This paper focuses on the model of the Signal In Space bi-ases in the Integrity Support Message (ISM) and Advanced RAIM (ARAIM) user algorithm. Signal distortions, satel-lite orbit and clock missmodelling, antenna phase center offsets, inter-frequency biases and other satellite related errors generate biases in the ranging error distributions which must be included in the error model and handled in the integrity algorithm. The scope of this paper is to investigate the design of these biases in the ISM in order to optimize the user availability and integrity performance. The study of present ARAIM algorithms shows situations where the conservatism of the biases model can degrade significantly the user availability. One of the reason is related to the assumption that the signs of these biases are unknown. An investigation on the methodology to handle the signs is provided. An improved approach is presented using this information and modelling it in both ISM and ARAIM algorithm. This approach allows to improve the user availability and integrity performance by reducing the protection levels. The user performance improvement was confirmed during an experimentation activity performed with service volume simulations. Finally the performance improvement was confirmed with an analysis on real data collected during a GPS-Galileo dual frequency flight campaign performed in March 2015.
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