This paper presents a URA/SISA analysis to support ARAIM based on a time-dependent statistical characterization of orbit and clock errors observations. For each individual GPS and Galileo satellites, by comparing precise orbits to broadcast ephemeris data, this work computes the Signal in Space Range Error (SISRE) which needs to be overbounded by the URA/SISA value included in the ISM. Over seven years of service history data for GPS and four months for Galielo are computed in this analysis, showing that range error is manly driven by satellite's clock performance. Satellites that historically have presented well behaved (Tsisre are equipped with stable clocks which display small error dispersion. A particular example is provided through GPS SVN65/PRN24 whose Cesium clock worsens satellite's performance as compared to the rest of the GPS block IIF equipped with Rubidium clocks. Time- dependent results show that orbit and clock error distributions are not zero mean on a monthly basis, although they do not exhibit a significant bias in a long-term scale. This fact might open the possibility of a short-time adaptable bnom and (Jura overbounding parameters contained in the ISM.