# Predicting C/N0 as a Key Parameter for Network RTK Integrity Prediction in Urban Environments
## 4 Predicting Signal Strength
### Classification of the Signal Reception Conditions by Ray-Tracing  
A signal (ray) may fall into one out of four categories as shown in Figure 6[27]. Line-of-sight (LOS) is the case where the satellite is only directly visible from the antenna position. Non-line-of-sight (NLOS) reception happens if the signal impinges the antenna in a way other than direct path, which can be either a reflection or diffraction. In this study, by NLOS we mean specular reflection only. Diffraction events are mentioned separately. If the signal comes to the antenna both along the direct path and also from a reflection, then it is called a multipath. In a blocked situation, the signal is halted from reaching the antenna by any means, usually a large block of building or mountain. It should be mentioned that here, only single reflections are considered, so the case of blocked signals may also contain situations of multiple reflections. Diffraction can occur in all the four cases (LOS, NLOS, MP and blocked).  
Figure 5: Signal strength variation for GPS and GLONASS L1 signals shown for Round 5. The color code in the bottom of the figure indicate the solution type: fixed (green), float (olive), code (yellow) and navigated (red), cf. Figure 2. The way-points A to K are as defined in Figure 1a.  
This signal reception classification has become possible due to 3D city models. The precise geometric information which is provided by the coordinates of the building models is divided into several polygons. The standard CityGML (Geography Markup Language) Level of detail 2 (LoD2) which is used here includes the details of the ridge or former rooftops of the buildings which are typical in German cities. In this study, the 3D building model Level of detail 2 (LoD2) provided by the city Hannover is used. The 3D city model is divided into 1 km by 1 km, gml files. Based on the coordinates of the experiment, a large enough area is selected to include all the buildings that might be influential in the ray-tracing. A nearby skyscraper which is not included in the selected area is additionally considered because although it is quite far, it has direct influence on the signals. Coordinate transformations are performed as indicated in [38].  
Satellite coordinates and their temporal evolutions are known for every constellation by almanacs in advance, while more precise information is broadcast in real time from the satellites through navigation messages.  
The last type of information needed for the classification is the position of the moving receiver. For prediction purposes, the user position can be obtained from a path planning, assuming that a specific trajectory should be driven a given moment in time. Results of [28; 39] show that the ray-tracing classification is preserved for a road section of few meters. In this study, in order to ease the direct comparison with the measurements, the coordinate time series of the reference trajectory are used that were obtained by tightly coupling the GNSS and IMU data.  
Knowing the coordinates of the satellite and the receiver antenna, the ray tracing is performed [40]. For diffraction, the third Fresnel ellipsoid is considered as the obstruction criteria, cf. [41].