPS navigation system should be controlled within a certain limit so as not to affect the work of other electronic devices inside the vehicle.
The main application of car GPS navigation system
The simplicity of operation of a car GPS navigation system is a goal pursued by the designer. In fact, this also involves a safety issue because the driver is not allowed to drive while watching the electronic map. Therefore, the designer considers developing speech and speech recognition technology for the application requirements of the car navigation system. The user sends a command by voice instead of the button operation to make the navigation system complete the corresponding work. The navigation system replaces the image text by voice and sends a signal to the driver. Or instruction. For example, the Toyota Vios navigation system is a navigation system with voice technology, which can be voiced at places such as turning intersections.
The key to the promotion and application of automotive GPS navigation systems is electronic maps. When a user purchases a vehicle equipped with a navigation system, he or she can also get a CD-ROM with an electronic map. After the user opens the vehicle navigation system, the CD-ROM must be inserted into the optical disc drive of the navigation device. When the system needs road information, it will be obtained on the CD-ROM. This is the case, for example, in the Toyota Vios DVD navigation system, which has a DVD drive under the passenger seat that reads the electronic map DVD disc.
With the urban and rural construction and road changes, the electronic map CD will be replaced regularly. However, the latest version of the electronic map is not guaranteed to be accurate. In response to this situation, the car navigation system needs to add a certain auxiliary support system.
Currently, in areas where car navigation systems are widely used (such as Europe, the United States, and Japan), call centers or public transportation information stations established by automobile companies or other commercial companies send short messages or the latest traffic information sent by radio stations through GSM mobile communication. For the navigation device on the car, the information of the electronic map on the car can be corrected at any time.
China has strict regulations on the publication of maps, and it is no exception to electronic maps. Because it is different from general electronic publications, its standard, policy, confidentiality and technicality are the scientific expressions of national maps and administrative divisions. Therefore, electronic maps must be reviewed by the State Bureau of Surveying and Mapping before they can be published. At present, the DVD voice electronic navigation system of the Vios car also specifically states that the electronic navigation system is “supported and approved by the State Bureau of Surveying and Mappingâ€. In order to adapt to the need for the car navigation device to change the map scale according to the driving area, route, automatic or manual (ie, the electronic map zooms in or out), the map used by the car navigation system must be recorded in a vector manner.
At present, some "automobile GPS navigation systems" installed by some domestic automobile operators are mainly dispatched and monitored. Unlike the car navigation systems described above, vehicles in these systems do not require vehicle independence except for GPS antennas and receivers. Navigation equipment (including electronic maps), the vehicle keeps in touch with the company through the vehicle communication equipment. Because the navigation process of such systems is actually done manually, it is not a car GPS automatic navigation system.
to sum up
With the development of automotive electronics technology, the car GPS navigation system will make people's life more perfect and convenient. Xiaobian through the collection and sorting, simple summary of its composition, working principle, key indicators and applications.
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