## 5 Static Field Test  
To further evaluate the performance of the ASTKF method in a real environment, a static field test was conducted. The static test site was selected in an open, outdoor area, and the test environment is shown in Figure 9. In the test, the RF front-end was used to record the GPS L1 C/A code signal, and the signal length was approximately 5 min. The vector software receiver was used for post-processing analysis. During the field test, satellites 10, 12, 14, 20, 21, 24, 25 and 32 were visible. Figure 10 shows a sky plot of the visible GPS satellites. The ASTKF and two popular methods (NWPR and VSM) were used to estimate C/N\({}_{0}\) for comparative analysis. The averaging time of 0.5 s was selected as the empirical value of the averaging time.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**0.5 s Averaging Time**} & \multicolumn{2}{c}{**1 s Averaging Time**} & \multicolumn{2}{c}{**3 s Averaging Time**} & \multicolumn{2}{c}{**5 s Averaging Time**} \\ \cline{2-9}  & **Mean** & **Std** & **Mean** & **Std** & **Mean** & **Std** & **Mean** & **Std** \\  & **(dB-Hz)** & **(dB-Hz)** & **(dB-Hz)** & **(dB-Hz)** & **(dB-Hz)** & **(dB-Hz)** & **(dB-Hz)** & **(dB-Hz)** \\ \hline
**NWPR** & 9.88 & 6.22 & 10.30 & 5.69 & 11.19 & 4.30 & 11.71 & 3.20 \\ \hline
**VSM** & 10.51 & 4.42 & 10.93 & 3.57 & 11.64 & 2.18 & 11.90 & 1.58 \\
**ASTKF** & 13.94 & 2.03 & 14.09 & 1.71 & 14.25 & 1.29 & 14.32 & 1.07 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Weak signal C/N\({}_{0}\) estimation statistics.  
Figure 9: Static real signal acquisition environment.  
As can be seen from Figure 10, satellite 10 has the highest elevation angle, and the signal reception quality is satisfactory. In a short time (5 min), the signal power and \(C\!/\!N_{0}\) do not change substantially. Figure 11 presents its \(C\!/\!N_{0}\) estimation results. According to Figure 11, in the real signal environment, the ASTKF estimation results are less volatile than the NWPR and the VSM estimation results. Figure 12 is a box diagram of the \(C\!/\!N_{0}\) estimation results of satellite 10 at various averaging times. According to Figure 12, the ASTKF method achieves higher accuracy (the result distribution is more concentrated). Moreover, the ASTKF estimation error with an averaging time of 0.5 s is less than that of the NWPR and the VSM with an averaging time of 5 s. These results are consistent with the conclusions of the simulation tests.  
As Figure 10 shows, satellite 24 has a low elevation angle, and the signal is susceptible to interference and attenuation. Satellite 24 is selected for evaluation of the estimation performance of the ASTKF method when the signal condition in the environment is poor. Figure 13 shows that the trends in the ASTKF and the other two methods are consistent. At 100-200 s, the signal power and \(C\!/\!N_{0}\) show wide ranges of changes. The ASTKF method can correctly track this change in \(C\!/\!N_{0}\). The test results demonstrate that in the real environment, the ASTKF method realizes satisfactory estimation performance on signals of poor quality.  
Figure 11: Satellite 10 \(C\!/\!N_{0}\) (0.5 s averaging time).  
Figure 10: Sky plot of GPS satellites for the static field test.