Quantitative analysis of GNSS performance under railway obstruction environment

Abstract

GNSS (Global Navigation Satellite Systems) have been widely applied in railway applications as timing, passenger information, track survey, and also safety relevant applications as train localization for train control purposes on lines like Chicago-Detroit Line, Qinghai-Tibet railway line, etc. However, due to the continuous dynamic movement of the locomotive, it goes through various environments as tunnels, bridges, mountainous areas, urban canyons, etc. GNSS performance is affected and thus degraded in some GNSS signal reception constrained environment scenarios, which leads the localization result not meeting the required accuracy and safety requirements. With the dynamic movement of the locomotive, the measurement results are not abundantly enough. It is quite necessary to investigate the methodology for GNSS performance evaluation in signal reception constrained environments using both simulation and field test results together. This will reduce the test complexity and also provide the possibility to compare between different parameters. In this paper, a 3-km-long railway environment scenario in Qinghai-Tibet railway is selected and modelled with signal reception limitations recorded several times in the on-board log files. After an investigation of the terrain structure of the environment, a hill near the railway tracks namely mountainous environment is causing the “half sky” signal blocking scenario. The terrain is modelled using Google SketchUp. Then, the model data is converted into QualiSIM, which is a matlab-based tool to provide the possibility to understand the sky plot of the location according to almanac and relevant information. The satellite visibility, satellite geometry and signal propagation trajectories are calculated and illustrated in QualiSIM. Using the output from QualiSIM, the locomotive operation route, 3D environment “half sky” model and signal propagation trajectory are all configured and simulated using Spirent GSS8000 system. The GNSS signals are repeated using Spirent GSS8000 together with Spirent SimGEN software. A UBLOX M8N and Unicore UB370 GNSS receivers were used to test and compare the performance of both receivers under the modelled mountainous environment. The accuracy levels of both receivers are compared by satellite visible number, HDOP (Horizontal Dilution of Precision) and position error. The measurement errors are illustrated using error ellipse, which shows the different accuracy level and the reliability level for the two receivers. Comparing the recorded results of open sky and different levels of obstruction, GNSS performance can be gradually degraded when the degree of obstruction is increased.