## 4 Conclusions  
The 2-h onboard GPS data collected by Chang'E-5T probe are explored and analyzed in this contribution. The observation quality, mainly including the number of observed satellite, C/N0 level, PDOP as well as C1 code errors are evaluated in detail. It is found that on average the onboard GNSS receiver can track 7-8 GPS satellites per epoch for Chang'E-5T. For over 71.6% of the observations, their C/N0 values are higher than 31 dB-Hz, and for 31.8% over 34 dB-Hz. The PDOP values are significantly related to the spacecraft altitude, reaching about 20 at 50,000 km altitude and better than 10 below 30,000 km. Since the C1 and L1 observations are almost free from the ionosphere and troposphere delays, the C1 code error is calculated by differencing the C1 and L1 observations directly while the L1 ambiguities is removed by averaging. The resultant overall C1 code errors are 4.15 m. For C1 observations with higher C/N0 levels, i.e., 36 dB-Hz, the precision can be better than 2 m. Although increased C/N0 threshold can improve overall data quality and produce smaller code errors, the available number of GPS observation is greatly reduced and the resulting PDOP values are increased.  
OD is carried out for Chang'E-5T using the C1 and L1 observations and the Helmert variance component estimation method is investigated to determine the weights of code and carrier phase observations. The OD precision is firstly evaluated by overlap comparison, which indicates an orbit consistency is about 17 m in 3D RMS. The RMS of L1 and C1 residuals are 74 mm and 4.58 m, respectively, showing good consistency with the C1 code errors statistics. Furthermore, the OD precision is analyzed by screening GPS data with different C/N0 thresholds of 28 dB-Hz, 31 dB-Hz and 34 dB-Hz. The results indicate that the OD precision for Chang'E-5T is mainly dominated by the number of visible GPS satellites. For the cases of C/N0 thresholds of 28 dB-Hz and 31 dB-Hz, the orbit differences are below 10 m. However, higher C/N0 threshold of 34 dB-Hz results in significant orbit precision degradation to 51 m, which should be primarily due to data volume decrement. Therefore, we suggest that the received C/N0 minimum should be designed to be less than 31 dB-Hz for onboard receiver in SSV, preferably less than 28 dB-Hz.  
We need to further research in some aspects to improve the accuracy and autonomy of navigation architectures in future lunar exploration missions for the various mission phases. First, comprehensive utilization of GPS, GLONASS, BeiDou and Galileo system should be taken into account to improve OD accuracy by increasing the number of available satellites and reducing PDOP values. In addition, the integration of GNSS with other state-of-the-art space navigation sensors like IMU and Doppler radar altimeter is expected to achieve a high degree of autonomy and robustness of navigation. Second, a receiver with higher performance, particularly the capability of receiving weak signals, should be developed to provide GNSS navigation to the lunar explorers at the distance of 400,000 km and even farther in deep space. Third, the data processing algorithm should be adopted to further improve the accuracy and autonomy of navigation solutions using existing and future GNSS signals, such as real-time enhanced filtering and weak signal and low C/N0 data processing algorithms.  
This research is partially supported by the National Natural Science Foundation of China (Grant Nos. 41674004, 41404032, 41574030), the Fundamental Research Funds for the Central Universities (No. 2042016 kf0185) and the Natural Science Foundation of Hubei Province (2014 CFB168). [PERSON] performed the experiments and wrote the article. [PERSON] provided the initial idea for this study and supervised the article. [PERSON] wrote the article. [PERSON] supervised the experiments. [PERSON] prepared and analyzed the data and drew figures.