## 6 Discussion and Conclusions  
With the rapid development of LEO satellites, the joint positioning method of LEO and GNSS has been extensively studied. However, as in most studies, it just stays at the level of observed value fusion. To improve the performance of GNSS receivers under ultra-weak signal conditions, we propose a novel receiver structure with an LEO-augmented GNSS system, in which LEO-assisted GNSS signal acquisition and tracking methods are adopted. By utilizing the LEO satellite to realize Doppler positioning, this method initializes the position and time of the GNSS receiver and effectively reduces the acquisition and tracking search range of the receiver. Through the ephemeris information broadcasted by the LEO satellite, GNSS receivers can eliminate the impact of bit reversal and increase the coherent integration time. We carry out theoretical analysis and simulation experiments on the proposed acquisition and tracking scheme. Results show that the acquisition efficiency is improved by 170 times, acquisition sensitivity is improved by 8 dB, and the tracking sensitivity is improved by 4 dB compared with the traditional standalone receivers. Concerning LEO positioning, we only use the existing Iridium Doppler positioning as an example. Our proposed LEO-augmented GNSS receiver structure is based on the LEO  
Figure 16: Performance comparison of estimated Doppler error.  
Figure 15: Performance comparison of frequency tracking.  
positioning. The information LEO provided is independent of specific LEO constellations. Moreover, we choose the positioning error range within 1 km, which is wide enough. The only difference is that the LEO is assumed to transit GNSS ephemeris in this work. Thus, the presented results are generic with other LEO constellations if the LEO satellite is transmitting the GNSS ephemeris.  
This paper mainly discusses the high-sensitivity benefits of an LEO-assisted GNSS receiver, which is considered to be used in the ultra-weak signal environment, such as the strong wideband jamming background. Our next research work will focus on the performance of the structure under high-precision and high-dynamic applications. For high-precision applications, the PLL or FLL-assisted PLL should be used to get the carrier phase. With regard to the high dynamic applications, the GNSS/INS coupled system will be considered to reduce the impact of the receiver motion. In addition, with the development of LEO positioning technology, LEO satellite constellations with multiple satellites can effectively improve the initial positioning and timing accuracy, which means that the augmented effect will be further increased.  
Conceptualization, L.C. and J.Z.; methodology, L.C. and W.G.; software, L.C. and Y.D.; validation and writing--original draft preparation, L.C. and W.G. All authors have read and agreed to the published version of the manuscript.  
This research is funded by the National Natural Science Foundation of China, grant number 41604021 and grant number 41974038; the Fundamental Research Funds for the Central Universities, grant number 2042019 kf0215.  
Data Availability Statement: Data sharing not applicable.  
The authors declare no conflict of interest.