Beidou and GPS, positioning in the end who is stronger

- Apr 17, 2018-

Beidou and GPS, positioning in the end who is stronger?


As a member of the Global Navigation Satellite System (GNSS) Club, navigation and positioning is a basic function. First look at how the positioning function compares:


The process of receiver using satellite positioning can be selected in a two-dimensional abstraction to select at least four points (satellites) on a blackboard, with each point as a center, each drawing a circle with chalk (receiver-to-satellite distance measured). The overlapping area of handwriting is the result of positioning. Because of various kinds of errors, the result of measuring each satellite is more like a piece of chalk, not a line without width.


The thickness of the chalk is affected by various error mechanisms. Compass and GPS are mainly different in the accuracy of the atomic clock on the star. The second generation of the Beidou uses the cesium atomic clock. The GPS uses the helium atomic clock to combine the cesium atomic clock. GPS is at this point. Better, that is, chalk traces more detailed. However, now the Beidou third-generation satellites use hydrogen atomic clocks. The frequency accuracy has increased from 5×10-13 to 6×10-14, and the frequency stability has increased from 2×10-14 (1d) to 6×10-15 (1d). Compared with the second-generation satellites, it has increased by an order of magnitude, reached the world's leading level, and the positioning accuracy will be further improved.


In addition to the influence of chalk thickness, obviously the more dots are drawn, that is, the more satellites there are, the smaller the overlap area is, that is, the more accurate the positioning, but if these points are concentrated on one side, or even overlap, The overlapping areas of the chalk traces they draw will become very large, whereas the more dispersed these points, the lower the chalk trace overlap, which means that the receiver will obtain more accurate positioning results. This parameter to measure the position distribution is the so-called PDOP value, the lower the better the distribution, the more accurate the positioning result.


When the Beidou generation really positioned slowly and had poorer accuracy, it all happened that after more than ten years, there were people who felt that the Beidou was not a good thing. They said that there were "less satellites," "still under construction," and "the accuracy was far worse than GPS." ?


In fact, starting from the second generation of the Beidou, the constellation was designed for the Asia Pacific region. Unlike the US MEO-based model, the GEO+IGSO+MEO model was adopted. The advantage is that fewer satellites are used to quickly achieve high accuracy in key service areas. The disadvantage is that high-latitude regions have poor reception effects for GEO, the precision factor is too large, and the other side of the earth cannot receive it. With the increase of Beidou satellites, These problems will be solved later.


Even so, today, when the three generations have not been used yet, look at the current accuracy of the Beidou. Let's choose the northernmost point in China: 52.4476° north latitude. The official Beidou 2nd generation service area is only available. Between the northern and southern latitudes of 55°, what is the visibility of Beidou satellites 24 hours a day in this position?




As can be seen, there are at least eight Beidou satellites with elevation angles above 10° available throughout the day.


Contrast to the current GNSS club "first brother" GPS, satellite visibility here:




Did you find that the number of visible satellites is not as good as Beidou? The feeling is whether it is possible to engage in big news such as "GPS was defeated in China by the Big Dipper..."


Of course, blind arrogance is also not desirable. There are always some fake news about Beidou’s “secondary killing GPS”. It is not good for the public to know the Beidou. In addition to using the security of its own navigation system, GNSS clubs who can not kill anybody who is good, not long ago China and the United States have signed the "Beidou and GPS signal compatibility and interoperability Joint Statement", mutual backup and more harmonious ... ...


Just now that there are many BeiDou satellites, but the positioning can not only look at the number of stars and eat the loss of the constellation set. The elevation angle of the BeiDou GEO received here is too low and the geometric distribution is poor, so the accuracy factor is poor:




The Beidou geometric accuracy factor reaches a maximum of 6.0 and fluctuates. Compared to GPS, the maximum is 4.4.




This is because the Beidou GEO satellites are suspended above the equator. The elevation angle received here is less than 30°. The inclination of the IGSO satellite orbit is 55°. This is also the head of the “8”-shaped trajectory, so the 1 and 4 quadrants In most areas, there are basically no stars. The combination of these factors leads to a large DOP value for the Beidou, but it is still not a problem to meet the 10m accuracy.




 So you have to say here that BeiDou is not as good as GPS. Of course, with the three generations of networks, this gap will soon be gone and a few more MEOs will be better.... The GPS trajectory distribution of this place is below. :




Next, erroneously select a point in southern China: 26.6940° north latitude and 109.9688° east longitude. This coordinate is visually measured in Hunan. How is Beidou satellite visibility?




At least ten stars are visible, then look at the GPS:




Still the same as before, the difference between North and South is not significant. The emphasis is on the DOP value. Can it be a war? The Beidou DOP value is as follows:




The maximum does not exceed 3.4, then take a look at GPS:




Can ignore the particularly high point, the basic is still within 4.4, but obviously the Beidou's smoother!


The reason why we can maintain a good position distribution throughout the day is because we can receive Beidou GEO satellites and IGSO satellite signals steadily here. MEO satellites are only a minority, rather than GPS, which is composed of MEOs. The change will continuously change the satellites involved in positioning, and Beidou will not introduce the introduction of large numbers of errors in the calculation of the jumps and low elevation angles caused by the replacement of positioning satellites. The lower the satellite elevation angle is, the more air the signal passes through. The more errors are introduced, the more signal fading, and the more affected by multipath and other factors.


Here, Beidou’s GEO satellites can be seen all day, and the IGSO satellites can be seen in more than half of the time. The MEO satellites do not stay in one place, but turn their own circles around the earth, so they will repeat continuously and repeatedly. From the low elevation angle into the field of view, rise to a certain height, and then reduce the cycle of the elevation angle out of the field of vision.


After considering these factors, the Beidou is definitely no worse than GPS. Overall, in the current Beidou second-generation service area, Beidou’s all-day visible stars exceed GPS, and most areas in the constellation distribution are also better than GPS, but taking into account the differences in the precision of atomic clocks and the distribution of constellations in high and middle latitudes. It is said that Beidou is comparable to GPS or reliable, and it is also in line with actual measurement results. However, now that the construction of three generations of networks is underway, the number of Beidou satellites is steadily increasing, and the most critical atomic clock problem has also been solved in the third generation. By that time, we will not be humble!