Inside GNSS Media & Research

JUL-AUG 2019

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Page 50 of 67 J U L Y / A U G U S T 2 0 1 9 Inside GNSS 51 where denotes the matrix of bus boundary mentioned earlier. is a 3x2 transform matrix. Aer the trans- formation, satellites and building sur- face boundary can be presented in the same coordinate. Line segment rep- resents the building surface boundary corresponding to line segment as shown in Figure 7. en, the azimuth and the elevation angles for point E, and F can be calculated in the Skyplot respectively. STEP III: NLOS Correction Based on Detected TEBs Considering satellites' elevation angle, azimuth angle and building boundary information (elevation and azimuth angles in Skyplot), satellite transmis- sions blocked by building are detected. en, NLOS correction is implemented with a NLOS error model consequent- ly. In terms of the measurements from a GNSS receiver, each pseudorange mea- surement is written as follows: (4) where is the geometric range between the satellite and the GNSS receiver. denotes the satellite clock bias. indi- cates the receiver clock bias. repre- sents the ionospheric delay distance; indicates the tropospheric delay dis- tance. represents the errors caused by the multipath effects, NLOS recep- tions, and receiver noise. We focus on mitigating the NLOS errors. e NLOS error model proposed in the paper by Hsu, 2018 is expressed in Figure 8. e expected signal transmission route is expressed as dash blue line in Figure 8. represents the distance from receiver to the building. represents the ele- vation angle of GNSS signal. Assuming the building is vertical to the ground and GNSS signal reflection satisfied the law of reflection. As a result, the NLOS error can be calculated based on the azimuth angle, elevation angle and the distance from the receiver to the build- ing causing the reflection. e process of NLOS correction is summarized in detail in Algorithm 3 in Wen, et alia. (5) FIGURE 7 Skyplot of GNSS satellites and detected building surface boundary. Green and red circles and the nearby numbers indicate satellites and corresponding PRNs. Line segment indicates the boundary of the buidling boundary. STEP IV: GNSS Positioning Based on Corrected Pseudorange Measurement In this step we implemented GNSS WLS based on the corrected NLOS and healthy pseudorange measure- ments. Measurements with low eleva- tion angle and SNR are more likely to be a contaminated GNSS signal, such as the multipath or NLOS, due to the ref lection, blockage, and diffraction. us, proper thresholds need to be set to exclude the unhealthy measurements. e weighting scheme follows the sug- gestions from Herrera, et alia in the paper in Additional Resources. Experiment Setup and Result Experiments are conducted in a typical urban canyon of Hong Kong, and the experimental scene is shown in Figure 9. e Skymask in the right-hand side dem- onstrates the degree of urbanization. In the experiment, a receiver is used to col- lect raw GPS and BeiDou measurements, while a 3-D LiDAR sensor is employed to provide the real-time 3-D point clouds scanned from the surroundings. Both the receiver and the 3-D LiDAR are installed on the top of an experiment vehicle, which can be seen in le-hand side of Figure 7. e data were collected FIGURE 8 NLOS correction model in term of (left) elevation angle and (right) Azimuth angle point of view.

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