A GPS/GNSS RTK (Real-Time Kinematic) base station is a fixed GNSS receiver with a known position that provides correction data to one or more rover receivers to achieve high-precision positioning.
If you’re interested in setting up your own GNSS RTK base station, you can check out LOCOSYS RTK-M300 ( 4G-LTE ) system. They also offer a range of RTK receiver boards that can receive RTK corrections.