Atmospheric ducting is a phenomenon where radio waves are bent and trapped between layers of the atmosphere with different refractive indices. This allows radio signals to propagate much further than normal line-of-sight distances, sometimes hundreds or even thousands of kilometers beyond the horizon.
AIS signals typically have a range of 20-40 nautical miles under normal conditions, limited by the curvature of the Earth. However, under ducting conditions, these signals can be received at much greater distances, which explains why some AIS stations can occasionally pick up ships that are far beyond their expected range.
Our ducting analysis uses the following atmospheric parameters from the Global Forecast System (GFS):
The analysis calculates the Modified Refractivity (M) profiles using these parameters, then identifies areas where the gradient dM/dh is negative, indicating potential ducting conditions.
The ducting analysis is updated every 6 hours when new GFS data becomes available. The last update time is shown in the status section at the top of the page.