Design Of The GNSS-based Train Positioning Unit And Reliability And Safety Index Analysis

GNSS (Global Navigation Satelite System) based train positioning unit featuring extensive coverage area, high positioning accuracy and low maintenance cost can meet the demand for continuous postioning of modern railway. It is especially suitable for ’low density lines’and reprents major development trend of future train positioning technology. Research on GNSS-based train positioning technology has been done by several research institutes in abroad as well as in China. However, no sysmetic research has been done on the reliability and safety issues which are critical to the system. Based on this situation, a GNSS based train postioning unit is designed in this paper. Then research is done on the reliability and safety performance of the postioning unit by using the Fault Tree Analysis method and PDS (Reliability of computer based safety systems) method. The specific research works as follows:(1)The research status at home and abroad of GNSS-based train positioning unit is summarized. Through demand analysis of the positioning unit, the RAMS demand indexes of the positioning unit are determined. In order to obtain the desired reliability and safety performance, the2-out-of-2redundancy structure is adopted in the hardware structure design of the positioning unit. Meanwhile, the failure rates of the components used in the positioning unit are calculated, which will be used in the quantitative anaylisis of the system safety and reliability.(2)Research on the reliability of the designed postioning unit is done by using Fault Tree Analysis method. Thaking the event that the positioning unit could not perfrom as the top event of the fault tree, the structure of the fault tree is designed and qualitative and quantitative anaylisis are done. Meanwhile, through calculating the probability importance degree and structure importance degree of the bottom events, the modules/components with high probability importance and structure importance degree that have greater impact on the reliability of the positioning unit are determined and comprehensive improvement measures of the system reliability are proposed.(3)The probabilily of failure per hour (PFH) of the designed GNSS-based postioning unit, which are key evaluation indexes of hardware Safety Integrity Level (SIL), are calculated using PDS method. A further analysis of random hardware failure safety integrity level of components is done to indentify the modules, which have greater impact on the hardware SIL.