It is believed that many people have encountered inaccurate positioning when taking a taxi when using mobile phones, and the driver cannot find anyone when coming to pick up; When we made an appointment to meet, it was clear that the map was displayed at the same point, but we could not touch each other; The navigation system was too late when driving and missed the exit.
If you use the ultra precise positioning mobile phone Xiaomi 8, the above problems will be solved. As the world's first smartphone supporting dual band GPS, Xiaomi 8 can achieve ultra precise positioning.
As official positioning technology partners, Baidu, Gaode and Tencent, the three major map manufacturers, also give high praise to Xiaomi 8, which is called a breakthrough innovation and milestone progress in the map navigation industry.
GPS development history and basic positioning principles
GPS， The full name of Global Positioning System is a space based radio navigation system that provides uninterrupted positioning, navigation and timing services worldwide. As long as anyone has a receiver, the GPS system can provide him with location and time information. GPS consists of three parts: satellites orbiting the earth; Monitoring stations set up on the ground; And user owned receivers such as smart phones.
The predecessor of GPS is the 1958 US Navy research, which uses the meridian satellite system based on Doppler frequency shift technology. It is also the first successful satellite navigation system in the world, laying the foundation for the development of GPS. In 1973, GPS was officially established in the US Department of Defense, and the development process was divided into three stages. The first stage is the feasibility study. In 1978, the first GPS experimental satellite was launched. In this stage, four experimental satellites were launched. The second stage is the comprehensive development and test stage. From 1979 to 1984, seven more test satellites were launched and receivers for various purposes were developed. The third stage is the practical networking stage, which began in 1985. In 1989, the first GPS working satellite was successfully launched. In 1991, GPS became famous in the Gulf War. At the same time, the miniaturization of the GPS receiver was successful. In 1995, the United States announced that GPS was in full operation.
GPS satellites send signals from space that can be received and recognized by mobile phones, including the current position coordinate information of the satellite, and the precise time of the GPS satellite itself corrected by atomic clocks and relativistic effects. Assuming that the time between the satellite and the mobile phone is precisely synchronized, after receiving the signal from the satellite, the mobile phone subtracts the launch time carried in the signal from its own time, and then multiplies the ideal transmission speed of electromagnetic waves, namely the speed of light, to calculate the distance between the mobile phone and the satellite.
Since the position of the satellite is precisely known, we have obtained the distance from the satellite to the mobile phone previously. Using the distance formula in three-dimensional coordinates and the coordinates of three satellites, we can form three equations to solve the position of the observation point (X, Y, Z). However, since this conclusion assumes that each satellite is precisely synchronized with the time reference of the mobile phone, in fact, a fourth satellite must be added to calibrate the time. At present, there are 31 GPS satellites orbiting the earth at an altitude of 20200 km above the ground in a 12 hour cycle, making it possible to observe more than 4 satellites at any point on the ground at any time.
Limitations of L1 single frequency GPS mobile phones
As mentioned earlier, the key to GPS positioning is to obtain the distance between the mobile phone and the satellite. To calculate the distance, you must know the propagation time of the signal. This data is calculated by the mobile phone. The key is how long it takes for the GPS signal to reach the mobile phone after transmission.
1、 Limited accuracy of L1 band GPS signal
If only 1575.42MHz GPS signal in L1 band is used for positioning, its bandwidth is small, only 1MHz. That is to say, the transmission time of a minimum cycle is 1/106s=1 μ s, which is 300 meters when multiplied by the speed of light. If the L1 band GPS signal is compared to a ruler used to measure the distance between satellite and mobile phone, its "minimum scale" is 300 meters, which is the theoretical range of ranging error of a single L1 band satellite. This limitation of L1 band will lead to inaccurate positioning no matter in open areas or cities with many high-rise buildings.
2、 L1 band GPS signal is vulnerable to reflection interference
In the scene of numerous buildings, when positioning in cities, rivers, lakes and seas, metal, water, glass, etc. are all good reflectors of GPS signals. In the above scenarios, using GPS positioning, the mobile phone will receive one or more signals reflected by the surrounding terrain in addition to the signals transmitted in a straight line from the GPS satellite. In this way, multiple signals received by the mobile phone will be superimposed, which will cause the direct signal with the shortest distance to become invisible, thus reducing the accuracy of distance calculation between the satellite and the mobile phone, and even leading to the disappearance of satellite signals in serious cases. This effect is called multipath effect in physics.
It can be seen that in the figure below, because the accuracy of L1 frequency band signal is 300 meters, the direct signal and reflection signal are overlapped, and the mobile phone will think that the actual received signal is gray superposition signal, causing positioning deviation.
3、 L1 band GPS signal is vulnerable to the ionosphere
In open areas, the main reason for inaccurate positioning is the refraction of GPS signals caused by the ionosphere in the atmosphere, which increases the transmission time. Ionosphere in the atmosphere is full of ions and electrons under the irradiation of sunlight, which has a serious impact on GPS signals, such as electromagnetic waves. It is found that the refraction effect caused by the ionosphere is inversely proportional to the square of the signal bandwidth, that is, the lower the bandwidth, the greater the impact caused by the ionosphere.
L1 plus L5 dual frequency GPS, Xiaomi 8 realizes ultra precise positioning
The dual frequency GPS function enables Xiaomi 8 to use the GPS signals of L1 and L5 bands to locate at the same time. GPS signals in L5 band are more complex than those in L1 band, and both mobile phones and satellites need higher costs. Previously, L5 band was mostly used for industrial applications with high precision requirements, such as oil and gas exploration and aviation safety industries. Xiaomi 8 was the first consumer product to apply dual frequency GPS.
1、 L5 band GPS signal has higher precision
Since the bandwidth of GPS signal in L5, i.e. 1176.45MHz band, is up to 10MHz, 10 times that of L1 band, and the minimum cycle transmission time is 0.1 μ s, the positioning and ranging error of a single satellite is also reduced to 30m.
2、 L5 band GPS signal is not vulnerable to reflection interference
Therefore, in the scenarios with serious multi-path effects such as high-rise buildings, it is more difficult to stack the L5 frequency band signal direct reflection used by Xiaomi 8, thus reducing the impact on positioning accuracy. As shown in the figure below, the blue and purple reflected signals can hardly affect the green direct signal, and Xiaomi 8 can obtain more accurate positioning.
3、 L1 and L5 dual frequency comparison can eliminate the influence of ionosphere
As for the refraction effect caused by the ionosphere, first of all, the bandwidth of L5 band is 10 times that of L1 band. According to the calculation that the error caused by the ionosphere before is inversely proportional to the square of the bandwidth, the influence of the ionosphere on GPS signals in L5 band is one percent of that in L1 band. Secondly, because the ionosphere has different effects on L1 and L5 frequency band signals, dual frequency GPS equipment such as Xiaomi 8 can not rely on other factors. By comparing the delay of two channels of signals, the error caused by the ionosphere is eliminated by calculation, and the GPS positioning accuracy is further improved.
Xiaomi 8 dual frequency GPS measurement: a street for single frequency mobile phones
It can be seen from the actual measurement that when Xiaomi 8, which uses dual frequency GPS, navigates around the island under the space bridge, the positioning points almost follow the track route, while the single frequency GPS mobile phone, because of the signal interference and obstruction of the viaduct, causes the positioning points to drift significantly, and even once entered the surrounding building area, which is mainly the limitation of the single frequency GPS mobile phone in complex urban scenes, Similarly, the actual measurement of Jianwai SOHO is the same.
Similarly, when performing single point positioning at the Space Bridge, it can be seen that only the positioning point of Xiaomi 8 of the three test mobile phones almost coincides with the actual position. At present, most single frequency GPS mobile phones have different degrees of deviation. That is to say, when users use Didi and other platforms to hail, it is easy to mark the wrong location, which leads to the problem that drivers and passengers cannot find each other.
Dual frequency satellite positioning, the future trend
As mentioned earlier, due to such factors as size and cost, dual frequency GPS equipment, even if commercially available, is mostly used in professional fields or military fields with higher accuracy requirements. However, in recent years, the popularity of automatic driving and other technologies has also put forward higher requirements for GPS accuracy. IEEE of the International Institute of Electrical and Electronics Engineers predicted that with the continuous progress of the overall GPS solution, the mobile phones for ordinary consumers in the future will gradually popularize dual band GPS, and eventually make the GPS positioning accurate to 30 cm.
In addition to GPS from the United States, there are GLONASS from Russia, Beidou from China, Galieo from the European Union, and QZSS from Japan. Xiaomi 8 supports all the above mainstream satellite positioning systems. Among these systems, Galieo supports E1 and E5a bands compatible with GPS L1 and L5; As a supplement to GPS, the L1 and L5 frequency bands of QZSS are the same as those of GPS.
At present, there are 30 positioning satellites supporting L1+L5 dual band in the world, and the satellites launched in the future will also support dual band. It can be seen that in order to achieve higher positioning accuracy, it is an inevitable trend in the future to support L1+L5 dual frequency receivers, whether satellite or smart phone receivers. At present, in order to save Lu Chi, we are the first to support the dual frequency GPS, Galieo, and QZSS systems. Xiaomi 8, with ultra high positioning accuracy, can be said to take the lead.
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