# Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite
## 4 In-Orbit Performance Evaluation Results
### Navigation Augmentation Signal Noise Evaluation
The C/N0 changes as the elevation angle change and the highest C/N0 is only 46.4 dB-Hz, which is significantly lower than the theoretical C/N0, suggesting that the radio frequency circuit and the antenna need to be further optimized. There is a sudden drop in the second half of the observation window, most likely to be is caused by signal obstruction since the sudden observation noise change also presents in subsequent experiments. The figure indicates that the overall standard deviation is 6.4 m. The observation noise generally dependent to the elevation angle, but the observation noise dramatically increases at low elevation case due to the signal obstructions and multipath effect. For the elevation higher than 20\({}^{\circ}\) case, the observation noise is reduced to 3.2 m. It is also noticed that there is a constant bias in the statistics, which is caused by the satellite clock drift. Since the code tracking loop of the receiver is not aided with the carrier phase information, it is vulnerable to the multipath effect. Considering the large acceleration in the line-of-sight direction between the satellite and the receiver, it is still a challenge to improve the code noise from the tracking  
Figure 9: Representative sky plot of the Wuhan station shows the tracks of Luojia-1A and GPS/Beidou satellites over a 10 min pass. loop design, but there is no doubt that the pseudorange precision can be improved significantly with better understand of LEO signal characteristics. Luojia-1A is capable of transmitting dual frequency ranging signals simultaneously. The geometry-free (GF) combination of the pseudorange and carrier phase measurements is formed to evaluate the inter-frequency consistency. The geometry-free combination is defined as:  
\[ \begin{split} P_{\text{GF}}=P_{1}-P_{2}=I_{1}-I_{2}+2\epsilon_{pi} \\ L_{\text{GF}}=L_{1}-L_{2}=I_{1}-I_{2}+(\lambda_{1}N_{1}-\lambda_ {2}N_{2})+2\epsilon_{pi}\end{split}  \tag{7}\]  
where and are the geometry-free combination of the pseudorange and carrier phase measurements. The GF combination eliminated most errors except the ionosphere and the ambiguities. The ionosphere delay is also significantly mitigated and the ambiguities are constant over time. Therefore, the GF combination can be used to roughly evaluate the measurement noise. The GF combination of the pseudorange and the carrier phase measurements from Luojia-1A are presented in Figure 11. The standard deviation of the GF combination of the Luojia-1A pseudorange is 4.7 m. Assuming that \(P_{1}\) and \(P_{2}\) are independent, then the standard deviation of the pseudorange measurement is 3.3 m (1\(\sigma\)). The observation noise presents strong correlation as the elevation angle The GF combination of the carrier phase measurement is presented in the lower panel of Figure 11, which indicates that the standard deviation is 1.8 cm. Due to large acceleration in the line-of-sight direction (up to 77 m/s\({}^{2}\)) between the LEO and receiver, it is difficult to balance the sensitivity and the tracking loop precision. We still attempt to optimize the tracking loop parameters of our ground receiver to obtain better accuracy. Figure 10: The pseudorange noise of Luojia-1A H1 signal.