9 major differences between Beidou positioning and GPS positioning technology
Beidou satellite construction is an important part of China's strategic cause. Of course, national defense security is the most important reason for building Beidou. In a battlefield where geographical information is so important, it is necessary to have its own intellectual property rights and products. Although the civilian use of GPS is free in China, if the United States shuts down or uses its "SA policy" to "trick" GPS during the war, it will undoubtedly be fatal to countries that rely on GPS at that time. GPS jammer
The Beidou satellite started in 1992 and achieved regional coverage in 2012. It can be said that Asian countries provide services, and now it has gone abroad and is welcomed by Southeast Asian countries. This article aims to carefully compare the differences between Beidou and GPS from a technical point of view.
1. Tri-frequency signal
Beidou uses tri-frequency signals, while GPS uses dual-frequency signals, which is the latecomer advantage of Beidou. Although GPS launched the first tri-frequency satellite on May 28, 2010, it will take several years until all GPS satellites are aged and scrapped and replaced with tri-frequency satellites. The past few years have been Beidou's dominant period. The tri-frequency signal can better eliminate the influence of high-order ionospheric delay, improve positioning reliability, enhance data preprocessing capabilities, and greatly improve the efficiency of ambiguity fixation. Moreover, if there is a problem with a frequency signal, the traditional method can be used to use the other two frequencies for positioning, which improves the reliability of positioning and the ability to resist interference. Beidou is the world's first satellite navigation system that provides tri-frequency signal services.
2. Active positioning and passive positioning
Active positioning means that the receiver itself needs to transmit information and communicate with satellites, and passive positioning does not. The active positioning of the Beidou generation, the active positioning technology can complete the positioning with only two satellites, but it needs the support of the DEM (Digital Elevation Model) database of the information center and participates in the calculation. It is retained on the second generation of Beidou, but it is not used as the main positioning method. The Beidou II uses passive positioning, which is the same as GPS. It does not require the information center to participate in the calculation, and active positioning is used as a supplementary function.
The advantage of this function is that when the quality of satellites you observe is very poor and the number is small (theoretically, passive positioning requires at least 4 satellites to solve the four unknown parameters of XYZ and time, and more are actually required), you can still position. This function will be more useful for emergency situations. For example, in the valley, the observation conditions are very poor, and it is very important to know the approximate location. The downside is that your location information will be exposed during the war.
The need for the information center to participate in the solution is due to "limited resources". For example, a Beidou generation handheld can be located every 60 seconds, and cannot be located frequently to ensure that the information center cannot be overloaded. But the main reason why the Beidou generation cannot be used by civilians is not because this.
The first generation of Beidou is called the Beidou Satellite Experimental System (RDSS), and the second generation of Beidou is called the Beidou Satellite Navigation System (RNSS). "First generation, second generation" is for convenience. As you can tell from the name, the Beidou generation is just an internal experiment to test whether our theories and technologies are feasible, how accurate the positioning is, and then make follow-up improvements. The original intention of the design was not intended for civilian use at all.
3. Short message communication service
This is the original function of China's satellite navigation, and it is very practical. During the Wenchuan Earthquake in 2008, the only communication method in the earthquake area was the Beidou generation. It is no surprise that this feature has been retained in the second generation. However, this function is also limited in capacity, so it is not suitable for daily communication, but more suitable for emergency communication. Based on this function, another advantage of Beidou is that it can not only know where I am, but also let others know where you are. This feature is useful for calling for help.
4. Domestic monitoring
The satellite positioning system generally consists of three parts: the space constellation part, the ground monitoring part and the user receiver part. Among them, the ground surface monitoring part is composed of three major parts: monitoring station, main control station, and injection station.
The GPS system builds 5 monitoring stations, 1 main control station and 3 injection stations around the world to ensure the operation of the satellites. These stations are all located in the United States and distributed evenly around the world. Including the continental United States of America, Guam and Hawaii in the Pacific Ocean, Diego Garcia in the Indian Ocean, and the Ascension Islands in the Atlantic Ocean. China cannot build monitoring stations all over the world, so when designing the Beidou system, China must take into account that the ground monitoring part can only be built in China to ensure the normal operation of the entire system. It is not impossible to build a station overseas, but even if it is built, it will only play a role in improving accuracy, and it must not be used as a control function. This was originally a disadvantage of Beidou. The domestic monitoring was forced out, and there were no other options, but now it has become Beidou's security advantage, and it does not need to be controlled by other countries.
Today, the first land remote sensing satellite data receiving station outside China, the "Arctic Station", will start construction in Sweden this year and is expected to be completed in two years. China will build its first overseas satellite tracking station in Argentina in South America. From South America to the North Pole, China's satellite industry has started a globalization model.
5. Open step by step
GPS must be used after the entire system is built. Currently, the 14 Beidou satellites in orbit use 5 geostationary orbit (GEO) satellites, 5 inclined geosynchronous orbit (IGSO) satellites, and 4 medium altitude circular orbit (MEO) satellites. The Beidou constellation plan was hard-won. Academician Xu said that there were several plans participating in the competition at that time. The revision and argumentation of the plan alone lasted for three years. The final plan cannot be said to have no shortcomings, but it is definitely the best among all the plans. good one. Closer to home, with the support of this innovative space constellation, the Beidou satellite navigation system was officially put into operation in the Asia-Pacific region on December 27, 2012 after only launching 16 satellites. This is conducive to accelerating the commercialization of Beidou, further improving the subsequent system, and accelerating the maturity of the Beidou industry chain. After all, the biggest market for Beidou must be China. Let the Beidou system develop in the Asia-Pacific region for a few years first, let the chips mature for a few years, and it will be relatively easy to promote it to the world. Moreover, the utilization efficiency of satellites in the Asia-Pacific region is definitely higher, and it is more worthy of priority investment.
6. Partial strengthening and gradual maturity
Theoretically, the global positioning accuracy of GPS is equivalent. The Beidou system has been specially strengthened for China and its surrounding areas, and the geometric conditions of domestic satellites are relatively good. The accuracy of single-point positioning depends on two aspects: one is the accuracy of observations, and the other is the spatial geometric distribution of the observed satellites. In navigation, DOP is used to represent the contribution of satellite spatial graphics, including: spatial DOP GDOP, position DOP PDOP, time DOP TDOP, planar DOP HDOP, vertical DOP VDOP, relative positioning geometry Decrease of precision factor RDOP. As the Beidou global system gradually matures and the DOP becomes smaller and smaller, it is only a matter of time before its positioning accuracy in China and surrounding areas exceeds that of GPS. "Gradual maturity" is not an excuse, but technological and theoretical progress.
(1) Increase in the number of satellites.
The GPS design uses 21+3 satellites, that is, 21 working satellites and 3 spare satellites. At present, GPS has actually used 32 satellites. The more satellites, the more redundant data will be obtained, the more reliable the data, and the smaller the DOP value. There are only 16 Beidou satellites now. When the number of Beidou satellites increases, more observation data will be obtained, and the accuracy improvement is inevitable. At present, Beidou chips generally support GPS. There may be the following reasons. The first is to supplement the accuracy of the Beidou system, and the second is to open up the market (in the beginning, only Beidou was supported and no one used it). In this case, the chip is more complex, the power consumption is higher, and the development is more difficult. But it can’t be said that it’s all bad. At present, compatibility with different systems is also the development trend of the industry. GLONASS chips are generally compatible with GPS, and Beidou chips also support GLONASS or even three systems. Compatible systems, more data redundancy, higher precision, and small DOP, this function will also become an advantage for Chinese chip manufacturers.
To add, currently Meizu MX4, MX4pro, Mi 4, Huawei G7, Samsung S5, and NOTE4 all support GPS, GLONASS, and Beidou. It can be seen that compatibility with the three major navigation systems is the general trend. I believe that in the near future, there will be more and more terminals compatible with Beidou.
(2) Correction model optimization.
The errors related to the signal propagation path are: tropospheric refraction error, ionospheric refraction error delay error, multipath effect, and earth rotation effect error. There is no way to completely eliminate these errors, they can only be continuously reduced. The Klobuchar model used to improve the delay error of the ionospheric refraction error is based on the long-term meteorological observation data to construct an empirical formula for the ionospheric refraction changing with time. To put it bluntly, it is a guess, but it has a level, data support, and a smart guess. After it comes out, it will be tested and verified. If it works well, keep it, and if it doesn’t work, continue to change it. The ionosphere and troposphere in different regions of the world are different. These formulas are constructed based on foreign observation data, and they will naturally be worse when used in China. We need to give Beidou more time to accumulate observation data and wait for the development or optimization of more A corrected model for the Chinese region.
(3) The satellite orbit accuracy is improved.
There must be a certain gap between the actual orbit of the satellite and the designed orbit. The principle of pseudo-range positioning is: use the distance resection method to determine the three-dimensional coordinates of the receiver antenna. Only when the satellite orbit accuracy improves the positioning accuracy will it be high. The orbit of the satellite is fitted through the observation data of the monitoring station. The longer the observation time, the more data accumulated, and the more accurate the fitted orbit. Beidou lacks foreign observation data, so the orbit accuracy is relatively high in the Asia-Pacific region, and the orbit accuracy abroad will be relatively poor. To make up for this shortcoming also needs to give Beidou time. Mention geostationary orbit (GEO) satellites again. GEO satellites cannot be completely stationary relative to the earth, and will drift to a certain extent. However, there is only one geosynchronous orbit, and resources are very scarce. Internationally, this orbit is divided into small and small circular arcs. Satellites can only move within the allocated range, otherwise they may collide with other satellites, so every once in a while You need to adjust the GEO satellite position. At present, the adjustment of Beidou adopts the pulse method, which can only adjust the position of the satellite according to the whole number of times, not a few tenths of times, so there may be situations where one more time is too much, and one less time is not enough. In the subsequent launch of GEO satellites, the adjustment satellites will switch to continuous ones, spray as much as you want, and enhance satellite control capabilities and accuracy. Once the adjustment is made, the previous observation data will be invalidated, and the data needs to be accumulated again. During the satellite adjustment period, that satellite was disabled, because we don't know its exact location, and it took a few days to re-orbit. But fortunately, the number of GEO satellites is relatively small (5), and orbit determination is easier and faster than the other two satellites. This situation does not exist for the other two satellites. Over time, the accuracy of the fitted orbit will naturally improve until it is exhausted.
7. Positioning accuracy
The positioning accuracy of the Beidou system has been increased from 25m horizontally and 30m elevationally to the current level of 10m, and the elevation is 10m. The speed measurement accuracy has been increased from 0.4 meters per second to 0.2m. Performance is comparable to GPS. Academician Xu said in his lecture that their actual measurement accuracy (calculated according to the medium error) can reach the level of 4-5m, and the accuracy level of 5-6m in elevation. Academician Xu said that the Beidou could achieve such accuracy when it was first put into use, which they did not even expect when they designed the Beidou system. They are already very satisfied, and Beidou still has a lot of room for improvement, and the accuracy can be further improved.
The above-mentioned 10m accuracy, many people think it should be the average accuracy of the Asia-Pacific region. It should be noted that the plane accuracy and elevation accuracy of Beidou are basically equivalent, while the horizontal accuracy of the GPS system is indeed good, but its elevation accuracy is a weakness, which is much worse than the horizontal accuracy, generally 1.5 to 2 times.
The accuracy of GPS positioning can reach mm level, which is achievable, but it cannot be discussed outside the constraints. Locate the signal jammer and find Dazhan Electronics. There are many forms of satellite positioning methods. According to the state of the user's satellite measurement equipment in operation, it can be divided into static positioning and dynamic positioning. If the position of the reference point is different, it can be divided into absolute positioning and relative positioning. The difference technology is based on the principle of synchronous co-orbit, using the reference station of known points to calculate the correction information, and then send it to the rover, and then correct the instantaneous position of the rover. This is a technology for dynamic measurement, which improves the positioning accuracy from 10-40m to less than 3m. Accuracy reaching the mm level should be absolute positioning under static long-term high-quality observation conditions. To explain in detail, static means to build a special house and a fixed observation pier. At this time, the accuracy of the tripod is not enough, and it is easy to be moved. For a long time, that is, 24 hours, 365 days of uninterrupted observation, it is necessary to ensure that there is a power supply, and the requirements are very high. The power cannot be cut off, and there must be a backup power supply. High-quality observation conditions are to have no electromagnetic interference, no high-rise building shielding, no one can approach the GPS antenna at will, no calm water nearby (there will be multi-path effects), and no large hillsides. Indispensable is a high-precision, high-stability, high-quality GPS receiver and other ancillary facilities (save, process data, etc.). To meet these conditions, we can only build a permanent high-precision observation station in a rural area with certain conditions away from the city. It is not easy to meet such harsh conditions, and the construction and operation costs are very high. If Beidou observes in this way, the accuracy must not be 10m. Don’t just say that it is better than Beidou just by hearing a data. Please explain the observation conditions. In particular, the GPS system uses the WGS-84 coordinate system, and the Beidou uses the CGCS2000 coordinate system, so the values of the two cannot be directly compared, and coordinate conversion is required, and coordinate conversion generally brings about loss of accuracy. Accuracy can be directly compared in their respective coordinate systems without coordinate conversion.
8. Promote the upgrading of the entire manufacturing industry
(1) The GPS chip is so easy to use, why don’t we work hard to make progress if the chip of the Beidou system is not easy to use? The answer is definitely no.
With the completion of the Beidou system, Chinese chip manufacturers have arrived in spring! Although there is currently a gap, it is entirely possible that Chinese chip manufacturers will finally have the opportunity to compete with foreign chip manufacturers in Beidou chips. On the contrary, it is almost impossible for domestic chip manufacturers to produce GPS chips to compete with foreign manufacturers. This is called breaking the monopoly of GPS. Experts predict that by 2020, the Beidou satellite navigation market alone will reach an annual output value of 400 billion yuan, with a compound annual growth rate of more than 40%.
Beidou is very profitable, and the country has created opportunities for you. How to get this money depends on each person's ability. If you look at the Beidou Industrial Parks that are blooming all over the country, you will know how rare the opportunity is. The whole system country is only responsible for the space constellation and the ground monitoring part. These two parts cost a lot of money, have high technical requirements, and are unique. The system performance indicators mainly depend on these two parts. The user acceptance part is mainly left to the market. This part determines the ease of use of the navigation system and the vitality of the ecological chain, etc., but this part can be participated by everyone, and the substitution is very strong. Products that cannot improve their own competitiveness will eventually be replaced.
(2) A large part of the reason why the accuracy of the Beidou system is not high enough is that China's atomic clocks are not good enough.
In satellite guidance and positioning, the time system is of great significance. When the tracking station determines the orbit of the satellite, when the error of the satellite position is required to be less than 1cm, the corresponding time error should be less than 2.6μs (1 microsecond = 10-6 seconds ); if the measured distance error is required to be less than 1cm, the measurement error of the signal propagation time should be less than 0.03ns (1 nanosecond = 10-9 seconds). Ningbo Dazhan Electronic Technology Co., Ltd. Compared with foreign products, China's atomic clocks are large in size, heavy in weight, and their accuracy is an order of magnitude worse. This kind of high-tech technology is embargoed to China abroad, and we can only rely on ourselves. why not? Because for a while, atomic clocks could be bought from abroad, and the cost was not high compared to their own development, and the quality was very good, so the domestic development of atomic clocks was directly abandoned. When China said it wanted to build a navigation system, foreign countries immediately imposed an embargo on China. At this time, China quickly started the development of atomic clocks. It is a pity that the development of atomic clocks in China has been suspended for several years. Although the original technology is not advanced, it is also Barely keeping up with the world trend, but now it has lagged a lot. If China insists on developing its own atomic clock, the current Beidou system will be more accurate. If it weren't for the Beidou system, China's atomic clock technology would be even more tragic.
Nine, the construction speed is fast
Europe proposed the construction of the GALILEO system as early as February 10, 1999, and launched the first experimental satellite in 2005. On April 27, 2008, the second experimental satellite was launched, and the progress was delayed by five full years compared to the original plan. , launched the third and fourth satellites in October 2012. These four satellites form a network and initially play the role of precise positioning on the ground.
The first Beidou satellite was launched on April 14, 2007, and the 16th Beidou satellite was launched on October 25, 2012. This is the last satellite of the Beidou regional network. The regional network of the Beidou navigation project was successfully completed. In 2013 On December 27, 2019, two system documents, "Beidou System Open Service Performance Specification (Version 1.0)" and "Beidou System Space Signal Interface Control Document (Version 2.0)" were officially released. This is the symbol of Beidou's official commercial use. You can develop your own products based on these two files.
Beidou and GALILEO have a competitive relationship (overlapping) in frequency use. According to the international legal principle of "whoever uses it first gets it", China launched three "Beidou" second-generation satellites in 2009, officially using this frequency, and The European Union did not even shoot all three of its experimental satellites. Lose the battle and lose ownership of the frequency.
The 9 advantages of Beidou are listed above, of course, the problem of Beidou is not avoided. In addition to the few foreign monitoring stations, GEO satellites with both advantages and disadvantages, the ones not mentioned above are:
(1) The quality of the satellite is not high and its lifespan is relatively short.
The lifespan of GPS satellites is generally 8 years, and the lifespan of Beidou satellites is about 5 years. Among the 16 satellites that have been launched, 2 old satellites have failed. The work of replacing satellites is done by the state and does not affect users at all. At present, this problem has been basically solved, and the life of the new satellite has been extended.
(2) Chip prices are higher.
This is due to the production characteristics of the chip. The investment in the research and development of the chip is very large. Once the development is successful, the subsequent production cost will be relatively low. Beidou has just been put into use, and there are still relatively few users, so the price is high. When it is mass-produced, the cost will be diluted.
From November 17th to 21st, 2014, the 94th meeting of the Maritime Safety Committee of the International Maritime Organization (IMO) was held in London, England, and the navigation safety circular for the approval of the Beidou satellite navigation system was reviewed and approved, marking the official launch of the Beidou satellite navigation system. Become an integral part of the global radio navigation system and obtain international legal status for maritime applications. The Beidou satellite navigation system has become the third global satellite navigation system after the Global Positioning System (GPS) and the GLONASS satellite navigation system (GLONASS), serving the world's navigation users. This will surely drive the internationalization and industrialization of Beidou satellite navigation system in the field of navigation. After being recognized by IMO, my country will continue to comprehensively promote the formulation and revision of standards, norms, and guidance documents of international technical organizations such as the International Electrotechnical Commission, the International Organization of Navigational Lights, the International Maritime Radio Technical A full range of applications in the maritime sector.