GPS World, July 2015

PERSONAL NAVIGATION Software Hardware Integration Figure 2 illustrates an example of a 3D building model established in this way The 3D building map contains a very small amount of data for each building in comparison to that of the 3D graphic application For our purposes the file only contains the frame data of each building instead of the detail polygons data This basic 3D building map is utilized in the simulation of ray tracing Our version of the ray tracing method does not consider diffractions or multiple reflections because these signals occurred under unfavorable conditions Here we utilize only the direct path and a single reflected path The developed ray tracing simulation can be used to distinguish reflected rays and to estimate the reflection delay distance Our research work assumes that the surfaces of buildings are reflective smooth planes that is mirrors Therefore the rays in the simulation obey the laws of reflection In the real world the roughness and the absorption of the reflective surface might create a mismatch between the ray tracing simulation and the real propagation Here we ignore this effect as the roughness of the building surface is much smaller than the propagation distance The OPENING FIGURE 1 shows an example of the GNSS signal propagation using ray tracing and a 3D building map Red green and white lines denote the LOS path reflected paths and the NLOS paths respectively In this environment a conventional positioning method such as weighted least squares WLS usually estimates the position on the wrong side of street as shown in the red balloon With the aid of 3D building model and ray tracing the map based positioning method is able to provide a result close to the ground truth Map Based Pedestrian Positioning The flowchart of the 3D city building model based particle filter is shown in FIGURE 3 This method first implements FIGURE 3 Flowchart of the particle filter using 3D city building models a particle filter to distribute position candidates particles around the ground truth position In Step 2 when a candidate position is given the method can evaluate whether each satellite is in LOS multipath or NLOS by applying the ray tracing procedure with a 3D building model According to the signal strength namely carrierto noise ratio C N 0 the satellite could be roughly classified into LOS NLOS and multipath scenarios If the type of signal is consistent between C N 0 and ray tracing classification the simulated pseudorange of the satellite for the candidate will be calculated In the LOS case simulated pseudoranges can be estimated as the distance of the direct path between the satellite and the assumed position In the multipath and NLOS cases simulated pseudoranges can be estimated as the distance of the reflected path between the satellite and the candidate position via the building surface Ideally if the position of a candidate is located at the true position the difference between the simulated and measured pseudoranges should be zero In other words the simulated and measured pseudoranges should be identical Therefore the likelihood of each valid candidate is evaluated based on the pseudorange difference between the pseudorange measurement and simulated pseudorange of the candidate which is simulated by 3D building models and ray tracing Finally the expectation of all the URA index range accuracy URA 3Dmap index 1 24 m 34 m 1 75 100 2 34 m 485 m 2 50 75 3 485 m 685 m 3 25 50 4 685 m 965 m 4 10 25 5 965 m 1365 m 5 5 10 6 1365 m 240 m 6 5 TABLE 1 The definition of URA and URA3Dmap used in this article Percent of the valid candidate GPS World July 2015 www gpsworld com 38