User Integrity Risk Calculation at the Alert Limit without Fixed Allocations

There are two major concepts to calculate the integrity for the user. Either it can be calculated which error magnitude has to be assumed to be compliant with a given integrity risk value (protection level concept), or the integrity risk can be directly calculated at the Alert Limit (AL). For the first concept fix allocations have to be established for each failure mechanism (e.g. 2% horizontal and 98% vertical like WAAS allocation) and HPL respectively VPL result. However, this has to be reconsidered if a the combined integrity risk value is specified (cf. Galileo SRD). Furthermore, user geometries which lead to high HPL are different from those which lead to high VPL values. In case of WAAS this does not have to be considered due to the fact that just 2% are allocated for the horizontal case and due to the fact that WAAS has compared to Galileo very relaxed availability requirements (cf. unavailability of 5% for SBAS vs. 0.5% for Galileo), so that the decrease in availability due to this fixed split can be tolerated. In case of Galileo four failure mechanisms need to be considered, horizontal, vertical and for each of them fault-free and one undetected error. Following the first concept, a fix allocation for each of the four failure mechanisms has to be specified, but the user geometries for which each mechanism might occur are totally different. Therefore, a WAAS similar concept would result in unrealistically high performance allocations with a huge impact on ground infrastructure and system cost due to the fact, that each of two mechanisms requires almost the same allocation. It seems more feasible to calculate the integrity risk for each failure mechanism at the AL and to compare the sum of all four contributions with the required integrity risk. This approach corresponds to the second concept. This paper outlines the described user integrity concept without fix allocations. Furthermore, detailed simulations are presented to verify the enhanced availability performance.