Quantitative Analysis of GNSS Performance under Railway Obstruction Environment
Debiao Lu, Shuxian Jiang, Baigen Cai, Wei Shangguan, Beijing Jiaotong University, China
GNSS has been widely applied in railway applications as timing, passenger information, track survey, and also train localization for train control purposes on lines like Chicago–Detroit Line, Qinghai-Tibet railway, etc. However, due to the dynamic movement of the locomotive, it goes through various environments as tunnel, bridges, mountainous, urban, etc. GNSS performance is affected and maybe degraded in some GNSS signal reception constrained environment scenarios, and thus cannot meet the accuracy demands then further safety requirements when the signal reception is poorly received. Train localization performance requirement specifications based on GNSS has not been standardized yet, let alone the quantification of the performance in various environment scenarios. Due to the fact that the locomotive is always on the route, the actual performance is hard to estimate using a simple environment model. Many of the tests have been carried out at the test site. It is quite necessary to investigate the method for GNSS performance evaluation in the GNSS reception constrained environments using both simulation and real tests together. This will reduce the test complexity and also provide the possibility to compare between real and simulated results.
In this paper, a railway station called XiaoNanChuan and a 3 km long track near the station in Qinghai-Tibet railway line is selected and modelled due to the signal reception limitations which was recorded several times in the locomotive on-board unit action records. From the terrain information of the environment, this is a mountainous scenario which has a hill quite near the track. The terrain is first fitted using the Google SketchUp. Then, the model data is converted and modelled in a Matlab-based Tool named QualiSIM. The QualiSIM provides the ability to simulate the satellite visible number information, HDOP information and also the signal reception trajectory as direction-finding of the propagation of the route. This provides the possibility to understand the satellite availability and signal propagation routes.
The 3D model and signal propagation trajectory information is documented and configured using Spirent SimGEN software using the signal blocking and pseudorange ramp tools inside the software. The GNSS signals are repeated using Spirent GSS 8000 together withSpirent SimGEN. Two receivers are tested under the modelled environment scenario. A UBLOX M8N receiver and Unicore UB370 receiver were used to test and compare the performance of both receivers under the mountainous environment. The accuracy levels of both receivers are compared supported by satellite visible number information, HDOP and UERE. The measurement errors are modelled using the 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 degrees of obstruction, GNSS performance can be gradually degraded when the degree of obstruction is increased.