Plain English Meaning
MSAS is Japan's version of a system that fixes small errors in GPS signals so aircraft flying in that part of the world can rely on GPS for more accurate navigation, including instrument approaches.
Full Definition
A Japanese satellite-based augmentation system (SBAS) operated by Japan that improves the accuracy, integrity, and availability of GPS signals over Japan and surrounding regions. MSAS uses ground reference stations to monitor GPS satellites, calculate corrections, and broadcast those corrections to aircraft through the MTSAT geostationary satellite, allowing GPS to be used for more precise navigation and .