This paper presents a nominal range error analysis to support ARAIM based on a statistical study of orbit and clock errors observations for Galileo F-NAV Open Service. Nominal range error is characterized by the socalled Signal-in-Space Range Error (SISRE) which is computed by comparing precise reference orbits to broadcast ephemeris for each individual satellite. This work processes Galileo constellation service data from March 2015 to July 2016 accounting for the progressive satellite addition during the constellation expansion. Given the undergoing maturation of the Galileo ground segment along with the pending declaration of the Full Operational Capability, the nominality of the error is assessed by disregarding the 1% of the worst-case samples. Results will address the differences in the nominal range error performance among Galileo satellites highlighting their clear dependence on the satellite clock error behavior. Furthermore, this work evaluates on a monthly basis the evolution of error performance throughout the monitoring period showing a substantial enhancement in the past months in nominal range error. Results advocate for flexible overbounding values contained in the Integrity Support Message (ISM) that are able adapt throughout the constellation evolution along its deployment (i.e. monthly update rate as proposed by Online Architecture).