# GNSS Signal Distortion Estimation: A Comparative Analysis of L5 Signal from GPS II and GPS III
## 3 Results Analysis
### Signal Analysis
Ideally, the analog channel amplitude and group delay should be constants within the main lobe bandwidth. However, there were different degrees of jitter in the group delay and amplitude affected by the analog device. It can be seen from the figure that, in terms of amplitude and group delay characteristics, the consistency of the analog channel in GPS III satellites was better than that in GPS II satellites. To further analyze the details, the standard deviations of the amplitude and group delay within different bandwidths were considered. It can be seen from Figure 12 that, under different bandwidths, the amplitude standard deviations of BIII were better than those of BIIF. In the 5 MHz bandwidth, the amplitude standard deviations of BIII satellites were approximately 0.1, whereas those of the BIIF satellites fluctuated between 0.1 and 0.55. In the 8 MHz bandwidth, the amplitude standard deviation of the BIII satellite was in the interval 0.13-0.16, whereas, for BIIF, the interval was 0.13-0.65 under the same conditions. In the 10 MHz bandwidth, the amplitude standard deviations of BIII were approximately 0.25, whereas, for BIIF satellites, they fluctuated between 0.13 and 0.80. Similarly, Figure 13 describes the standard deviations of group delay in different bandwidths. Figure 12: Standard deviations of amplitude under different bandwidths. Figure 13: Standard deviations of group delay under different bandwidths. As indicated by Figure 13, unlike the amplitude characteristics, the standard deviations of the group delay presented a more complex characteristic. In the 5 MHz bandwidth, the group delay standard deviations of the BIII satellites were in the interval 1.0-1.8, whereas, for BIIF satellites, they fluctuated between 0.9 and 2.3. In the 8 MHz bandwidth, the group delay standard deviations of the BIII satellite were in the interval 1.4-2.2, whereas, for BIIF, they ranged between 1.7 and 3.5. In the 10 MHz bandwidth, the amplitude standard deviations of the BIII and BIIF satellites were in the intervals 3.5-4.2 and 3.0-5.4, respectively. According to the above analysis of the amplitude and group delay, it is evident that the analog channel was more strongly affected by the group delay. Additionally, the navigation payload transmission channel performance of the GPS III system L5 signal was improved, as compared with that of GPS II, regardless of the amplitude or the group delay characteristics. The S-curve bias (SCB) is an index for evaluating the ranging bias of navigation signals [11]. Ideally, the navigation payload transmission channel SCB should be 0. However, the navigation payload transmission channel was affected by various digital and analog devices, which inevitably introduced ranging errors. In addition, the ranging deviations in satellites were different due to the analog devices used, and this error could not be deducted by differentiation. According to the above estimated digital and analog distortions, this study provides a ranging error reference for user receivers. For subsequent analyses, the receiver unilateral bandwidth setting range was 6-12 MHz, the correlator interval was within 0.5 chips, and the absolute maximum value of SCB was considered, as shown in Table 2. Figures 14 and 15 show the SCBs for BIIF and GPS BIII for different user receiver settings, respectively. Table 2 shows the statistical results of SCB for all satellites. As shown in the table, the SCB of the GPS II signal fluctuated in the range 0.027-0.034 m. By contrast, the SCB of the GPS III signal was concentrated at 0.01 m, indicating that the ranging bias was improved by approximately 0.02 m. Therefore, the user positioning accuracy can be effectively improved by estimating the transmission channel of the navigation payload and providing the user with the resulting ranging deviation.