# Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite
## 5 Conclusions and Future Work  
This paper presents the initial results of the Luojia-1A satellite navigation augmentation system in-orbit validation. Wuhan University carried out the experiment of signal augmentation with the LEO platform. Luojia-1A satellite is capable of determining its own orbit and time standard in real-time and broadcasting its own dual frequency ranging signals. The ground assessment indicates that the phase noise of the payload is comparable with the GNSS signals, although a low-cost OCXO is  
Figure 12: Representative pseudorange and carrier phase measurement residuals of the Luojia-1A with error sources corrected by empirical models.  
employed. During the ground test, the precision of the pseudorange measurement and the carrier phase measurement achieves 2.6 m and 0.013 m, respectively. The in-orbit verification indicates that the observation noise of the onboard dual-frequency receiver is 0.31 m and 0.44 m for the Beidou B1 and B2 measurements. B2 has poorer measurement noise due to its lower C/N0. Equipped with two GNSS receiving antenna on the opposite direction, Luojia-1A satellite is capable of tracking GNSS signals down to \(-20^{\circ}\) elevation, which has potential applications in GNSS occultation. The internal precision of the broadcast ephemeris is about 0.1 m, which does not include the systematical bias. The in-orbit stability of the clock reaches \(3\times 10^{-10}\) and clock drift is observed during the in-orbit test. As a low-cost oscillator, its stability meets the expectation. The precision of the observed pseudorange measurement is assessed with the time differenced approach, and the precision of the navigation augmentation signals reaches 3.3 m for the H1 signal. With the dual-frequency measurement, the precision of the geometry-free combination of the pseudorange and carrier phase measurement reaches 1.8 cm. However, the pseudorange measurements suffer from severe multipath impact, which needs to be improved in the future. Finally, the correctness of the navigation augmentation is validated by calculating the residuals from the empirical models. After removing the error sources, the residuals of the augmentation signals are stable over a long period. After correcting the error sources with the empirical models, the precision of the pseudorange residuals is around 3.6 m. However, the remaining pseudorange residuals still present elevation-dependent biases, which need to be further studied. The precision of the carrier phase is limited by the clock phase noise of the ground receiver, which attained about 22 cm accuracy in the current stage. With a higher quality clock, centimeter-level accuracy carrier phase measurements can be expected. Generally, the assessment results verified that the methodology of the Luojia-1A satellite's navigation augmentation system is practically feasible and the augmentation signals from Luojia-1A is useable. However, there is still a great deal of work to do in the future, including improving ground receiver performance, modeling the time synchronization errors, refining the error source models, and carrying out more experiments to investigate the benefit of combining GNSS/LEO for positioning and navigation applications. With better understanding of the LEO navigation augmentation signals, higher precision and better performance can be expected.  
[PERSON] wrote the manuscript. [PERSON], [PERSON] and [PERSON] inspired the idea and proofread the manuscript. [PERSON], [PERSON], [PERSON] and [PERSON] helped with the data collection. [PERSON], [PERSON] and [PERSON] helped in performing the data analysis. This research is support by the National Natural Science Foundation of China (NSFC 41704002, 91638203), China Postdoc Science Foundation (2017M620337) and the Fundamental Research Funds for the Central Universities. We gratefully thank [PERSON] from the University of California, San Diego for polishing the manuscript. The authors declare no conflict of interest.