They have a GPS receiver in a fixed, known location. They measure the received signal and from the variations infer corrections for ionospheric effects. They are part of the GPS network.
No. These are the receivers from japan’s GEONET.[1]
“Geospatial Information Authority of Japan (GSI) operates GNSS CORSs that cover Japanese archipelago with over 1,300 stations at an average interval of about 20km for crustal deformation monitoring and GNSS surveys in Japan.” [2]
Basically the government of Japan pepered their country with GPS base stations and they let researchers use the data from them. This is just a novel use of that data.