# BeiDou Augmented Navigation from Low Earth Orbit Satellites
## 5 Conclusions  
Currently, the Global Navigation Satellite System (GNSS) mainly use the satellites in Medium Earth Orbit (MEO) to provide position, navigation, and timing (PNT) service. The weak navigation signals limit its usage in deep attenuation environments, and make it easy to interference and counterfeit by jammers or spoofers. Moreover, being far away to the Earth results in relative slow motion of the satellites in the sky and geometric change, making long time needed for achieved centimeter positioning accuracy. By using the satellites at lower earth orbit (LEO) as the navigation satellites, these disadvantages can be addressed.  
LEO satellites (with altitudes from 160 km to 2000 km) are 20 times closer to the Earth than GNSS at MEO, leading to stronger radio signals on the ground for navigation. The swift motion of LEO provides the rapid geometric change, which resulting quicker PDOP change and rapid convergence  
Figure 6: Time series of kinematic PPP positioning errors of CENT station in 1 h after the convergence of PPP for BeiDou-3 (**a**) and BeiDou-3 as well as LEO combined solutions (**b**), respectively.  
for integer ambiguity. We compared and analyze this theoretically. We have simulated the BeiDou-3 constellation consisting three GEO, three IGSO, and 24 MEO satellites and LEO navigation system consisting of a constellation of 120 satellites in 10 orbit planes with inclination of 55 degrees in a nearly circular orbit (eccentricity about 0.000001) at an approximate altitude of 975 km to assess the integrated navigation performance of LEO constellation. The results confirm the theoretical analysis, and demonstrate the satellite visibility and PDOP can be improved. More importantly, the accuracy and convergence of PPP can be improved and significantly reduced to about 1 min.  
Although the simulation used in this study is used for augmenting BeiDou-3 with LEO constellation. The LEO constellation can also be used as independent navigation system, as Iridium does. However, as too close to the Earth, the ground converge of LEO satellites is less than one-tenth of that of MEO ones. Hence, hundreds of LEO satellites would be needed to match the coverage of GNSS. Thanks to the development of commercial communication LEO constellation, it is promising and possible to integrate the navigation components to the communication satellites to provide independent navigation from LEO. Currently, many plans have been proposed to achieve this around the world, such as Chinese Hongyun and Hongyan. Once it has been implemented, the real-time navigation with cm level accuracy will be achieved easily, and definitely significantly change our daily life.  
[PERSON] and [PERSON] wrote the main manuscript text. [PERSON] conducted the experiments. [PERSON] and [PERSON] conceived of the idea and reviewed the experimental methods and results. All authors reviewed the manuscript.  
This work was supported by the Shandong Provincial Natural Science Foundation, China (Grant number ZR2018 PD006); Scientific Research Foundation of Shandong University of Science and Technology for recruited Talents (Grant number 2017 RCJJ072).  
The authors declare no conflict of interest.