# Predicting C/N0 as a Key Parameter for Network RTK Integrity Prediction in Urban Environments
## 4 Predicting Signal Strength
### Predicting Satellite Visibility  
In Figure 7a, the signal visibility for GPS is shown as predicted by the ray-tracing algorithm. The color codes indicate the signal reception classes, cf. Figure 6. Green is LOS, purple is NLOS, yellow shows multipath and gray indicates diffraction. Here, the gray color implies that the satellite is in a blocked situation but the signal arrives from diffraction. The small red vertical bars indicate the start and end epochs of signal interruptions.  
The continuity of the real code and phase signals for GPS constellation are shown in Figure 7b,c respectively. While PRN 14 has been predicted mostly in a diffracted mode, it has not been observed in real data, neither in code nor carrier phase. Maybe this is due to the fact that for the diffraction calculation, the third Fresnel ellipsoid has been considered. This can imply that probably lower-order Fresnel ellipsoids are more realistic to be used. There are many interruptions in the code signal in PRNs 31 and 32; but the carrier phase  
Figure 6: Classification of GNSS signal reception conditions. (**a**) Satellite signals may experience one of the four situations: LOS, NLOS, multipath or blocked. Diffraction can occur in addition to any of the aforementioned cases. (**b**) Exemplary skyplot with corresponding visibility status for every azimuth and elevation. This skyplot belongs to a point in the middle of the north–south oriented street near way-point E (cf. Figure 1a).  
signal has been observed less for these two satellites. This can imply the difficulty of maintaining the carrier phase in more interrupted signals rather than code.  
Figure 8a-c, depict the predicted, real code and real phase visibility, respectively, for GLONASS system. As it can be seen in the prediction part, PRNs 5, 7, 8, 14, 15, 21 and 22 are predicted to be visible, while in reality, PRNs 5, 7, 14, 21 and 22 for code and 7, 14, 21 and 22 for phase are observed.  
The confusion matrix between predicted satellite visibility and real code and phase observations for GPS and GLONASS for the fifth round are reported in Table 1. This table shows the extent to which the predicted visibility complies with real data. There are two values in each cell. The first one without parentheses is the percentage of the total number of satellites meaning 32 for GPS and 24 for GLONASS, while the value in parentheses indicates the percentage referring only to the predicted satellites meaning PRNs 1, 3, 4, 14, 17, 19, 21, 22, 31, and 32 for GPS and PRNs 5, 7, 8, 14, 15, 21, and 22 for GLONASS. As an example, in 18.69% of the epochs of the fifth round (3031 epochs), the predicted visibility is the same as the real phase visibility for the GPS constellation (all 32 satellites). This value for only the predicted visible satellites is 59.81%. These values can be visible in the upper right panel of Table 1. In this panel, considering all satellites, 90.56% (18.69 + 71.87) of the epochs are correctly predicted (true positive and true negative); while considering the predicted satellites, a total of 77.61% (59.81 + 17.80) of the epochs are correctly predicted. This means that the total visibility status of the satellites are well predicted, but in the details of the visibility for predicted PRNs, the situation is more difficult.  
Figure 7: Predicted and observed GPS satellite visibility exemplarily shown for the fifth round of the experiment: (**a**) The predicted GPS visibility, the LOS, NLOS, MP and diffracted signals are depicted in green, purple, yellow and gray color, respectively. (**b**) The real visibility of code GC1C signal. (**c**) The real visibility of the GL1C signal. The real data are exemplary from the Altus receiver. Short vertical red lines indicate the signal start or interruptions. The way-points A to K are as defined in Figure 1a.