National Ocean Service (NOS) Stations for Great Lakes. The National Water Level Observation Network is a key part of the NOAA Tsunami Warning System and the NOAA Storm Surge Warning System. NWLON stations support Physical Oceanographic Real-Time Systems (PORTS) in major ports and harbors. NWLON stations have standard configurations of water level sensors (including backup sensors), backup and primary data-collection platforms, solar panel power, GOES satellite radios, and telephone modems. Sensors are calibrated and vertically referenced to nearby networks of benchmarks. The data continuity, the vertical stability and careful referencing of NWLON stations have enabled the data to be used to estimate relative sea-level trends for the nation.