Deployment of global navigation satellite system (GNSS) buoy system as an array in a wide ocean is expected as a powerful tool for monitoring tsunami and ocean bottom crustal deformation in the region [1]. IoT non-terrestrial network (NTN) [2] is crucial to realize the system since the buoys are deployed in the ocean. In the GNSS buoy array, although data size sent from each buoy is small, it requires to accommodate more than 10 buoys while the satellite link in available. In this case, grant-free access, which brings advantages of low latency and small signaling overhead, is expected. In this work, the use of uplink based on non-orthogonal multiple access (UL-NOMA) [3] is evaluated as grant-free access in the GNSS buoy array. We propose a method of generating a low-density signature of NOMA by buoy's location (latitude and longitude) for reducing the signaling overhead and latency.