Abstract:
Navigation Satellite System (GNSS) plays a crucial role in urban positioning from which various consumer applications could benefit from, i.e., intelligent transport systems, vehicle lane control, navigation for blinds, and location based services. The key performance requirements of these services are availability and accuracy. Availability refers to the number of the satellites visible to a GNSS receiver, while accuracy of urban positioning depends largely on the quality of signals received from those visible satellites and also the geometry of them.
The aforementioned availability and accuracy are decided by urban environment that presents three major challenges in the computation of the visibility of and signals received from a satellite: Firstly, tall buildings, elevated structures such as overpasses, and elevated railways, may block signals from the satellites. Secondly, signals may be multi-reflected by flat, glass or mental surrounding surfaces. Thirdly, when signals are partially blocked by a surrounding obstacle, edge-diffraction may occur. All of these may either make the satellites invisible or distort signals from satellites due to the non-line-of-sight (NLOS) signals, i.e., reflection and diffraction signals, and multipath signals, received by the receiver, thus make the positioning difficult and even impossible. This phenomenon is also called the urban canyon effect.
To evaluate signal propagation thus availability of GNSS satellites in urban environments, a ray-tracing based modelling technique is proposed that can predict the LOS, NLOS and multipath signals together with 3D city model. The time evolution of key parameters describing the GNSS constellation, i.e., the position of satellites, computed through the Simplified General Perturbations (SGP4) model with the broadcast Almanac data in Two-Line Element Set (TLES) format. A ray-tracing based modelling technique is presented to predict the satellite visibility in an urban environment that is represented with 3D model. The developed technique is validated against in-site measurements in urban environment and can provide insights into mitigation of NLOS and multipath effects in urban canyon positioning. It can be used to analyze the visibility of GNSS satellites on roads city-wide in a city. The original city model is represented with CityGML models that then be converted to STL surface models for the calculation. Laborious and time-consuming in-site measurements are therefore avoided. With the simulated results, places where the satellite visibility is poor can be identified and necessary measures can be taken accordingly.