The SBAS L1 signal was designed in the late 90s in order to provide ranging plus integrity and correction data from Geo-stationary satellites to the only available core GNSS constellation and signal at that time, GPS L1 C/A. The SBAS L1 signal design was specifically optimized for a well determined set of system characteristics of that epoch: GPS constellation number of satellites, satellites atomic clock offset and drift characteristics at that epoch, GPS L1 C/A signal selectivity availability (SA), targeted approach procedures with vertical guidance (APV I), etc [1][2]. Therefore, the signal characteristics such as message content, implemented channel code, number of channel components, useful bit rate, etc., were selected accordingly to the fulfillment of these needs [1][2]. Nowadays, the majority of these system characteristics/targets have changed or evolved: SA has been disabled, new atomic clocks are much more stable than the old ones, and approaches with vertical guidance with a decision height down to 200 ft (LPV 200) are targeted. Moreover, a new GNSS signal in the L5 band, GPS L5, has been designed specifically for civil aviation requirements. Finally, more core GNSS constellations (with its own civil aviation oriented signals such as GALILEO E5) are being and will be deployed in the upcoming years. Therefore, the design of a new SBAS signal, called L1/L5, which includes the future multi-constellation horizon, the advantages provided by the new L5 civil oriented signals exploitation and which takes into account all the system characteristics/targets evolutions is mandatory.