The SBAS, satellite-based augmentation system, is an international standard navigation service with continental coverage. The SBAS is defined in the GNSS SARPS (standards and recommended practices) established by the ICAO (international civil aviation organization) which prescribes RF signal characteristics, message contents, and user receiver processing procedure. The SBAS provides integrity information, differential correction information, and ranging signal by broadcast from geostationary satellite. Among them, the integrity information is the most important for air navigation users, while the differential correction information improves positioning accuracy within a few meters. All of SBAS systems available up to now, i.e., US WAAS, European EGNOS, and Japanese MSAS, provide augmentation information regarding only GPS. In order to provide the better availability to users, it is a natural way to extend the SBAS to support the multi-constellation environment. In fact, the SARPS has already defined GLONASS signal specifications as an additional GNSS core system. Additionally, it has also defined SBAS to be able to augment GLONASS. According to the current specification, in fundamental, both GPS and GLONASS satellites can be augmented by the single SBAS simultaneuosly. For investigation of potential problems and possible solutions, the authors made an upgrade to the existing SBAS testbed developed by the ENRI so to be capable of augmenting both GPS and GLONASS simultaneously. The testbed runs with RINEX observations taken from GEONET, a nationwide observation network in Japan. The output SBAS message stream is tested compatibility with multi-constellation environment and evaluated for its performance using a single-frequency multi-constellation (SFMC) user-domain receiver software. The authors have confirmed that the performance of SBAS is improved by using an additional core satellite system, i.e., GLONASS, especially when satellite visibility is limited. In summary, the authors investigated potential problems and solutions on realizing a multi-constellation SBAS based on the current standards and confirmed them through their SBAS testbed upgraded to be capable of augmenting both GPS and GLONASS. The multi-constellation SBAS is tested and evaluated by using both single- and multi-constellation user-domain receiver software. It was confirmed that the multi-constellation SBAS has capability of providing improved availability.